© 2011 by jericho lynn moll. all rights reserved
TRANSCRIPT
© 2011 by Jericho Lynn Moll. All rights reserved.
SELF-SEALING OF THERMAL FATIGUE AND MECHANICAL DAMAGE IN FIBER-REINFORCED
COMPOSITE MATERIALS
BY
JERICHO L. MOLL
DISSERTATION
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering
in the Graduate College of the University of Illinois at Urbana-Champaign, 2011
Urbana, Illinois
Doctoral Committee:
Professor Nancy R. Sottos, Chair and Director of Research Professor Paul V. Braun Professor Jianjun Cheng Professor Scott R. White
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ABSTRACT
Fiber reinforced composite tanks provide a promising method of storage
for liquid oxygen and hydrogen for aerospace applications. The inherent thermal
fatigue of these vessels leads to the formation of microcracks, which allow gas
phase leakage across the tank walls. In this dissertation, self-healing
functionality is imparted to a structural composite to effectively seal microcracks
induced by both mechanical and thermal loading cycles. Two different
microencapsulated healing chemistries are investigated in woven glass
fiber/epoxy and uni-weave carbon fiber/epoxy composites.
Self-healing of mechanically induced damage was first studied in a room
temperature cured plain weave E-glass/epoxy composite with encapsulated
dicyclopentadiene (DCPD) monomer and wax protected Grubbs’ catalyst healing
components. A controlled amount of microcracking was introduced through
cyclic indentation of opposing surfaces of the composite. The resulting damage
zone was proportional to the indentation load. Healing was assessed through
the use of a pressure cell apparatus to detect nitrogen flow through the
thickness direction of the damaged composite. Successful healing resulted in a
perfect seal, with no measurable gas flow. The effect of DCPD microcapsule size
(51 µm and 18 µm) and concentration (0 - 12.2 wt%) on the self-sealing ability
was investigated. Composite specimens with 6.5 wt% 51 µm capsules sealed
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67% of the time, compared to 13% for the control panels without healing
components.
A thermally stable, dual microcapsule healing chemistry comprised of
silanol terminated poly(dimethyl siloxane) plus a crosslinking agent and a tin
catalyst was employed to allow higher composite processing temperatures. The
microcapsules were incorporated into a satin weave E-glass fiber/epoxy
composite processed at 120⁰C to yield a glass transition temperature of 127⁰C.
Self-sealing ability after mechanical damage was assessed for different
microcapsule sizees (25 µm and 42 µm) and concentrations (0 – 11 vol%).
Incorporating 9 vol% 42 μm capsules or 11 vol% 25 μm capsules into the
composite matrix leads to 100% of the samples sealing. The effect of
microcapsule concentration on the short beam strength, storage modulus, and
glass transition temperature of the composite specimens was also investigated.
The thermally stable tin catalyzed poly(dimethyl siloxane) healing
chemistry was then integrated into a [0/90]s uniweave carbon fiber/epoxy
composite. Thermal cycling (-196⁰C to 35⁰C) of these specimens lead to the
formation of microcracks, over time, formed a percolating crack network from
one side of the composite to the other, resulting in a gas permeable specimen.
Crack damage accumulation and sample permeability was monitored with
number of cycles for both self-healing and traditional non-healing composites.
Crack accumulation occurred at a similar rate for all sample types tested. A 63%
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increase in lifetime extension was achieved for the self-healing specimens over
traditional non-healing composites.
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ACKNOWLEDGEMENTS
I would like to acknowledge many people who have contributed their
time and efforts toward the work presented in this dissertation. Many thanks to
my advisor, Prof. Nancy Sottos for her guidance, advice and compassion as I
struggled through the last five years, and to my co-adviser, Prof. Scott White, for
his encouragement and optimism as well as advice especially in the area of
composites manufacturing. I’d also like to thank the additional members of my
committee, Prof. Paul Braun and Prof. Jianjun Cheng for their time, input and
suggestions. To both past and present AMS members, in particular Dr. Amit
Patel, Dr. Chris Mangun, Dr. Ben Blaiszik, Dr. Mike Keller, Amanda Jones,
Cassandra Kingsbury, Brett Beiermann, Sen Kang and Kevin Hart, I thank you for
your help, support and suggestions in lab. You all are my friends and co-workers
and my experience here at U of I would not have been the same without you.
Finally, I would also like to acknowledge the Air Force Office of Scientific
Research and NASA for funding.
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TABLE OF CONTENTS
LIST OF TABLES ...................................................................................................... viii LIST OF FIGURES ...................................................................................................... ix CHAPTER 1: INTRODUCTION ................................................................................... 1
1.1 Motivation ............................................................................................. 1 1.2 Self-healing Polymers ............................................................................ 3 1.3 Self-healing Composites ........................................................................ 5 1.4 Dissertation Overview ........................................................................... 8 References .................................................................................................. 9
CHAPTER 2: SELF-HEALING OF MECHANICAL DAMAGE IN A LOW TG FIBER-REINFORCED COMPOSITE.................................................................. 12
2.1 Introduction ........................................................................................ 12 2.2 Manufacturing .................................................................................... 12
2.2.1 Preparation of Sample Components ................................... 12 2.2.2 Sample Types ....................................................................... 16 2.2.3 Sample Preparation ............................................................. 17
2.3 Sample Testing .................................................................................... 18 2.4 Damage Area Calculation .................................................................... 21 2.5 Results ................................................................................................. 22
2.5.1 Sealing Performance ............................................................ 22 2.5.2 Damage Characterization .................................................... 25
2.6 Summary and Conclusions .................................................................. 30 References ................................................................................................ 31
CHAPTER 3: SELF-HEALING OF MECHANICAL DAMAGE IN A HIGH TEMPERATURE CURED STRUCTURAL COMPOSITE ..................................................... 32
3.1 Abstract ............................................................................................... 32 3.2 Introduction ........................................................................................ 32 3.3 Materials and Methods ....................................................................... 35
3.3.1 Microencapsulated Healing Agent Preparation .................. 35 3.3.2 Sample Types ....................................................................... 40 3.3.3 Sample Preparation ............................................................. 41 3.3.4 Mechanical Testing .............................................................. 43
3.4 Results ................................................................................................. 45 3.4.1 Sealing Performance ............................................................ 45 3.4.2 Effect of Microcapsule Concentration on Mechanical
Properties ............................................................................. 46
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3.5 Discussion ............................................................................................ 50 3.6 Conclusions ......................................................................................... 50 References ................................................................................................ 51
CHAPTER 4: SELF-HEALING OF THERMAL FATIGUE DAMAGE IN A CARBON FIBER-
REINFORCED COMPOSITE.................................................................. 53 4.1 Introduction ........................................................................................ 53 4.2 Experimental Methods and Procedure ............................................... 55
4.2.1. Microcapsule Preparation ................................................... 55 4.2.2 Layup and Sample Types ...................................................... 57 4.2.3 Coefficient of Thermal Expansion Measurement ................ 58 4.2.4 Cryogenic Cycling and Permeability Testing ........................ 59
4.3 Results and Discussion ........................................................................ 63 4.3.1 Microcrack Accumulation .................................................... 63 4.3.2 Coefficient of Thermal Expansion ........................................ 66 4.3.3 Sealing Performance ............................................................ 68
4.4 Conclusions ......................................................................................... 70 References ................................................................................................ 71
CHAPTER 5: SUMMARY AND FUTURE WORK ....................................................... 74 5.1 Summary ............................................................................................. 74 5.2 Future Work ........................................................................................ 75 References ................................................................................................ 78
AUTHOR’S BIOGRAPHY ......................................................................................... 79
viii
LIST OF TABLES
2.1 Summary of resin components for various sample types .............................. 16 2.2 Sealing summary for self-healing and control specimens .............................. 25 3.1 Summary of sample types and components .................................................. 41 4.1 Summary of sample types and components .................................................. 57
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LIST OF FIGURES
1.1 Carbon fiber/epoxy composite cryogenic tank ................................................ 2 1.2 Carbon fiber/epoxy composite with a crack initiated by thermal cycling ........ 2 1.3 Composite face sheet due to core delamination in a helicopter rotor blade .. 3 1.4 Schematic of microcapsule based self-healing ................................................. 4 1.5 Schematic of microcapsule based healing in a fiber-reinforced composite .... 6 1.6 Image of hollow glass fibers embedded in a carbon fiber composite .............. 7 2.1 Optical image of 51 µm diameter DCPD microcapsules ................................. 13 2.2 Flow chart of UF/DCPD microencapsulation method..................................... 14 2.3 Histograms of UF/DCPD microcapsules for stirring rates of a) 1000 rpm and b)
1800 rpm ......................................................................................................... 14 2.4 Histograms of a) Grubbs catalyst in wax spheres manufactured at a stir rate
of 2500 rpm and b) plain wax spheres also manufactured with a stir rate of 2500 rpm ......................................................................................................... 15
2.5 Optical image of wax protected Grubbs’ catalyst microspheres .................... 16 2.6 Schematic of side view of sample layup ......................................................... 17 2.7 Schematic of indention apparatus to induce controlled damage in composite
laminates ......................................................................................................... 18 2.8 Schematic of pressure cell setup .................................................................... 19 2.9 Pressure evolution for representative sealed and control samples ............... 20 2.10 Digital image of a type 2 control sample with manual tracing around the
damaged region ............................................................................................. 21 2.11 Comparison of type 1 control samples with self-healing samples containing
varying capsule sizes and concentrations: a) percentage of samples sealing completely, b) initial leakage rate in nonsealed samples, and c) sealing efficiency. All samples were damaged with a peak force of 410 N. Error bars represent one positive standard deviation. .................................................. 23
2.12 Composite damage area as a function of damage load, capsule size and capsule concentration. Error bars represent one standard deviation ......... 26
2.13 Comparison of crack damage in 6.5 wt%, 51 µm diameter capsule type 2 control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c). ............................. 27
x
2.14 Comparison of crack damage in 12.2 wt%, 18 µm diameter capsule type 2 control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c). ............................. 29
3.1 Dual microcapsule self-healing concept. a) Both types of microcapsules are dispersed into epoxy resin. b) A crack propagates in the resin, rupturing both types of microcapsules causing the encapsulated components to wick into the crack plane. c) Healing components mix and polymerize in the crack. The new polymerized material bonds the crack faces together, effectively healing the matrix. ....................................................................................................... 35
3.2 Flow chart of HOPDMS/PDES encapsulation procedure ................................ 36 3.3 Histograms of microcapsules used in this study: a) HOPDMS/PDES
microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 25 µm, b) HOPDMS/PDES microcapsules prepared at an agitation rate of 1500 rpm with an average diameter of 42 µm, c) hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 31 µm and d) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 41 µm.. .................. 37
3.4 Scanning electron microscope (SEM) images of a) 25 µm HOPDMS microcapsules prepared at an agitation rate of 2000 rpm and b) 41 µm DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm ………………………………………………………………………………………………….………………..38
3.5 Flow chart of DBTL/hexylacetate or hexylacetate only microencapsulation procedure. ....................................................................................................... 39
3.6 Flow chart of the rinsing procedure for DBTL/hexylacetate or hexylacetate only microcapsules.......................................................................................... 40
3.7 Schematic of composite layup components ................................................... 42 3.8 A polished cross-sectional image of an undamaged region of an 8H satin
weave composite sample. .............................................................................. 42 3.9 A polished tiled cross-sectional image of a control 2 sample with 11 vol% 25
μm capsules showing the percolating crack damage running from one side of the sample to the other. ................................................................................. 43
3.10 Summary of self-sealing results for composite samples with multiple sizes and concentrations of HOPDMS/PDES microcapsules dispersed in the matrix.. .......................................................................................................... 46
3.11 Composite short beam strength as a function of microcapsule concentration. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation. ....................................................................................... 47
3.12 Representative short beam shear loading curves for composite samples with varying microcapsule concentrations. Vc is capsule volume fraction in the composite ................................................................................................ 47
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3.13 Comparison of storage modulus values with expected model predictions. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation ............ 48
3.14 Changing glass transition temperature with microcapsule concentration in fiber reinforced composite and epoxy samples. Microcapsule volume fractions are reported in the resin only, not accounting for the inclusion of fiber reinforcement. Horizontal line represents the Tg of epoxy resin combined with HOPMDS monomer. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation ................................................................. 49
4.1 Histogram of microcapsules used in this study: a) HOPDMS/PDES microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 29 ± 15 µm, b) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 44 ± 16 µm .......... 56
4.2 Representative plot of the change in sample thickness with temperature curve................................................................................................................ 59
4.3 Schematic of thermal fatigue apparatus ........................................................ 60 4.4 Approximate temperature profiles through the thickness of composite
samples for a) heating to 35⁰C and b) cooling to -196⁰C ............................... 61 4.5 Illustration showing how nitrogen gas can escape through a thermally
damaged composite sample in an unmodified pressure cell ......................... 62 4.6 Schematic of the pressure cell apparatus. a) Sealed sample chamber with
sample and rubber o-rings. b) Complete test apparatus with the sealed sample chamber inserted in the pressure cell................................................ 63
4.7 Optical image of a polished cross-section of a control 1 sample with the plies and fiber orientations labeled ........................................................................ 64
4.8 Microcrack density with number of thermal cycles in a) plies 1 and 4 and b) plies 2 and 3 .................................................................................................... 64
4.9 Representative optical micrographs of thermally induced cracks. a) Microcrack in ply 4 in a control 1 sample after 110 thermal cycles. b) Microcrack in ply 2 and 3 in a control 2 sample after 310 thermal cycles. c) Delamination running between plies 3 and 4 in a control 2 sample after 310 thermal cycles ................................................................................................. 65
4.10 Comparison of coefficient of thermal expansion of epoxy resin with expected rule of mixtures values. Error bars represent one standard deviation ...................................................................................................... 67
4.11 Optical images of delamination running between plies after 2500 thermal cycles in a) control 1 and b) control 2 samples .............................................. 68
4.12 Representative output pressure, after 10 minutes of testing, of a control 1 sample with number of thermal cycles .......................................................... 69
4.13 Percentage of samples sealed with number of thermal cycles .................... 70
1
CHAPTER 1
INTRODUCTION
1.1 Motivation
Fiber reinforced polymer matrix composites are used in a wide variety of
applications due to their excellent strength to weight ratio and the ability of the
engineer to tailor specific mechanical properties into the composite. Unfortunately,
composite materials are susceptible to matrix cracks, as well as large-scale
delaminations that run between composite layers.
Matrix cracking is particularly problematic in composite cryogenic tanks. Figure
1.1 shows an image of a carbon fiber composite tank which is underdevelopment as a
fuel storage container in liquid oxygen and nitrogen for aerospace applications [1]. The
coefficient of thermal expansion mismatch between the carbon/epoxy layer leads to the
generation of high stresses during thermal cycling. These stresses initiate transverse ply
cracks, which can percolate, rendering the composite walls permeable.
Figure 1.2 shows a typical transverse ply crack due to thermal cycling of a
carbon/epoxy composite. Previous studies have investigated the leakage rate of
gaseous fuel through damaged composite tank walls. Peddiraju et al. predicted
numerically how the leakage rate of cryogens varies with composite ply number,
thickness and damage opening in the material [2]. Other studies measured the effect of
2
mechanical and thermal damage on the leakage rate of hydrogen gas through a
reinforced composite material [3]. Bechel et al. quantified how the density of thermally
induced cracks and resulting permeability increase with the number of thermal cycles
[4-6].
Figure 1.1 Carbon fiber/epoxy composite cryogenic tank.
Figure 1.2 Carbon fiber/epoxy composite with a crack initiated by thermal cycling.
3
Composite sandwich structures are used widely for aerospace applications such
as airplane rudders, ailerons and flaps. The face sheets of these sandwich structures are
susceptible to cracking due to mechanical fatigue or low velocity impact such as a tool
drop which can induce microcracking and increase water absorption into the core
material. Moisture absorption not only increases the overall weight of the composite
but can cause the face sheet/core bond to weaken leading to delamination (Figure 1.3)
and ultimately, premature composite failure [7].
Figure 1.3 Composite face sheet due to core delamination in a helicopter rotor blade [8].
1.2 Self-healing Polymers
The introduction of self-healing functionality provides a method of reversing the
damage in polymeric materials and improving the mechanical properties of the part [9].
Self-healing has been successfully demonstrated in thermosetting [10], fiber reinforced
composites [11-13], elastomers [14], coatings [15, 16], as well as self-healing adhesives
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[17]. A comprehensive review of self-healing was recently published by Blaiszik et al.
[18]. The basic concept for autonomic healing as originally proposed by White et al. is
shown schematically in Figure 1.4 [9]. A solid catalyst and an encapsulated monomer
are dispersed in a polymeric matrix material. Healing is triggered by crack damage
which causes the capsules to rupture. Monomer is wicked into the crack plane due to
capillary forces and contacts the embedded catalyst, initiating a polymerization
reaction. The new polymerized material bonds the crack faces together, effectively
healing the matrix.
Figure 1.4 Schematic of microcapsule based self-healing [9].
While most of the literature has focused on healing of mode I cracks, a few
recent investigation have demonstrated the ability to heal puncture damage and
prevent gas from flowing through the material. Beiermann et al. introduced
Microcapsule
Catalyst
Crack
Healing agent
Polymerized healing agent
5
encapsulated poly(dimethyl siloxane) (PDMS) based healing components into the PDMS
layer of a polyurethane/nylon/PDMS laminate [19]. Damage induced a
polycondensation reaction between the encapsulated healing components effectively
filling the puncture and regaining the ability to seal. Kalista et al. reported a self-healing
poly(ethylene-co-methacrylic acid) copolymer capable of autonomic molecular
rearrangement which was able to maintain pressure up to 3 MPa after projectile
puncture damage [20]. Self-healing is inherent in ionomers, which have a two-step melt
elastic recovery and inter chain diffusion self-healing mechanism.
1.3 Self-healing Composites
In fiber reinforced composites, the healing components (microcapsules and
catalyst) must be sequestered in the matrix rich regions of the material, shown
schematically in Figure 1.5 for woven fiber architecture. Kessler et al. incorporated
dicyclopentadiene (DCPD) microcapsules and solid particles of Grubbs’ catalyst into
graphite epoxy composites [11]. Using a width tapered double cantilevered beam
specimen geometry, they demonstrated a 38% recovery of interlaminar fracture
toughness after healing at room temperature, and a 66% recovery after healing at 80⁰C.
Patel et al. used a similar healing chemistry composed of encapsulated DCPD and wax
protected Grubbs’ catalyst. Wax protection was used to prevent deactivation of the
catalyst by amines present in the epoxy matrix [21, 22]. Patel reported a reduction in
crack length in self-healing samples and the ability to withstand greater impact energies
6
before a reduction in residual compressive strength was observed [23]. Moll et al.
demonstrated the ability to self-seal and prevent gas phase leakage across a composite
containing microencapsulated DCPD and wax protected Grubbs’ catalyst [24]. Using an
encapsulated epoxy resin and latent imidazole curing agent in the matrix, Yin et al.
demonstrated a recovery of interlaminar fracture toughness after the application of
heat (130 °C for 1 hour) [25, 26]. Although this was not autonomic self-healing, the
method did heal the cracks formed in the matrix material.
Figure 1.5 Schematic of microcapsule based healing in a fiber-reinforced composite [23].
Bond and co-workers developed an alternative healing strategy to recover
mechanical properties and visualize the damaged region of a composite [13, 27-37].
Hollow glass fiber reinforcement with an outer diameter of 60 µm was embedded into
glass and carbon fiber reinforced composites (Figure 1.6). These hollow glass fibers
7
were filled with either uncured epoxy resin or curing agent, which upon fracture, were
delivered to the damaged region of the sample [29, 30, 38]. Residual strengths of 87%
were reported after self-healing of impact damage. In an alternate application,
fluorescent dye was incorporated into the composite material with the healing
components. The dye wicked into the cracks in the material enabling visual assessment
of damage in the sample [13, 28].
Figure 1.6 Image of hollow glass fibers embedded in a carbon fiber composite [29].
Employing a vascularized approach, Bond et al. incorporated 1.5 mm PVC tubing
into the core of a composite sandwich structure [33, 36]. These tubes were used to
deliver healing agent to the damaged foam core and shown to recover the flexural and
compressive strength after impact of the sandwich structure composite. Bond and
Trask also developed a method to embed solder wires into advanced composites which
were later removed by the application of heat to create a network of channels for
healing agent delivery [31].
0.10 mm
8
1.4 Dissertation Overview
The work described in this dissertation focuses on the manufacture, testing and
sealing performance of self-healing fiber reinforced composite materials with low and
high glass transition temperatures. Self-healing of mechanical and thermal fatigue
damage is investigated along with the influence of microcapsules on the mechanical
properties of the composite material.
Chapter 2 describes the sealing ability of a self-healing low temperature cured (and
low Tg) glass fiber reinforced composite. In this dissertation, self-sealing is defined as
the ability of a material to heal a percolating crack network in such a way that gas
cannot pass through the sample.
This includes the manufacturing of the healing components and the composite
specimens along with the testing procedure and results. The influence of microcapsule
concentration on sealing results and damage area are also discussed.
In chapter 3, a fully cured high Tg self-healing composite is described. Included is
a description of the thermally stable healing chemistry, manufacturing of the
encapsulated components and composite samples along with the influence of
microcapsule concentration on the sealing results, Young’s modulus and interlaminar
properties of the specimens.
Chapter 4 describes the thermal fatigue damage of carbon fiber/epoxy
composites with microcapsules embedding in the matrix. The accumulation of crack
damage with number of thermal cycles along with the ability of these composites to
self-healing thermal damage is discussed.
9
Finally, in chapter 5 overall conclusions and results of this dissertation are
summarized and compiled. Suggestions for future work and directions are also
included.
References [1] R. Heydenreich, “Cryotanks in future vehicles,” Cryogenics, vol. 38, no. 1, pp. 125-130,
Jan, 1998. [2] P. Peddiraju, J. Noh, J. Whitcomb et al., “Prediction of cryogen leak rate through
damaged composite laminates,” Journal of Composite Materials, vol. 41, no. 1, pp. 41-71, Jan, 2007.
[3] R. Grenoble, and T. Gates, “Hydrogen Gas Leak Rate Testing and Post-Testing Assessment of Microcrack Damaged Composite Laminates,” in AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Newport, Rhode Island, 2006.
[4] V. T. Bechel, M. B. Fredin, S. L. Donaldson et al., “Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 7, pp. 663-672, 2003.
[5] V. T. Bechel, J. D. Camping, and R. Y. Kim, “Cryogenic/elevated temperature cycling induced leakage paths in PMCs,” Composites Part B-Engineering, vol. 36, no. 2, pp. 171-182, 2005.
[6] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.
[7] H. S. Choi, and Y. H. Jang, “Bondline strength evaluation of cocure/precured honeycomb sandwich structures under aircraft hygro and repair environments,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 9, pp. 1138-1147, Sep, 2010.
[8] Airwolf_Aerospace. March 3, 2011, 2011; www.airwolfaerospace.com. [9] S. R. White, M. M. Caruso, and J. S. Moore, “Autonomic healing of polymers,” Mrs
Bulletin, vol. 33, no. 8, pp. 766-769, Aug, 2008. [10] E. N. Brown, N. R. Sottos, and S. R. White, “Fracture testing of a self-healing polymer
composite,” Experimental Mechanics, vol. 42, no. 4, pp. 372-379, Dec, 2002. [11] M. R. Kessler, and S. R. White, “Self-activated healing of delamination damage in woven
composites,” Composites Part a-Applied Science and Manufacturing, vol. 32, no. 5, pp. 683-699, 2001.
[12] M. R. Kessler, N. R. Sottos, and S. R. White, “Self-healing structural composite materials,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 8, pp. 743-753, 2003.
[13] J. W. C. Pang, and I. P. Bond, “'Bleeding composites' - damage detection and self-repair using a biomimetic approach,” Composites Part a-Applied Science and Manufacturing, vol. 36, no. 2, pp. 183-188, 2005.
[14] M. W. Keller, S. R. White, and N. R. Sottos, “A self-healing poly(dimethyl siloxane) elastomer,” Advanced Functional Materials, vol. 17, no. 14, pp. 2399-2404, Sep, 2007.
[15] S. H. Cho, S. R. White, and P. V. Braun, “Self-Healing Polymer Coatings,” Advanced Materials, vol. 21, no. 6, pp. 645-+, Feb, 2009.
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[16] K. S. Toohey, N. R. Sottos, and S. R. White, “Characterization of Microvascular-Based Self-healing Coatings,” Experimental Mechanics, vol. 49, no. 5, pp. 707-717, Oct, 2009.
[17] G. Miller, “Self-Healing Adhesive Film for Composite Laminate Repairs on Metallic Structures,” Aerospace Engineering, University of Illinois, Urbana-Champaign, Illinois, 2007.
[18] B. J. Blaiszik, S. L. B. Kramer, S. C. Olugebefola et al., "Self-Healing Polymers and Composites," Annual Review of Materials Research, Vol 40, Annual Review of Materials Research, pp. 179-211, 2010.
[19] B. A. Beiermann, M. W. Keller, and N. R. Sottos, “Self-healing flexible laminates for resealing of puncture damage,” Smart Materials & Structures, vol. 18, no. 8, Aug, 2009.
[20] S. J. Kalista, “Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture,” Mechanics of Advanced Materials and Structures, vol. 14, no. 5, pp. 391-397, 2007.
[21] J. D. Rule, E. N. Brown, N. R. Sottos et al., “Wax-protected catalyst microspheres for efficient self-healing materials,” Advanced Materials, vol. 17, no. 2, pp. 205-+, Jan, 2005.
[22] G. O. Wilson, J. S. Moore, S. R. White et al., “Autonomic healing of epoxy vinyl esters via ring opening metathesis polymerization,” Advanced Functional Materials, vol. 18, no. 1, pp. 44-52, Jan, 2008.
[23] A. J. Patel, N. R. Sottos, E. D. Wetzel et al., “Autonomic healing of low-velocity impact damage in fiber-reinforced composites,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 3, pp. 360-368, Mar, 2010.
[24] J. L. Moll, S. R. White, and N. R. Sottos, “A Self-sealing Fiber-reinforced Composite,” Journal of Composite Materials, vol. 44, no. 22, pp. 2573-2585, Oct, 2010.
[25] T. Yin, M. Z. Rong, J. S. Wu et al., “Healing of impact damage in woven glass fabric reinforced epoxy composites,” Composites Part a-Applied Science and Manufacturing, vol. 39, no. 9, pp. 1479-1487, Sep, 2008.
[26] T. Yin, L. Zhou, M. Z. Rong et al., “Self-healing woven glass fabric/epoxy composites with the healant consisting of micro-encapsulated epoxy and latent curing agent,” Smart Materials & Structures, vol. 17, no. 1, Feb, 2008.
[27] C. Y. Huang, R. S. Trask, and I. P. Bond, “Characterization and analysis of carbon fibre-reinforced polymer composite laminates with embedded circular vasculature,” Journal of the Royal Society Interface, vol. 7, no. 49, pp. 1229-1241, Aug, 2010.
[28] J. W. C. Pang, and I. P. Bond, “A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility,” Composites Science and Technology, vol. 65, no. 11-12, pp. 1791-1799, Sep, 2005.
[29] R. S. Trask, G. J. Williams, and I. P. Bond, “Bioinspired self-healing of advanced composite structures using hollow glass fibres,” Journal of the Royal Society Interface, vol. 4, no. 13, pp. 363-371, Apr, 2007.
[30] R. S. Trask, H. Rwilliams, and I. P. Bond, “Self-healing polymer composites: mimicking nature to enhance performance,” Bioinspiration & Biomimetics, vol. 2, no. 1, pp. P1-P9, Mar, 2007.
[31] R. S. Trask, and I. P. Bond, “Bioinspired engineering study of Plantae vascules for self-healing composite structures,” Journal of the Royal Society Interface, vol. 7, no. 47, pp. 921-931, Jun, 2010.
[32] G. Williams, R. Trask, and I. Bond, “A self-healing carbon fibre reinforced polymer for aerospace applications,” Composites Part a-Applied Science and Manufacturing, vol. 38, no. 6, pp. 1525-1532, 2007.
11
[33] H. R. Williams, R. S. Trask, and I. P. Bond, “Self-healing composite sandwich structures,” Smart Materials & Structures, vol. 16, no. 4, pp. 1198-1207, Aug, 2007.
[34] H. R. Williams, R. S. Trask, A. C. Knights et al., “Biomimetic reliability strategies for self-healing vascular networks in engineering materials,” Journal of the Royal Society Interface, vol. 5, no. 24, pp. 735-747, Jul, 2008.
[35] H. R. Williams, R. S. Trask, P. M. Weaver et al., “Minimum mass vascular networks in multifunctional materials,” Journal of the Royal Society Interface, vol. 5, no. 18, pp. 55-65, Jan, 2008.
[36] H. R. Williams, R. S. Trask, and I. P. Bond, “Self-healing sandwich panels: Restoration of compressive strength after impact,” Composites Science and Technology, vol. 68, no. 15-16, pp. 3171-3177, Dec, 2008.
[37] G. J. Williams, I. P. Bond, and R. S. Trask, “Compression after impact assessment of self-healing CFRP,” Composites Part a-Applied Science and Manufacturing, vol. 40, no. 9, pp. 1399-1406, Sep, 2009.
[38] R. S. Trask, and I. P. Bond, “Biomimetic self-healing of advanced composite structures using hollow glass fibres,” Smart Materials & Structures, vol. 15, no. 3, pp. 704-710, Jun, 2006.
12
CHAPTER 2
SELF-HEALING OF MECHANICAL DAMAGE IN A LOW TG FIBER-REINFORCED COMPOSITE
2.1 Introduction
In this chapter, we describe a self-healing plain weave E-glass epoxy composite with the
healing components, microencapsulated dicyclopentadiene (DCPD), and paraffin wax coated
Grubbs’ catalyst dispersed throughout the matrix. Healing is assessed through use of a pressure
cell apparatus to detect nitrogen flow through the thickness direction of a damaged composite.
Successful healing resulted in a perfect seal, with no measureable gas flow. A controlled
amount of microcracking is introduced through cyclic indentation of opposing surfaces of the
sample. The resulting damage zone is proportional to the indentation load. We investigate the
effect of DCPD microcapsule size and concentration on the self-sealing ability of plain weave E-
glass epoxy composites.
2.2 Manufacturing
2.2.1 Preparation of Sample Components
The healing agent, DCPD (Alpha Aesar) monomer was distilled, stabilized with
150 ppm p-t-butylcatechol and encapsulated in a urea-formaldehyde (UF) shell (Figure
13
2.1). Figure 2.2 contains a diagram of the encapsulation procedure. Sealing
performance was investigated for two microcapsule sizes which were varied by
controlling the stir rate. The resulting histograms of microcapsule size distribution are
shown in Figure 2.3. The larger capsules, 51 µm in diameter, were manufactured by the
oil in water emulsion in situ polymerization technique described by Brown et al. [1]. The
smaller capsules, 18 µm in diameter, were manufactured in a similar fashion with a few
procedural modifications. The amount of ethylene-maleic anhydride (EMA) was
increased by 50% from the amount specified by Brown et al. [1]. The additional amount
of stabilizing agent was needed because the surface area of the DCPD/water interface in
the emulsion increases with decreasing droplet size. After completion of the
encapsulation procedure, the excess EMA was eliminated by centrifuging the capsule
solution in deionized water three times, decanting off the water after each cycle. The
capsules were spray dried into a dry, free flowing white powder.
Figure 2.1 Optical image of 51 µm diameter DCPD microcapsules.
14
Figure 2.2 Flow chart of UF/DCPD microencapsulation method.
Figure 2.3 Histograms of UF/DCPD microcapsules for stirring rates of a) 1000 rpm and b)
1800 rpm.
Grubbs’ catalyst in wax microspheres and plain wax microspheres were
manufactured in a method previously outlined by Rule et al. [2]. A histogram showing
15
the size distribution of both of these components is in Figure 2.4. As received first
generation Grubbs’ catalyst (Aldrich) was dissolved in benzene in a method previously
outlined by Jones et al. [3] and freeze-dried and protected from deactivation by primary
amines [4] in the epoxy matrix curing agent by paraffin wax. The 11 µm diameter
Grubbs’ catalyst, shown in Figure 2.5, and wax spheres were centrifuged three times in
water to remove EMA, the emulsion stabilizing agent. The resulting solution was frozen
in liquid nitrogen and freeze dried for approximately 48 h, resulting in a dry powder.
Catalyst-free wax microspheres with an average diameter of 26 µm, were also produced
by this protocol for control samples.
Figure 2.4 Histograms of a) Grubbs catalyst in wax spheres manufactured at a stir rate of
2500 rpm and b) plain wax spheres also manufactured with a stir rate of 2500 rpm.
16
Figure 2.5 Optical image of wax protected Grubbs’ catalyst microspheres.
2.2.2 Sample Types
Self-healing composite panels were fabricated along with two types of
corresponding control panels. The components in each of these sample types are
summarized in Table 2.1. Self-healing samples consisted of epoxy resin, fiber
reinforcement, microcapsules, and wax protected catalyst microspheres. The first type
of control specimens were made of epoxy resin and fiber reinforcement only and
contained no self-healing components (microcapsules or wax microspheres). The second
type of control specimens were comprised of epoxy resin, fiber reinforcement,
microcapsules, and wax microspheres without catalyst.
Table 2.1 Summary of resin components for various sample types.
Sample type Epoxy Resin
E-glass reinforcement
Wax protected catalyst
Wax micro-
spheres
UF/DCPD micro-
capsules
Self-healing X X X - X Control 1 X X - - - Control 2 X X - X X
17
2.2.3 Sample Preparation
Composite panels were manufactured using a hand lay-up and compression
molding technique similar to the method outlined by Kessler et al. [5]. A side view of
the layup schematic can be seen in Figure 2.6. The composite reinforcement consisted
of four 150 mm by 200 mm plain weave E-glass (340 g/m2) plies. We selected Epon 862
epoxy resin with Epikure 3274 curing agent mixed at a ratio of 100:40 as the matrix. The
dry capsules and catalyst spheres were gently mixed into the resin followed by
degassing under vacuum for at least twenty minutes. The layup was compacted under
170 kPa at 35 ⁰C for 72 h, creating samples with an average fiber volume fraction of
0.35. Self-healing and control (type 2) samples were prepared with varying capsule
concentrations.
Figure 2.6 Schematic of side view of sample layup.
18
2.3 Sample Testing
Each composite panel was sectioned into twelve 45 mm by 45 mm samples. We
developed an indentation protocol to induce consistent amounts of damage in the
composite samples. A schematic of the indentation apparatus is shown in Figure 2.7.
The square composite specimens were simply supported on a ring with an inner
diameter of 24 mm and a height of 4 mm. The samples were damaged by driving a
Vicker’s diamond indenter tip into both sides of the laminate with a predetermined
velocity and load. The indenter tip was positioned above the middle of the sample and
was cyclically driven into the surface of the specimen ten times on each side. All
samples, unless otherwise noted, were damaged with a maximum load of 410 N at a
velocity of 150 µm/s. After the damage cycle, the samples were allowed to heal
overnight at 30 ⁰C before testing continued.
Figure 2.7 Schematic of indention apparatus to induce controlled damage in composite
laminates.
19
Self-sealing ability was evaluated using a pressure cell apparatus to quantify
nitrogen flow through the damaged composites. The pressure cell (Figure 2.8) was
designed for prior experiments by Beiermann et al. to study healing of puncture damage
in PDMS laminates [6]. In the cell, the sample served as a barrier between a pressurized
chamber and the atmosphere. Pressure on both sides of the sample was monitored
with time during the experiment.
Figure 2.8 Schematic of pressure cell setup [7].
Laminates were placed in the sample chamber and securely clamped between
two O-rings to eliminate leakage from the cell. A computer controlled regulator (EP211-
X60-5V, Omega) was used to ramp the applied pressure to the cell to 276 kPa. Pressure
transducers were placed on either side of the sample. The first transducer measured
the input pressure applied to the sample by the regulator. The second transducer
measured the output pressure on the opposite side due to nitrogen flow through the
sample. If no output pressure was detected then the specimen was fully healed or there
20
was insufficient damage to create an interpenetrating crack network that allowed
nitrogen to flow through the specimen. Hence, healing of damage was assessed by the
change in pressure recorded by the second transducer. The input pressure and the time
evolution of the output pressure measured for representative control type 1 and type 2
samples along with a fully sealed self-healing sample are plotted in Figure 2.9.
Figure 2.9 Pressure evolution for representative sealed and control samples.
Three different criteria were used to evaluate the self-sealing ability of
composite specimens, the percentage of fully healed samples, the initial slope, and the
sealing efficiency (ηs). A sample was considered fully healed if the output pressure did
not increase by more than 0.07 kPa after thirty minutes of testing. In addition, for
samples that leaked we characterized the leakage rate by calculating the initial slope of
the pressure evolution curve. A large slope corresponded to a high gas flow rate, while
a small slope corresponded to a sample with low gas flow. A sealing efficiency was also
calculated for all samples after thirty minutes of testing,
21
30min
( )1
( )
outS
in t
P t
P t
(2.1)
where pressure readings are taken after thirty minutes. A sealing efficiency of zero
corresponded to a final output pressure at thirty minutes equal to the input pressure
(no healing), while a fully healed sample yielded an efficiency of unity.
2.4 Damage Area Calculation
In both types of composite control samples, the opaque damage area was easily
identified. As shown in Figure 2.10, the damaged region was manually traced using
photo editing software. The damage area was then averaged over all samples of a given
type. In self-healing samples it was not possible to accurately identify the damage area
using this method due to the dark purple color of the Grubbs’ catalyst embedded in the
specimens.
Figure 2.10 Digital image of a type 2 control sample with manual tracing around the
damaged region [7].
22
2.5 Results
2.5.1 Sealing Performance
The effects of capsule size and concentration on self-sealing results were
investigated for 51 µm and 18 µm diameter capsules at concentrations of 12.2 wt%, 6.5
wt% and 2.7 wt% in resin with wax protected catalyst at concentrations of 2.4 wt%, 2.6
wt% and 2.7 wt% respectively. The total amount of wax protected catalyst was kept
constant, but the overall concentration changed due to differences in the mass of
microcapsules added. Figure 2.11 compares the sealing performance of self-healing
specimens with varying microcapsule concentrations and diameters to that of the type 1
control samples (no microcapsules or catalyst). All samples in this data set (see Table
2.2) were damaged at a maximum load of 410 N. The 12.2 wt% and the 6.5 wt%, 51 µm
diameter capsule self-healing samples performed better than the type 1 control samples
in all three of the sealing evaluation categories. The 6.5 wt% specimens with 51 µm
diameter capsules had the highest percentage sealed, the lowest leakage rate and the
highest sealing efficiency. In contrast, none of the 6.5 wt% specimens with 18 µm
diameter capsules fully sealed, although their leakage rate was significantly lower and
sealing efficiency significantly higher than type 1 control samples.
23
Figure 2.11 Comparison of type 1 control samples with self-healing samples containing
varying capsule sizes and concentrations: a) percentage of samples sealing completely, b) initial leakage rate in nonsealed samples, and c) sealing efficiency. All samples were damaged with a peak force of 410 N. Error bars represent one positive standard deviation [7].
24
Specimens with 2.7 wt% 51 µm diameter capsules and both concentrations of 18
µm diameter capsules have a low healing percentage, but a lower leakage rate and
higher sealing efficiency than type 1 controls. The sealing data for these specimens
suggests that insufficient amounts of healing agent are delivered to the damaged region
for full sealing of crack networks.
The sealing performance of type 2 control specimens was also investigated and
compared to corresponding self-healing specimens with various capsule concentrations
for the 51 µm diameter specimens. For all three capsule concentrations investigated,
the self-healing samples performed better than the respective type 2 control samples.
The effect of peak damage load on the sealing results was also investigated by
damaging 6.5 wt% 51 µm capsule type 2 control and self-healing samples at a lower
peak load of 380 N (instead of the standard 410 N used for all other samples). A
summary of the sealing results for these samples is provided within Table 2.2. All of the
self-healing samples sealed fully in these tests (100%), compared to 25% for the control
samples. This result indicates that there is a maximum peak load below which 100%
sealing can be achieved for a given capsule size and concentration.
25
Table 2.2 Sealing summary for self-healing and control specimens.
Sample Type
Capsule Diameter
(m)
Capsule Concentration (wt% in resin)
Peak Load (N)
Percent Healed
(%)
Leakage Rate
(kPa/min)
Sealing Efficiency,
s
Self-healing 51 12.2 410 58 9.1 0.81 Self-healing 51 6.5 410 67 9.2 0.47 Self-healing 51 2.7 410 17 50.9 0.52
Control-2 51 12.2 410 8 17.6 0.43 Control-2 51 6.5 410 0 83.1 0.14 Control-2 51 2.7 410 8 87.6 0.31
Self-healing 18 12.2 410 8 21.4 0.40 Self-healing 18 6.5 410 0 15.1 0.42
Control-1 N/A 0 410 13 111.7 0.21 Self-healing 51 6.5 380 100 N/A 1.00
Control-2 51 6.5 380 25 12.2 0.77
2.5.2 Damage Characterization
To further understand the relationship between capsule size, concentration and
sealing performance, we characterized the area of the damage zone in both types of
control samples and compared the crack density in cross-sections of both healed and
type 2 control specimens. The damage zone area of the control samples was analyzed
from digital specimen images acquired after indentation (Figure 2.10). The damage
area, plotted in Figure 2.12, increased with increasing capsule concentration and size.
Although increasing the capsule concentration increases the amount of DCPD delivered
to the crack, it also increases the amount of crack damage, suggesting that an optimum
capsule concentration exists for a given amount of damage. For the self-healing
samples with 51 µm diameter capsules, the best sealing performance was achieved with
6.5 wt% capsule specimens. The higher capsule concentration (12.5 wt%) specimens led
to a larger amount of damage which could not be filled as effectively with capsules of
26
this size. The lower capsule concentration (2.7 wt%) specimens have less damage, but
insufficient amounts of healing agent for complete sealing. Similarly, samples with 18
µm capsules had less damage area than the 51 µm capsule samples, but the percentage
of samples which sealed was substantially lower. Thus, smaller capsules do not deliver a
sufficient amount of DCPD to seal the damage.
Figure 2.12 Composite damage area as a function of damage load, capsule size and
capsule concentration. Error bars represent one standard deviation [7].
The crack network was evaluated by comparing polished cross-sections of
damaged regions in the composite specimens. Tiled scanning electron microscope
images are shown in Figures 2.13 and 2.14 for type 2 control and self-healing samples
with 6.5 wt% 51 µm capsules and 12.2 wt% 18 µm capsules, respectively. The cracks
have been manually highlighted in white within the image of the type 2 control sample
in a) and the self-healing sample in c). Only the highlighted cracks are shown for the
control sample in b) and the self-healing sample in d). Although cracks are still present
27
in the self-healing sample, enough of the cracks have healed to disrupt the percolation
of crack damage, thus successfully sealing the sample.
Figure 2.13 Comparison of crack damage in 6.5 wt%, 51 µm diameter capsule type 2
control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c) [7].
28
As revealed in Figures 2.13 and 2.14, cracks in the composite samples run
predominately between tows through resin rich regions in the composite, rupturing
microcapsules in this area. The region directly under the indenter tip has the largest
crack separation, which decreases radially. The self-healing cross-sections (Figure 2.13
c, d and Figure 2.14 c, d) reveal a noticeable reduction in the number of cracks and a
smaller crack separation when compared to the type 2 control samples (Figure 2.13 a, b
and Figure 2.14 a, b). Cracks with smaller separation are more easily healed due to the
smaller volume of DCPD needed to fully seal the damage.
29
Figure 2.14 Comparison of crack damage in 12.2 wt%, 18 µm diameter capsule type 2
control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c) [7].
30
2.6 Summary and Conclusions
A self-healing fiber reinforced polymer matrix composite was achieved by
incorporating urea-formaldehyde encapsulated dicyclopentadiene monomer and wax
protected Grubbs’ catalyst into the matrix of a glass fiber reinforced epoxy composite
using a wet lay-up technique. The specimens were damaged by repeated indentation at
prescribed loads. The connectivity of cracks in the damaged composite was evaluated
by applying pressurized nitrogen gas to one side of the sample and monitoring the
output pressure on the opposite side of the specimen.
Protocols were established to evaluate the self-sealing capacity of these
composite specimens by monitoring the percentage of samples that fully healed, initial
leakage rate, and the sealing efficiency. The effects of capsule size and concentration on
the self-sealing performance of the samples were evaluated. Specimens with 6.5 wt%
51 µm capsules performed the best in all three sealing assessment categories.
Although the addition of microcapsules increases the fracture toughness of the
matrix [8], increasing capsule concentration and size leads to an increase in damage
area in composites with both microcapsules and paraffin wax microspheres. The
optimum capsule concentration for a given capsule size depends on the size of the
damage volume compared to the volume of healing agent delivered.
Normal composite samples (type 1 controls) were tested and compared to self-
healing samples. Specimens with 12.2 wt% and 6.5 wt% 51 µm capsules outperformed
the normal composites in all three sealing categories. All self-healing samples,
regardless of capsule size or concentration, show lower leakage rate and higher sealing
31
efficiency compared to normal composites. Hence, optimized self-healing composites
show great potential to seal non-catastrophic crack damage in fiber reinforced
composite materials.
References [1] E. N. Brown, M. R. Kessler, N. R. Sottos et al., “In situ poly(urea-formaldehyde)
microencapsulation of dicyclopentadiene,” Journal of Microencapsulation, vol. 20, no. 6, pp. 719-730, Nov-Dec, 2003.
[2] J. D. Rule, E. N. Brown, N. R. Sottos et al., “Wax-protected catalyst microspheres for efficient self-healing materials,” Advanced Materials, vol. 17, no. 2, pp. 205-+, Jan, 2005.
[3] A. S. Jones, J. D. Rule, J. S. Moore et al., “Catalyst morphology and dissolution kinetics of self-healing polymers,” Chemistry of Materials, vol. 18, no. 5, pp. 1312-1317, Mar, 2006.
[4] G. O. Wilson, J. S. Moore, S. R. White et al., “Autonomic healing of epoxy vinyl esters via ring opening metathesis polymerization,” Advanced Functional Materials, vol. 18, no. 1, pp. 44-52, Jan, 2008.
[5] M. R. Kessler, and S. R. White, “Self-activated healing of delamination damage in woven composites,” Composites Part a-Applied Science and Manufacturing, vol. 32, no. 5, pp. 683-699, 2001.
[6] B. A. Beiermann, M. W. Keller, and N. R. Sottos, “Self-healing flexible laminates for resealing of puncture damage,” Smart Materials & Structures, vol. 18, no. 8, Aug, 2009.
[7] J. L. Moll, S. R. White, and N. R. Sottos, “A Self-sealing Fiber-reinforced Composite,” Journal of Composite Materials, vol. 44, no. 22, pp. 2573-2585, Oct, 2010.
[8] E. N. Brown, N. R. Sottos, and S. R. White, “Fracture testing of a self-healing polymer composite,” Experimental Mechanics, vol. 42, no. 4, pp. 372-379, Dec, 2002.
32
CHAPTER 3
SELF-HEALING OF MECHANICAL DAMAGE IN A HIGH TEMPERATURE CURED STRUCTURAL COMPOSITE
3.1 Abstract
A two capsule healing chemistry, shown to be stable to 150⁰C is used to self-heal
crack damage in a fiber-reinforced composite with a Tg of 127⁰C. The healing system is
comprised of silanol terminated poly(dimethyl siloxane) plus a crosslinking agent and a
tin catalyst. Sealing of mechanical damage is assessed through the use of a pressure cell
apparatus to detect nitrogen flow through a damaged composite. Successful healing
results in a perfect seal, with no measurable gas flow. The effect of microcapsule size
and concentration on self-sealing ability and the effect of microcapsule concentration
on the short beam strength, storage modulus and Tg were investigated.
3.2 Introduction
Microcracking in advanced composites leads to a reduction in stiffness and an
increase in permeability. In cryogenic tanks, microcracks form during thermal cycling
due to the coefficient of thermal expansion mismatch between the fibers and the
33
matrix. When a sufficient density of microcracks form a percolating network of cracks
through the thickness of the composite, cryogens begin to leak through the tank wall
[1]. In sandwich structures mechanical fatigue or low velocity impact can also induce
microcracking and an increase in water absorption into the core material, which not
only increases the overall weight of the composite, but can also cause delaminations
between the face sheet and core [2].
Stacking sequence and fiber architecture affect the permeability of composites
during and after thermal cycling due to the development of thermal stresses in the plies
[3-5]. In an effort to minimize leakage caused by cryogenic cycling and low velocity
impacts, McVay et al. embedded flexible barrier layers within a composite and
demonstrated a 290% improvement over composites without the barrier layers [6].
Beiermann et al. introduced encapsulated poly(dimethyl siloxane) (PDMS) based healing
components into the PDMS layer of a polyurethane/nylon/PDMS laminate [7]. Damage
induced a polycondensation reaction between the encapsulated healing components
effectively filling the puncture and regaining the ability to seal. Kalista et al. reported a
self-healing poly(ethylene-co-methacrylic acid) copolymer capable of autonomic
molecular rearrangement which was able to maintain pressure up to 3 MPa after
projectile puncture damage [8]. Self-healing is inherent in ionomers, which have a two-
step melt elastic recovery and inter chain diffusion self-healing mechanism. Moll et al.
demonstrated the ability to self-heal 67% of samples and prevent gas phase leakage
across a composite containing microencapsulated DCPD and wax protected Grubbs’
catalyst [9]. Due to limitations of thermal stability of the healing chemistry, these
34
composites could not be processed at elevated temperature and their final glass
transition temperature was approximately 60⁰C.
Here we present a thermally stable dual microencapsulated self-healing
chemistry which has previously been used as a healing agent in epoxy and bulk PDMS [7,
10, 11]. The dual microcapsule self-healing concept is shown in Figure 3.1. Both types
of microcapsules are dispersed in the epoxy matrix (a) and are ruptured upon crack
damage (b) triggering the release of healing agents into the crack plane by capillary
forces. Once the healing agents mix by diffusion, polymerization in the crack plane (c)
bonds the crack faces together, effectively healing the matrix. Capsule A contains a
poly(dimethyl siloxane) monomer and cross-linker while capsule B contains a tin
catalyst. Since this healing chemistry is stable until 150⁰C, the composite is cured at
121⁰C for 8 hours, resulting in a composite glass transition temperature (Tg) of
approximately 127⁰C.
35
Figure 3.1 Dual microcapsule self-healing concept. a) Both types of microcapsules are dispersed into epoxy resin. b) A crack propagates in the resin, rupturing both types of microcapsules causing the encapsulated components to wick into the crack plane. c) Healing components mix and polymerize in the crack. The new polymerized material bonds the crack faces together, effectively healing the matrix.
3.3 Materials and Methods
3.3.1 Microencapsulated Healing Agent Preparation
A thermally stable healing chemistry was achieved by the encapsulation of a
PDMS monomer and a catalyst in two separate types of microcapsules. Type A
microcapsules were comprised of a mixture of 53 wt% hydroxyl end-functionalized
polydimethylsiloxane (HOPDMS) with the balance polydiethoxysiloxane (PDES)
encapsulated in a urea-formaldehyde shell. Type B microcapsules contained a 50:50
36
mixture of dibutyltin dilaurate (DBTL) catalyst from Gelest and hexylacetate from Sigma
Aldrich, which acts as a carrier solvent, encapsulated in a polyurethane shell.
Urea-formaldehyde microcapsules containing HOPDMS and PDES were
manufactured using an emulsion in situ polymerization method identical to the one
described by Mangun et al. [10] for S21 HOPDMS. A flow chart outlining the
manufacturing procedure is shown in Figure 3.2. Microcapsules with average diameters
of 25 ± 10 μm (2000 rpm) and 42 ± 20 μm (1500 rpm), were produced by varying the
agitation rate. Representative histograms for each agitation rate are provided in Figure
3.3. Figure 3.4 a) shows a scanning electron microscope image of HOPDMS/PDES
microcapsules manufactured at 2000 rpm.
Figure 3.2 Flow chart of HOPDMS/PDES encapsulation procedure.
37
Figure 3.3 Histograms of microcapsules used in this study: a) HOPDMS/PDES
microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 25 µm, b) HOPDMS/PDES microcapsules prepared at an agitation rate of 1500 rpm with an average diameter of 42 µm, c) hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 31 µm and d) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 41 µm.
38
Figure 3.4 Scanning electron microscope (SEM) images of a) 25 µm HOPDMS
microcapsules prepared at an agitation rate of 2000 rpm and b) 41 µm DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm.
Polyurethane microcapsules containing only hexylacetate (Figure 3.4 b) or a
50:50 mixture of hexylacetate and DBTL were manufactured using an oil in water
emulsion interfacial polymerization similar to the one described by Mangun et al. [10]
with a few notable exceptions. A flow chart outlining the encapsulation procedure is
shown in Figure 3.5. In a 600 mL beaker 2.5 g gum arabic (Sigma-Aldrich) was dissolved
in 100 mL of deionized water under agitation at 1100 rpm. In a separate container 17 g
hexylacetate or 10 g hexylacetate and 10 g DBTL were combined along with 2.5 g
Desmodur L75 polyurethane prepolymer (Bayer) and added to the gum arabic solution
at 85⁰C. This solution was mixed for 1 hour before the agitation rate was decreased to
200 rpm and the temperature was adjusted to room temperature for 1 hour. Next, 50
mL of deionized water and 25 mL of 2.5wt% ethylene maleic anhydride solution
(Zeeland Chemical) were added to the mixture while the agitation rate was kept
constant and the temperature was adjusted to 55⁰C for 4 hours. The procedure for
rinsing the microcapsules is outlined in Figure 3.6. Excess surfactant was eliminated
39
from the aqueous capsule solution by centrifuging four times for 30 minutes at 1000
rpm, discarding the supernatant and replacing with deionized water after each
centrifugation cycle. Finally, the solution was frozen in liquid nitrogen and placed on a
freeze drier to sublime off the water resulting in discrete, free flowing microcapsules.
Figure 3.5 Flow chart of DBTL/hexylacetate or hexylacetate only microencapsulation procedure.
40
Figure 3.6 Flow chart of the rinsing procedure for DBTL/hexylacetate or hexylacetate only microcapsules.
3.3.2 Sample Types
Three types of samples were fabricated for this study, the components of each
are summarized in Table 3.1. Control 1 samples were comprised of epoxy resin and E-
glass reinforcement (no microcapsules). Control 2 samples contained not only the
epoxy resin and E-glass reinforcement, but also HOPDMS/PDES microcapsules and
hexylacetate microcapsules. Finally self-healing samples were composed of epoxy resin,
E-glass reinforcement, HOPDMS/PDES microcapsules and DBTL/hexylacetate
microcapsules. Control 2 samples were designed to mimic the mechanical behavior of
self-healing samples, but without the ability to polymerize the healing agents.
41
Table 3.1 Summary of sample types and components.
Control 1 Control 2 Self-healing
Epoxy resin Epon 862/Epikure W Epon 862/Epikure W Epon 862/Epikure W
Reinforcement 8H satin weave E-
glass 8H satin weave E-
glass 8H satin weave E-
glass
Type A capsules - HOPDMS/PDES HOPDMS/PDES
Type B capsules - hexylacetate DBTL/hexylacetate
3.3.3 Sample Preparation
All composite samples were manufactured with Epon 862/Epikure W (Miller-
Stephenson), at a mix ratio of 100:26.4, epoxy resin and an 8 harness satin weave E-
glass (Style 7781, Fibre Glast) reinforcement. The microcapsules were mixed (10:1 by
weight, HOPDMS/PDES:DBTL/hexylacetate or hexylacetate) into the unreacted epoxy
resin followed by 30 minutes of degassing at 100⁰C. A wet layup procedure was used
with the plies sequenced (0,90,0)s, where 0 represents the warp direction. A schematic
of the layup procedure is shown in Figure 3.7. A Teflon plate was placed under the
sample to create a smooth surface finish from which the sample could be easily
removed. The composite sample was laid up in a layer by layer format with liquid epoxy
resin wetting out each ply. The composite thickness (1.92 mm – 2.03 mm) was
controlled by steel spacers placed around the periphery and a porous steel plate was
placed on top allowing resin to bleed though the plate and into bleeder cloths above. As
pressure was applied, the sample compacted until the porous plate came into contact
with the steel spacers, thus dictating the sample thickness and fiber volume fraction.
The entire layup was cured under 1.2 kPa at 121⁰C for 8 hours.
42
Figure 3.7 Schematic of composite sample layup components.
Figure 3.8 shows a polished cross section of a control 2 sample. The composite is
free of voids and the microcapsules are evenly distributed in the matrix rich regions. The
microcapsule concentrations were calculated based on polished cross-sectional images
such as Figure 3.8. Fiber tows running both parallel and perpendicular to the plane of
the image are visible.
Figure 3.8 A polished cross-sectional image of an undamaged region of an 8H satin
weave composite sample.
43
3.3.4 Mechanical Testing
After the composite panels were manufactured, they were sectioned into
smaller 45 mm x 45 mm samples. Damage was introduced into these specimens by
cyclically (10 times per side) driving an indenter tip into both sides of the sample to a
maximum load of 690 N. The complete details of this damage protocol are contained
both in chapter 2 and in Moll et al. [9]. Figure 3.9 shows a tiled cross-sectional optical
image of a control 2 sample containing 11 vol% 25 µm capsules after the damage
protocol with a large crack spanning the entire thickness of the sample.
Figure 3.9 A polished tiled cross-sectional image of a control 2 sample with 11 vol% 25
μm capsules showing the percolating crack damage running from one side of the sample to the other.
After damaging the samples, they were allowed to heal for 12 hours before
testing them for leaking in a pressure cell apparatus [7, 9] to assess sealing capability.
During this test, the composite permeability was measured by applying a pressurized
gas to one side and monitoring the pressure on the opposite side. A sample was
considered fully healed if the output pressure did not increase by more than 70 Pa over
the 30 minute test.
44
Short beam shear experiments were performed on samples of approximately 4
mm x 25 mm. Prior to testing, the samples were conditioned for at least 48 hours at
23⁰C and 50% relative humidity. Short beam shear tests were performed according to
ASTM standard D2344 [12]. Samples were loaded in 3-point bending at a rate of 1
mm/min with a 12 mm span between bottom supports. The load and displacement
data were recorded. Short beam strength is calculated from,
0.75sbs mPF
b h
(3.1)
where Pm is the peak load, b is the sample width, and h is the thickness.
Dynamic mechanical analysis (DMA) was also performed on samples to obtain
the storage modulus and Tg following ASTM standard D5023-07. After conditioning at
23⁰C and 50% relative humidity for a minimum of 48 hours, the samples were loaded
into a TA Instrument RSA III DMA in 3-point bending with a span of 25 mm. The
temperature of the sample was ramped from 20⁰C to 160⁰C at a rate of 3⁰C/min. The
storage and loss moduli were recorded as a function of temperature. The glass
transition temperature (Tg) is reported based on the peak in the tangent of the phase
angle.
45
3.4 Results
3.4.1 Sealing Performance
The effect of microcapsule size and concentration on the self-sealing
performance of the composite samples was investigated for microcapsule diameters of
25 μm and 42 μm. Figure 3.10 compares the sealing performance of various
microcapsule concentrations and sizes to that of the control samples. Although the
control 1 samples did not have the ability to heal, 14% of the samples did not leak due
to insufficient damage. Self-healing samples with 8 vol% 25 μm microcapsules sealed
75% of the time, compared to control 2 samples which sealed only 29% of the time. The
small percentage of control 2 samples that seal is likely attributed to the presence of the
viscous HOPDMS liquid in the crack network. Increasing both size and concentration of
microcapsules (9 vol% 42 µm and 11 vol% 25 µm microcapsules), resulted in 100% of the
self-healing samples sealing compared to 0% for the representative control 2 samples.
The delivery of more healing agents by increasing capsule size and/or concentration
improves healing performance. The decrease in sealing performance for control 2
samples is due to the increased crack damage associated with these samples, as was
previously shown by Moll et al. [9].
46
Figure 3.10 Summary of self-sealing results for composite samples with multiple sizes
and concentrations of HOPDMS/PDES microcapsules dispersed in the matrix.
3.4.2 Effect of Microcapsule Concentration on Mechanical Properties
The effect of microcapsule concentration on the short beam strength (SBS),
storage modulus and Tg were investigated for composite samples with 0, 4, 7 and 15
vol% 25 µm capsules. We found that the short beam strength decreased as the
concentration of capsules in the composite increased, as summarized in Figure 3.11.
Samples without any microcapsules present (control 1) have a short beam strength of
45 MPa, while samples with 15 vol% microcapsules have a short beam strength of 33
MPa, corresponding to a 27% decrease. Representative loading curves for all
microcapsule concentrations tested are plotted in Figure 3.12. Samples without
microcapsules failed mostly in bending with very little interlaminar cracking. As the
microcapsule concentration increased, the failure mode changed to interlaminar shear
with more interlaminar cracking. As seen in Figure 3.12, the loading curves have a larger
47
plateau region as the microcapsule concentration increases. The initial slopes of the
loading curves also decreased with increased capsule concentration.
Figure 3.11 Composite short beam strength as a function of microcapsule concentration.
Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.
Figure 3.12 Representative short beam shear loading curves for composite samples with
varying microcapsule concentrations. Vc is capsule volume fraction in the composite.
48
The storage modulus at 25⁰C was measured for microcapsule concentrations
ranging from 0 to 15 vol% (Figure 3.13). The storage modulus decreased by 15% with
the addition of 15 vol% microcapsules. Experimental values are compared with the
model developed by Naik et al. [13-17] for satin weave composites in Figure 3.13. The
matrix properties were modified by assuming the microcapsules did not contribute to
the matrix stiffness. The model over predicts the composite modulus indicating that the
microcapsules may also interrupt the woven fiber architecture, reducing the overall
stiffness of the composite.
Figure 3.13 Comparison of storage modulus values with expected model predictions.
Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.
The Tg of fiber-reinforced composites and non-reinforced epoxy resin samples
was also measured for varying microcapsule concentrations (Figure 3.14). As the
microcapsule concentration increased, the Tg decreased slightly for both the neat resin
49
and the composite. To determine if the decrease in Tg was caused by the capsule
components leaching out of the microcapsules and plasticizing the matrix, we prepared
additional samples of epoxy resin and 0.55 wt% HOPDMS mixed in directly into the
epoxy matrix. No drop in Tg was observed with the addition of HOPDMS. Similar trends
have been reported for the addition of particulates in composites by Crowson et al. [18],
who showed a decrease in the Tg of a glass bead reinforced epoxy resin with increasing
bead concentration. Also, Yuan et al. [19] found that in a glass fiber reinforced epoxy
composite with epoxy resin filled microcapsules the Tg of the composite decreased with
increasing microcapsule concentration.
Figure 3.14 Changing glass transition temperature with microcapsule concentration in
fiber reinforced composite and epoxy samples. Microcapsule volume fractions are reported in the resin only, not accounting for the inclusion of fiber reinforcement. Horizontal line represents the Tg of epoxy resin combined with HOPMDS monomer. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.
50
3.5 Discussion
A high Tg structural composite capable of healing crack damage and sealing off
nitrogen gas flow up to at least 275 kPa was achieved with a dual microcapsule system.
This is the first demonstration of microcapsule based autonomic self-healing in a fully
cured, high Tg material. Previously, only 67% of composite samples were capable of
withstanding a pressure of 275 kPa with a low Tg matrix, cured at room temperature [9].
The current PDMS based healing chemistry has achieved sealing in 100% of high Tg
composite samples when 9 vol% 42 µm capsules or 11 vol% 25 µm capsules are
dispersed in the matrix. This healing system has shown excellent sealing ability,
however the addition of microcapsules led to some decreases in mechanical
properties. The presence of some larger microcapsules (ca. 100 um) that were not
sieved or improper rinsing of the capsules may have contributed to change in
mechanical properties. In the future, it will be necessary to optimize composite
processing as well as the size and amount of the microcapsules to achieve healing, but
minimize the reduction in mechanical properties.
3.6 Conclusions
A fully cured, high glass transition temperature (127⁰C) E-glass reinforced self-
healing composite with self-sealing functionality was achieved by the incorporation of a
thermally stable healing chemistry comprised of a hydroxyl end-functionalized
polydimethylsiloxane (HOPDMS) and cross-linking agent polydiethoxysiloxane (PDES)
catalyzed by dibutyltin dilaurate (DBTL). Encapsulated healing components (25 – 42 µm
51
average diameter) were incorporated into the matrix rich regions of the sample during
fabrication.
The effect of microcapsule concentration on short beam strength, and storage
modulus were investigated for composite samples with capsule concentrations from 0 –
15 vol%. At a concentration of 15 vol%, a 27% decrease in the short beam strength was
noted while the storage modulus was reduced by 15%. The glass transition temperature
(Tg) of the fiber-reinforced composite and plain epoxy resin was also reduced slightly,
ca. 3°C, with the addition of microcapsules.
Composite specimens sealed 100% of the time when 9 vol% 42 µm or 11 vol% 25
µm capsules were added. Hence, self-sealing composites have shown the potential to
prolong the lifetime of cryogenic tanks and composite face sheets by eliminating
leakage of cryogenic fuel and reducing water uptake into the sandwich structure core.
References [1] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for
cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.
[2] H. S. Choi, and Y. H. Jang, “Bondline strength evaluation of cocure/precured honeycomb sandwich structures under aircraft hygro and repair environments,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 9, pp. 1138-1147, Sep, 2010.
[3] T. Yokozeki, T. Ogasawara, T. Aoki et al., “Experimental evaluation of gas permeability through damaged composite laminates for cryogenic tank,” Composites Science and Technology, vol. 69, no. 9, pp. 1334-1340, Jul, 2009.
[4] S. Choi, and B. V. Sankar, “Gas permeability of various graphite/epoxy composite laminates for cryogenic storage systems,” Composites Part B-Engineering, vol. 39, no. 5, pp. 782-791, 2008.
52
[5] H. Kumazawa, T. Aoki, and I. Susuki, “Influence of stacking sequence on leakage characteristics through CFRP composite laminates,” Composites Science and Technology, vol. 66, no. 13, pp. 2107-2115, Oct, 2006.
[6] A. C. McVay, and W. S. Johnson, “Permeability of Various Hybrid Composites Subjected to Extreme Thermal Cycling and Low-velocity Impacts,” Journal of Composite Materials, vol. 44, no. 12, pp. 1517-1531, Jun, 2010.
[7] B. A. Beiermann, M. W. Keller, and N. R. Sottos, “Self-healing flexible laminates for resealing of puncture damage,” Smart Materials & Structures, vol. 18, no. 8, Aug, 2009.
[8] S. J. Kalista, “Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture,” Mechanics of Advanced Materials and Structures, vol. 14, no. 5, pp. 391-397, 2007.
[9] J. L. Moll, S. R. White, and N. R. Sottos, “A Self-sealing Fiber-reinforced Composite,” Journal of Composite Materials, vol. 44, no. 22, pp. 2573-2585, Oct, 2010.
[10] C. L. Mangun, A. C. Mader, N. R. Sottos et al., “Self-healing of a high temperature cured epoxy using poly(dimethylsiloxane) chemistry,” Polymer, vol. 51, no. 18, pp. 4063-4068, Aug, 2010.
[11] S. H. Cho, H. M. Andersson, S. R. White et al., “Polydimethylsiloxane-based self-healing materials,” Advanced Materials, vol. 18, no. 8, pp. 997-+, Apr, 2006.
[12] ASTM-D2344, "Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates," 2006.
[13] N. K. Naik, and P. S. Shembekar, “ELASTIC BEHAVIOR OF WOVEN FABRIC COMPOSITES .3. LAMINATE DESIGN,” Journal of Composite Materials, vol. 26, no. 17, pp. 2522-2541, 1992.
[14] N. K. Naik, and V. K. Ganesh, “PREDICTION OF ON-AXES ELASTIC PROPERTIES OF PLAIN WEAVE FABRIC COMPOSITES,” Composites Science and Technology, vol. 45, no. 2, pp. 135-152, 1992.
[15] N. K. Naik, and P. S. Shembekar, “ELASTIC ANALYSIS OF WOVEN FABRIC LAMINATES .2. MIXED COMPOSITES,” Journal of Composites Technology & Research, vol. 15, no. 1, pp. 34-37, Spr, 1993.
[16] N. Naik, Woven Fabric Composites, p.^pp. 1-23, 73-93, Lancaster, Pennsylvania: Technomic Publishing Company, Inc., 1994.
[17] P. S. Shembekar, and N. K. Naik, “ELASTIC ANALYSIS OF WOVEN FABRIC LAMINATES .1. OFF-AXIS LOADING,” Journal of Composites Technology & Research, vol. 15, no. 1, pp. 23-33, Spr, 1993.
[18] R. J. Crowson, and R. G. C. Arridge, “ELASTIC PROPERTIES IN BULK AND SHEAR OF A GLASS BEAD REINFORCED EPOXY-RESIN COMPOSITE,” Journal of Materials Science, vol. 12, no. 11, pp. 2154-2164, 1977.
[19] L. Yuan, A. Gu, G. Lian et al., “Novel Glass Fiber-reinforced Cyanate Ester/Microcapsule Composites,” Journal of Composite Materials, vol. 43, no. 16, pp. 1679-1694, 2009.
53
CHAPTER 4
SELF-HEALING OF THERMAL FATIGUE DAMAGE IN A CARBON FIBER-REINFORCED COMPOSITE
4.1 Introduction
Autonomous self-healing of thermal fatigue damage is achieved in a [0/90]s
carbon fiber/epoxy composite. The healing chemistry is the thermally stable dual
microcapsule system comprised of a tin catalyst and a poly(dimethyl siloxane) monomer
with a cross-linker described in chapter 3. Sealing ability is evaluated with the pressure
cell apparatus to detect nitrogen flow through the damaged composite.
Under thermal fatigue, fiber-reinforced composite materials suffer from the
formation of small microcracks due to the coefficient of thermal expansion (CTE)
mismatch between the fiber reinforcement and the polymer matrix [1, 2].
Microcracking is especially relevant in carbon fiber/epoxy composites where the CTE of
carbon fiber in the longitudinal direction is negative and the CTE of epoxy is positive.
Thermally induced microcracks can lead to composite permeability, hindering their use
in cryotanks [3]. Bechel et al. quantified the density of thermally induced cracks and the
resulting permeability increase with the number of thermal cycles in various carbon
fiber/polymer composites constituents and layup geometries [4-6]. Kumazawa et al.
54
investigated the influence of stacking sequence on the permeability of a bi-axially
loaded carbon fiber composite [7].
Microcrack damage in fiber-reinforced composites is particularly difficult to
detect using non-destructive methods. Only modest success at damage detection has
been achieved using approaches such as line scanning thermography [8], magnetic
resonance detection [9, 10], acoustic emission [11-13] dielectric variations [14, 15], and
electric resistance changes in carbon fiber [16, 17] and nanotube reinforced composites
[13, 18, 19]. Each of these methods has shortcomings associated with them such as
sensitivity of detection, depth of detection, and practicality of use [20].
Self-healing provides a promising method to reduce crack propagation due to
mechanical and thermal fatigue in fiber-reinforced composite materials and extend the
life expectancy of the part. These materials have the ability to heal crack damage due to
mechanical fatigue in bulk polymers through the incorporation of healing agent filled
microcapsules into epoxy [21-24] and poly(dimethyl siloxane) (PDMS) [25] resin. Brown
et al. [21] reported a 213% life extension at a moderate ∆K1 and Jin et al. [24] described
complete crack arrest in an epoxy adhesive using dicyclopentadiene microcapsules and
Grubbs’ catalyst healing chemistry. Keller et al. saw a 24% reduction in torsion fatigue
crack growth in a PDMS resin with an encapsulated PDMS healing agent [26].
Here we investigate the microencapsulated thermally stable tin catalyzed
poly(dimethyl siloxane) healing chemistry, described in chapter 3, to self-heal thermally
induced cracks in a carbon fiber/epoxy composite. This healing chemistry has been used
previously to heal crack damage in epoxy [27, 28], polymer coatings [29] and in bulk
55
PDMS [30]. The effect of microcapsule incorporation on the accumulation of thermally
induced transverse microcracks in the composite and the effect of microcapsule
concentration on CTE of the epoxy resin was also investigated.
4.2 Experimental Methods and Procedure
4.2.1 Microcapsule Preparation
The healing chemistry in this study is composed of hydroxyl end-functionalized
poly(dimethyl siloxane) (HOPDMS) with polydiethoxysiloxane (PDES) acting as a cross-
linker in a reaction catalyzed by dibutyltin dilaurate (DBTL). The HOPDMS, PDES and
DBTL were purchased from Gelest. The HOPDMS and PDES were encapsulated together
(53wt% HOPDMS) in a urea-formaldehyde shell. The encapsulation procedure (Figure
3.2) is identical to the one outlined by Mangun et al. [28]. The microencapsulation
mixture was agitated at a rate of 2000 rpm, producing microcapsules with an average
diameter of 29 ± 15 µm (Figure 4.1 a).
The DBTL catalyst microcapsules, with an average diameter of 44 ± 16 µm
(Figure 4.1 b), were prepared by an interfacial polymerization method (Figure 3.5)
similar to the technique described in Mangun et al. [28] with a few notable
modifications. A polyurethane prepolymer, Desmodure L75, (Bayer) was dissolved in 10
g DBTL along with 10 g hexylacetate, used as a carrier solvent, and poured into a
mixture of 100 mL deionized water and 2.5 g gum arabic (Sigma-Aldrich) mixing at 1100
rpm. This solution was mixed for 1 hour at 85⁰C before the stir rate was reduced to 200
56
rpm and the temperature control was turned off for an additional hour. Next, 50 mL of
deionized water and 25 mL of 2.5 wt% ethylene maleic anhydride (Zeeland Chemical)
solution were added to the aqueous microcapsule mixture and the temperature was
adjusted to 55⁰C for 4 hours. After the reaction was completed, the microcapsules were
cleaned by centrifuging four times for 30 minutes at 1000 rpm, as described in chapter
3. After each centrifuging cycle the aqueous portion was removed and replaced with
deionized water. Finally, the microcapsule solution was frozen in liquid nitrogen and
placed on a freeze drier to remove the water. Control microcapsules were
manufactured in an identical method using only 17 g hexylacetate, instead of the 50:50
mixture of DBTL and hexylacetate.
Figure 4.1 Histogram of microcapsules used in this study: a) HOPDMS/PDES microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 29 ± 15 µm, b) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 44 ± 16 µm.
57
4.2.2 Layup and Sample Types
Three types of samples were investigated. The components of each are listed in
Table 4.1. The matrix material for all sample types was Epon 862/Epikure W (Miller-
Stephenson), with a unidirectional carbon fiber fabric (2583-C, Fibre Glast)
reinforcement. All composites were laid up in a [0/90]s geometry with a total sample
thickness of 1.75 ±0.03 mm. Control 1 samples consisted of only the epoxy resin and
carbon fiber reinforcement (no microcapsules). In addition to the epoxy resin and
carbon reinforcement, control 2 samples also contained HOPDMS/PDES and
hexylacetate microcapsules. Finally, self-healing samples were comprised of epoxy
resin, carbon reinforcement, HOPDMS/PDES microcapsules and DBTL/hexylacetate
microcapsules. Control 2 specimens were not able to heal because of the absence of
the DBTL catalyst.
Table 4.1 Summary of sample types and components. Control 1 Control 2 Self-healing
Epoxy resin Epon 862/Epikure W Epon 862/Epikure W Epon 862/Epikure W
Reinforcement Uni Carbon Uni Carbon Uni Carbon
Ply Sequence [0/90]s [0/90]s [0/90]s
Type A Capsules (vol%)
- HOPDMS/PDES (9 vol%)
HOPDMS/PDES (9 vol%)
Type B Capsules (vol%)
- Hexylacetate (1 vol%)
DBTL/Hexylacetate (1 vol%)
All composite samples were manufactured with Epon 862/Epikure W (Miller-
Stephenson), at a mix ratio of 100:26.4, epoxy resin and a unidirectional carbon fiber
(Style 2583-C, Fibre Glast) reinforcement. The microcapsules were mixed (10:1 by
weight, HOPDMS/PDES:DBTL or hexylacetate) into the unreacted epoxy, which was
58
degassed for 45 minutes at 100⁰C to eliminate trapped air which could lead to voids in
the final composite sample. A wet layup procedure was used with the plies sequenced
[0/90]s, where 0 represents the warp direction. A schematic of the layup procedure is
shown in Figure 3.7 in chapter 3. A Teflon plate was placed under the sample to create
a smooth surface finish from which the sample could be easily removed. The initial
overall microcapsule concentration in the resin was 4 wt%, but the final volume fraction
of microcapsules in the composite was 0.09. The composite sample was laid up in a
layer by layer format with liquid epoxy resin wetting out each ply. The composite
thickness was controlled by steel spacers placed around the periphery and a porous
steel plate was placed on top allowing resin to bleed though the plate and into bleeder
cloths above. As pressure was applied, the sample compacted until the porous plate
came into contact with the steel spacers, thus dictating the sample thickness (1.85 ±
0.03 mm) and fiber volume fraction (0.4). The entire layup was cured under 1.2 kPa at
121⁰C for 8 hours. During the application of pressure, the top plies compacted first,
resulting in these plies having a lower thickness than the bottom plies.
4.2.3 Coefficient of Thermal Expansion Measurement
The coefficient of thermal expansion (CTE) was measured for plain epoxy
samples with varying concentrations of microcapsules. The samples were cured in 8
mm diameter cylinders and were sectioned in 4 mm increments with parallel sides. The
specimens were placed between a parallel plate fixture in a rheometer with a constant
normal force of 0.5 N held on the sample. The temperature was increased at a rate of 5
59
⁰C/min from 25⁰C to 120⁰C. The change in displacement with increasing temperature
was recorded. A representative change in displacement with increasing temperature
plot for epoxy resin with 3.5 vol% HOPDMS/PDES microcapsules is shown in Figure 4.2.
The CTE of the entire composite was not measured because the concentration of
microcapsules was easier to control in epoxy only samples.
Figure 4.2 Representative plot of the change in sample thickness with temperature curve.
4.2.4 Cryogenic Cycling and Permeability Testing
After the composite panels were manufactured, they were sectioned into square
samples approximately 45 mm X 45 mm. Each was polished on two, perpendicular sides
and examined under an optical microscope to verify that no cracks were present prior to
thermal cycling.
A schematic of the cryogenic cycler is shown in Figure 4.3. The samples were
placed in a wire basket and were repeatedly cycled between 35⁰C for 8 minutes and
60
liquid nitrogen (-196⁰C) for 8.5 minutes. Approximate sample through thickness
temperature profiles with time for heating and cooling are plotted in Figure 4.4 a) and b)
respectively. The laminates heat up to the ambient oven temperature (35⁰C) in
approximately 8 minutes, and then cool to liquid nitrogen temperature approximately 5
minutes after being submerged. These temperature profiles were calculated by solving
the one dimensional heat equation with transient boundary conditions approximated by
monitoring the surface temperature of the samples with time. After several cycles, the
samples were examined using optical microscopy to determine the number of
transverse microcracks in each ply. Next, the samples were placed in a desiccator for 4
hours to ensure all condensation was eliminated before testing for leaking.
Figure 4.3 Schematic of thermal fatigue apparatus.
61
Figure 4.4 Approximate temperature profiles through the thickness of composite samples for a) heating to 35⁰C and b) cooling to -196⁰C.
Sample permeability was evaluated in a pressure cell apparatus designed for
previous experiments [30, 31]. In this setup, nitrogen was flowed into the cell with a
controlled pressure (275 kPa) recorded by a pressure transducer. If a percolating
network of cracks was present in the sample, nitrogen gas would flow completely
through the sample and result in an increased output pressure on the opposite side of
the sample. This output pressure was measured and recorded by a second pressure
62
transducer. All tests were run for 10 minutes, which was sufficient time to see the
development of the output pressure evolution with time curve, but short enough to be
practical for laboratory experiments.
Prior experiments tested sample permeability after a localized damage event.
The crack networks were confined to the center of the sample, and were completely
encompassed by o-rings on both sides of the sample. Thermal fatigue damage is not
localized, and therefore the o-rings would not completely contain the nitrogen leaking
through the sample as shown in Figure 4.5. To eliminate this problem, a new sample
chamber was fabricated for the pressure cell to contain the nitrogen escaping through
the sides of the sample. A diagram of the removable sealed sample chamber and the
pressure cell is shown in Figure 4.6. The sample chamber consisted of two aluminum
plates with an o-ring sealed against each side of the sample, and a third larger o-ring
completely surrounding the sample. After the samples were tested for leaking, the
process of cycling and testing was repeated until all samples leaked.
Figure 4.5 Illustration showing how nitrogen gas can escape through a thermally
damaged composite sample in an unmodified pressure cell.
63
Figure 4.6 Schematic of the pressure cell apparatus. a) Sealed sample chamber with
sample and rubber o-rings. b) Complete test apparatus with the sealed sample chamber inserted in the pressure cell.
4.3 Results and Discussion
4.3.1 Microcrack Accumulation
Periodically throughout the cryogenic cycling process, the samples were
examined to record the number of transverse microcracks in each ply with number of
thermal cycles. Figure 4.7 shows an optical micrograph of a cross-section of a control 1
sample with each of the plies and fiber orientations labeled. The crack density with
number of thermal cycles for each ply is plotted in Figure 4.8. Representative images in
Figure 4.9 illustrate the types of cracks in the composite samples. Thermally induced
microcracks span the entire thickness of the ply as shown in Figure 4.9 a) and b).
64
Figure 4.7 Optical image of a polished cross-section of a control 1 sample with the plies and fiber orientations labeled.
Figure 4.8 Microcrack density with number of thermal cycles in a) plies 1 and 4 and b)
plies 2 and 3.
65
Figure 4.9 Representative optical micrographs of thermally induced cracks. a)
Microcrack in ply 4 in a control 1 sample after 110 thermal cycles. b) Microcrack in ply 2 and 3 in a control 2 sample after 310 thermal cycles. c) Delamination running between plies 3 and 4 in a control 2 sample after 310 thermal cycles.
Transverse cracking appeared first in the top ply due to a combination of a more
rapid temperature change experienced by the outer plies and the higher fiber volume
fraction in the top ply due to uneven sample compaction during cure. A representative
crack in ply 4 in a control 1 sample is shown in the optical micrograph in Figure 4.9 a).
As displayed in Figure 4.8 a), the cracking in ply 4 begins quickly, after as few as 5 cycles.
After 1500 cycles, the crack initiation rate decreases, and the number of transverse
microcracks remains essentially constant for the remainder of testing. Cracks began to
66
appear in ply 1 after 600 cycles, for all specimen types. The crack growth rate was much
slower than ply 4, but continued to increase modestly throughout the test. Figure 4.8 b)
summarizes the accumulation of transverse cracks for the inner plies. A representative
optical image of a transverse crack crossing plies 2 and 3 and intersecting embedded
microcapsules after 310 cycles is displayed in Figure 4.9 b). Typically, a single transverse
crack will simultaneously go through plies 2 and 3. Cracks begin to appear in these plies
after approximately 100 thermal cycles and continue to increase in number through the
testing period. Delaminations are also present in the interlaminar regions of the
composite samples. An optical image of a delamination in a control 2 sample running
between plies 1 and 2 after 310 cycles is shown in Figure 4.9 c).
4.3.2 Coefficient of Thermal Expansion
Thermally induced microcracks in composite materials are caused by the
coefficient of thermal expansion (CTE) mismatch between the fiber reinforcement and
the epoxy resin. Since changes in CTE can alter the cracking behavior, the effect of
microcapsules in resin on the CTE was investigated. Epoxy samples were made with
HOPDMS/PDES microcapsule concentrations varying from 0 vol% to 15 vol%. The
resulting experimental and predicted CTE values based on the rule of mixtures are
plotted in Figure 4.10. The CTE at 15 vol% capsules was 88 x 10-6 /⁰C, 91% higher than
that of the neat resin (46 x 10-6 /⁰C). Although this is a relatively large increase, these
results follow the rule of mixtures for microcapsules with CTE values the same as that of
bulk PDMS (300 x 10-6 /⁰C). The CTE of carbon fiber in the longitudinal direction is -0.5 x
67
10-6 [32]. The larger difference between the CTE of the microcapsule filled epoxy and
the CTE of the carbon fibers, may have lead to an increase in the localized thermal
stresses in the composite.
Ply level thermal stresses can lead to delaminations between fiber plies as seen
in Figure 4.11 for control 1 and control 2 samples after 2500 cycles. Delaminations
appeared after fewer cycles in samples with microcapsules than in samples without
microcapsules. This is likely caused by a number of factors associated with the addition
of microcapsules into a composite including a reduction in interlaminar strength
(chapter 3) and an increase in local thermal stresses. Possible additional sources could
be poor bonding of the epoxy matrix to the HOPDMS/PDES microcapsules or improper
composite sample manufacturing.
Figure 4.10 Comparison of coefficient of thermal expansion of epoxy resin with
expected rule of mixtures values. Error bars represent one standard deviation.
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Figure 4.11 Optical images of delamination running between plies after 2500 thermal
cycles in a) control 1 and b) control 2 samples.
4.3.3 Sealing Performance
Composite sample permeability and crack density (Figure 4.8) increased with
thermal cycles. Figure 4.12 plots the output pressure, measured at 10 minutes of testing
after various amounts of thermal cycles, for a representative control 1 sample. This
particular specimen began to leak after approximately 1225 thermal cycles. As the
number of cycles continued to increase, so did the permeability. After about 1600
cycles the rate of increase in output pressure decreased as the output pressure
approached the value of the input pressure (275 kPa). There was a small decrease in
the output pressure after 1850 cycles, which is likely due to the sample not being
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perfectly centered in the sealed sample test chamber. A trend similar to Figure 4.12 was
also observed for both control 2 and self-healing samples.
Figure 4.12 Representative output pressure, after 10 minutes of testing, of a control 1
sample with number of thermal cycles.
All samples were thermally cycled and tested in the pressure cell to determine
the number of cycles each sample was able to withstand before leaking. In all samples,
no leaking occurred until a crack network had propagated through all four plies. As
revealed in Figure 4.8 a) transverse microcracks occurred last in ply 1, making it the
critical ply to trigger leaking. Control 1 and control 2 samples were not capable of self-
sealing and therefore leaked as soon as a percolating crack network extended through
all plies. Self-healing samples accumulated transverse microcracks at a similar rate to
both the control 1 and control 2 samples (Figure 4.8), but leaked after a higher number
of cycles as seen in Figure 4.13. Control 1 samples survived an average of 1027 thermal
cycles before they started to leak, while control 2 samples survived only 717 cycles.
Self-healing samples remained leak-free for an average of 1675 cycles, prolonging the
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lifetime of these samples by 63% (649 cycles) over traditional composite samples
(control 1) and 134% (959 cycles) over samples with microcapsules but no catalyst
(control 2). The reduction in sample lifetime seen in control 2 samples is likely due to
the larger CTE mismatch between the resin and the carbon fiber reinforcement as
discussed previously.
Figure 4.13 Percentage of samples sealed with number of thermal cycles.
4.4 Conclusions
Self-healing samples have shown the ability to prolong the lifetime of composite
materials subjected to thermal cycling. Crack accumulation in each ply was tracked as a
function of number of thermal cycles. Composite samples with and without
microcapsules accumulated cracks at a similar rate. Leaking was not observed in any
samples until all four plies had cracked. Transverse microcracks appeared first in the
top ply and last in the bottom ply, which was likely caused by a combination of varying
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ply thicknesses as a result of the manufacturing method and thermal shock experienced
by the outer plies. The thinnest surface ply (ply 4), developed cracks first.
The effect of microcapsule concentration on the coefficient of thermal expansion
(CTE) of the epoxy matrix was investigated for samples with capsule concentrations
ranging from 0 to 15 vol%. The CTE of epoxy increased with increasing microcapsule
concentration by approximately 90% over this concentration range. An increase in CTE
resulted in a larger mismatch between the carbon fibers and the epoxy matrix, possibly
contributing to delaminations between plies which form after a lower number of cycles
and causing the control 2 samples to leak before the other sample types.
Overall, self-healing samples accumulated cracks at a similar rate as both types
of control samples, but survived an average of 959 more thermal cycles than controls
with microcapsules (control 2) and 650 more cycles than controls without microcapsules
(control 1). Hence, self-healing composites have the ability to seal thermally induced
microcracks formed in fiber reinforced composite materials. This technology shows
great promise toward improving thermal fatigue performance of composite storage
vessels.
References [1] N. L. Hancox, “Thermal effects on polymer matrix composites: Part 1. Thermal cycling,”
Materials & Design, vol. 19, no. 3, pp. 85-91, Jun, 1998. [2] M. C. Lafarie-Frenot, “Damage mechanisms induced by cyclic ply-stresses in carbon-
epoxy laminates: Environmental effects,” International Journal of Fatigue, vol. 28, no. 10, pp. 1202-1216, Oct, 2006.
[3] R. Heydenreich, “Cryotanks in future vehicles,” Cryogenics, vol. 38, no. 1, pp. 125-130, Jan, 1998.
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[4] V. T. Bechel, M. B. Fredin, S. L. Donaldson et al., “Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 7, pp. 663-672, 2003.
[5] V. T. Bechel, J. D. Camping, and R. Y. Kim, “Cryogenic/elevated temperature cycling induced leakage paths in PMCs,” Composites Part B-Engineering, vol. 36, no. 2, pp. 171-182, 2005.
[6] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.
[7] H. Kumazawa, T. Aoki, and I. Susuki, “Influence of stacking sequence on leakage characteristics through CFRP composite laminates,” Composites Science and Technology, vol. 66, no. 13, pp. 2107-2115, Oct, 2006.
[8] O. Ley, S. Chung, J. Schutte et al., "Application of line scanning thermography for detection of interlaminar disbonds in sandwich composite structures."
[9] G. LaPlante, A. E. Marble, B. MacMillan et al., “Detection of water ingress in composite sandwich structures: a magnetic resonance approach,” Ndt & E International, vol. 38, no. 6, pp. 501-507, Sep, 2005.
[10] A. E. Marble, G. LaPlante, I. V. Mastikhin et al., “Magnetic resonance detection of water in composite sandwich structures,” Ndt & E International, vol. 42, no. 5, pp. 404-409, Jul, 2009.
[11] R. M. Measures, “SMART COMPOSITE STRUCTURES WITH EMBEDDED SENSORS,” Composites Engineering, vol. 2, no. 5-7, pp. 597-618, 1992.
[12] G. Caprino, and R. Teti, “RESIDUAL STRENGTH EVALUATION OF IMPACTED GRP LAMINATES WITH ACOUSTIC-EMISSION MONITORING,” Composites Science and Technology, vol. 53, no. 1, pp. 13-19, 1995.
[13] L. M. Gao, E. T. Thostenson, Z. G. Zhang et al., “Coupled carbon nanotube network and acoustic emission monitoring for sensing of damage development in composites,” Carbon, vol. 47, no. 5, pp. 1381-1388, Apr, 2009.
[14] A. A. Nassr, and W. W. El-Dakhakhni, “Damage Detection of FRP-Strengthened Concrete Structures Using Capacitance Measurements,” Journal of Composites for Construction, vol. 13, no. 6, pp. 486-497, Nov-Dec, 2009.
[15] A. A. Nassr, and W. W. El-Dakhakhni, “Non-destructive evaluation of laminated composite plates using dielectrometry sensors,” Smart Materials & Structures, vol. 18, no. 5, May, 2009.
[16] Z. H. Xia, and W. A. Curtin, “Damage detection via electrical resistance in CFRP composites under cyclic loading,” Composites Science and Technology, vol. 68, no. 12, pp. 2526-2534, Sep, 2008.
[17] A. E. Zantout, and O. I. Zhupanska, “On the electrical resistance of carbon fiber polymer matrix composites,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 11, pp. 1719-1727, Nov, 2010.
[18] B. R. Loyola, V. La Saponara, and K. J. Loh, “Characterizing the Self-Sensing Performance of Carbon Nanotube Enhanced Fiber-Reinforced Polymers,” in The International Society for Optical Engineering, 2010.
[19] K. J. Kim, W.-R. Yu, J. S. Lee et al., “Damage characterization of 3D braided composites using carbon nanotube-based in situ sensing,” Composites: Part A, vol. 41, no. 10, pp. 1531-1537, 2010.
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[20] K. Diamanti, and C. Soutis, “Structural health monitoring techniques for aircraft composite structures,” Progress in Aerospace Sciences, vol. 46, no. 8, pp. 342-352, Nov, 2010.
[21] E. N. Brown, S. R. White, and N. R. Sottos, “Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite - Part II: In situ self-healing,” Composites Science and Technology, vol. 65, no. 15-16, pp. 2474-2480, Dec, 2005.
[22] E. N. Brown, S. R. White, and N. R. Sottos, “Fatigue crack propagation in microcapsule-toughened epoxy,” Journal of Materials Science, vol. 41, no. 19, pp. 6266-6273, Oct, 2006.
[23] A. S. Jones, J. D. Rule, J. S. Moore et al., “Life extension of self-healing polymers with rapidly growing fatigue cracks,” Journal of the Royal Society Interface, vol. 4, no. 13, pp. 395-403, Apr, 2007.
[24] H. Jin, G. M. Miller, N. R. Sottos et al., “Fracture and fatigue response of a self-healing epoxy adhesive,” Polymer, vol. 52, no. 7, pp. 1628-1634, Mar, 2011.
[25] M. W. Keller, S. R. White, and N. R. Sottos, “A self-healing poly(dimethyl siloxane) elastomer,” Advanced Functional Materials, vol. 17, no. 14, pp. 2399-2404, Sep, 2007.
[26] M. W. Keller, S. R. White, and N. R. Sottos, “Torsion fatigue response of self-healing poly(dimethylsiloxane) elastomers,” Polymer, vol. 49, no. 13-14, pp. 3136-3145, Jun, 2008.
[27] S. H. Cho, H. M. Andersson, S. R. White et al., “Polydimethylsiloxane-based self-healing materials,” Advanced Materials, vol. 18, no. 8, pp. 997-+, Apr, 2006.
[28] C. L. Mangun, A. C. Mader, N. R. Sottos et al., “Self-healing of a high temperature cured epoxy using poly(dimethylsiloxane) chemistry,” Polymer, vol. 51, no. 18, pp. 4063-4068, Aug, 2010.
[29] S. H. Cho, S. R. White, and P. V. Braun, “Self-Healing Polymer Coatings,” Advanced Materials, vol. 21, no. 6, pp. 645-+, Feb, 2009.
[30] B. A. Beiermann, M. W. Keller, and N. R. Sottos, “Self-healing flexible laminates for resealing of puncture damage,” Smart Materials & Structures, vol. 18, no. 8, Aug, 2009.
[31] J. L. Moll, S. R. White, and N. R. Sottos, “A Self-sealing Fiber-reinforced Composite,” Journal of Composite Materials, vol. 44, no. 22, pp. 2573-2585, Oct, 2010.
[32] I. M. Daniel, and O. Ishai, Engineering Mechanics of Composite Materials, 2nd ed., p.^pp. 375: Oxford University Press, 2006.
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CHAPTER 5
SUMMARY AND FUTURE WORK
5.1 Summary
Self-healing composites provide a promising method to solve the problem of
leaking in composite storage vessels and moisture ingression in composite sandwich
structures caused by impact or fatigue. Self-healing ability interrupts the percolating
crack network that causes these problems and prolongs the lifetime of the composite.
A test protocol was established using a pressure cell apparatus, to monitor and
evaluate the ability of a sample to hold pressure up to 275 kPa. Composite samples
cured at room temperature with imbedded encapsulated dicyclopentadiene and
Grubbs’ catalyst were capable of sealing mechanically induced crack damage in 67% of
samples with 6.5 wt% 51 µm capsules compared to 13% of traditional composites
without capsules. The effect of microcapsule concentration and size on the damage
area of composite control samples without catalyst was investigated. It was found that
damage area increased with both increasing capsule size and concentration.
A high temperature cured self-healing structural composite with a glass
transition temperature of 127⁰C was manufactured. A thermally stable healing
chemistry was used to ensure the self-healing ability would remain active after the
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composite was cured at 121⁰C for 8 hours. These composite samples were capable of
sealing mechanically induced crack damage in 100% of samples with 9 vol% 42 µm and
11 vol% 25 µm microcapsules. The effect of microcapsule concentration on short beam
strength, storage modulus and glass transition temperature were investigated for
composite samples. Short beam strength was reduced by 27% and storage modulus
was reduced by 15% in samples with 15 vol% microcapsules when compared to
specimens without microcapsules. The glass transition temperature of the fiber
reinforced samples was reduced by approximately 3 ⁰C when the concentration of
microcapsules in the resin was 32 vol%.
A cross-ply carbon fiber/epoxy self-healing composite was manufactured and
cryogenically cycled a total of 2500 times. Transverse microcrack damage in each of the
four plies was monitored with cycle number. The effect of microcapsule concentration
on the coefficient of thermal expansion (CTE) of epoxy was investigated. The CTE of
epoxy samples with 15 vol% microcapsules increased by 90% over plain epoxy samples.
Self-healing specimens were able to withstand an average of 63% more thermal cycles
before leaking than traditional composite samples without microcapsules.
5.2 Future Work
There are several challenges that need to be addressed before self-healing
composites can be commercially viable. Most of these challenges center on the need
for new and improved processing methods. The wet hand layup technique employed in
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this study can only be used to make flat plates, while actual composite parts for aircraft
or storage vessels have much more complex geometries. There is also a need to
improve processing such that the thicknesses of all plies are equal. Variations in ply
thickness correspond to differences in fiber volume fraction, which dictate many
mechanical properties of the composite. Manufacturing self-healing pregreg would
allow us to take great steps toward solving these problems. Prepreg is composed of
fiber reinforcement coated by partially cured resin which is sold in thin, flat, flexible
sheets. These sheets can be stacked together to form layups with desired fiber
orientations. Incorporating microcapsules into prepreg would enable us to make
samples with plies of consistent thicknesses, dictated by the thickness of the prepreg
and slightly more complex composite shapes.
Self-healing prepreg can be made by adding an additional component to
traditional wet prepregging setups. In this process, fibers are run into a resin bath and
collected on a mandrel where the epoxy is allowed to partially react before it is chilled
to slow the cure process. To add microcapsules to this prepreg, fiber would be run
through an aqueous microcapsule solution and dried prior to the resin bath. It has been
shown previously that microcapsules will adhere to fibers after dipping in an aqueous
capsule bath [1].
The use of smaller (1-10 µm diameter) microcapsules would be useful in
optimizing composites capable of healing thermal fatigue damage. Unlike mechanical
damage, thermal fatigue damage has a relatively small crack separation (<5 µm), which
can be healed using smaller scale capsules. These microcapsules will be easier to
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incorporate into prepreg using the process outlined above. Finally, the use of smaller
microcapsules will allow better dispersed of capsules in each ply, instead of being
sequestered primarily to the interlaminar regions of the composite making it possible to
heal transverse cracks as well as delaminations.
Additional healing chemistries should be evaluated for self-healing composites.
As mentioned in Chapter 4, the current poly(dimethyl siloxane) system significantly
increases the coefficient of thermal expansion (CTE) of epoxy. Microcapsules containing
other monomers may not cause such a drastic increase in CTE, reducing the thermal
strains in the composite. One potential chemistry to evaluate is an epoxy healing
system. This two capsule system would be comprised of amine capsules and epoxy
capsules. While the microcapsules themselves may still increase the CTE of the epoxy,
the healed material would have the same CTE as the bulk matrix.
Finally, before self-healing composites become mainstream, they need to be
evaluated for long term environmental stability. Testing should be done to determine
how the composites and encapsulated components are affected by prolonged exposure
to non-ambient temperatures and humidities. The composite healing and mechanical
properties should be investigated prior to and post exposure. Analysis of microcapsules
can be done using thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) to determine if components have leached out of the microcapsules or
become inactive over time.
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References [1] B. J. Blaiszik, M. Baginska, S. R. White et al., “Autonomic Recovery of Fiber/Matrix
Interfacial Bond Strength in a Model Composite,” Advanced Functional Materials, vol. 20, no. 20, pp. 3547-3554, Oct, 2010.
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AUTHOR’S BIOGRAPHY
Jericho Lynn Moll was born in Browerville, Minnesota on December 12, 1982. She was
raised in Anchorage, Alaska where she attended Grace Christian School. After
graduating in May, 2001 she enrolled at Hope College in Holland, Michigan. In May,
2005 she graduated with a B.A. in Chemistry and a B.S in Engineering with an emphasis
in Chemical Engineering. After graduation, Jericho moved to Vladivostok, Russia for
nine months to volunteer at Vladivostok Homeless Children’s Rehabilitation Center, a
Russian non-profit organization. In the fall of 2006 she enrolled in the Materials Science
and Engineering Department at the University of Illinois at Urbana-Champaign, where
she worked for Prof. Nancy Sottos on self-sealing composites. In May, 2008 she was
awarded her Masters of Science degree.