Y-Stage
Collection Fiber
ExcitationFiber
Laser Probe
X-Stage
Z-Stage
Tripod Mount
Objective
0 5000 10000 150000
500
1000
1500
2000
2500
NRSCA
RSCA
Untreated
Motivation
Objective
Al2O3 for carbon fiber
composite reinforcement
SCAs for improved dispersion
Future Work
References and
Acknowledgements
Stress mapping methods with applied loading
Stress analysis and mechanical behavior of HCFRPs under
tensile loading
Mechanical Analysis of Hybrid Carbon Fiber Reinforced with Alumina Nanoparticles via PiezospectroscopyAlex Selimov
1, Ryan Hoover
1, Valentina Villegas
1, Albert Manero
1, Peter Dackus
2, Declan Carolan
2, Ambrose Taylor
2, and Seetha Raghavan
1
1Mechanical and Aerospace Engineering, College of Engineering and Computer Science, University of Central Florida, Orlando, FL 32816, USA
2Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.
Al2O3
R
R
R
O
O
O Si X
Epoxy m
atrix
Hydrolyzable reactivegroups (O) which reactwith Alumina molecules(R) to form covalentbonds.
Organofunctionalgroup (X) whichbonds to theepoxide group inthe polymermatrix
To investigate the stress sensitive properties of hybrid reinforced carbon fiber composites and manufacturing steps for their improvement.
4A2
4T2
2A
2E
Ground State
R2~14430cm-1
R1~14403cm-1
Excitation of chromium impurities in aluminum oxide causes
emission of stress sensitive R-lines
Cr3+
Wavenumber(cm-1)
Inte
nsi
ty (
Arb
itra
ry)
R1 Δv
R2 Δv
Unstressed stateStressed state
Alumina when excited with a laser has a characteristic emission pattern which relates to the energy state of
the chromium ion impurities present in the crystal structure. The spectral emissions of alumina shift with
applied stress. This is a linear relationship and is described by the following equation:
Where Δv is the shift in wavenumber of the spectrum, σii is the applied hydrostatic stress, and Πii is the
hydrostatic PS coefficient which is measured experimentally. Using this relationship, applied stress to
alumina can be calculated. This allows for the understanding of material behavior in multicomponent
systems in which alumina is used.
Schematic for manufacturing of HCFRP samples through RIFT
Carbon Fiber stack
Loaded resin
Vacuum Reservoir
Epoxy Resin
Alumina Particles Vacuum is used to inject resin to create void free composite
Stress sensitivity is enabled in carbon
fiber samples through the inclusion
of alumina nanoparticles within the
epoxy resin used to bind the carbon
fiber sheets. The samples tested are
manufactured using a method known
as RIFT (resin infusion under
flexible tooling) which provides a
matrix free of voids and enables high
volume fraction of filler.
Schematic explaining Silane Coupling Agents (SCAs)
Silane coupling agents
(SCAs) are chemicals which
act as bridges between
organic and inorganic
materials. In the case of the
HCFRPs investigated, SCAs
were analyzed for their effect
on dispersion which
ultimately affects mechanical
behavior.
Forc
e (k
N)
0
2
4
6
8
10
12
14
16
18
20
0
0.5
1
1.5
2
2.5
3
PS stress map
DIC strainmap
Time
Stre
ss (M
Pa)
σa
Simplified schematic of PS equipment explaining various systems required.In order to take optical stress measurements using Piezospectroscopy, various experimental systems are
required to be operated in conjunction. This includes simultaneous spectral collections using
spectrograph and CCD devices and application of loading using an electromechanical load frame.
Load profile applied to carbon fiber samples with labeled stress and strain
measurement points
Load was applied to the samples in 1.05kN steps. Each step was held to enable PS stress measurements.
Strains were captured only during the ramping periods due to laser interference with the DIC camera. This
enabled a full stress and strain profile of the samples with the applied loading patterns.
Improvement of tensile strength due to hybrid reinforcements
The tensile strength of hybrid
materials is improved due to the
presence of a secondary inhibitor to
crack propagation. In hybrid
materials both the fiber and
nanoparticles deflect cracks
preventing early failure. In these
materials the failure mode tends to be
total failure of the material due to the
large presence of inner microcracks.
Epoxy matrixCarbon fiber reinforcement
Alumina nanoparticle reinforcement
Deflection of crack by alumina nanoparticles Deflection of
crack by carbon fiber
Experimental Parameters for HCFRP sample tested
The experimental parameters for the carbon fiber samples are outlined in the table above. The samples
had end tabs applied post manufacturing. These tabs were made of aluminum and were attached to the
carbon fiber using Loctite® EA 9394 . Sample dimensions were 10x100x4mm and the central
mapping dimensions were 10x50mm. The spectral dimensions were 32x16 resulting in a higher
resolution in the x-direction.
The experimental setup is shown above with the labeled DIC and PS collection
systems. Data was collected on both sides of the samples in conjunction with applied
loading in order to determine stress and strain patterns.
Combined experimental setup for DIC and PS
measurements
Dispersion maps of 12wt% carbon fiber sample with various SCA
treatments
0
8 Biaxial strain
(e1+e2) (m
m/m
)
-400
0
400
Comparison of stress and strain maps from PS and DIC for 10wt% reveal earlier detection of high stress zones with PS
Comparison of PS and DIC stress and strain maps reveal an advantage of using alumina particle reinforcements to enable stress sensitivity. Columnar stress concentrations are seen which highlight the ability of
PS to detect internal load transfer behavior.[1] An explanation for the origins of these stress concentrations are provided below. It must be noted that while DIC is not able to detect this sub-surface behavior, the
agreement of final stress and strain maps validate initial stress map results.
Intensity concentrations relate to location of stress concentration zones
for sample.Columnar particle concentrations are seen through the intensity maps, where higher intensity
correspond to a higher local concentration of particles. Higher concentration of composite
inclusions are known to relate to higher average matrix stress.[2] This stress is amplified through
loading which results in the visible stress concentration zones. This relationship enables
predetermination of failure zones based on particle concentrations.
Intensity distributions for different surface treated filler
R1 Intensity (AU)
Pix
el C
ou
nts
Initial work has been conducted on
the improvement of filler
dispersion through the use of the
silane coupling agent surface
treatments. It is seen that reactive
silane coupling agent treatments
(RSCA) improve dispersion when
compared to the untreated sample
at 12wt% of alumina filler. This is
seen by the narrower and lower
intensity peak of the RSCA
sample. A legend is shown below.
Untreated NRSCA RSCA
High intensity hot spots indicate areas of
high alumina concentration
Improved dispersion in
RSCA sample seen from
lower intensity peak
position combined with
small distribution width
Schematic representation of hybrid carbon
fiber composite structure reinforced with
alumina nanoparticles.
Addition of stress-sensitive
particulate reinforcements allows for
early crack detection.
Laser source.
Intensity (Counts) Stress(MPa)
0
12
Inten
sity (C
ou
nts x 1
03)
High resolution local stress measurements will allow for the detection of
particle-matrix debonding in HCFRPs with different particle treatments
Future work will involve the
dispersion characterization of
hybrid carbon fiber samples
with reinforcement surface
treatment at various volume
fractions. Mechanical properties
of each composite will be
characterized along with failure
behavior in order to understand
optimal manufacturing methods
for these materials.
Early detection of crack
Measurement of crack growth until composite failure
Surfacetreatmentsprovide bettermechanicalproperties andcrack resistance
Columnar stress concentrations seen in stress maps indicating sub-
surface load transfer behavior.
Validation of stress maps with DIC through matching of stress and
strain behavior
PS shift calibrated with DIC biaxial strain in small region
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Biaxial Strain e1+e2 (mm/m)
Peak s
hif
t (c
m-1
)
Using a small homogenous region, the PS response of the sample can be calibrated to
strain by plotting the peak shift versus biaxial strain. The beginning of yielding can be
seen through the lowered peak shift response at higher strains. As the sample returns
to zero load there is a residual positive shift which indicates a reduction in the
compressive residual stress in the particles due to a breakdown of the matrix-particle
interface.
Yielding proposed to result
from breakdown of particle-
matrix interface
Residual positive peak
shift shows relaxation
from compressive
residual stress.
Weight % Single point collection time
Total number of stress points
Spectral Resolution
Max load achieved
10wt% 300ms 512 312µm 18.8kN
Speckle pattern on backside for DIC
strain measurements
Front side used for PS stress
measurements. Side used had
higher intensity response.Stress sensing capabilities enabled through the use of next generation hybrid
carbon fiber composites
Carbon fiber composites are highly suitable materials for aerospace applications due to their light weight
and high strength. However, the improvement of carbon fiber composites is limited due to the necessary
tradeoffs in mechanical properties when balancing the ratio of fibers to epoxy matrix. The reinforcement of
carbon fiber composites with ceramic particles provides hybrid properties which will be characterized for
high strength and stress sensing applications.
[1] Freihofer, G., Dustin, J., Tat, H., Schülzgen, A., & Raghavan, S. (2015). Stress and structural damage sensing piezospectroscopic
coatings validated with digital image correlation. AIP Advances, 5(3), 037139.
[2] Clyne, T. W., & Withers, P. J. (1995). An introduction to metal matrix composites. Cambridge University Press
This material is based upon work supported by the National Science Foundation under Grant No. CMMI 1130837. This work is also
supported by the University of Central Florida s EXCEL program and Duke energy. The authors of this work would like to thank
Yangyang Qiao of Dr. Gou s Research group for his help in obtaining the DIC strain measurements. We would also like to thank Dr.
Gregory Freihofer for providing continuing support for the experimentation and analysis performed in this work.
Homogenous
region for
calibration
-300
-230
-140
-70
0
70
140
230
300
35 MPa
315 MPa
630 MPa
5 10 15 20 25 30
0
2
4
6
8
10
12
142000
4000
6000
8000
10000
12000
14000
16000
Peak shift columnar stress concentrations clearly noticed corresponding to intensity concentrations
Columnar particle concentrations detected through intensity distributions
5 10 15 20 25 30
02
46
8
1012
14
Δ𝑣 = 1
3Π𝑖𝑖𝜎𝑖𝑖
PS stress
map
DIC strain
map
Carbon fibercoupon in tension
Photo-luminescent data collection
DIC camera forstrain measurements
R1 Intensity (AU)
Pix
el C
ou
nt Good dispersion
Poor dispersion
