Fractographic Observation of Various Loading Modes of Fibre Reinforced Laminates

Rosa Marat-Mendes1,3,a and Manuel de Freitas2,3,b

1Escola Superior de Tecnologia de Setúbal, Rua do Vale de Chaves, Estefanilha,

2910-761 Setúbal, Portugal

2Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

3ICEMS, Instituto Superior Técnico, TULisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.

a rosa.marat@estsetubal.ips.pt (corresponding author), b mfreitas@dem.ist.utl.pt

Keywords: Fractographic analysis; Glass fibre epoxy; Interlaminar fracture; Delamination.

Abstract One of the major disadvantages of laminated composites is their tendency to delaminate.

Unidirectional glass/epoxy laminates have been tested under static conditions by the use of fracture

mechanics. Mode I, mode II, mixed mode I-II, mode III and mixed mode II-III tests were

performed. Double cantilever beam (DCB), end-notched flexure (ENF), mixed-mode bending

(MMB) and edge crack torsion (ECT) specimens were used.

Scanning electron microscopy technique was used to identify distinguishing fractographic features

and to establish the differences between the various modes of fracture after specimens testing. The

propagated orientation of the delamination could be specified from the patterns of fracture surface.

Scanning electron micrographs of fractured surfaces showed that the most predominant

fractographic features in mode I and mode II are the large amount of fibre pull-out and the cusps

markings respectively. In the MMB specimen the fracture surfaces are characterized by fibre

breakage under shearing with fractures localized in the resin with cusps having an orientation of 90º

(mode II) and also fractures localized in the resin and along the resin/fibre interface (mode I).

Mode III characterization concluded that some limited mixed mode II-III seems to be present for

ECT specimen on delamination initiation and growth, but a large majority of mode III delamination

is present.

1. Introduction

Delamination is one of the most common reasons for failure of composite laminates and can cause

major reductions in the stiffness or strength of a structure. However, delamination can be relatively

difficult to detect and quantify since they constitute sub-surface damage. The present paper

concentrates on the fractographic characteristics of delamination in laminates under different known

loading modes. Fractographic techniques can identify damage initiation and subsequent directions

of propagation. In metallic materials the fractographic analysis techniques are well established and

are a valuable diagnostic technique for failure analysis of components and structures failed in

service. Similar studies have been carried out in composite materials but only at a small extent are

available.

It is possible to evaluate, based on fractographic analysis, how the composite structures failed and

also the nature of the applied loads/modes. In order to study the delamination failure of composite

materials, experimental studies were carried out to characterize the modes of failure.

Various damage mechanisms can readily be established and the modes of failure or the mixity of

different modes may also be calculated. The known modes of loading have been applied by using

standard fracture mechanics testing methods. Specifically, these included double cantilever beam

(DCB) [1], end-notch flexure (ENF) [2], mixed-mode bending (MMB) [3] and edge crack torsion

(ECT) [4-5] test coupons under static loading conditions.

Materials Science Forum Vols. 730-732 (2013) pp 337-342Online available since 2012/Nov/12 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.730-732.337

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2. Experimental

Fracture toughness specimens were subjected to a fractographic examination following testing in a

variety of loading modes. The DCB, ENF and MMB specimens were manufactured from 24-ply

laminates with a unidirectional stacking sequence of [0]24. The ECT specimens were manufactured

from 28-ply laminates with a stacking sequence of [90/(+45/-45)3/(-45/+45)3/90]s. An initial

artificial delamination was simulated along the midplane by means of a non-adhesive 12.7 µm

Teflon®

insert.

The fibre/resin system was UE400REM/ET443 which consists of an epoxy resin reinforced with E

glass fibres. The applied loading modes are given in Table 1 together with the particular test

methods and the stacking sequence that were used.

Table 1. Fracture modes and respective methods used for fracture testing.

Fracture mode Coupon test method Stacking

Mode I DCB [0]24

Mode II ENF [0]24

Mixed Mode I-II MMB [0]24

Mixed Mode II-III ECT [90/(+45/-45)3/(-45/+45)3/90]s

Mode III ECT [90/(+45/-45)3/(-45/+45)3/90]s

The specimens have been mounted into the test fixtures and quasi-static tests were conducted under

displacement control at a crosshead displacement rate of 0.5 mm/min on an INSTRON3369 load

frame under deflection-controlled loading. Load-displacement plots, P-δ, were recorded during the

tests. Prior to fracture testing the initial crack length was marked on one side of the specimen edge

and all fracture tests were performed from the end of the (straight) insert without pre-cracking the

specimen.

Samples were cut from the fractured specimens using a diamond-coated saw after experimental

tests. Several samples were taken from both surfaces of each specimen. The areas considered to be

most interesting were at the end of the insert (i.e. the start of the crack) and the area around the

crack tip.

Each sample was then mounted on an aluminium disc and its edges coated with conductive paint

and gold coating. A Hitachi S-2400 scanning electron microscope was used. Micrographs of various

features were taken from different regions of several samples of each fracture specimen. The results

of the SEM examination are presented for each mode in the following sections. In all micrographs,

the white arrow indicates the globally induced crack propagation direction, and the magnification is

given.

3. Results and Discussion

3.1. Mode I and Mode II

In the DCB specimen a pure mode I delamination exists and a typical SEM image of the fracture

surface is generally smooth with few fibre fractures and the failure mechanism is characterized by

fractures localised mainly in the resin and along the resin/fibre interface [6]. SEM images of the

opened surfaces in the starter region (Fig. 1) clearly shows broken fibres pulled out from the resin

as an evidence of fibre bridging. Also the appearance of scarps parallel to the crack growth

direction and textured microflow appears in the mode I fracture surface.

Some authors have reported “cusp pattern” or “hackle pattern” structures in mode I, mode II and

mixed modes [7] where it is believed that cusp markings’ are developed by shear stresses.

Fig.2 shows an area where the matrix developed a higher degree of deformation away from the

starter film. This picture shows a sporadic “cusp pattern” characteristic of shear deformation. The

cusp alignment changes as the mode II component is introduced and increases [6].

338 Advanced Materials Forum VI

In the ENF specimen a pure Mode II delamination exists and fracture surfaces shown in Fig. 3 were

formed by the characteristic cusp pattern, usually observed after Mode II failure. Mode II failure

mechanism consisted of fibre breakage under shearing and is characterized by fractures localised in

the resin with many cusps having an orientation of approximately 90º to fibre orientation. These

features appear as inclined platelets on the surface as shown in Fig. 3b).

a) b)

Figure 1 – Typical mode I fracture surface in E glass fibre/epoxy near the starter film:

a) (x20); b) (detail x100). (Delamination propagates from top to bottom).

Figure 2 – Mode I fracture surface in E glass fibre/epoxy away from starter film (x400).

(Delamination propagates from top to bottom).

a) b)

Figure 3 – Typical mode II fracture surface in E glass fibre/epoxy near the starter film:

a) (x100); b) (detail x400). (Delamination propagates from top to bottom).

3.2. Mixed Mode I-II

The MMB test is a widely accepted test method to characterize the fracture toughness [3]. This test

allows delamination under a range of combinations of mode I and mode II loadings. In the MMB

specimen a mixed mode I-II delamination exists and a more complex failure mechanism occurs.

The fracture surfaces are characterized by fibre breakage under shearing (Mode II), by fractures

localised in the resin with cusps having an orientation of 90º and also fractures localised in the resin

and along the resin/fibre interface (Mode I), Fig. 4. The presence of cusps at 45º, are in fact scarps,

Broken Fibres

Fibres at 0º Scarps

Textured microflow

Fibres at 0º Cusps

Fibres at 0º 90º Cusps

Materials Science Forum Vols. 730-732 339

which are angled because they have initiated at the fibres and extended into the surrounding matrix.

The angle is associated with the cracks having a component in the direction of crack growth and

perhaps associated with the large resin-rich sites adjacent to the fibres [6].

Figure 4 – Toughness versus mode mixity for E glass fibre/epoxy showing the change in fracture

morphology (x100 and detail x400). a) 25%GII/GC; b) 33%GII/GC; c) 48%GII/GC; d) 86%GII/GC.

4. Mode III and Mixed Mode II-III

In the ECT specimen, pure Mode III delamination co-exists with mixed mode II-III. Fracture

surfaces of the ECT specimens have been studied in three different positions along the specimen

length. Fracture surfaces of the ECT specimens have been analysed in three different positions:

(position a) loading side localized upper the starter film; (position b) middle of the specimen;

(position c) loading side localized outside the starter film.

A schematic of the SEM locations of the fracture surfaces of the ECT specimens are presented in

Fig. 5.

Figure 5 – Schematic of the SEM locations of the ECT sample.

(Crack propagates from top to bottom)

0

0,2

0,4

0,6

0,8

1

1,2

0% 20% 40% 60% 80% 100%

GC=

GI+

GII

[kJ

/m2]

GII/GC [%]

a) d)

b) c)

Pure Mode II

Pure Mode I

Scarps 0º fibres

Broken fibres

Textured

Microflow 45º Cusps

90º Cusps

45º Cusps

a)

b)

c)

d)

b=

38

mm

ã

(a) (b) (c)

L=89mm 34

Teflon

90º

0º

340 Advanced Materials Forum VI

Fig. 6a) shows the SEM image in the side of the loading pin (position a in Fig. 5). This figure

illustrates that the fracture surfaces of the ECT specimens are very different from the fracture

surfaces shown earlier in the DCB, ENF and MMB specimens. In this case the cusps are not

perpendicular to the surrounding fibres; they are rotated in clockwise approximately 45º to the fibre

orientation, which indicates mixed mode II-III fracture and also a presence of whilst long cusps

(ribbons) which are associated with Mode III. This morphology implies a shear component that is

not parallel to the fibre direction. One means to consider this is to treat it as a local Mode III

component to the delamination growth.

Figure 6 – ECT fracture surface: a) position a (x100); b) position b (x100); c) position c (x100)

(Crack propagates from top to bottom).

Fig. 6b) shows the SEM image in the middle of the specimen (position b in Fig. 5) where it is

shown that the fracture surface of the ECT specimen in this position is very different from the one

that appears in Fig. 6a). In this figure the appearance of long ribbons is more evident, which

indicates the morphology of cusps, oriented parallel to the fibres, indicating a pure Mode III

delamination. It can also be seen that the cusps are less defined with smoother fracture features

which indicates some evidence of Mode I, but negligible Mode II fracture.

Fig. 6c) illustrates the SEM image in the loading side localized outside the starter film (position c in

Fig. 5) where it can also be observed the presence of cusps rotated anticlockwise in around 45º to

the fibre direction indicating a mixed mode II-III fracture which are similar to the fracture

observation in the side of the loading upper the starter film (position a in Fig. 5) and also long

ribbons associated to pure mode III. When compared the fracture surfaces shown in Fig. 6 with the

G distribution also shown in Fig. 6), these helps to verify that the ECT specimen present a mode III

fracture and also some mixed mode II-III in both sides of the specimen. It can be also observed that

in the middle of the specimen away from both ends the ECT specimen presents pure mode III

fracture.

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0% 20% 40% 60% 80% 100%

GC

[kJ

/m2]

x/L

Mode I

Mode II

Mode III

a) b) c)

-45º

Cusps

Long

Ribbons

+45º

Cusps

Long

Ribbons

90º

Fibres Long

Ribbons

Materials Science Forum Vols. 730-732 341

5. Conclusions

The present investigation has examined the fractographic features associated with delamination in

unidirectional laminates of E glass fibre/epoxy composite. Pure mode I, pure mode II, and mixed

modes I-II and II-III of loading have been considered under static condition. This entitled the use of

DCB, ENF, MMB and ECT fracture mechanics coupons.

The predominant fractographic feature found in mode I was broken fibres and pull-out from fibre

bridging. In mode II testing the most predominant morphology was the cusp pattern. The fracture

surface of the mixed mode I-II testing are characterized by fibre breakage under shearing (mode II)

and fractures localized in the resin and along the resin/fibre interface (mode I). In mixed mode II-III

the most predominant morphology were the cusps with 45º of orientation to the fibre alignment

(mixed mode II-III) and long ribbons (mode III) indicating the presence of some mode III fracture

in both sides of the specimen and also some mixed mode II-III. The fractographic feature of ECT

shows also pure mode III loading in the middle of the specimen away from both ends.

This study allowed to characterize the delamination surfaces occurred due to different types of

applied loading and may be considered a valuable way to help studies of engineering failure

analysis of components and structures made of composite materials.

References

[1] ASTM D 5528 – 01, Sandard test method for mode I interlaminar fracture toughness of

unidirectional fiber-reinforced polymer matrix composites: American Society for Testing and

Materials. Annual Book of ASTM Standards. (2001)

[2] P. Davies (Ed.), ESIS Protocol for Interlaminar Fracture Testing of Composites, ESIS-Polymers

& Composites Task Group (1993)

[3] ASTM D 6671 – 06, Standard test method for mixed mode I-mode II interlaminar fracture

toughness of unidirectional fiber reinforced polymer matrix composites: American Society for

Testing and Materials. Annual Book of ASTM Standards. (2006)

[4] R. Marat-Mendes, M. Freitas, Characterisation of the edge crack torsion (ECT) test for the

measurement of the mode III interlaminar fracture toughness. Engineering Fracture Mechanics.

76 (2009) 2799-2809.

[5] J.G. Ratcliffe, Characterization of the edge crack torsion (ECT) test for mode III fracture

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National Research Council (2004)

[6] E.S. Greenhalgh, Failure analysis and fractography of polymer composites. Woodhead

Publishing Ltd, 2009, pp. 164-227.

[7] J. Bonhomme, A. Argüelles, J. Viña, I. Viña, Fractography and failure mechanisms in static

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Polymer Testing. 28 (2009) 612-617.

342 Advanced Materials Forum VI

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Fractographic Observation of Various Loading Modes of Fibre Reinforced Laminates

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