Damage development and progression was monitored by acoustic emission during pin loading of a hole in a quasi-isotropic Hexcel F593/T300 W2G 190 carbon fibre reinforced epoxy composite. Analysis of the acoustic emissions showed that failure was initiated by fibre matrix debonding, followed by fibre fracture, and subsequent matrix cracking. Sections taken through the specimens at varying intervals between the initial acoustic emission and final catastrophic failure confirmed this sequence of events. Good agreement was obtained between the load at the onset of failure and that predicted by finite element modelling

Carr, D.

Crosky, A.

Kelly, D.

Li, R.

Schmidt, L.

Ujjin, R.

University of Queensland eSpace

SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
Damage Development During Pin Loading Of A Hole In A Quasi-Isotropic 
Carbon Fibre Reinforced Epoxy Composite* 
R. Ujjin1, A. Crosky2, L. Schmidt2, D. Kelly1, R. Li1 and D. Carr3 
1School of Mechanical Engineering, University of New South Wales, NSW 2052 
2School of Materials Science and Engineering, University of New South Wales, NSW 2052 
3Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234 
 
 
 
ABSTRACT: Damage development and progression was monitored by acoustic emission during pin loading 
of a hole in a quasi-isotropic Hexcel F593/T300 W2G 190 carbon fibre reinforced epoxy composite. 
Analysis of the acoustic emissions showed that failure was initiated by fibre matrix debonding, followed by 
fibre fracture, and subsequent matrix cracking. Sections taken through the specimens at varying intervals 
between the initial acoustic emission and final catastrophic failure confirmed this sequence of events. Good 
agreement was obtained between the load at the onset of failure and that predicted by finite element 
modelling. 
1 INTRODUCTION  
This work was undertaken to establish the onset of damage during bearing loading to evaluate the 
predictions of a numerical analysis. In previous work, the authors carried out post-mortem 
microstructural examination of carbon fibre/epoxy composite specimens loaded to varying levels of 
the ultimate bearing failure load [Ujjin et al., 2003]. This revealed that the principal failure modes 
were fibre kinking, fibre fracture, fibre-matrix shearing, and matrix cracking, but it was difficult to 
establish the precise load for failure initiation.  
 Acoustic emission (AE) provides a nondestructive investigative technique for monitoring 
damage development in real-time during an actual test. It has been used in previous composite 
testing programs to monitor damage such as creep damage [Toshiyuki, 1997], delamination [Park, 
2001], and damage development during tensile and compressive testing [Ireman et al., 2000, 
Ageorges et al., 1999]. Moreover, different types of microfailure have been found to give different 
acoustic signatures and these can be used to identify the different types of damage as they occur.  
 In the present study, acoustic emission monitoring, coupled with post-mortem microstructural 
analysis, was employed to evaluate the micro-failure modes of quasi-isotropic and near quasi-
isotropic laminates carbon fibre/epoxy laminates. The objectives of this work were to determine the 
point of failure initiation and the occurrence of subsequent individual micro-cracking events. It was 
also hoped that an identification of the types of damage that occurred in the laminate would be 
made possible. In order to discriminate between the different AE waveforms and their 
corresponding failure modes, it was necessary that differences in amplitude and energy, along with 
the load-deflection response of the loading pin, were measured and recorded throughout the 
duration of the test. 
 Microbuckling theory, in conjunction with the finite element analysis program MSC.MARC, 
was used to predict failure. This procedure has been used previously to successfully predict bearing 
failure initiation in non-woven 0º/90º composite laminates [Wu and Sun, 1998]. It focuses on the 
normal stress (σxx) and out-of-plane shear stress (τxz) in pin-loaded laminates. The pin-loaded 
analysis was used, since it represents the worst case service condition where the original clamping 
produced by a close tolerance bolt in a fastened joint is lost during service.  
 
                                                 
*This research was undertaken as part of the research program of the Cooperative Research Centre for Advanced 
Composite Structures Ltd. 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
2 EXPERIMENTAL 
2.1 Specimens preparation and test procedure 
A series of quasi-isotropic carbon fibre/epoxy laminate panels were tested. The panels were made 
from Hexcel Composites W2G190 prepreg, a bi-directional woven carbon fibre fabric prepreg, 
impregnated with F593 epoxy resin. Laminates of three different thicknesses, constructed from 24, 
16 and 5 plies of prepreg, each ply being 0.21-0.24 mm thick, were examined with two different 
hole diameters of 10 mm and 20 mm being used. Equal amounts of 0º/90º and ±45º plies were used 
in the lay up of the panels except in the case of the 5 ply specimens. All specimens were cured in an 
autoclave at 180°C using the cure cycle recommended by the prepreg manufacturer. Specimens 
were fabricated with the stacking sequences shown in Table 1. 
 
Table 1. Laminate configurations 
 
 Cured specimens were trimmed to approximately 160 mm in length and 70 mm in width. A 20 
mm (or 10 mm) hole was drilled 70 mm from the end of each panel along the centerline. The 
samples were then sectioned across their width through the centre of the hole with the use of a 
water-cooled circular diamond saw. Following cutting, the bearing area was ground with 600 grit 
SiC paper to ensure that the surface of the hole was smooth. Grinding was carried out dry, using a 
cylindrical steel mandrel around which a single layer of SiC paper had been wrapped. The mandrel 
was mounted in a drill press with the panel in the horizontal plane to ensure that the surface of the 
hole was ground perpendicular to the surface of the panel. Each sample was checked for 
smoothness prior to testing using a low power light microscope.  
 A conventional pin loading setup was used to perform the bearing tests on an Instron 1185 
universal testing machine. The lower edge of the specimen was clamped in a steel fixture with 
adjustable steel support plates to prevent global buckling. The load was applied through 20 mm (or 
10 mm) hardened steel pins. The specimens were loaded at a rate of 0.5 mm/min. Figure 1 shows a 
diagram of the experimental setup with the AE probe and monitor attached.  
 
Figure 1. Schematic diagram of AE system and experimental setup 
 
Testing procedure  
A set of 6 specimens was first tested to failure to determine the ultimate failure strength and to 
obtain the approximate load levels at which acoustic emissions occurred. For subsequent tests, the 
testing was stopped immediately after the 1st, 2nd or 3rd AE signal event. These, together with the 
panels tested to final failure, provided a set of samples with various levels of damage. Figure 2 
shows the pin loading setup on the Instron 1185 testing machine. 
Laminate Stacking sequence 
24 Ply [±45º 0º/90º ±45º 0º/90º 0º/90º ±45º 0º/90º ±45º ±45º 0º/90º ±45º 0º/90º]s 
16 Ply [±45º 0º/90º ±45º 0º/90º 0º/90º ±45º 0º/90º ±45º]s 
5 Ply [±45º 0º/90º 0º/90º 0º/90º ±45º] 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Figure 2. Pin loading setup on testing machine 
 
2.2 Acoustic Emission 
Acoustic emission has been verified as one of the few successful techniques for detecting failure as 
it occurs in a laminated composite [Fowler, 1986]. With the application of load onto a composite 
material there are many mechanisms through which acoustic emissions may be generated. These 
include resin cracking, fibre fracture, debonding, and interlaminar cracking [Geoff, 1994]. AE 
techniques can offer real-time information on the characteristics of damage events and fracture of 
the material [Toshiyuki, 1997]. Analysis of the amplitude distributions provides some guidance; 
peaks in the spectrum can be associated with specific types of damage events as well as ultimate 
failure. The AE technique was therefore considered beneficial for the detection of damage initiation 
generated during this testing program. AE events may also provide information on the nature of 
damage events that occur during testing.  
 The system used for failure detection was MISTRAS 2001 AEDSP-32/16 from Physical 
Acoustics Corporation. This system is a computerised multi-channel system for recording, 
processing and analysing AE signals. The system consists of a PC with a supplementary data card, 
the MISTRAS software, AE piezoelectric transducers, and pre-amplifiers. An AE sensor was 
attached to the specimen using a vacuum grease couplant. The sensor output was amplified at a pre-
amplifier. The threshold level was set to 45 dB. The signal was fed into an AE signal processing 
unit where the AE parameters were analysed using in-built software. The AE signal amplitude and 
event durations were recorded over the elapsed time. The AE waveforms were then analysed for the 
characteristic frequencies using a Fast Fourier Transform (FFT).  FFT frequency spectra can be 
used as fingerprints allowing discrimination of the differing failure modes. 
3 NUMERICAL ANALYSIS 
3.1 Microbuckling failure model 
Microbuckling in composites generally results from a geometric instability rather than a material 
failure due to overstressing [Staab, 1999]. The fibre buckling mechanism can be defined as fibre 
instability followed by a decreased capability of the fibres to carry load, with the final result being 
matrix failure through overstressing. There are two distinct modes of fibre buckling in laminar 
analysis: extension (symmetric) and shear (antisymmetric) modes. The shear mode usually occurs at 
lower loads and is the relevant mode here. In the shear mode, fibres are assumed to buckle with the 
same wavelength and are considered to be in phase with each other, as shown in Figure 3(a). In this 
case, matrix deformation is predominantly in shear [Staab, 1999]. Compressive stress in a fibre due 
to in-phase buckling leads to the formation of kink zones, with the consequence of deformation of 
the brittle fibres. Microbuckling of fibres leading to the formation of kink zones with excessive 
deformation is shown in Figure 3(b). The physical damage [Wu and Sun, 1998] is shown in Figure 
3(c)  
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Figure 3. (a) Fibre microbuckling modes in shear (antisymmetric) (b) Schematic of kink band geometry  
(c) Micrograph of pin-contact damage [Wu and Sun, 1998] 
 
 In previous studies [Sun and Jun, 1993, Jun, 1993], an approach from Rosen’s energy method 
[Rosen, 1965] was adopted to derive the elastic fibre microbuckling solution through the use of the 
Euler-Bernoulli beam-column theory with an elastic foundation. The results for the predicted 
critical compressive stress for the in-phase (antisymmetric) mode was identical to that reported 
previously by other workers [Daniel and Ishai, 1994]. The critical compressive stress can be defined 
as:  
1
m
cr
f
G
c
σ = −                     (1) 
 In the present study, a fibre-microbuckling model, previously proposed work [Sun and Jun 1994] 
was adapted to predict contact failure initiation. As in previous work [Wu and Sun, 1998], the 
assumption was made that fibre buckling occurs in the through thickness plane. This then permitted 
evaluation of θ, the ratio of the out-of-plane shear stress, τxz, to the normal stress, σxx, from finite 
element analysis. 
/xz xxθ τ σ=                     (2) 
 
3.2 Finite element modelling 
The finite element software MSC.MARC was used to perform the 3-D non-linear pin contact 
analysis. The average normal stress σxx and out-of-plane shear stress τxz in the critical volume were 
both obtained from the model. These were then used to construct the average stress curve for initial 
failure prediction. The pin-contact model is shown in figure 4. 
  
Figure 4. Schematic diagram for 3-D finite element model 
4 RESULTS 
4.1 Finite element model results 
The predicted values of the initial bearing failure load are shown in Table 2. According to the 
microbuckling theory [Wu and Sun, 1998], bearing failure initiation is most likely to occur at the 
load level where the load-displacement curve exhibits a second non-linearity. Generally, this is 
between 70-80 % of the ultimate load.  As can be seen from Table 2, good agreement was obtained 
between the load at the onset of failure and that predicted by non-linear contact analysis in the finite 
element modeling using the microbuckling criterion, for all three laminates for both pin diameters. 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Table 2. Initial bearing failure prediction results  
 
 
 
 
 
 
 
 
 
 
 
4.2 Acoustic emission analysis 
While the principal aim of the acoustic emission monitoring was to determine the point of damage 
initiation, it was also found that different failure phenomena occurred during the subsequent course 
of testing. The first signal consistently appeared with the same waveform and spectrum patterns. 
For a specific failure mechanism, different AE event spectra were plotted and were again 
consistently represented by the same pattern. Therefore, it was considered possible to use the 
frequency spectrum of the AE events as fingerprints characteristic of the physical events from 
which the AE originated, as reported previously [Ageorges et al., 1999].  
 During the test the number of hits, duration, amplitude and energy were monitored using the AE 
system. The failure mechanism determination was then based on the amplitude and width of the AE 
events within the time domain and frequency spectrum. In the present study, the first signal 
occurred at an AE amplitude within the 30-50 dB range. The associated characteristic signal 
appeared in the range of 100-300 kHz. This damage initiation was considered to be fibre matrix 
debonding [Park et al., 2002].  The second group range was between 50-80 dB. The spectrum 
associated with this event showed sharp peaks at around 300 kHz. The corresponding failure mode 
was considered to be fibre breakage [Ageorges et al., 1999]. Once the sample was close to final 
failure, a wider range of frequencies, in the 0-800 kHz range, was observed and this was considered 
to be matrix cracking. A combination of the first and second types of waveform occurred 
simultaneously just before final fracture which was accompanied by a high acoustic emission of 
around 100 dB. The three distinct AE signals are shown in figure 5. 
 
 
                              (a)                                                               (b)                                                               (c) 
Figure 5. AE waveforms and their FFT: (a) Interfacial failure, (b) Carbon fibre break and (c) Matrix cracking. 
 
 
Panel type 24 Ply 24 Ply 16 Ply 16 Ply 5 Ply 
Thickness (mm) 5.38 5.38 3.63 3.59 0.88 
Pin diameter (mm) 20 10 20 10 20 
Peak load (kN) 53.01 27.92 32.82 17.64 7.11 
Experiment 42.20 21.26 24.92 13.58 5.41 Damage initiation 
load (kN) FEA 39.37 20.27 23.66 13.02 5.08 
Experiment 79.6 76.1 75.9 77.0 76.0 % of peak load 
FEA 74.3 72.6 72.1 73.8 71.4 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
4.3 Microstructural examination 
After testing, the specimens were sectioned to determine the internal failure modes associated with 
each load level and the corresponding AE signal. Following cutting, the sections were polished and 
examined using a light microscope. 
 The fracture mechanisms arising from the pin loading contact were correlated with the AE events 
according to the load level. The corresponding failure modes can be seen in Figure 6. Figure 6(a) 
shows fibre breakage at a load level between 70-80% of ultimate load. Figure 6(b) shows shear 
bands, matrix cracking and microbuckling as the load was increased from 80-100% of the ultimate 
load.  
 
 (a)                                         (b) 
Figure 6. Micrograph of sectioned specimens; (a) Carbon fibre breakage, (b) Matrix cracking. 
5 CONCLUSIONS 
The present work demonstrates that acoustic emission signals can provide an indication of bearing 
failure initiation in a quasi-isotropic carbon fibre reinforced epoxy composite. Bearing failure 
initiation prediction with the use of a microbuckling criterion has been demonstrated. From the 
predicted results, it was found that the failure predictions for all types of laminates were within 5% 
of the theoretically expected values. 
 Acoustic emission (AE) monitoring, combined with Fast Fourier Transform (FFT) frequency 
analysis, provides an efficient way to detect damage initiation and monitor subsequent microfailure 
events.  In addition, the FFT spectra of the AE events were shown to exhibit characteristic features 
for specific types of microfailure thus allowing for use as failure mechanism fingerprints. 
ACKNOWLEDGEMENTS 
R.Ujjin acknowledges the receipt of a PhD scholarship from the Cooperative Research Centre for 
Advanced Composite Structures Ltd.  
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Damage Development During Pin Loading Of A Hole In A Quasi-Isotropic Carbon Fibre Reinforced Epoxy Composite

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Damage Development During Pin Loading Of A Hole In A Quasi-Isotropic Carbon Fibre Reinforced Epoxy Composite

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