Comparison of acoustic emission parameters for fiber breakage and de-lamination failure mechanisms in carbon epoxy composites

Acoustic emissions (AE) generated by a structure under stressed condition provide a passive method to understand the flaw growth phenomenon. In complex structures, such as composites, characterisation of AE signals generated by various failure mechanisms enhances such understanding. Sample level tests have been carried out on carbon epoxy unidirectional laminate in longitudinal direction of fiber to study the AE characteristics of different failure mechanisms, namely,  fiber breakage and inter layer de-lamination. The AE parameters such as amplitude, energy, duration, rise time and signal strength have been acquired and analysed using various correlation plots. The fiber breakage is represented by high energy and longer duration hits with an amplitude of above 90 dB. The de-lamination mechanism is producing AE hits of medium energy of about 1000 units and long duration up to 106 µs. The duration per unit energy and the rise time per unit energy graphs portray a vivid picture of the occurrence of fibre breakage and delamination. 
 
 Key words: Carbon-epoxy composites, failure modes, acoustic emission (AE).


INTRODUCTION
Composites are widely being used as structural members in the aerospace industry, because of its high specific strength, stiffness and good corrosion resistance.Structural integrity assessment and quality control of composite structures have been a challenging task.Experience has shown that using non-destructive testing (NDT) for structural integrity assessment has greatly improved the quality and performance of composite systems.Non destruction evaluation (NDE) of composite structures is complex in terms of testing and data interpretation owing to its anisotropy and nonhomogeneity.
Currently, Acoustic Emission (AE) testing is found to be a reliable and cost effective non-destructive tool for use with composite structures for on line structural health monitoring (American Society for Non-destructive Testing, 2005).The rapid release of strain energy at localized stress concentration points of microscopic or macroscopic defects within the structure under strain generates acoustic emissions.By mounting piezoelectric *Corresponding author.E-mail: malolanv@yahoo.comAuthor(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License  Epoxy content, Eq/Kg 5.0 -5.9 6 Volatile content, by weight 0.75% (maximum) transducers on the structure, emissions are detected and sent to AE data acquisition system for recording and processing.This technique can monitor the dynamic behaviour of the flaws within the stressed structure and provides information with respect to classification and location of flaws as well as damage severity.High sensitivity, less preparation time, cost effectiveness, global coverage and online testing features are the merits of AE technique when compared with conventional NDT techniques.
There are many research papers describing the various types of damage mechanisms in composite structures (Chen et al., 1992;Suzuki et al., 2000).The predominant failure modes observed in composites are matrix cracking, fiber breakage and de-laminations (Giordano et al., 1998).During testing of composites, different failure modes occur simultaneously, thereby giving rise to different AE signals.Therefore, the challenge lies in skilful data interpretation.Extensive testing on specially designed specimens for characterising different failure modes is essential before adapting the AE technique for complex structures.
AE data has been generated at sample level for carbon epoxy composite laminates and the same has been analysed with respect to different correlation plots by comparing various parameters such as amplitude, energy, signal strength, duration, and rise time.These parameters have been used for characterising fibre failure and de-lamination mechanisms; thereby, the differentiating features of these two failure mechanisms have been studied.Tensile testing of unidirectional T700 carbon epoxy specimens with fibres in longitudinal (parallel to tensile axis) is used for evaluating the fibre breakage.Three point bend testing of unidirectional laminate specimens with fibre parallel to the bending plane is employed for evaluation of inter layer de-lamination phenomenon.

EXPERIMENTAL PROCEDURE
T700 carbon fiber laminates were prepared using LY556 based epoxy resin with HY5200 hardener.The aforementioned combination is most suitable for preparation of advanced composites by filament winding method due to its longer pot life at process temperature and better mechanical properties.The specifications of the carbon fiber T-700, Epoxy resin LY556 and Hardener (HY 5200) are shown in Tables 1 to 3.

Test specimen preparation
High temperature cured uni-directional T-700 carbon fibre laminates were prepared using filament winding process where the roving is wetted with the epoxy resin and wound over a rotating diamond shaped mandrel.The specially designed diamond shaped mandrel for the purpose is shown in Figure 1a.Finally, wet winding of carbon fiber (LY556+HY5200) system was followed by curing process in electrical oven.The detailed specimen preparation  scheme is as shown in Figure 1b.
The tensile specimens made from the unidirectional laminate having fibres loaded in the longitudinal direction (same fibre and load directions) is designated as UDL(T) and are of size 250 × 15 × 2 mm.Aluminium tabs are bonded to the tensile specimens with high shear strength adhesive to facilitate the gripping in universal testing machine (UTM).The bending specimens made from the unidirectional laminate and having fibres loaded in the longitudinal direction (same fibre and load directions) is designated as UDL(B) and are of size 85 × 30 × 4 mm.Tensile and bend test specimens were as per the AS TM standards (ASTM Standard 2000, 2003) and are as shown in Figure 2a and b.

Experimental setup
M/s Instron make, 100 KN UTM with closed loop screw driven system was used for carrying out tensile testing and three point bend testing.M/s PAC, USA, make AE system is used for on-line monitoring.M/s PAC make, R15D model resonant piezoelectric transducers are used with external preamplifier for sensing the acoustic emissions from the specimen.The following AE settings have been used for the test:

Test data
The failure modes of the specimens after the tests are as shown in Figure 4.The AE test data for all the specimens is summarised in Table 4.
The observations on the failure modes of the specimens and AE test data are as follows: Fibre breakage is the principal failure mode in the longitudinal tensile specimens [UDL(T)] and hence they have shown the highest failure loads.Matrix cracking is dominant in the early phase of loading cycle which is then taken over by fibre breakage mechanism in latter part with scant presence of the other failure mechanisms like   de-lamination.These specimens have shown very large number of AE hits, high energy and signal strength.The duration of the AE hits are higher attributing to the higher energy content.
The longitudinal specimens [UDL(B)] in three point bend test have shown the failure loads much lower than that of UDL(T) specimens.The bending load is taken by the fibres with de-lamination as the principal failure mechanism.Accordingly the number of AE hits, energy and signal strength are lower than that of UDL(T) specimens.The durations recorded in UDL(B) specimens are much higher than those recorded with UDL(T) specimens indicating that the principal failure mode of inter layer de-lamination possesses the characteristic feature of medium energy and signal strength with very long durations.
Though the range of absolute rise time is higher in case of UDL(T) specimens, the parameter of rise time per unit energy is much shorter when compared with UDL(B) specimens.Hence, the fibre breakage mechanism shows sharp rise time when compared with de-lamination.

AE correlation plots
The AE data for all types of specimens has been post

AE amplitude and cumulative signal strength vs. normalised load plots
The amplitude of an AE hit is the highest point of the  range of 0.9 to 1.0 only).In comparison with UDL(T) specimens, UDL(B) specimens have portrayed less AE due to the fact that, global matrix cracking is less in bending mode as compared to the tensile mode testing.
The hits with amplitude more than 90dB are very few varying from 3 to 5 across the specimens and mostly they are towards the end of the loading cycle which is attributed to isolated fibre failures.

Cumulative amplitude distribution plots
The cumulative amplitude distribution curve helps in identifying the dominant failure mechanisms present in the failure of the given specimen based on number of slopes (Ativitavas, 2002).Figure 6 shows the typical cumulative amplitude distribution plots for both the specimens.The cumulative amplitude distribution curves  presence of fibre breakage mechanism.The higher energy fibre breakage hits are dominant after about 70 to 80% of the loading.The AE hits of fibre breakage mechanism have contributed significant signal strength.The de-lamination failure mechanism in UDL(B) specimens has exhibited medium energy AE hits which are around 1000 units.

Acoustic signal duration plots
The duration of acoustic signal is the time between the  to 10 6 µs.The higher duration hits are more significant in number in UDL(B) specimens.The same is evident from the duration distribution plots.

Duration and rise time per unit energy vs. load plots
Duration and rise time per unit energy plots for both the specimens are as shown in Figures 11 and 12, respectively.These plots have been made for the AE hits with more than 3000 units of energy and higher than 90 dB amplitude representing fibre failure and lower than 90 dB with duration longer than 10 4 µs representing de-lamination.The plots show that UDL(T) specimens contributed dominant fibre failures and UDL(B) specimens contributed dominant de-laminations (Kim and Som, 1984).It can also be inferred that fibre failure hits are with shorter duration and sharper rise time when compared with de-lamination.

Conclusions
The experiments were carried out on carbon-epoxy T700 UD laminate longitudinal specimens by tensile testing and three points bend testing.AE signatures corresponding to failure mechanisms of fibre breakage 11a) UDL(T) Specimen 11b) UDL(B) Specimen  The fibre breakage AE hits are with sharp rise time compared to de-lamination mechanism.The comparison of AE parameters for both the failure mechanisms is shown in Table 5.
Figure 1.(a) Diamond shaped Mandrel used for laminate preparation.(b) Process flow chart for laminate preparation.
Threshold: 40 dB Peak definition time [PDT]: 20 µs Hit definition time [HDT]: 50 µs Hit lock time [HLT]: 300 µs The AE test set up for tensile testing of UDL(T) specimens and three point bend testing of UDL(B) specimens in UTM is as shown in Figure 3.The specimens are subjected to loading gradually up to failure.The load versus displacement/strain and load versus AE were measured simultaneously.Six specimens have been tested in each category.

Fig. 4
Fig.4 Failure modes of the test specimens

Figure 4 .
Figure 4. Failure modes of the test specimens.

Figure 5 .
Figure 5. Amplitude and cumulative signal strength vs. normalised load plots.

Figure 11 .
Figure 11.Duration per unit energy versus load plots.

Figure 12 .
Figure 12.Rise time per unit energy versus load plots.

Table 1 .
Specifications of carbon fiber T 700.

Table 4 .
AE test data for different failure mechanisms