El SEV[ER PII: S0963-8695(98)00002-4

NDT&E International, Vol. 31. No. 3, pp. 157-163, 1998 © 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0963-8695/98 $19.00+0.00

Failure mechanisms analysis of 2D carbon/carbon using acoustic emission monitoring Y. Z. Pappas a, Y. P. Markopoulos a, V. Kostopoulos a'*

aApplied Mechanics Laboratory, Mechanical and Aeronautical Engineering Department, University of Patras, University Campus, 265 00 Patras, Greece

Received 8 September 1997; revised 18 December 1997

The present work aims toward the application of an innovative methodology for the analysis of acoustic emission (AE) activity monitored during the quasistatic tensile loading of centre-hole carbon/carbon. An in-house developed algorithm is proposed, which utilises the results of unsupervised pattern recognition classification of AE data. Correlation between clusters and specific material failure modes was achieved, using algorithm results and cluster activation parameters. During the analysis a dependence of AE activity on the hole diameter was observed. © 1998 Elsevier Science Ltd. All rights reserved

Keywords: acoustic emission, pattern recognition, ceramic matrix composites, cluster activation

Introduction

Ceramic matrix composites (CMCs) are ideal structural materials for high temperature applications, since they keep their excellent mechanical properties at elevated tempera- ture. In addition, CMCs appear to have remarkably lower notch sensitivity compared to monolithic ceramics due to their built-in stress redistribution mechanisms [1'21. How- ever, better understanding of damage behaviour and failure mechanism development within the structure of CMCs under quasistatic and dynamic loading is necessary in order to design with these materials and improve their response to applied stresses.

According to recent literature 1~'21, in the presence of strain concentration sites such as holes and notches, CMCs may exhibit three different fracture modes/classes of behaviour under mode I remote loading. Carbon/carbon (C/C) composites have been classified as class III CMC material type 1~'21, which means that during their tensile loading an extensive shear damage band occurs normal to the notch direction, emanating from the edge of the notch/hole and leading to a diminishing stress concentration factor ahead of it.

Among other non-destructive methods, which have been applied for damage monitoring in composite materials

* Correspondence to V. Kostopoulos.

during the last decade, acoustic emission (AE) has been proved one of the most efficient. However, only limited work has been published concerning CMCs t3"41 in the direction of a global approach of fracture process, mainly based on continuous AE monitoring of the loading procedure and the qualitative discussion of the raw data. Moreover, only in exceptional cases[5 71 are some preliminary results reported, which try to correlate AE activity with specific failure mechanisms.

On the other hand, the use of AE monitoring as a damage evaluation method has been applied in the case of organic matrix composites, and results associating AE data with specific damage modes have been presented [8-1°1. These results are mainly refereed to the correlation of the amplitude distribution to specific materials failure modes, but this single parameter analysis has been proven very poor for a reliable damage mode identification.

Therefore, in order to increase the reliability of AE data analysis for fracture mechanism identification in the case of composites, a multiparameter approach is necessary. Based on this idea, a new analysis algorithm is proposed contain- ing descriptor selection procedures, validation techniques, filtering and statistical analysis of AE data, taking into account the stochastic character of AE data and the knowledge of the failure mechanisms appearing in CMCs based on microscopical fractographic examination. Thus,

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the correlation between clusters and different failure mechanisms was achieved. Furthermore, strong evidence was detected according to which the monitored AE activity in the case of centre-hole samples is affected by the hole size in a very complicate way, due to the scattering of the produced elastic waves.

The application of AE data analysis algorithm shows a high reliability for identification of damage mechanisms devel- oped during the quasistatic tensile loading of centre-hole C/C samples, due to AE multivariable analysis performed. This innovative approach seems to be promising for the analysis of AE data monitored from notched composites.

Data analysis procedure

Unsupervised pattern recognition analysis was applied to separate different fracture mechanisms using a set of AE parameters/descriptors. The new in-house developed algo- rithm, consisting of three parts, was used for the analysis of AE monitored signals during the quasistatic tensile loading of centre-hole C/C material [71. For the application of the proposed algorithm, the following assumptions has been made:

(1) all AE descriptors are random variables [6"11,~2]; (2) each AE descriptor exhibits Gaussian distribution

during the clustering[ ~11; (3) each material failure mechanism is characterised by a

single cluster in an unique way and each cluster may characterise at least one failure mechanism, whenever this is possible I5 7r; and

(4) all the failure mechanisms are continuously active during loading with variable intensity TM.

It has been proven that the degree of fulfilment of the above assumptions affects the effectiveness of the developed algorithm. However, within the algorithm a control feedback loop has been built which verifies the validity of the above assumptions and gives the analysis a dynamic character.

Moreover, special care has been taken for noise reduction from the AE set of data under consideration. To that end, some special techniques have been used according to the literature 161, based on the correlation between AE para- meters of amplitude and counts, together with the proposed correlation of all the AE monitored parameters in the form of 2D and 3D graphs, supported by statistical filtering (first part of the algorithm) [71.

In order to proceed to the first part of the algorithm, the question which must be answered is: how many and which AE descriptors will be used during the clustering phase'? In the present analysis, candidate parameters were the amplitude (A), rise time (RT), energy (E), duration (D) and counts (C). Their physical meaning is extensively presented in literature [~31.

The choice of the descriptors will be based on the following demands: limited correlation between the chosen descrip- tors (correlation factor less that 0.6), and optimum classification of the AE data set.

The latter is well defined by many authors '~'~fl~l and it requires after the clustering procedure one type of statistical distribution (not necessarily Gaussian) for each AE para- meters, low parameter variation and reasonable population of events in each cluster.

In order to meet the physical engineering needs of the problem, a special clustering optimising 'trial and error" procedure was accomplished in the proposed algorithm, concerning the selection of descriptors and the number of the clusters.

Furthermore, it was assumed that the number of clusters is governed by the most likely activated (in the sense of AE) failure mechanisms, as was expected.

After the optimum classification of AE data (second part of the algorithm), the data set for every cluster is known. The next step of the algorithm consists of the determination of a very important variable of data analysis, the cluster activation. Activation of a cluster in time is defined as the time derivative of the normalised counts of a cluster. Combining the variation of the activation in time of each cluster and its characteristic AE parameters to the micro- mechanical observation of the material failure mechanisms, one may correlate in a very reliable way clusters and failure mechanisms (third part of the algorithm). Nevertheless, the introduced variable of the cluster activation in time (CAT) represents the progress of each material failure mechanism, at each stage of loading procedure, which is very important in practice.

On the other hand, a number of researchers' 1.2.4,14 16r have extensively discussed the failure mechanisms, which are developed in the case of 2D C/C composites, in the presence of notches and holes under tensile loading. A schematic and systematic presentation of the successive failure modes appearing during the quasistatic loading of 2D C/C in the presence of strain concentration sites is given in Figure 1.

Experimental procedure

Description of the material

A commercially available 2D carbon/carbon was examined. The laminate was reinforced by orthogonally woven eight- harness satin fabric, stacked together in a symmetrical (0°/ 90 ° ) way. The fabric contained high tensile modulus fibres.

- 3 The bulk density of the 2D C/C composite was 1.49 g cm and its total porosity was less than 10%.

Testing procedure

Straight strip specimens 225 mm long, 25.4 mm wide and 3 mm thick (the original plate thickness) were cut out from the 2D C/C composite plate, using a diamond bladed low speed saw. Tapered glass/polyester end-tabs were used for the gripping area of the specimens. Diamond drills were used to prepare the holes. In order to avoid delamination at the hole edge during drilling of the circular holes, two acrylic plastic sheets were placed at both sides of the

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Failure mechanisms analysis of 2D carbon/carbon

specimens. The centre-hole specimens were non- destructively inspected using ultrasonic scanning (immer- sion technique in a specially developed glass jug filled with high viscosity gel coupler, 5 MHz transducer) and those with damaged areas around the hole were rejected.

All tests were carried out on a closed loop servohydraulic testing machine equipped with a hydraulic gripping system at room temperature in air. The tensile tests were conducted using displacement control, and it was applied a crosshead velocity of 0.1 mm rain ~. Strain measurements were peflormed using an extensiometer. During the tests, acoustic emission activity was monitored using a 150 kHz resonant transducer, which was coupled with medium viscosity vacuum grease and held in place with adhesive tape. Finally, AE events were traced by a Physical Acoustic Corporation© (PAC) SPARTAN AT system and the parameters (A, RT, E, D and C) of each event were stored. During the AE monitoring, the following settings have been applied: total amplification was 45 dB, threshold level was 50 dB, peak definition time was 30 #sec and a high pass filter with a 100 kHz cutoff frequency.

Results and discussion

Five groups of specimens were tested. Each group consisted of three specimens with the same hole diameter. The results 0 of the tensile loading of each group are summarised in Table 2.5

4 1. According to these results, centre-hole 2D C/C compo- sites present low notch sensitivity, as was expected for a 6

8 class III CMC material. The formation of the shear band

before the notch provides the stress redistribution and relaxes the stress intensity. A more detailed observation of the stress-strain curves of 2D C/C laminates containing centre holes 171 shows that the greater the hole diameter, the less the inelastic strain appeared, and the more AE activity was monitored for the same strain level. For hole diameter greater than 2.5 ram, the inelastic strain appeared to be limited.

Application of pattern recognition algorithm

In the present work, the k-means pattern recognition (PR) method was used at the proposed algorithm, which minimises the sum of the distances between the clusters, reiteratively. Among the advantages of this method are its simplicity and its ability to treat large populated data sets. Its main disadvantage is the problem of trapping in local minimum, which imposes the need of stable physical intuition and the systematic testing approach.

Table 1 Stress-strain results and AE response of centre-hole 2D C/C composites

d Oul t (sd) Max ~ (sd) Total AE (mm) (MPa) (%) cumulative

counts

173.6 (1.12) 0.357 (0.0094) 29,210 150.5 (1.23) 0.277 (0.0099) 29,100 137.2 (0.27) 0.237 (0.0064) 30,243 114.6 (1.29) 0.214 (0.0092) 28,284 99.5 (1.18) 0.174 (0.0090) 29,490

Figure 1

Tensile Loading

Weak fiber/matrix Interface Low shear strength

+ Shear Band Dominated Matrix Cracking

" d t - - - ~ I ~ - Tunneling Intralaminar Matrix Cracking ~ I "~ in the Shear Band Zone V Mechanism

Matrix Cracking in 90 Plies

, + + Single Fiber Failure Multiple Matrix Crack Bridging Single Fiber Bridging

with Failed and Intact Fibers of Matrix Cracking

""- Long Debonding J

~ - . Matrix Cracking in Fiber Boundles / . /

Matrix Peel-off z.. ~"

Interface Fail (Short Debonding) V

Frictional Sliding V

Fiber Pull-out T

Multi Fiber Failure

v Final Failure

Failure mechanisms of 2D C/C composites containing notches or holes, under tensile loading

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For each specimen group (which represents a different hole diameter), the procedure for the parametric identification of independent AE descriptors was applied successfully, using as a lower correlation level of 60% (correlation factor < 0.6). It was found Ivl that for 2D C/C material and for all five specimen groups, the optimum AE descriptors number was five, namely amplitude, energy, duration, counts, rise time. Using these descriptors, the clustering methodology revealed five clusters. The properties of these clusters for every specimen group are given in Ref. 171

According to the clustering results, there was a considerable overlapping among clusters. Moreover, the distribution of AE parameters for some clusters and some specimen groups after the clustering procedure were not Gaussian (the ideal and easier case), which demands a more careful examina- tion of the further data analyses that have been extensively discussed Ivl.

The cumulative counts for each one of the five clusters as a function of the applied stress are presented in Figures 2 -4 for the case of virgin material specimen group and the specimen groups that contain holes of 4 and 8mm, respectively. As have been reported earlier [6'71, character- istic load intervals representing different stages of damage accumulation could be defined by the slope changes of each cluster curve. A careful examination of the above figures shows that clusters 2 and 5 have significant correlation mainly at the stage of their generation and this becomes more important with increasing the hole diameter. On the other hand, cluster 2 seems to play a dominant role for the complete AE activity of each specimen group and maintains the same form for different hole diameters. Thus, it probably identifies a characteristic fracture mechanism of 2D C/C material such as the formation of the shear band. Finally, clusters 3 and 4 are well correlated in the case of damaged specimens as is shown in Figures 3 and 4, providing that these two clusters correspond to strongly related fracture mechanisms.

The clusters activation in time for each one of the five clusters is presented in Figures 5-7 for the undamaged group of samples and for the specimen groups that contain holes of 4 and 8 mm diameter, respectively. According to these plots, the difference between each cluster AE response is strongly related to hole size. More precisely, it has been observed that the greater the hole diameter, the more delayed the major activation of some clusters appeared. Thus, using these plots of activation of each cluster, together with the variation of its cumulative counts as a function of the applied stress, the reliability of material failure mechanism identification using AE approach increases.

At this point it is important to mention that, according to algorithm results, there was a tendency to decrease the mean value of AE parameters of the same cluster, increasing the hole diameter. Assuming that the mean values of each AE parameter could provide an accurate representation of the characteristics of each cluster [5 71, then the AE response of C/C composites for a given hole diameter, concerning all AE parameters, could be reconstructed using the mean values for each cluster. Figure 8 shows the results of this

reconstruction for every hole diameter. According to this figure, a shifting to lower values is observed, for the values of the parameters C, D and E, which represent the 40-45% of the relative value of the undamaged virgin specimen. The shifting for the RT parameter is about 30%, while A remains

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Figure 2 Cumulat ive counts vs appl ied load after classification of AE response for virgin 2D C/C material

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Figure 4 Cumulat ive counts vs appl ied load after classification of AE response fo r 2D C/C material contain ing a hole of d = 8 mm

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Failure mechanisms analysis of 2D carbon/carbon

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Figure 5 Normalised clusters act ivat ion in t ime for v i rg in 2D C/C material

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161

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Figure 8 Variation of the normalised mean value of AE parameters for each hole diameter

almost unaffected. This shifting of the AE parameters to lower values in the presence of a hole is the result of a combination of two different effects:

(1) the presence of the damage induced in the material at lower load level, which dissipates a greater part of the emitted AE energy (activity), since the generated elastic waves travel through a damaged material struc- ture; and

(2) the scattering of the emitted elastic waves at the different hole boundaries (for the different hole dia- meters), which induces different travel paths of the AE signals within the specimen structure. The lack of

material in the area of the holes also contributes to the above statement.

After a certain hole diameter ( ~ 6 mm), the mean value of all AE parameters increases slightly, since, as has been shown [41, the width of the shear band decreases. Further investigation is needed in that area in order to explain completely the above stated behaviour.

I d e n t i f i c a t i o n of 2 D C / C fa i lu re m e c h a n i s m s

The most difficult step of the proposed algorithm (third part), and not always feasible, is the identification of material failure mechanisms which produce the AE activity, and their correlation to the defined clusters.

As has been mentioned earlier, C/C material is characterised as a class III material, due to the weak fibre/matrix interface, and the lower shear strength compared to the tensile one. Thus, C/C is a fibre failure dominated material. Matrix cracks, extended within the shear band zone, combined with fibre/matrix debonding and single fibre failures, which leads to fibre pullout and frictional sliding, consist of the main stress redistribution mechanisms that offer notch insensitivity to the material.

The following quantities have arisen during the application of the proposed methodology of AE data analysis:

(1) time of each cluster first major activation (FAT), detect- able by AE;

(2) corresponding stress for FAT;

162

Failure mechanisms analysis of 2D carbon/carbon

(3) variation of clusters activation for different hole diameter; (4) population of each cluster; and (5) distributions of AE parameters for each cluster.

Taking into account the variation of the above quantities 171, each cluster could be correlated with sufficient confidence to some of the reported earlier 2D C/C failure mechanisms. These relations are:

(1) cluster l events are connected to short fibre~matrix debonding;

(2) cluster 2 events are related to shear band dominated matrix cracking, including also events related to intra- laminar matrix cracking;

(3) cluster 3 events are produced by single fibre failures, probably including events related to long fibre~matrix debonding;

(4) cluster 4 events represent ,frictional sliding and fibre pullout; and

(5) cluster 5 events are related to multifibre,failures.

Conclusions

Identification of centre-hole 2D carbon/carbon failure mechanisms during tensile loading was performed using an unsupervised pattern recognition approach of acoustic emission monitored signals. A new in-house developed algorithm for AE data analysis was proposed in order to increase reliability and accuracy of the pattern recognition approach. The results of the clustering procedure were con'elated in many ways to the active (in AE sense) failure mechanisms. The presence of the hole and the formed shear band affects the propagation of the elastic waves and the resulting AE signals, since they are scattered at the hole and the crack boundaries. Thus, the hole diameter influences the AE activity, imposing a shifting to the values of AE parameters. The new approach of cluster activation has in time proven very helpful for establishing a cluster failure mechanisms relation.

References

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3 (;orman, M., Acoustic emission in 2D carbon-carbon coupons in tension. J. Camp. Mat., 1991, 25, 703 714.

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12 Botten, S., Use of pattern recognition as a method to evaluate acoustic emission signals. Am. Soc. Mech. Eng., 1993, 257. 256-262.

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