Mechanical Behavior and Acoustic Response of Carbon/Carbon Composite with Different Densities

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1 Mechanical Behavior and Acoustic Response of Carbon/Carbon Composite with Different Densities Arie Bussiba *, Romana Piat, Thomas Boehlke, Rami Carmi, Igal Alon, Moshe Kupiec Nuclear Research Centre Negev, P.O. Box 9, Beer-Sheva, 9 (Israel) Institute of Engineering Mechanics, University of Karlsruhe (TH), Postfach 69, 76, Karlsruhe (Germany) * Corresponding author. busarie@bezeqint.net Abstract. C/C composites with different densities, produced by chemical vapor infiltration (CVI) are tested mechanically under quasi-static loading and tracked simultaneously with Acoustic Emission (AE) technique. Threshold mechanical values varying with composite density are detected by AE activity which indicates the damage onset. Three stages in damage accumulation profile up to fracture are found in terms of AE response. Similarity in profile and threshold value are found between the cumulative AE counts vs. strain and crack density vs. strain, predicted by the micro mechanical model, indicating the importance of monitoring structural integrity by the AE method. Wave s analysis points out on four possible failure mechanisms: multilayer cracking, breaking of fiber bundles, interfacial matrix de-bonding and micro-crack growth.. Introduction Great efforts are being devoted to developing highly reliable carbon fiber reinforced carbon matrix composites due to their unique physical and mechanical properties, especially at high temperature service up to C []. Thus, it is of crucial importance to explore influences of structural and morphological parameters during the optimization of the manufacturing process on the micro-failure mechanisms which control the macroscopic mechanical properties []. Moreover, enhancing and integrating such composites in load bearing engineering components requires special attention as to the assessment and ensuring of structural integrity as well as life prediction procedures. From the material and engineering point of view, in the future these parallel activities will increase the potential of such composites to be good candidates with respect to structural materials for aerospace and other applications. In the current study we apply the AE technique among the powerful non-destructive evaluation (NDE) methods [], to characterize mechanical properties, damage initiation and growth in C/C composites with different densities. Thereby, we emphasize on two aspects: First, on using AE as NDE technique for monitoring and for the control of the fabrication process in order to assure the quality of composites and components. Second, applying AE data (such as threshold values, type of damage accumulation etc..) in order to improve the reliability of structural integrity evaluation during service life. In addition, using the advanced analysis of AE data including waveform processing using Fast Fourier transform (FFT), Short-Time Fourier Transform (STFFT) [] and Wavelet can point out the active micro failure mechanisms and highlight any possible sequence of events during the fracture process. Finally, the work concludes with a comparison to a micro-mechanical model developed for some composites with the AE experimental data.. Material and Experimental Methods The tested C/C composites were produced by isothermal, isobaric CVI process, which consists of a synthesis at high temperature reactor of carbon particles from hydrocarbon gas and their deposition on carbon fiber fetls (CCKF, Sintec, Germany). The temperature used was 95 C with a total pressure of kpa, and methane has been used as the precursor gas. The infiltration was carried out at the Institute for Chemical Technology of the University of Karlsruhe in a vertical gap reactor. The carbon particles were deposited in the form of a pyrolytic carbon matrix around the fibers resulting in a layered morphology [5]. The fiber volume content was about % (of the total volume) and the diameter of the fiber was of the order of µm. The felts were produced using a different infiltration time:, 5 and hours, obtaining matrices with a densities of. (high density- HD),.5 (medium density-md) and.5 (low density-ld) g/cm, respectively. Further details on the infiltration procedure are given elsewhere [6]. The typical microstructure of the HD composite is shown in Figure a, and the layered structure around the fibers was observed at the fracture surface using scanning electron microscopy (Figure b). Flexural stress and fracture toughness tests were conducted on uniform (5 x 7 x 55 mm) and notched beams with V-notch radius of µm and length of mm, using four point bending (inner and outer span of and mm) set-up. The deflection was measured by a sensitive extensometer (.5mm/mm full scale) and the crack opening displacement was measured by the crack opening gauge with full scale of.mm. Quasi-static loading with

2 velocity of.5 mm/min was carried out using a computer controlled machine with a load cell of kn. The flexural properties were calculated according to the standard expressions given in ASTM D79-95a, and the fracture toughness was calculated following the expression specified in ASTM E99-9. The AE technique was used to detect the early damage during loading and was quantified by means of mechanical properties noted as threshold parameters. AE activity was tracked continuously to characterize the damage accumulation profile up to fracture. The AE data such as counts, amplitude, duration, peak frequency and more, was analyzed in order to highlight any possible fracture sequence events. This was also based on waveforms analyses by applying FFT and ST-FFT functions in order to indicate any potential active failure mechanisms (fibers fracture, matrix and interface cracking, micro-crack growth). More details on the AE set-up sensors, amplification and threshold values are given in [7]. The crack path was characterized by an optical microscopy, and the micro-failures modes were classified using a scanning electron microscopy. b) Figure : Polarized light microscopy micrographs showing microstructure of the HD composite, the layered morphology around the fibers as revealed after fracture.. Experimental Results Figure illustrates the typical mechanical behavior and the corresponding AE response in terms of counts rate in both flexural and toughness tests. As shown, there is an almost linear behavior characterizing the flexural stress vs. strain curve, however, the AE counts rate intensified as the stress increases up to the fracture (Figure a). The threshold strain (e th ) and the equivalent stress ( ) are noted and represent the early damage progress monitored by AE. The crack resistance curve shows a linear portion accompanied by discontinuous slow crack growth (Figure b). These irregularities manifested by the crack extension, arrest and re-occur alternately up to fracture. In contrast to the gradual increase in the AE counts rate in the former case, here the rate is almost constant at the linear portion of the curve followed by peaks in the AE counts (dashed arrows in Figure b), whenever localized crack growth occurs. This type of AE profile points out the damage accumulation mechanism of discrete crack jump. Again, the onset of the damage in the presence of notch is marked by the threshold crack opening displacement (COD th ) with the related stress intensity factor (K Ith ) HD e th AE counts rate (x ) K (MPa.m / ) MD K Ith COD th...6. COD (mm) Figure : Mechanical and acoustic responses for selected density composites in: flexural test, fracture toughness test. K Ilin AE counts rate (x )

3 In addition to the damage onset detected by AE activity, the damage evolution profile with loading progress may be reflected by AE cumulative counts for both tests, as shown in Figure. While an almost linear mechanical response is well observed in the flexural mode, an exponential behavior is obtained for the AE cumulative display (Figure a). Actually, there is an intersection point where the damage behavior is being changed from a linear to a major exponential shape. For the notched one, this type of AE response is characterized up to the linear portion of the curve (denoted by K Ilin ), and the deviation from this behavior is due to the initiation of an incremental micro crack growth. Thus, the damage evolution detected by AE data in this composite with different densities follows the exponential function of the strain or COD. 5 MD Intersection point Cumulative AE counts (x ) K (MPa.m / ).6.. LD K Ilin...6 COD (mm) 5 Cumulative AE counts (x 5 ) Figure : Damage evolution profile in terms of AE cumulative counts for different C/C composites in: flexural test, fracture toughness test. σf, σth (MPa) σ f /σ f /σ f KIC, ΚIth (MPa.m / ).6... K Ith K IC K Ith /K IC KIth/KIC Density (g/cm ) Density (g/cm ) Figure : Threshold and critical mechanical values vs. composite density in: flexural test, fracture toughness test Figure depicts and summarizes the influence of the composite density on the threshold and critical mechanical values as well as on the normalized display for both tests. As demonstrated, any decrease in composite density causes a significant decrease of the flexural stress (σ f ), of the critical toughness property as well of the threshold values. In a normalized representation ( /σ f, K Ith /K IC ), for both tests, a decrease in density results in a significant decrease in the threshold stress value and in the resistance to the damage onset. For example, the normalized value for the LD is close to zero which points out that almost no threshold is detected.

4 . Discussion The importance of threshold values in fatigue or stress corrosion cracking in metallic materials is well documented and recognized. In composite materials, it is more complicated to detect any thresholds by means of mechanical testing. Alternately, the use of AE as NDE method indicates clearly the existence of threshold values, below of which no damage is being initiated. Actually, three stages of damage evolution in terms of cumulative AE counts can be noticed from the curve profile shown in Figure 5a. The first one ends with a threshold stress as noticed also in other composite materials []. Here, no AE activity is noticed followed by a reversible mechanical behavior. The second stage is almost linear and ends with a significant jump in the cumulative AE counts (denote by two dashed arrows), a phenomenon which indicates a damage intensification by the coalescence process of micro-defects or cracks, followed by a third stage with an exponential-type behavior up to the final fracture. A similar type of damage accumulation behavior was reported for Glass fiber epoxy [9]. In addition, the threshold parameters for the C/C composites are not accompanied by a measurable change in the stiffness property, and the AE activity is the only means of tracking threshold mechanical value. However, in PVC/glass-fiber composite [9] depicted in Figure 5b, the threshold is associated with linear deviation and a stiffness decreasing with further loading. Namely, there is an agreement of the threshold behavior in both mechanical and acoustic responses, and here the latter is complimentary evidence. MD "Jump" ΙΙΙ...6 Ι ΙΙ... Cumulative AE counts (x ) Stress (MPa) 5 PVC- wt% Glass....6 Strain (%) Cumulative AE events (x ) Figure 5 : The three stages in damage accumulation for C/C MD composite, threshold stress and deviation from linearity in stress-strain curve for PVC/glass-fiber composite. LD Density level.6 KHz (c) Flexural Stress (M Pa) Peak frequency (M Hz) FFT magnitude (V).. KHz 9 KHz 55 KHz 6 Frequency (KHz) Amplitude ( -7 V) Multilayered cracking Interfacial matrix debonding Micro-crack growth Bundle fibers break-up KHz 55 KHz 6 Time (µsec) khz 9 KHz Figure 6 : AE waves analyses in LD composite: stress and peak frequency with its density vs. strain, FFT with frequency domain, (c) ST-FFT with frequency and time domains data. Typical global and local analyses of AE data are presented in Figure 6, and some argumentations on the active microfailure mechanisms are being derived with the conjunction of fracture surface tracking. The peak frequencies (PFs) density and the stress vs. the flexural strain are shown in Figure 6a. Four main PFs are observed up to fracture with a different level of density. The dominant one appears at khz, and its density increases as the strain increases. The second one appears at khz and its density increases moderately as approaching to the final fracture. The third one appears at 9 khz where it remains almost unchanged in the density up to fracture, and the final one begins to be dominant at 55 khz at the later stage of loading. The FFT analysis of an AE wave taken close to fracture shows four

5 characteristic frequencies in the order from low to high, as mentioned above. During the domain display using ST- FFT, the order of the appearance of frequencies are:, 9, 55, KHz (Figure 6b). From the above results we can assume that four dominant micro-mechanisms controlled the fracture process in the LD composite. At the early stage of loading, low and medium frequencies are dominant and maybe related to the cracking of the multilayered matrix (Figure 7a) and interface matrix de-bonding, respectively. With further loading, the dominant frequency is related to the fiber failure (Figure 7b), and the high frequency maybe associated with the propagation of micro-cracks (Figure 7c). The occurrence sequence of the mentioned micro-failure mechanisms is shown in Figure 6c, whereas the multilayered cracking is the first to be active, followed by matrix de-bonding, micro-cracks, and finally, by bundle fibers failure. A comparison with the micro-mechanical model developed by Talreja [] is suggested for inter-laminar cracking in composite laminates. Talreja determined that the damage parameter for a bi-linear stress strain curve in terms of crack density follows an exponential behavior of the strain, which is shown schematically in Figure a. This prediction was found experimentally as well as in the current study for the LD composite with an almost bi-linear curve (Figure b). As shown, the micro-mechanical model predicts also a threshold strain e which has also been detected in the AE experimental data. (c) Figure 7 : Micro-failures mechanisms: multilayered cracking, bundles fiber failure, (c) micro crack growth. σ Stress Bi-Linear curve η Cracks Density... e Strain e Figure : Comparison between the predicted damage profile and threshold parameter to the AE experimental data Talreja micro-mechanical model [], LD C/C composite. 5. Conclusion The current research emphasizes the efficiency of the AE method in detecting very early damage in C/C composite which can be quantified by means of threshold values. This finding is important from the structural integrity point of view, where conventional methods failed to detect and evaluate damage initiation. In addition, the AE data characterizes the damage accumulation profile up to fracture. By using AE wave s analysis, one can as well point out the dominant failures mechanisms and the sequence events during the fracture process. Acknowledgement Financial support from the Deutsche Forschungsgemeinschaft (DFG), Bonn, Germany under the Project No. BO66/- is gratefully acknowledged. 6 LD Cumulative AE counts (x )

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