2140 Journal of the American Ceramic Society-Mei et al Vol. 90. No. 7 decreases with increasing cycles. The decreasing rate of AE energy reveals that the tested composite is certain to attain a steady state during which the initiation of the new cracks and/or the propagation of the previous cracks will be terminated tran- sitorily for that specific applied stress and given temperature 24000 gradient conditions. If the applied stress or temperature gradient AT is increased, more cracks would be formed and propagate. and the dynamic equilibrium between the internal microstruc ures and the external applied conditions would be disturbed until reaching a new equilibrium. Additionally, noise is unavoid- 9600 able for AE measurement and cannot be removed completely during testing. Thus, the accumulated AE energy still exhibits a 4800 ing trend after 15 cycles, although its stepwise form becomes considerably weak In addition to the above qualitative evidence, it is interesting to discuss quantitatively the damage evolution during thermal Thermal cycling Number, N cycles. An energy percentage in a certain domain between N, and n, is define Fig. 10. umulated acoustic emission energy versus thermal cycle number N curve for the C/SiC composites during the thermal cycles. En noticeable that these results imply that the main changes of the matrix structure(matrix cracking) should appear in the temper- ture intervals of the ae events. The greater the microstructural changes, the stronger the ae events and the more severe the Where En, Etotal refer to the AE energy at cycle n and the total Figure 10 presents an entire accumulated AE energy versus accumulated energy. Thus, the energy percentages per five cy thermal cycle number N curve for the composite specimen dur les’ interval are31.5%N∈[0,527.6%N∈[5,10]22.9% ing testing. The stepwise increasing AE energy is found to hav ∈[10.15]10.2%N∈[5,20].and7.8%N∈[20,25], respec a decay rate as N proceeds, and increases rather modestly ively. The AE energy before 15 cycles (i.e, 60% of the maxi- cycles. More importantly, the incremental amount of mum 25)has reached 82% of the nergy nergy per one cycle decreases with increasing N and the that the major damage takes place the initial cycles form of the AE energy almost disappears after Furthermore, the decreasing energy percentage implies that 15 cycles. This is in good agreement with the critical cycle num- the newly created damage pre-cycle diminishes gradually with ber Ne, whose TCC strain rate is zero as determined in Fig. 5. N, and eventually becomes saturated when the critical value Thus, the ae data also confirm the notion that matrix damage arrive b Fig. 11. Micrographs showing initiation and evolution of the coating cracks, as well as the matrix cracks, of the C/SiC composites with thermal cycles. be determined as: (a)1. 8 mm- after five cycles, (b)3.5 mm- after 10 cycles. (c)4 I after 15 cycles, and (d)5.1 mm-Iafternoticeable that these results imply that the main changes of the matrix structure (matrix cracking) should appear in the temper￾ature intervals of the AE events. The greater the microstructural changes, the stronger the AE events and the more severe the damage induced. Figure 10 presents an entire accumulated AE energy versus thermal cycle number N curve for the composite specimen dur￾ing testing. The stepwise increasing AE energy is found to have a decay rate as N proceeds, and increases rather modestly after 15 cycles. More importantly, the incremental amount of the AE energy per one cycle decreases with increasing N and the stepwise form of the AE energy almost disappears after 15 cycles. This is in good agreement with the critical cycle num￾ber Nc, whose TCC strain rate is zero as determined in Fig. 5. Thus, the AE data also confirm the notion that matrix damage decreases with increasing cycles. The decreasing rate of AE energy reveals that the tested composite is certain to attain a steady state during which the initiation of the new cracks and/or the propagation of the previous cracks will be terminated tran￾sitorily for that specific applied stress and given temperature gradient conditions. If the applied stress or temperature gradient DT is increased, more cracks would be formed and propagate, and the dynamic equilibrium between the internal microstruc￾tures and the external applied conditions would be disturbed until reaching a new equilibrium. Additionally, noise is unavoid￾able for AE measurement and cannot be removed completely during testing. Thus, the accumulated AE energy still exhibits a slow increasing trend after 15 cycles, although its stepwise form becomes considerably weak. In addition to the above qualitative evidence, it is interesting to discuss quantitatively the damage evolution during thermal cycles. An energy percentage in a certain domain between N1 and N2 is defined as PE ½N1;N2 ¼ PN2 n¼N1 En Etotal % (9) Where En, Etotal refer to the AE energy at cycle n and the total accumulated energy. Thus, the energy percentages per five cy￾cles’ interval are 31.5% NA[0, 5], 27.6% NA[5, 10], 22.9% NA[10, 15], 10.2% NA[15, 20], and 7.8% NA[20, 25], respec￾tively. The AE energy before 15 cycles (i.e., 60% of the maxi￾mum 25) has reached 82% of the total energy, indicating that the major damage takes place within the initial cycles. Furthermore, the decreasing energy percentage implies that the newly created damage pre-cycle diminishes gradually with N, and eventually becomes saturated when the critical value Nc arrives. Fig. 10. Accumulated acoustic emission energy versus thermal cycle number N curve for the C/SiC composites during the thermal cycles. Fig. 11. Micrographs showing initiation and evolution of the coating cracks, as well as the matrix cracks, of the C/SiC composites with thermal cycles. The crack density can be determined as: (a) 1.8 mm1 after five cycles, (b) 3.5 mm1 after 10 cycles, (c) 4.9 mm1 after 15 cycles, and (d) 5.1 mm1 after 25 cycles, respectively. 2140 Journal of the American Ceramic Society—Mei et al. Vol. 90, No. 7