Fatigue damage accumulation in 3-D SiC/SiC composites 213 fatigue limit)then the rate of increase of the mate- response monitoring of the material the more pro- rial damping for all the first three eigenfrequencies nounced the measured variation is remains high but slightly lower compared to one appearing during the first stage, with the 3.2 Correlation with acoustic emission results exception of the damping monitored for the first As has already been mentioned, during fatigue eigenfrequency experiments at h maximum applied load After the threshold of 150 kcycles, all the sur- levels, continuous AE monitoring was applied. riving specimens appeared with a higher rate of Then, pattern recognition techniques were applied increase for the material damping as they approach and the events which are related to fibre failure the end of their life. This was valid for all the have been separated in order to be correlated to the eigenfrequencies. What it is very interesting to stiffness degradation presented in the former Sec- notice here is that the increase of damping during tion. More precisely, the applied pattern recogni he first 25 kcycles is of the order of 170%, while tion technique provides a new analysis algorithm the maximum damping increase for loading up to of ae data and has been proposed elsewhere. 3It 0-7 of UTS was 320% and for loading up to 0.75 of contains descriptor selection procedures, validation UTS was 500%. Comparison of these values to the steps, filtering and statistical analysis of AE data, relative variation of effective dynamic modulus of taking into account the stochastic character of AE elasticity, indicates that a loss factor or equiva- events and the failure mechanisms appearing in the lently the damping of the relative eigenmodes is a specific material under consideration. a key point much more sensitive parameter to fatigue damage of the used algorithm is the cluster activation in compared to the effective modulus of elasticity. time informing which increases the reliability of Furthermore, the fact that during the last stage correlation between clusters and failure mechan of fatigue life(for maximum applied stress 0-75 isms. The corresponding results are given in Figs 5 of UTS), modal damping and the corresponding and 6 loss factor has a higher rate of increase That Both Figs 5 and 6 represent the AE events, could be easily explained by the development of which correspond to fibre breakage, as a function fibre failures which will finally lead the fatigue of the fatigue cycles for maximum applied stress sample to failure and could be used as an indicator of 0.7 and 0.75 respectively. The broken lines to the stage of damage, forming a predictor for the both figures indicate the degradation of the nor on-coming material failure due to fatigue. The malise effective dynamic modulus versus fatigue failure fibres create a new friction surface and cycles establish new energy dissipation mechanisms, In general, these types of diagrams, have a bath which are activated during the transverse vibration type shape that could be derived in three different of the specimen and are monitored as damping regions:(i)a Burn-in phase, (ii)a Steady-state region and (iii)a Burn-out stage. Regarding the Table 2 summarises the maximum normalised Burn-in phase (in both Figures up to 25 kcycles),it variations measured by using the dynamic response accounts for fibre fracture events during the appli monl itoring during the fatigue of 3-D SiC/Sic cation of fatigue mean load and the first cycles of composites under two different maximum applied the fatigue test. The steady-state phase represents stress levels an increase of damage. which weakens the material According to these results, in general, damping structure, causing degradation of its performance coefficient is a much more sensitive damage ind cator compared to the effective dynamic modulus of elasticity and additionally the higher the eigen- Fatigue test for omux/ut 07 frequency and the eigenmode used for the dynami Table 2. Dynamic response monitoring of 3-D SiC/SiC of amax/oudt Maximum applied (Exx o)(nxx/nxxo) ue cycles nIck Steady-state f2:0.9 3-012 f3:0.89 Number of Fatigue keycles 0-75 2716 f:0-85 Fig. 5. Correlation between AE activity related to fibre frac- f3:0-77 6-084 ture and normalised effective dynamic modulus for fatigue loading of 3-D SiC/Sic up to 0.7 of UTSfatigue limit) then the rate of increase of the mate￾rial damping for all the ®rst three eigenfrequencies remains high but slightly lower compared to the one appearing during the ®rst stage, with the exception of the damping monitored for the ®rst eigenfrequency. After the threshold of 150 kcycles, all the sur￾viving specimens appeared with a higher rate of increase for the material damping as they approach the end of their life. This was valid for all the eigenfrequencies. What it is very interesting to notice here is that the increase of damping during the ®rst 25 kcycles is of the order of 170%, while the maximum damping increase for loading up to 0.7 of UTS was 320% and for loading up to 0.75 of UTS was 500%. Comparison of these values to the relative variation of e€ective dynamic modulus of elasticity, indicates that a loss factor or equiva￾lently the damping of the relative eigenmodes is a much more sensitive parameter to fatigue damage compared to the e€ective modulus of elasticity. Furthermore, the fact that during the last stage of fatigue life (for maximum applied stress 0.75 of UTS), modal damping and the corresponding loss factor has a higher rate of increase. That could be easily explained by the development of ®bre failures which will ®nally lead the fatigue sample to failure and could be used as an indicator to the stage of damage, forming a predictor for the on-coming material failure due to fatigue. The failure ®bres create a new friction surface and establish new energy dissipation mechanisms, which are activated during the transverse vibration of the specimen and are monitored as damping increase. Table 2 summarises the maximum normalised variations measured by using the dynamic response monitoring during the fatigue of 3-D SiC/SiC composites under two di€erent maximum applied stress levels. According to these results, in general, damping coecient is a much more sensitive damage indi￾cator compared to the e€ective dynamic modulus of elasticity and additionally the higher the eigen￾frequency and the eigenmode used for the dynamic response monitoring of the material the more pro￾nounced the measured variation is. 3.2 Correlation with acoustic emission results As has already been mentioned, during fatigue experiments at both maximum applied load levels, continuous AE monitoring was applied. Then, pattern recognition techniques were applied and the events which are related to ®bre failure have been separated in order to be correlated to the sti€ness degradation presented in the former Sec￾tion. More precisely, the applied pattern recogni￾tion technique provides a new analysis algorithm of AE data and has been proposed elsewhere.13 It contains descriptor selection procedures, validation steps, ®ltering and statistical analysis of AE data, taking into account the stochastic character of AE events and the failure mechanisms appearing in the speci®c material under consideration. A key point of the used algorithm is the cluster activation in time informing which increases the reliability of correlation between clusters and failure mechan￾isms. The corresponding results are given in Figs 5 and 6. Both Figs 5 and 6 represent the AE events, which correspond to ®bre breakage, as a function of the fatigue cycles for maximum applied stress of 0.7 and 0.75 respectively. The broken lines in both ®gures indicate the degradation of the nor￾malised e€ective dynamic modulus versus fatigue cycles. In general, these types of diagrams, have a bath￾type shape that could be derived in three di€erent regions7 : (i) a Burn-in phase, (ii) a Steady±state region and (iii) a Burn-out stage. Regarding the Burn-in phase (in both Figures up to 25 kcycles), it accounts for ®bre fracture events during the appli￾cation of fatigue mean load and the ®rst cycles of the fatigue test. The steady-state phase represents an increase of damage, which weakens the material structure, causing degradation of its performance Table 2. Dynamic response monitoring of 3-D SiC/SiC of composites max/ult Maximum applied fatigue cycles (kcycles) (Exxc,e/Exxc,e0) min (nxxe /nxxe 0) max 0.7 1000 f1: 0.96 2.213 f2: 0.93 3.012 f3: 0.89 4.258 0.75 174 f1: 0.89 2.716 f2: 0.85 5.342 f3: 0.77 6.084 Fig. 5. Correlation between AE activity related to ®bre frac￾ture and normalised e€ective dynamic modulus for fatigue loading of 3-D SiC/SiC up to 0.7 of UTS. Fatigue damage accumulation in 3-D SiC/SiC composites 213