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40TH ANNIVERSARY 3.1.3. Summary of observations of simple cross-ply Nicalon/CAS laminates Damage due to thermal shock in simple cross-ply lami- nates was described and quantified in detail in this sec tion The main damage mechanism was found to be matrix cracking Matrix cracks advanced parallel to the horizon tal in transverse plies and at right angles to the horizontal in longitudinal plies. They were deflected at fibre-matrix interfaces at every quenching temperature investigated, so no fibre failures were observed The (90/0%)s laminate exhibited better resistance to thermal shock than the(0/90%s laminate. However, dam age in both laminates originated in the thick, central ply and then, at higher temperature differentials, extended to adjacent plies. Matrix cracks in both laminates were found to remain Figure 5 PMC bridging li thickness at△T=600°C. shallow, surface features irrespective of the severity of thermal shock loading. However, damage in the form of PMCs was more extensive than damage in the form of 550C, while all of them bridged it at ar=600C be. HMCs in both laminates, especially at higher quenching fore being arrested at the interface between 0 and 90 temperature differences plies(Fig. 5). At the highest temperature differentials (AT=700-8000C), some PMCs could be seen propa- gating a short distance inside the adjacent 90 plies A small number of short HMCs were almost evenly 3. 2. Multi-layer cross-ply Nicalon/CAS distributed between the TI plies at all temperature dif- laminates ferential investigated. The increase in AT resulted in a 3.2.1. The (0/90 l3s laminate moderate increase in their length in most cases The description of thermal shock damage on this laminate The depth and opening of both PMCs and is given with reference to the nomenclature of Fig. 7 HMCs were not altered by the application of For this Nicalon/CAS laminate AT =350°C. The form higher temperature differentials. They both remained of thermal shock damage observed was matrix crackin surface features throughout the temperature range which can be further divided into PMCs and HMCs. The investigated. fibres remained unaffected at all she OCKs The number of PMCs increased significantly for higher HMCs were the first form of damage observed after of HMCs shows only quenching through AT=350C. They were located ex- a moderate increase. The large difference in the rate clusively in 90 plies. Depending on the specimen un- of increase between the two types of matrix crack- der observation, these cracks emanated either from flaws, is evident in the graph of Fig. 6. It can be such as pores, and were contained inside the ply, or orig seen that at all ATs, about 2/3 of the total thermal inated from the edges of the ply and ran towards its cen shock damage is due to the formation and extension of tre. They were continuously deflected at successive fibre PMCS matrix interfaces PMCs were detected on the surfaces of thermally 1.8 shocked specimens, exclusively in 0 plies, after quench 1.6(90/0) SiC/CAS ing through AT=400oC. These cracks ran perpendicular to the horizontal (i.e. to the longitudinal fibres of the 0o 攴12 plies), leaving the fibres on their path unaffected, and ar- rested either at a fibre-matrix interface inside the ply or at 08 the interfaces between 00 and 90 plies 20.6 The evolution of both types of damage with increasing 04 FIl applied AT can be seen in the sequence of reflected light microscopy images of Fig. 8. At△T=350400° C only random HMCs could be gen 500 550 erally seen in the thick, central transverse ply (T1). How ever, a much longer crack was also evident in some spec Figure6 Crack densities of PMCs and HMCs and the total crack density imens quenched at this temperature differential. These at each AT Relevant trends for each damage mode are also shown. cracks were limited to the surface of the material40TH ANNIVERSARY Figure 5 PMC bridging L1 thickness at T = 600◦C. 550◦C, while all of them bridged it at T = 600◦C before being arrested at the interface between 0◦ and 90◦ plies (Fig. 5). At the highest temperature differentials (T = 700–800◦C), some PMCs could be seen propagating a short distance inside the adjacent 90◦ plies. A small number of short HMCs were almost evenly distributed between the T1 plies at all temperature differentials investigated. The increase in T resulted in a moderate increase in their length in most cases. The depth and opening of both PMCs and HMCs were not altered by the application of higher temperature differentials. They both remained surface features throughout the temperature range investigated. The number of PMCs increased significantly for higher Ts while the crack density of HMCs shows only a moderate increase. The large difference in the rate of increase between the two types of matrix cracking is evident in the graph of Fig. 6. It can be seen that at all Ts, about 2/3 of the total thermal shock damage is due to the formation and extension of PMCs. Figure 6 Crack densities of PMCs and HMCs and the total crack density at each T. Relevant trends for each damage mode are also shown. 3.1.3. Summary of observations of simple cross-ply Nicalon/CAS laminates Damage due to thermal shock in simple cross-ply laminates was described and quantified in detail in this section. The main damage mechanism was found to be matrix cracking. Matrix cracks advanced parallel to the horizontal in transverse plies and at right angles to the horizontal in longitudinal plies. They were deflected at fibre-matrix interfaces at every quenching temperature investigated, so no fibre failures were observed. The (90◦/0◦)s laminate exhibited better resistance to thermal shock than the (0◦/90◦)s laminate. However, damage in both laminates originated in the thick, central ply and then, at higher temperature differentials, extended to adjacent plies. Matrix cracks in both laminates were found to remain shallow, surface features irrespective of the severity of thermal shock loading. However, damage in the form of PMCs was more extensive than damage in the form of HMCs in both laminates, especially at higher quenching temperature differences. 3.2. Multi-layer cross-ply Nicalon/CAS laminates 3.2.1. The (0◦/90◦)3s laminate The description of thermal shock damage on this laminate is given with reference to the nomenclature of Fig. 7. For this Nicalon/CAS laminate Tc = 350◦C. The form of thermal shock damage observed was matrix cracking, which can be further divided into PMCs and HMCs. The fibres remained unaffected at all shocks. HMCs were the first form of damage observed after quenching through Tc = 350◦C. They were located exclusively in 90◦ plies. Depending on the specimen under observation, these cracks emanated either from flaws, such as pores, and were contained inside the ply, or originated from the edges of the ply and ran towards its centre. They were continuously deflected at successive fibrematrix interfaces. PMCs were detected on the surfaces of thermallyshocked specimens, exclusively in 0◦ plies, after quenching through T = 400◦C. These cracks ran perpendicular to the horizontal (i.e. to the longitudinal fibres of the 0◦ plies), leaving the fibres on their path unaffected, and arrested either at a fibre-matrix interface inside the ply or at the interfaces between 0◦ and 90◦ plies. The evolution of both types of damage with increasing applied T can be seen in the sequence of reflected light microscopy images of Fig. 8. At T = 350–400◦C only random HMCs could be generally seen in the thick, central transverse ply (T1). However, a much longer crack was also evident in some specimens quenched at this temperature differential. These cracks were limited to the surface of the material. 955