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40TH ANNIVERSARY Figure 7 The nomenclature used to describe damage due to thermal shock on a(0 /90%)3s laminate. At AT=400C PMCs appeared in the 0 plies(L1) In general, the application of higher ATs did not affect adjacent to TI while longer, random HMCs could again the morphology of PMCs. In contrast, HMCs located in be seen in Tl Damage, in the form of PMCs and HMCs, transverse plies at or close to the centreline of the face appeared in the 0 plies designated as L2 and the 90 plies became deeper and their opening, as well as their length, designated as T2, respectively, at AT=450C. At this increased significantly at the highest temperature differ temperature differential, a long HMC propagated along entials investigated(Fig 9) the central TI ply while in the t2 plies only individual PMCs were evenly distributed between the longitudinal HMCs appeared plies of the designation (i.e. Ll, L2 or L3). This was Similar patterns were observed at AT=500C. How- not exactly the case for HMCs as these were distributed in ever, almost all PMCs spanned the thickness of the L1, L2 a more random fashion, especially at the higher temper plies while some started to extend into the adjacent 90 ature differentials, between the pairs of transverse plies plies. In addition, some HMCs in the T2 plies connected (i.e. Tl, T2 or T3)depending on the specimen under in- to form longer cracks vestigation. At AT=600C all plies of this system sustained some The increase in PMc density with increasing shock form of thermal shock damage; TI contained a long, deep severity for each set of longitudinal plies (Ll, L2, L3)is HMC, T2 exhibited shorter and shallower HMCs, while shown in the graph of Fig. 10a. It is evident that crack individual, random HMCs could be seen in T3. In addi- density is always higher for the plies located towards the tion, all longitudinal plies (Ll, L2, L3)contained PMCs, centre of the sample surface, i.e. CDLI>CDL2>CDL3 at the number of which decreased on going from the cen- each AT investigated(CD: Crack Density). The rates of treline(C-C) towards the top or bottom edges of the increase of cracking in each set of plies are comparable surface Fig. 10b shows the change in HMC density with in- The application of even higher ATs(700-800oC)lead creasing temperature differential. A significant increase to an increase in the number of PMCs in the longitudinal in cracking can be observed, especially at the higher ther plies, although it again looked as if the plies closer to the mal shocks. The scatter in experimental data is larger at centreline had higher densities of these cracks than those the higher temperature differentials, which reflects the further away. In addition, some PMCs(especially in LI) randomness in the appearance and point of origin of long could be seen to extend into the adjacent transverse plies cracks in Tl and/or T2. TI and T2). HMCs followed a more random pattern. Comparison between PMCs and HMCs(Fig. 10b)re- There was always a long, deep crack that travelled along veals that the rate of increase in density of PMCs is much almost the full length of the ply in either Tl or T2. The higher than that of HMCs and, at high temperature differ- rest of these plies contained shorter and shallower cracks entials, PMCs are the major contribution to the total crack while the cracks located in T3, although continuously density. However, this graph fails to capture the signifi- increasing in number and length, failed to connect into cant differences in morphology between the two types of longer hmcs even at the highest at matrix cracking at△T≥600°C40TH ANNIVERSARY Figure 7 The nomenclature used to describe damage due to thermal shock on a (0◦/90◦)3s laminate. At T = 400◦C PMCs appeared in the 0◦ plies (L1) adjacent to T1 while longer, random HMCs could again be seen in T1. Damage, in the form of PMCs and HMCs, appeared in the 0◦ plies designated as L2 and the 90◦ plies designated as T2, respectively, at T = 450◦C. At this temperature differential, a long HMC propagated along the central T1 ply while in the T2 plies only individual HMCs appeared. Similar patterns were observed at T = 500◦C. However, almost all PMCs spanned the thickness of the L1, L2 plies while some started to extend into the adjacent 90◦ plies. In addition, some HMCs in the T2 plies connected to form longer cracks. At T = 600◦C all plies of this system sustained some form of thermal shock damage; T1 contained a long, deep HMC, T2 exhibited shorter and shallower HMCs, while individual, random HMCs could be seen in T3. In addition, all longitudinal plies (L1, L2, L3) contained PMCs, the number of which decreased on going from the centreline (C-C’) towards the top or bottom edges of the surface. The application of even higher Ts (=700–800◦C) lead to an increase in the number of PMCs in the longitudinal plies, although it again looked as if the plies closer to the centreline had higher densities of these cracks than those further away. In addition, some PMCs (especially in L1) could be seen to extend into the adjacent transverse plies (T1 and T2). HMCs followed a more random pattern. There was always a long, deep crack that travelled along almost the full length of the ply in either T1 or T2. The rest of these plies contained shorter and shallower cracks while the cracks located in T3, although continuously increasing in number and length, failed to connect into longer HMCs even at the highest T. In general, the application of higher Ts did not affect the morphology of PMCs. In contrast, HMCs located in transverse plies at or close to the centreline of the face became deeper and their opening, as well as their length, increased significantly at the highest temperature differentials investigated (Fig. 9). PMCs were evenly distributed between the longitudinal plies of the same designation (i.e. L1, L2 or L3). This was not exactly the case for HMCs as these were distributed in a more random fashion, especially at the higher temperature differentials, between the pairs of transverse plies (i.e. T1, T2 or T3) depending on the specimen under investigation. The increase in PMC density with increasing shock severity for each set of longitudinal plies (L1, L2, L3) is shown in the graph of Fig. 10a. It is evident that crack density is always higher for the plies located towards the centre of the sample surface, i.e. CDL1>CDL2>CDL3 at each T investigated (CD: Crack Density). The rates of increase of cracking in each set of plies are comparable. Fig. 10b shows the change in HMC density with increasing temperature differential. A significant increase in cracking can be observed, especially at the higher thermal shocks. The scatter in experimental data is larger at the higher temperature differentials, which reflects the randomness in the appearance and point of origin of long cracks in T1 and/or T2. Comparison between PMCs and HMCs (Fig. 10b) reveals that the rate of increase in density of PMCs is much higher than that of HMCs and, at high temperature differentials, PMCs are the major contribution to the total crack density. However, this graph fails to capture the signifi- cant differences in morphology between the two types of matrix cracking at T≥600◦C. 956