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40TH ANNIVERSARY 10m Figure 3 Photomicrograph of PMC in LI at AT=500C that arrests inside 0 ply could be seen traversing the thickness of ll at at 2.5 800C. while 1-2 longer HMCs ran along the length of O/90),SiC/CAS 2 The application of higher temperature differentials did not result in significant morphological changes in either HMCs or PMCs. both damage mechanisms remained sur- face features of small depth. At all temperature differen- tials PMCs were evenly distributed between the two Oo 0.5 plies termed LI Crack densities for HMCs and Pmcs were determined 450 in terms of crack length per unit area(mm/mm or mm-) 5005506 in order to allow a comparison to be made, as shown in Quenching Temperature Difference(C) at higher temperature differentials and form much longer each AT Relevant trends for each damage mode are also shown nsity Fig 4. The failure of small, individual HMCs to connect Figure 4 Crack densities of PMcs and HMcs and the total crack cracks results in only a moderate increase in crack density with increasing applied shock. By contrast, the density of The main mode of damage due to thermal shock on this PMCs increases at a higher rate. Although PMCs appear laminate was matrix cracking. Horizontal matrix cracks at higher AT, they constitute the larger percentage of (HMCs)developed parallel to the x-axis while perpen- the total damage accumulated at the higher temperature dicular matrix cracks(PMCs)could be seen running at differentials investigated Although an attempt was made to deduce differ- right angles to the x-axis. No damage to the fibres could quenching temperature differentials, the observed scatter PMCs were the first type of damage due to thermal in the measured crack density values did not allow safe shock to appear on the surfaces of this laminate. They conclusions to be reached were exclusively in the central, thick LI ply at AT 500oC. They did not affect the longitudinal fibres on their path and were arrested at fibre-matrix interfaces inside the 0o ply or at the interface between 0 and 90 plies. 3.1.2. The (90/0 s laminate HMCs or thermal debond appeared in the The description of this laminate is similar to that of the Tl plies of this laminate after quenching through (0% /90%)s laminate except that the central, thick ply is in AT=550C. Only a few of these cracks could be ob- the 0 configuration and designated Ll, while the adjacent served and they were deflected at successive fibre-matrix plies are in the 90 configuration and designated as Tl. interfaces. A major, long HMC could not be identified. Damage due to thermal shock was observed using opti- PMCs originating at AT= 500oC were few in num- cal microscopy after quenching through temperature dif- ber and did not penetrate deep inside the matrix ma- ferential higher than 500oC. Thus, the critical quenching terial. In addition, they did not span the full thick temperature differential for this laminate was determined ness of the Ll plies. Most of the PMCs could be seen tobe△Te=500°C. traversing the thickness of the central Ll ply at AT40TH ANNIVERSARY Figure 3 Photomicrograph of PMC in L1 at T = 500◦C that arrests inside 0◦ ply. could be seen traversing the thickness of L1 at T = 800◦C, while 1–2 longer HMCs ran along the length of T1. The application of higher temperature differentials did not result in significant morphological changes in either HMCs or PMCs. Both damage mechanisms remained surface features of small depth. At all temperature differentials PMCs were evenly distributed between the two 0◦ plies termed L1. Crack densities for HMCs and PMCs were determined in terms of crack length per unit area (mm/mm2 or mm−1) in order to allow a comparison to be made, as shown in Fig. 4. The failure of small, individual HMCs to connect at higher temperature differentials and form much longer cracks results in only a moderate increase in crack density with increasing applied shock. By contrast, the density of PMCs increases at a higher rate. Although PMCs appear at higher T, they constitute the larger percentage of the total damage accumulated at the higher temperature differentials investigated. Although an attempt was made to deduce different trends in crack density increase between successive quenching temperature differentials, the observed scatter in the measured crack density values did not allow safe conclusions to be reached. 3.1.2. The (90◦/0◦)s laminate The description of this laminate is similar to that of the (0◦/90◦)s laminate except that the central, thick ply is in the 0◦ configuration and designated L1, while the adjacent plies are in the 90◦ configuration and designated as T1. Damage due to thermal shock was observed using optical microscopy after quenching through temperature differentials higher than 500◦C. Thus, the critical quenching temperature differential for this laminate was determined to be Tc = 500◦C. Figure 4 Crack densities of PMCs and HMCs and the total crack density at each T. Relevant trends for each damage mode are also shown. The main mode of damage due to thermal shock on this laminate was matrix cracking. Horizontal matrix cracks (HMCs) developed parallel to the x-axis while perpendicular matrix cracks (PMCs) could be seen running at right angles to the x-axis. No damage to the fibres could be detected even at the highest temperature differentials investigated. PMCs were the first type of damage due to thermal shock to appear on the surfaces of this laminate. They were exclusively in the central, thick L1 ply at T = 500◦C. They did not affect the longitudinal fibres on their path and were arrested at fibre-matrix interfaces inside the 0◦ ply or at the interface between 0◦ and 90◦ plies. HMCs or ‘thermal debond’ cracks appeared in the T1 plies of this laminate after quenching through T = 550◦C. Only a few of these cracks could be observed and they were deflected at successive fibre-matrix interfaces. A major, long HMC could not be identified. PMCs originating at T = 500◦C were few in number and did not penetrate deep inside the matrix material. In addition, they did not span the full thickness of the L1 plies. Most of the PMCs could be seen traversing the thickness of the central L1 ply at T = 954