H Mei et al. Materials Science and Engineering A 460-461(2007)306-313 ac Exterior Fig 9. SEM micrograph a fractured fiber due to the constraint stress t between longitudinal (90 )and transverse(0 )fiber bundles in 2D C/SiC composites during thermal cycling. The external fatigue stress a is vertical. 8b, it appears that large quantities of fibers and/or bundles are that the longer fiber debonding and sliding during the testing pulled out together with the covered SiC matrix, and the pullout resulted eventually in the longer fiber pullout lengths in the next length of the fibers are longer for the braided 3D composites monotonic tension 500 um) than that for the 2D composites(100 um). Under The constraints between longitudinal(90%)and transverse thermal cycling and fatigue stress, the cyclic unloading and (0%)fiber bundles in 2D C/SiC composites are likely to enhance reloading increase the debonding length by decreasing interface physical destruction resulting from the cyclic thermal mismatch sliding resistance given by [15] Fig 9 shows that a bridging fiber was fractured by the con- (1) fiber bundles in 2D C/SiC composites during thermal cycin .y between longitudinal (90%)and transverse(0 It is easy for the matrix cracks to be formed at the intersec- where of is the fiber strength, d the diameter of the carbon fiber tions(crossovers)of the neighboring fiber bundles, where the (7 um as indicated later in Fig 9)and Le is the critical length thermal stress can be generated in the two perpendicular direc- of the broken fiber. Normally, it is thought that the fiber pullout tions (i.e, 90 and 0o)and the constrained thermal stress is length is equal to Lc/2 eventually relaxed by shearing the bridging fibers(Fig 9)along The cyclic fatigue stress or repetitive temperature could the propagating cracks For the braided 3D composites, all the reduce the sliding resistance tr of the interface by the friction and fibers are laid at a small angle(22)along the longitudinal wear effect between fiber and matrix. This process increased the axis. This fiber architecture is helpful for relaxation of thermal length of the debonded interface and enabled the broken fibers stress via deforming composites longitudinally and adjusting the to slide along the interfaces, leading to long fiber pullouts when braiding angle properly. As also illustrated in Fig. 4, it is actu- the fiber strength of is assumed to be constant(3.05 GPa)in ally observed that the 3D braided architecture exhibits better (1). As mentioned earlier in Fig 4, the braided 3D compos- deformability than the 2D architecture. As we know, the physi- ite have a larger strain incremental amount and strain rate than cal damage created by the cyclic thermal mismatch can facilitate the 2D composites during thermal cycling. It is not surprising fiber oxidation leading to mechanical degradation of the com-H. Mei et al. / Materials Science and Engineering A 460–461 (2007) 306–313 311 Fig. 9. SEM micrographs showing a fractured fiber due to the constraint stress τ between longitudinal (90◦) and transverse (0◦) fiber bundles in 2D C/SiC composites during thermal cycling. The external fatigue stress σ is vertical. 8b, it appears that large quantities of fibers and/or bundles are pulled out together with the covered SiC matrix, and the pullout length of the fibers are longer for the braided 3D composites (≈500m) than that for the 2D composites (≈100m). Under thermal cycling and fatigue stress, the cyclic unloading and reloading increase the debonding length by decreasing interface sliding resistance given by [15] τr = σfd 2Lc (1) where σf is the fiber strength, d the diameter of the carbon fiber (≈7m as indicated later in Fig. 9) and Lc is the critical length of the broken fiber. Normally, it is thought that the fiber pullout length is equal to Lc/2. The cyclic fatigue stress or repetitive temperature could reduce the sliding resistance τr of the interface by the friction and wear effect between fiber and matrix. This process increased the length of the debonded interface and enabled the broken fibers to slide along the interfaces, leading to long fiber pullouts when the fiber strength σf is assumed to be constant (≈3.05 GPa) in Eq. (1). As mentioned earlier in Fig. 4, the braided 3D compos￾ite have a larger strain incremental amount and strain rate than the 2D composites during thermal cycling. It is not surprising that the longer fiber debonding and sliding during the testing resulted eventually in the longer fiber pullout lengths in the next monotonic tension. The constraints between longitudinal (90◦) and transverse (0◦) fiber bundles in 2D C/SiC composites are likely to enhance physical destruction resulting from the cyclic thermal mismatch. Fig. 9 shows that a bridging fiber was fractured by the con￾straint stress τ between longitudinal (90◦) and transverse (0◦) fiber bundles in 2D C/SiC composites during thermal cycling. It is easy for the matrix cracks to be formed at the intersec￾tions (crossovers) of the neighboring fiber bundles, where the thermal stress can be generated in the two perpendicular direc￾tions (i.e., 90◦ and 0◦) and the constrained thermal stress is eventually relaxed by shearing the bridging fibers (Fig. 9) along the propagating cracks. For the braided 3D composites, all the fibers are laid at a small angle (∼22◦) along the longitudinal axis. This fiber architecture is helpful for relaxation of thermal stress via deforming composites longitudinally and adjusting the braiding angle properly. As also illustrated in Fig. 4, it is actu￾ally observed that the 3D braided architecture exhibits better deformability than the 2D architecture. As we know, the physi￾cal damage created by the cyclic thermal mismatch can facilitate fiber oxidation leading to mechanical degradation of the com-