S. Zhu et al. / Composites Science and Technology 59(1999)833-851 835 Experimental Curve 250 RT 0000 (Ec=Ep+Er) 150 1000°c Estimated 0.0010.002 0.003 Unloading line i Strain Fig 1. Monotonic tensile stress-strain curves of 2D Sic/SiC compo- site at room temperature and 1000 C in argon with a dis rate of 0.5 mm/min. Fig. 2. Schematic diagram showing the permanent strain(Ep)and recoverable strain(Er) and total strain(es) modulus calculated from the linear portion of the curve at 1000C are 100 MPa and 260 GPa, respectively, slightly higher than those at room temperature. It was reported that the room-temperature tensile behavior is rate-dependent [51]: higher strain rates lead to a lower Youngs modulus, higher proportional limit and higher 700 UTS [51] 1000C 600 Evans et al [10] found that the tensile stress/strain curve for 2D composites was quite closely matched by simply scaling down the stress for the ID curve by 1/2 The matrix cracks formed in 90 bundles evolve at lower stresses than cracks in ID composites. These crack 200 extend laterally into the 0 bundles and, finally, the fibers carry the load prior to composite failure. If we use the fracture load at room temperature, the 6080100120140160180200220 stress on"dry"fibers in 0 bundles can be calculated by counting the number of oo bundles in the cross section Stress (MPa) of a specimen and using 500 fibers in every bundle. The Fig. 3. Permanent strain measured by partial unloading versus stress term"dry"means that the contribution of the matrix at room temperature and at 1000 C in argon and 90. bundles is not considered. The stress calculated on dry fibers in 0 bundles is 1. 1 GPa, which is lower than the strength(3.5 GPa) of original SiC fiber. This temperature, although the threshold stress for producing implies that there is either processing damage or non- a permanent strain (as with the proportional limit)at uniform stress and strain states on fibers in the speci- 1000C is higher. Since the permanent strain is caused mens, which reduces the fiber strength. By comparing primarily by damage in the composite, the degradation the fiber strength and the Weibull modulus with those of the composite at 1000 C is more sensitive to the stress prior to incorporation into SiC/SiC composites, Eckel than that at room temperature and Bradt [60] found that fiber damage could occur The monotonic tensile fracture surfaces of the com- either during the weaving or during another stage of posites at ambient and high temperature revealed that composite manufacture. The pores and 2D woven tensile fracture occurred both in 0 and in 90 bundles architecture might certainly lead to non-uniform stress at both ambient and high temperature. The fiber pull-out and strain fields under an applied load [19, 54 length in 0 bundles at high temperature is greater than The permanent strain, Ep, i. e unrecoverable strain as that at room temperature. The greater fiber pull-out defined in Fig. 2 and measured by a repeated loading- length at 1000 C may be the reason for the higher UTS unloading method, as a function of the stress is shown and strain at UTS than those at room temperature in Fig 3. It can be seen that the increase in permanent The increase in fiber pull-out length with temperature strain with stress at 1000C is faster than that at room in argon is the reverse of what is shown by the results inmodulus calculated from the linear portion of the curve at 1000C are 100 MPa and 260 GPa, respectively, slightly higher than those at room temperature. It was reported that the room-temperature tensile behavior is rate-dependent [51]; higher strain rates lead to a lower Young's modulus, higher proportional limit and higher UTS [51]. Evans et al [10] found that the tensile stress/strain curve for 2D composites was quite closely matched by simply scaling down the stress for the 1D curve by 1/2. The matrix cracks formed in 90 bundles evolve at lower stresses than cracks in 1D composites. These cracks extend laterally into the 0 bundles and, ®nally, the ®bers carry the load prior to composite failure. If we use the fracture load at room temperature, the stress on ``dry'' ®bers in 0 bundles can be calculated by counting the number of 0 bundles in the cross section of a specimen and using 500 ®bers in every bundle. The term ``dry'' means that the contribution of the matrix and 90 bundles is not considered. The stress calculated on dry ®bers in 0 bundles is 1.1 GPa, which is lower than the strength (3.5 GPa) of original SiC ®ber. This implies that there is either processing damage or non￾uniform stress and strain states on ®bers in the speci￾mens, which reduces the ®ber strength. By comparing the ®ber strength and the Weibull modulus with those prior to incorporation into SiC/SiC composites, Eckel and Bradt [60] found that ®ber damage could occur either during the weaving or during another stage of composite manufacture. The pores and 2D woven architecture might certainly lead to non-uniform stress and strain ®elds under an applied load [19,54]. The permanent strain, "P, i.e. unrecoverable strain as de®ned in Fig. 2 and measured by a repeated loading± unloading method, as a function of the stress is shown in Fig. 3. It can be seen that the increase in permanent strain with stress at 1000C is faster than that at room temperature, although the threshold stress for producing a permanent strain (as with the proportional limit) at 1000C is higher. Since the permanent strain is caused primarily by damage in the composite, the degradation of the composite at 1000C is more sensitive to the stress than that at room temperature. The monotonic tensile fracture surfaces of the com￾posites at ambient and high temperature revealed that tensile fracture occurred both in 0 and in 90 bundles at both ambient and high temperature. The ®ber pull-out length in 0 bundles at high temperature is greater than that at room temperature. The greater ®ber pull-out length at 1000C may be the reason for the higher UTS and strain at UTS than those at room temperature. The increase in ®ber pull-out length with temperature in argon is the reverse of what is shown by the results in Fig. 1. Monotonic tensile stress±strain curves of 2D SiC/SiC compo￾site at room temperature and 1000C in argon with a displacement rate of 0.5 mm/min. Fig. 2. Schematic diagram showing the permanent strain ("p) and recoverable strain ("r) and total strain ("c). Fig. 3. Permanent strain measured by partial unloading versus stress at room temperature and at 1000C in argon. S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851 835