H Mei et aL. Materials Science and Engineering A 460-461(2007)306-313 1600 3D C/SIC 0.33 0.30 2D CSIC g150 024 100 060112011801240130013601420 Time(s) Fig. 3. Correlation of the cyclic temperature and the fatigue stress with the strain 0.00.1020.30.40.5060.70.80.91.0 of the braided 3D C/SiC composites subjected to thermal cycling and mechanical Tensile strain(%) Fig.2. Typical tensile stress-strain curves of 2D and braided 3D C/SiC com- due to loading/unloading followed by a time-dependent damage posites at room temperature with a loading rate of 0.001 mm/s strain baseline. As shown in Fig. 3, thermal strain of the tested specimen increases gradually, reaching a peak as the tempera- the onset of loading. Furthermore, the stress-strain relationship ture ascends to the upper limit of 1200.C, and then decreases tends to have apparent slope recovery above 150 MPa, indicat- with cooling back to the lower limit of 900CAs thermal cycles ing that the o fiber bundles in 2D architecture can enhance proceed, the quasi-triangular strain is repeated periodically with the transverse compression resistance to hinder the longitudinal a fixed range magnitude as the same period of 120s as the extension at higher stress level. It is interesting to note that near cyclic temperature, independent upon the external fatigue stress the top of the loading curves of the braided 3D composite, at [13, 14]. It is believed that the thermal strain range depends sig the point where a saturated matrix cracking state was believed nificantly on the temperature gradient AT. On the other hand to have been reached and no more matrix cracks and interfacial it can be also seen in Fig 3 that the mechanical fatigue stress debonding were believed to form, higher modulus fibers and provides a significant influence on the strain induced. It is wor- bundles split in a sudden manner leading to an apparent"stiff- thy to be noted that the stress-induced strain is repeated with ening"on the stress-strain curve On the other hand, the apparent the cyclic fatigue stress between 40 and 80MPa. No matter stiffening of the 3D braided composite is likely due to reorienta- how the temperature is high or low, the mechanical strain range tion of the fibers angle during loading. Initially the fibers are at an also always sustains a constant range magnitude, dependent angle substantially different from the stress axis. As the stress is only on the stress difference Ao and independent upon the applied, the braid may stretch in such a way that the fibers move cyclic temperature. Furthermore, the mechanical strains are reg- closer to the same orientation of the stress axis( see the details ularly distributed on the quasi-triangular thermal strain wave as in Fig. 8 of Ref [14)). As this happens, the fibers will carry the same period as the fatigue stress. Evidently, the mechan- more of the load and the matrix less, thus resulting in""stiffen- ical strain is considerably small in comparison to the thermal ing". Later, as fibers begin to fracture, accumulation of damage strain will cause the curve to bend over prior to failure. Nevertheless, The entire strain versus time curves for the 2d and braided right before the sudden failure of the braided 3D composite, the 3D C/Sic composites subjected to fatigue loading and thermal lope of the stress-strain curve decreases again. The average ten- cycling(N=50) in wet oxygen are compared in Fig. 4. The sile strengths and failure strains are 413.76 MPa,0.92% for the fitting strain of 2D composites varies approximately from the braided 3D composites, and 252. 45 MPa, 0.71%o for the 2D com- initial transient strain of 0.23% to the final nonreversible dam- posites, respectively. The average Youngs modulus obtained age strain of 0.35%, whereas for the 3d braided architecture by the linear fitting of the initial stress-strain curves from 0 to these two strain values are about 0. 21 and 0.47%o, respectively 60MPa is 142.85 GPa for the braided 3D and 91.23 GPa for 2d It should be noted that the two strain curves become parted from C/SiC composites each other after experiencing the initial nearly same transient increased stage. Compared with 2D architecture, the 3D braided 3.2. Strain response of the composites architecture composite has a larger strain in a higher strain rate under the same testing conditions Thus the deviation between Strain response of the 2D and braided 3D composites to ther- the 2D and braided 3D composite strain curves becomes greater mal cycling and mechanical fatigue stress is found to be the and greater with increasing thermal cycles. The fiber architec similar. Taking from an example of several braided 3D com- tures in the 2D and braided 3D fiber preforms must be considered posite samples, Fig. 3 indicates the correlation of the cyclic to be responsible for this result In 2D architecture, volume frac temperature and fatigue stress with the strain induced. It is tions of the longitudinal(90%)and transverse(0%)fibers are the apparent that the measured strain should be a combined result same, and equal to half the total fiber volume fraction. Fur- of thermal strain due to heating/cooling and mechanical strain thermore the longitudinal extension strain of the fibers can be308 H. Mei et al. / Materials Science and Engineering A 460–461 (2007) 306–313 Fig. 2. Typical tensile stress–strain curves of 2D and braided 3D C/SiC com￾posites at room temperature with a loading rate of 0.001 mm/s. the onset of loading. Furthermore, the stress–strain relationship tends to have apparent slope recovery above 150 MPa, indicat￾ing that the 0◦ fiber bundles in 2D architecture can enhance the transverse compression resistance to hinder the longitudinal extension at higher stress level. It is interesting to note that near the top of the loading curves of the braided 3D composite, at the point where a saturated matrix cracking state was believed to have been reached and no more matrix cracks and interfacial debonding were believed to form, higher modulus fibers and bundles split in a sudden manner leading to an apparent “stiff￾ening” on the stress–strain curve. On the other hand, the apparent stiffening of the 3D braided composite is likely due to reorienta￾tion of the fibers angle during loading. Initially the fibers are at an angle substantially different from the stress axis. As the stress is applied, the braid may stretch in such a way that the fibers move closer to the same orientation of the stress axis (see the details in Fig. 8 of Ref. [14]). As this happens, the fibers will carry more of the load and the matrix less, thus resulting in “stiffen￾ing”. Later, as fibers begin to fracture, accumulation of damage will cause the curve to bend over prior to failure. Nevertheless, right before the sudden failure of the braided 3D composite, the slope of the stress–strain curve decreases again. The average ten￾sile strengths and failure strains are 413.76 MPa, 0.92% for the braided 3D composites, and 252.45 MPa, 0.71% for the 2D com￾posites, respectively. The average Young’s modulus obtained by the linear fitting of the initial stress–strain curves from 0 to 50 MPa is 142.85 GPa for the braided 3D and 91.23 GPa for 2D C/SiC composites. 3.2. Strain response of the composites Strain response of the 2D and braided 3D composites to ther￾mal cycling and mechanical fatigue stress is found to be the similar. Taking from an example of several braided 3D com￾posite samples, Fig. 3 indicates the correlation of the cyclic temperature and fatigue stress with the strain induced. It is apparent that the measured strain should be a combined result of thermal strain due to heating/cooling and mechanical strain Fig. 3. Correlation of the cyclic temperature and the fatigue stress with the strain of the braided 3D C/SiC composites subjected to thermal cycling and mechanical fatigue. due to loading/unloading followed by a time-dependent damage strain baseline. As shown in Fig. 3, thermal strain of the tested specimen increases gradually, reaching a peak as the tempera￾ture ascends to the upper limit of 1200 ◦C, and then decreases with cooling back to the lower limit of 900 ◦C. As thermal cycles proceed, the quasi-triangular strain is repeated periodically with a fixed range magnitude as the same period of 120 s as the cyclic temperature, independent upon the external fatigue stress [13,14]. It is believed that the thermal strain range depends sig￾nificantly on the temperature gradient T. On the other hand, it can be also seen in Fig. 3 that the mechanical fatigue stress provides a significant influence on the strain induced. It is wor￾thy to be noted that the stress-induced strain is repeated with the cyclic fatigue stress between 40 and 80 MPa. No matter how the temperature is high or low, the mechanical strain range also always sustains a constant range magnitude, dependent only on the stress difference σ and independent upon the cyclic temperature. Furthermore, the mechanical strains are reg￾ularly distributed on the quasi-triangular thermal strain wave as the same period as the fatigue stress. Evidently, the mechan￾ical strain is considerably small in comparison to the thermal strain. The entire strain versus time curves for the 2D and braided 3D C/SiC composites subjected to fatigue loading and thermal cycling (N= 50) in wet oxygen are compared in Fig. 4. The fitting strain of 2D composites varies approximately from the initial transient strain of 0.23% to the final nonreversible dam￾age strain of 0.35%, whereas for the 3D braided architecture, these two strain values are about 0.21 and 0.47%, respectively. It should be noted that the two strain curves become parted from each other after experiencing the initial nearly same transient increased stage. Compared with 2D architecture, the 3D braided architecture composite has a larger strain in a higher strain rate under the same testing conditions. Thus, the deviation between the 2D and braided 3D composite strain curves becomes greater and greater with increasing thermal cycles. The fiber architec￾tures in the 2D and braided 3D fiber preforms must be considered to be responsible for this result. In 2D architecture, volume frac￾tions of the longitudinal (90◦) and transverse (0◦) fibers are the same, and equal to half the total fiber volume fraction. Fur￾thermore, the longitudinal extension strain of the fibers can be