tes science and te 1999)833-851 UTS of Hi-NicalonTM/SiC is similar to that of standard point is important to designers, since they cannot design SiC/SiC and enhanced SiC/SiC, but the strains at UTs components for high-temperature use on basis only of of Hi-Nicalon TM/SiC and enhanced SiC/SiC are much monotonic strength higher than that of standard SiC/SiC. The fatigue limit of the composite at room tempera The modulus calculated from the linear portion of the ture is much higher than the stress for the matrix urve is about 140 GPa, which is higher than that (90 cracking. This means that the composites can avoid the GPa)of enhanced SiC/SiC and lower than that (200 unsteady propagation of the matrix cracks induced by GPa)of standard SiC/SiC at 1300oC the first loading during cyclic fatigue at stresses up to the fatigue limit. In other words, the matrix cracks formed on first loading may propagate and also micro- 3. Cyclic fatigue cracking of the matrix may continue over several thou- sand cycles [69], but they are finally arrested and remain 3.1. Fatigue behavior of standard SiC/Sic stationary during subsequent cyclic loading. Fiber brid- ging is commonly thought to be the main reason for thi Fatigue behavior of Sic/SiC composites has been phenomenon the bridging force decreases the investigated over a decade [55-68]. The cyclic fatigue life stress intensity at crack tip. At stresses above the fatigue at room temperature and 1000oC in argon is shown in limit, cyclic fatigue fracture occurs accompanied by a Fig. 5. The stress-life curve at 1000oC can be divided modulus reduction with cycles [56, 62]. This reduction of into three regimes. One is the low cycle regime(< 104 modulus can be caused by the increase of either crack cycles), in which the stress exponent for fatigue life is number or crack length. Therefore, any factor(cyclic high and there is no evident difference in fatigue life loading, high temperature creep, oxidation, etc. )can between room temperature and high temperature, lead to a decrease of the modulus (life) of a specimen if although the slope of the curve at the latter temperature it increases crack number or crack length seems higher. The second regime is the rapid decrease in ruments on CMCs are fatigue life with stress at 1000oC for stresses lower than mostly applicable to unidirectional fiber reinforced 180 MPa. There is no second regime at room tempera- composites [70, 71]. For 2D woven SiC/SiC composite, ture. This means that the second regime is dependent on the basic elements are 0 bundles, 90 bundles and high-temperature effects. This will be discussed later. pores. Therefore, their interaction must be taken into The third regime exhibits a fatigue limit defined by the consideration to explain mechanical properties [72] specimens below it having a life over 10 cycles. The The cyclic fatigue fracture surfaces at room tempera fatigue limit at 1000 C is only 75 MPa, which is about ture and at high stresses(fatigue life <10 cycles )at 30% of UTS. The fatigue limit at room temperature is 000C are similar to the monotonic tensile fracture 160 MPa(about 80% of UTS) surfaces, shown in Fig. 6. However, cyclic fracture at Although the UTS and proportional limit at 1000c low stresses( fatigue life >10 cycles )at 1000 C mainly are higher than those at room temperature(Fig. 1), the occurred in 0 bundles, although one or two 90 bundle fatigue limit at 1000 C is much lower than that at room fail. This fracture morphology is associated with 2D temperature. This means that the fatigue limit is not woven structures with a large amount of pores proportional to the monotonic tensile strength. This There are three kinds of fracture modes of o bundles s schematically shown in Fig. 7. The first is that in which fracture of 0o bundles results from crack propa- gation in 90 bundles. In the second mode fracture occurs at a crossover point of 0%/90 bundles, and in the 1000°cin ● Rt in Air third the fracture is caused by shearing in the middle of 0 bundle owing to two cracks originating at the two nearest crossover points. The monotonic tension and cyclic fatigue at high stresses are composed largely of the first kind of fracture mode and occasionally, locally of the second kind. The cyclic fatigue at low stresses at 1000C is primarily of the second and third fracture 100 types, and in a few places of the first kind. Clusters of fibers with similar pull-out length in some 0 bundles 01102103104105106107108 sometimes exist in cyclic fatigue at 1000%C, when they fracture by the third kind of fracture mode. The change Cycles to Failure from stage I to II for fatigue at 1000 C(Fig. 5)agrees Fig. 5. Maximum tensile stress versus cycles to failure of 2D Sic/Sic with the fracture mode change(Figs. 6 and 7), which erature and1000°C. seems related to the maximum stressUTS of Hi-NicalonTM/SiC is similar to that of standard SiC/SiC and enhanced SiC/SiC, but the strains at UTS of Hi-NicalonTM/SiC and enhanced SiC/SiC are much higher than that of standard SiC/SiC. The modulus calculated from the linear portion of the curve is about 140 GPa, which is higher than that (90 GPa) of enhanced SiC/SiC and lower than that (200 GPa) of standard SiC/SiC at 1300C. 3. Cyclic fatigue 3.1. Fatigue behavior of standard SiC/SiC Fatigue behavior of SiC/SiC composites has been investigated over a decade [55±68]. The cyclic fatigue life at room temperature and 1000C in argon is shown in Fig. 5. The stress±life curve at 1000C can be divided into three reÂgimes. One is the low cycle reÂgime (<104 cycles), in which the stress exponent for fatigue life is high and there is no evident di€erence in fatigue life between room temperature and high temperature, although the slope of the curve at the latter temperature seems higher. The second reÂgime is the rapid decrease in fatigue life with stress at 1000C for stresses lower than 180 MPa. There is no second reÂgime at room tempera￾ture. This means that the second reÂgime is dependent on high-temperature e€ects. This will be discussed later. The third reÂgime exhibits a fatigue limit de®ned by the specimens below it having a life over 107 cycles. The fatigue limit at 1000C is only 75 MPa, which is about 30% of UTS. The fatigue limit at room temperature is 160 MPa (about 80% of UTS). Although the UTS and proportional limit at 1000C are higher than those at room temperature (Fig. 1), the fatigue limit at 1000C is much lower than that at room temperature. This means that the fatigue limit is not proportional to the monotonic tensile strength. This point is important to designers, since they cannot design components for high-temperature use on basis only of monotonic strength. The fatigue limit of the composite at room tempera￾ture is much higher than the stress for the matrix cracking. This means that the composites can avoid the unsteady propagation of the matrix cracks induced by the ®rst loading during cyclic fatigue at stresses up to the fatigue limit. In other words, the matrix cracks formed on ®rst loading may propagate and also micro￾cracking of the matrix may continue over several thou￾sand cycles [69], but they are ®nally arrested and remain stationary during subsequent cyclic loading. Fiber brid￾ging is commonly thought to be the main reason for this phenomenon, since the bridging force decreases the stress intensity at crack tip. At stresses above the fatigue limit, cyclic fatigue fracture occurs accompanied by a modulus reduction with cycles [56,62]. This reduction of modulus can be caused by the increase of either crack number or crack length. Therefore, any factor (cyclic loading, high temperature creep, oxidation, etc.) can lead to a decrease of the modulus (life) of a specimen if it increases crack number or crack length. The available theory and experiments on CMCs are mostly applicable to unidirectional ®ber reinforced composites [70,71]. For 2D woven SiC/SiC composite, the basic elements are 0 bundles, 90 bundles and pores. Therefore, their interaction must be taken into consideration to explain mechanical properties [72]. The cyclic fatigue fracture surfaces at room tempera￾ture and at high stresses (fatigue life <104 cycles) at 1000C are similar to the monotonic tensile fracture surfaces, shown in Fig. 6. However, cyclic fracture at low stresses (fatigue life > 104 cycles) at 1000C mainly occurred in 0 bundles, although one or two 90 bundles fail. This fracture morphology is associated with 2D woven structures with a large amount of pores. There are three kinds of fracture modes of 0 bundles, as schematically shown in Fig. 7. The ®rst is that in which fracture of 0 bundles results from crack propa￾gation in 90 bundles. In the second mode fracture occurs at a crossover point of 0/90 bundles, and in the third the fracture is caused by shearing in the middle of 0 bundle owing to two cracks originating at the two nearest crossover points. The monotonic tension and cyclic fatigue at high stresses are composed largely of the ®rst kind of fracture mode and occasionally, locally, of the second kind. The cyclic fatigue at low stresses at 1000C is primarily of the second and third fracture types, and in a few places of the ®rst kind. Clusters of ®bers with similar pull-out length in some 0 bundles sometimes exist in cyclic fatigue at 1000C, when they fracture by the third kind of fracture mode. The change from stage I to II for fatigue at 1000C (Fig. 5) agrees with the fracture mode change (Figs. 6 and 7), which seems related to the maximum stress. Fig. 5. Maximum tensile stress versus cycles to failure of 2D SiC/SiC composite at room temperature and 1000C. S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851 837