S. Mall,J.L Ryba Composites Science and Technology 68(2008)274-282 same at 550C from the previous study is also in agreement ature was lower than that at room temperature, the nor with its counterparts at 400C and 750C of this study. As malized stress at zero hour is not unity. The data, shown can be seen in Fig. 3 and Table 1, the ultimate tensile at 100 h, show those which did not fail up to 100 h which strength is reduced at the elevated temperatures relative was the run-out limit in this study. As can be seen in to that of room temperature. This reduction is almost same Fig. 4, both moisture and temperature have the detrimental in the temperature range used in this study(from 400 to effect on the stress rupture strength of the tested CMC sys 950C). The ultimate tensile strength at the elevated tem- tem. The composite survived up to at least 100 h at 400C peratures ranged from 74% to 84% of its counterpart at when loaded to a constant stress level of 72% and 60% of room temperature. However, there was one major differ- its ultimate tensile strength at room temperature under lab- nce in the initial linear portion of the stress strain curves oratory air and steam conditions, respectively. Their coun- between 950C and room temperature, 400C, 550C terparts at 750C and 950C were 52%(in air) or 49%(in and 750C. The stress strain curves at room temperature, steam) and 40%(in air) or 34%(in steam), respectively 400C, 550C and 750C show initially a linear portion Further, it is interesting to note that stress rupture strength up to about 200-225 MPa, which represents the portion generally reduced considerably due to exposure to moisture before any appreciable amount of matrix crack was devel- over a short duration oped in the composite. On the other hand, the stress strain 4 clearly shows that the moisture degraded the curve at 950C shows initial linear portion up to about CMC system at all three elevated temperatures used in this 100 MPa which is almost half of its counterpart at other study. This is elaborated in Fig. 5 where the interpolated temperatures. Thus, this CMCs system is degraded consid- normalized stress rupture strengths at 50 h for all six erably in this respect (i.e, appreciable amount of matrix ronments of this study from Fig 4 are plotted. This figure cracks had already developed when loaded up to compares the interpolated normalized stress rupture 100 MPa at 950C). The stress-strain curves after the strength at 50 h for all six test environments of this study knee-point show similar behavior at room and all elevated from Fig 4. This comparison suggests that stress rupture temperatures of this study, i.e., a continuously decreasing strength at 750C under both laboratory air and steam slope due to further matrix cracks and associated non-lin- conditions is relatively less than the expected degradation from 400C to 950C which is shown by the straight lines In other words, stress rupture strength at 750C in both 3.2. Stress rupture test test environments is less than the linearly interpolated val- ues between 400C and 950C. Thus, the tested Sic/sic Fig 4 shows stress rupture data in terms of the normal- CMC system with modified fiber/matrix interphase also ized stress versus time to failure relationships for both lab- performed relatively poor in the intermediate temperature oratory air and steam test environments. The y-axis of range(from 400 to 950C)compared to the outside of this figure is the applied stress in stress rupture tests was nor- range. This characteristic is similar to other SiC/SiC CMCs malized by the ultimate tensile strength at room tempera- systems as elaborated in the Introduction section ture. the data at zero hour in this figure are from the monotonic tensile tests conducted at each temperature, which have also been normalized. Since the ultimate tensile strength of the tested CMC system at each elevated temper S Air a Humid 素 0.6 400°c 708090100 950°c Fig 4. Stress rupture curves(normalized with room temperature ultimate Fig. 5. Estimated normalized rupture strength at 50 h at three tensile strengthsame at 550 C from the previous study is also in agreement with its counterparts at 400 C and 750 C of this study. As can be seen in Fig. 3 and Table 1, the ultimate tensile strength is reduced at the elevated temperatures relative to that of room temperature. This reduction is almost same in the temperature range used in this study (from 400 to 950 C). The ultimate tensile strength at the elevated tem￾peratures ranged from 74% to 84% of its counterpart at room temperature. However, there was one major differ￾ence in the initial linear portion of the stress strain curves between 950 C and room temperature, 400 C, 550 C and 750 C. The stress strain curves at room temperature, 400 C, 550 C and 750 C show initially a linear portion up to about 200–225 MPa, which represents the portion before any appreciable amount of matrix crack was devel￾oped in the composite. On the other hand, the stress strain curve at 950 C shows initial linear portion up to about 100 MPa which is almost half of its counterpart at other temperatures. Thus, this CMCs system is degraded consid￾erably in this respect (i.e., appreciable amount of matrix cracks had already developed when loaded up to 100 MPa at 950 C). The stress–strain curves after the knee-point show similar behavior at room and all elevated temperatures of this study, i.e., a continuously decreasing slope due to further matrix cracks and associated non-lin￾ear displacements. 3.2. Stress rupture test Fig. 4 shows stress rupture data in terms of the normal￾ized stress versus time to failure relationships for both lab￾oratory air and steam test environments. The y-axis of figure is the applied stress in stress rupture tests was nor￾malized by the ultimate tensile strength at room tempera￾ture. The data at zero hour in this figure are from the monotonic tensile tests conducted at each temperature, which have also been normalized. Since the ultimate tensile strength of the tested CMC system at each elevated temper￾ature was lower than that at room temperature, the nor￾malized stress at zero hour is not unity. The data, shown at 100 h, show those which did not fail up to 100 h which was the run-out limit in this study. As can be seen in Fig. 4, both moisture and temperature have the detrimental effect on the stress rupture strength of the tested CMC sys￾tem. The composite survived up to at least 100 h at 400 C when loaded to a constant stress level of 72% and 60% of its ultimate tensile strength at room temperature under lab￾oratory air and steam conditions, respectively. Their coun￾terparts at 750 C and 950 C were 52% (in air) or 49% (in steam) and 40% (in air) or 34% (in steam), respectively. Further, it is interesting to note that stress rupture strength generally reduced considerably due to exposure to moisture over a short duration. Fig. 4 clearly shows that the moisture degraded the CMC system at all three elevated temperatures used in this study. This is elaborated in Fig. 5 where the interpolated normalized stress rupture strengths at 50 h for all six envi￾ronments of this study from Fig. 4 are plotted. This figure compares the interpolated normalized stress rupture strength at 50 h for all six test environments of this study from Fig. 4. This comparison suggests that stress rupture strength at 750 C under both laboratory air and steam conditions is relatively less than the expected degradation from 400 C to 950 C which is shown by the straight lines. In other words, stress rupture strength at 750 C in both test environments is less than the linearly interpolated val￾ues between 400 C and 950 C. Thus, the tested SiC/SiC CMC system with modified fiber/matrix interphase also performed relatively poor in the intermediate temperature range (from 400 to 950 C) compared to the outside of this range. This characteristic is similar to other SiC/SiC CMCs systems as elaborated in the Introduction section. 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 10 20 30 40 50 60 70 80 90 100 Time (hr) Normalized Stress Steam Steam Steam Air Air Air 4000 C 7500 C 9500 C 4000 C 7500 C 9500 C 4000 C 7500C 9500C Fig. 4. Stress rupture curves (normalized with room temperature ultimate tensile strength). 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Temperature (C) Normalized Stress Air Humid 4000C 7500C 9500C Fig. 5. Estimated normalized rupture strength at 50 h at three temperatures. S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282 277