J. L Ryba/ Composites Science and Technology 68 (2008)274-282 specimen fractured or the test run-out limit occurred, which was set at 100 h in this study. Henceforth, tests performed in laboratory air or in 100% steam environment will be referred to as tests in air or steam or the in-air or in-steam 3. Results and discussion Generator 3.. Monotonic test Ahmir Enclosure Fig 3 shows the typical monotonic tensile stress strain curves at room and three elevated temperatures(400C, 750C and 950C)of the Syl-iBN/BN/SiC CMC system This figure also shows the corresponding curves at 550C and 750C for the same material from a previous study [15]. A summary of material properties from the mono- Fig. 2. Schematic of the heating chamber with enclosure for steam tonic tests are presented in Table 1. The monotonic tensile environment stress strain curves at 750C from the present and previous studies are in good agreement with each other, and the the laboratory air or in the steam environment. For the lat ter an alumina susceptor (i.e, enclosure), as shown sche matically in Fig. 2, was used. The specimen gage section was located inside the susceptor. A commercially available steam generator was used which measured and controlled the desired humidity of the steam. Steam was introduced 550°c[15 into the susceptor through a feeding tube as a continuous stream with a slightly positive pressure, expelling the dry air and thereby creating a 100% steam environment inside the susceptor. R-type thermocouples were initially glued on 2 a test specimen in order to calibrate the furnace temper- 2 zoo ature for the elevated temperature tests, both for the steam and dry conditions. This allowed calibration of the furnace itially and on the periodic basis during course of this study within +2C. Thus power setting of the furnace for a desired temperature was determined which was then used 950°c in the actual tests. Other than furnace calibration thermo- couples were not attached to the test speci Strain 23. Test details ig. 3. Monotonic tensile stress strain of this study and from Two types of tests were conducted; monotonic tension previous study[I5y and stress rupture. Two test environments, having either air or 100% steam conditions, were used at Table I 400°C,750°Cand950°C. Monotonic tensile tests were Test data summary conducted at these temperatures and also at room tempera- Test Young's Ultimate Normalized Failure Sourd ture to get baseline information of the tested CMC system. conditions modulus strength ultimate strain These were conducted in the laboratory air environment (GPa) (MPa) only since a previous study showed that monotonic tensile RT Present behavior of the Syl-iBN /BN/SiC CMC system was depen dent on the temperature, but not on the humidity [17]. This 400C 231 334 0.81 0.36 Present was expected since the test duration in monotonic tests was not long enough to cause any effect from moisture in the 750°C 0.79 0.38 Present monotonic tension tests. The monotonic tensile test 0.74 Present involved loading of the specimen under the load control mode with a loading rate of 3333 N/min Stress rupture 550C 233 [5] tests involved application of a prescribed load in a short 750C 235 335 0.81 0.36 [5] duration(l min)and then keeping it constant until the Normalized with the ultimate tensile strength at room temperaturethe laboratory air or in the steam environment. For the lat￾ter an alumina susceptor (i.e., enclosure), as shown sche￾matically in Fig. 2, was used. The specimen gage section was located inside the susceptor. A commercially available steam generator was used which measured and controlled the desired humidity of the steam. Steam was introduced into the susceptor through a feeding tube as a continuous stream with a slightly positive pressure, expelling the dry air and thereby creating a 100% steam environment inside the susceptor. R-type thermocouples were initially glued on to a test specimen in order to calibrate the furnace temper￾ature for the elevated temperature tests, both for the steam and dry conditions. This allowed calibration of the furnace initially and on the periodic basis during course of this study within ±2 C. Thus power setting of the furnace for a desired temperature was determined which was then used in the actual tests. Other than furnace calibration, thermo￾couples were not attached to the test specimens. 2.3. Test details Two types of tests were conducted; monotonic tension and stress rupture. Two test environments, having either laboratory air or 100% steam conditions, were used at 400 C, 750 C and 950 C. Monotonic tensile tests were conducted at these temperatures and also at room tempera￾ture to get baseline information of the tested CMC system. These were conducted in the laboratory air environment only since a previous study showed that monotonic tensile behavior of the Syl-iBN/BN/SiC CMC system was depen￾dent on the temperature, but not on the humidity [17]. This was expected since the test duration in monotonic tests was not long enough to cause any effect from moisture in the monotonic tension tests. The monotonic tensile test involved loading of the specimen under the load control mode with a loading rate of 3333 N/min. Stress rupture tests involved application of a prescribed load in a short duration (1 min) and then keeping it constant until the specimen fractured or the test run-out limit occurred, which was set at 100 h in this study. Henceforth, tests performed in laboratory air or in 100% steam environment will be referred to as tests in air or steam, or the in-air or in-steam tests. 3. Results and discussion 3.1. Monotonic test Fig. 3 shows the typical monotonic tensile stress strain curves at room and three elevated temperatures (400 C, 750 C and 950 C) of the Syl-iBN/BN/SiC CMC system. This figure also shows the corresponding curves at 550 C and 750 C for the same material from a previous study [15]. A summary of material properties from the mono￾tonic tests are presented in Table 1. The monotonic tensile stress strain curves at 750 C from the present and previous studies are in good agreement with each other, and the Fig. 2. Schematic of the heating chamber with enclosure for steam environment. 0 50 100 150 200 250 300 350 400 450 0 0.1 0.2 0.3 0.4 0.5 Strain % Stress (MPa) Room Temperature 4000 C 7500C 9500C 5500C [15] 7500C [15] Fig. 3. Monotonic tensile stress strain curves of this study and from previous study [15]. Table 1 Test data summary Test conditions Young’s modulus (GPa) Ultimate strength (MPa) Normalized ultimate strengtha Failure strain (mm/mm) Source RT 235 415 1 0.44 Present study 400 C 231 334 0.81 0.36 Present study 750 C 232 327 0.79 0.38 Present study 950 C 225 305 0.74 0.41 Present study 550 C 233 347 0.84 0.39 [15] 750 C 235 335 0.81 0.36 [15] a Normalized with the ultimate tensile strength at room temperature. 276 S. Mall, J.L. Ryba / Composites Science and Technology 68 (2008) 274–282