Effect of notches, specimen size, and fiber orientation on the monotonic tensile behavior of composites at ambient and elevated temperatures

The effect of circular center holes on the monotonic tensile stress-strain behavior and strength of a continuous fiber-reinforced oxide/oxide composite was investigated at ambient and elevated temperatures. The material used in this study consisted of 12 layers of un-coated plain weave fabric of Nextel™ 720, with either 0/90 or ±45° fiber orientation with respect to the loading direction, embedded in an aluminosilicate matrix. 100 and 200-mm long straight-sided specimens with center holes were tested for ratios of hole diameter to width (a/w) between 0.1 and 0.4, and width values between 3 mm and 25 mm. It was found that as-processed specimens with both fiber architectures were notch insensitive at ambient temperature, but exhibited mild notch sensitivity at elevated temperatures. It was also found that the strength of the material was retained after heat treatments at 1000°C, but that it decreased significantly after exposures at 1100°C for periods of time of 20 and 100 hours. The strength of test specimens with 0/90 fiber orientation was found to decrease with increasing width, but no size effects on strength were observed for specimens with ±45° fiber orientation. These results are discussed in relation to the microstructure of the material and its evolution at elevated temperatures.