Dynamic Fracture Strength and Toughness of Continuous Alumina Fiber-Reinforced Glass Matrix Composites at Elevated Temperature

An instrumented drop-weight test rig was used in the investigation of the dynamic fracture strength and toughness tests of continuous alumina fiber reinforced glass matrix composites, which were carried out at room and elevated temperatures using charpy-type specimen configuration, and with fiber orientation perpendicular (0 degrees) and parallel (90 degrees) to impact load direction. Increasing test temperatures, the results show a dependency of dynamic flexural strength, toughness, and impact energy for unidirectional 0 degrees and 90 degrees fiber orientation Al2O3-borosilicate composite up to 950 degrees C, whereas the Al2O3-96% silica composites showed little or no dependency on increasing test temperatures, before dropping off at temperatures higher than 750 degrees C. This type of behavior for Al2O3-borosilicate composite could be due to the suppression of matrix microcracking as temperature increases due to increased plasticity of the matrix, resulting in increased dynamic flexural strength and toughness values. Scanning electron microscope (SEM) investigations of fractured surfaces show that there are two identifiable major damage modes: fibrous at room temperature, as in Al2O3-borosilicate composites, where weaker fiber-matrix interfacial bond exists, due to the difference in thermal mismatch at room temperature resulting in reduced fiber-matrix bond strength, matrix microcracking and apparent fiber debonding occurring during the dynamic flexure and toughness tests, resulting in longer fiber pullout; nonfibrous at high temperatures, as in Al2O3-96% silica composites, this occurs as temperature increases, the thermal mismatch between fiber-matrix decreases leading to an increase in fiber-matrix interfacial bonding and thus increasing tendency to embrittlement. This resulted in changing failure mechanisms from fibrous at room temperature to nonfibrous at higher temperatures. The important variables of this test were discussed and its usefulness in comparing dynamic flexural, toughness, and impact resistant ceramic matrix composite (CMC) materials are demonstrated.