EFFECT OF ENVIRONMENTS ON THE FATIGUE CRACK PROPAGATION BEHAVIOR OF TITANIUM METAL MATRIX COMPOSITES

Mixed mode tensile and fatigue crack propagation behavior of the B(,4)C-B and SiC-B reinforced titanium metal matrix composites was studied by mechanical testing, fractography and Scanning Auger Spectroscopy for interface characterization. Influence of fiber/matrix interfaces on the mixed mode behavior was studied by modifying the chemical nature of the interface thru heat treatments. Reduction in longitudinal strength was observed due to oxide formation by the air heat treatment, and boride formation by the 885(DEGREES)C vacuum heat treatment. Little change in the transverse strength was observed due the heat treatments.Heat treated longitudinal composite specimens showed slower fatigue crack propagation rates than the as received specimens in humid air at R = 0.1. The slow down is due to secondary cracking and interface debonding. The transverse and 45(DEGREES) specimens showed a failure mechanism change from fiber splitting for the as received specimens to interfacial splitting for the heat treated specimens, in humid air at R = 0.1.Humid environments enhanced the mixed mode fatigue crack propagation rates in the as received titanium matrix composites at R = 0.1. The effect was more pronounced in the transverse and 45(DEGREES) specimens. A transition in failure modes from fiber splitting in humid air to interfacial splitting in dry environments was observed at a load ratio of 0.1. The transition took place at around 133 Pa water vapor pressure, where a steep rise in fatigue crack propagation rate was noticed. At R = 0.5, however no fiber splitting was observed in humid air. Fatigue crack closure load measurements revealed that closure loads were higher in humid air than in dry environments. The fiber splitting is thus modeled as an environmentally induced crack closure effect, where plastically deformed matrix sets up stress fields (radial and mode III stresses) on the brittle boron fibers weakened by the humidity.This research was sponsored by the Air Force Office of Scientific Research under contract No. 80-0052.Mixed mode fatigue crack propagation of the composites was self-similar for all the orientations. Entire mixed mode fatigue crack propagation of the as received B(,4)C-B/Ti-6Al-4V composites can be expressed by a single curve: da/dN (PROPORTIONAL) {(DELTA)G/G(,F((theta)))}('m) for all the fiber orientations. The failure strain energy release rate G(,F((theta))) can be analytically estimated with the help of a fracture mechanics model.