Effect of fiber orientation on fatigue crack propagation in short-fiber reinforced plastics

The influence of the fiber orientation on the crack propagation behavior was studied with single edge-notched specimens which were cut from an injection-molded plate of short-fiber reinforced plastics (SFRPs), at five fiber angles relative to the loading axis, i.e. θ = 0° (MD), 22.5°, 45°, 67.5°, 90° (TD). Macroscopic crack propagation path was nearly perpendicular to the loading axis for the cases of MD and TD. For the other fiber angles, the crack path was inclined because the crack tended to propagate along inclined fibers. In the relation between the crack propagation rate and the stress intensity factor range, ΔK, the propagation rate of fatigue cracks was slowest for MD, and increased with increasing fiber angle. When the crack propagation rate was correlated to ΔK/E (E = Young’s modulus), the relations for different orientations merged into a single relation. Based on the results of stress-ratio effect, it was concluded that the crack propagation rate was mainly controlled by ΔK at low rates and by the maximum stress intensity factor Kmax at high rates. For injection molded plates, the existence of the core layer accelerated crack propagation in MD direction, and decelerated in TD direction. Mechanisms of crack propagation were discussed based on microscopic observation of the near crack-tip region and fracture surfaces.