Effects of Material Orientation and Stress Ratio on Fatigue Crack Propagation in Short-Fiber Reinforced Plastics

Abstract

The effects of fiber orientation and stress ratio on the crack propagation behavior were studied with single edge-notched specimens which were cut from an injection-molded plate (IMP) of short carbon-fiber reinforced polyphenylene sulphide, 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, da/dN, and the stress intensity factor range, ∆K, the propagation rate of fatigue cracks was slowest for MD, and increased with increasing fiber angle. When da/dN was correlated to ∆K/E (E = Young's modulus), the relations for different orientations merged into a single relation. The core layer existing in IMP accelerated crack propagation in MD direction, and decelerated in TD direction. The da/dN vs ∆K/E relation of skin-layer plates is close to that for IMP. The effect of stress ratio becomes minimal when da/dN is correlated to the range of the energy release rate, ∆G = G max -G min (G max , G min = maximum, minimum energy release rates). The relation between da/dN and ∆G/E shows the least scatter for SFRP with different fiber orientations under different stress ratios. Key words:Fatigue crack propagation, Short-fiber reinforced plastics, Material orientation, Stress ratio, Fracture mechanics, Injection molding