Effect of test temperature on fatigue crack propagation in injection molded plate of short-fiber reinforced plastics

The crack propagation behavior was studied at 298K (RT), 343K, 373K, and 403K with center-notched specimens which were cut from an injection-molded plates of short carbon-fiber reinforced PPS at two fiber angles relative to the loading direction, i.e. θ = 0° (MD) and 90° (TD). Macroscopic crack propagation path was nearly perpendicular to the loading axis for both MD and TD. Microscopically, cracks in MD were blocked by fibers, circumvented fibers, and rarely broke fibers, showing zigzag path. For TD, the crack path was less tortuous following the fiber interface. The relation between crack propagation rate, da/dN, and stress intensity factor range, ΔK, at RT and 373K were similar for both MD and TD, while da/dN at temperatures above glass transition temperature, Tg (=360K), were two to three orders higher than that at temperatures below Tg. When compared at each temperature, da/dN was two orders lower in MD than in TD. At temperatures above Tg, inelastic deformation took place; the relation between load and displacement became nonlinear, accompanied by hysteresis loop expansion. When da/dN was correlated to the J-integral range, ΔJ, da/dN at four temperatures came closer for each case of MD and TD. Especially for the case of TD, the relations at four temperatures merge together. When compared at each temperature, da/dN of MD was lower than that of TD, even though the difference between MD and TD was smaller. According to SEM observation of fatigue fracture surfaces, many fibers were pulled out from the matrix on fatigue fracture surface of the skin layer of MD, and parallel fibers were observed on the fracture surface of TD. High temperature environment increased matrix deformation both in MD and TD, but did not change the fracture path or the micromechanism of fatigue crack propagation.