Variable amplitude fatigue behavior of neat and short glass fiber reinforced thermoplastics

Variable amplitude fatigue behavior of injection molded thermoplastics was investigated. A neat thermoplastic (polypropylene impact co-polymer) and a thermoplastic composite made of polybutylene terephthalate (PBT) with 30 wt% short glass fibers were used for the experimental study. Fatigue tests were conducted on both un-notched and notched specimens using plate-type specimens. For the notched specimens, a circular hole was drilled in the center of the specimens. Two-step loading (high-low and low-high) tests at two cycle ratios of 0.2 and 0.5 and stress ratios of R = 0.1 and −1 were conducted to investigate load sequence and mean stress effects, as well as the accuracy of linear damage rule. Periodic overload and service load history tests were also conducted to investigate the effect of overloads, cycling frequency, small stress cycles, and maximum stress level. A strengthening effect of increased frequency was observed for both materials and the effect of small stress cycles was found to be negligible. A general fatigue damage model along with the rainflow cycle counting method and linear damage rule were used for life predictions of variable amplitude service load history tests. More than 89% of the life predictions based on the modeling procedure used were within a factor of 3 of the experimental lives for both materials.