Fatigue of fiber-reinforced injection molded plastics. I: Stress-lifetime data

Glass-reinforced nylon 66 polyamide (PA), polybutylene terephthalate (PBT), and polycarbonate (PC) were injection molded in a ribbed plaque. Flat strips, tensile bars, and rib sections were cut from the plaque in the flow and crossflow directions and subsequently fatigue tested under both tensile and flexural loading conditions to obtain conventional fatigue S-N (stress versus number of cycles to fail) or Wohler curves. Both the fiber alignment and the loading mode strongly influenced the resultant S-N curve. As expected, longer fatigue life results when the imposed stress is parallel to the flow direction, which in turn parallels the predominant fiber orientation direction. Also, at the same stress level, it was observed that flexural loading resulted in a much longer fatigue life compared to tensile loading. Similar S-N data were obtained in flexure for different geometries, such as rib or flat cross sections, as long as the local fiber orientation was similar. Attempts to use an empirical relationship to rationalize predicted fatigue data based upon tensile or breaking strength were not successful. (C) 2004 Society of Plastics Engineers.