Fatigue of glass fiber reinforced injection molded plastics. II: Tensile versus flexural loading

Abstract A previous paper reported fatigue data for several glass fiber reinforced thermoplastic materials by measuring their S‐N, stress versus number of cycles to fail, behavior. It was demonstrated that both fiber orientation and loading mode, tensile versus flexural, strongly influence the resultant S‐N data. The orientation effects are considered real, reflecting the local fiber orientation distributions in the samples, which are determined by geometry, processing and material variables. However, the higher fatigue lives observed in flexural loading are considered an artifact. Specifically for this class of materials, it is concluded that the use of linear elastic beam bending theory equations to calculate bending stresses are inappropriate. It is shown that a plasticity‐based correction factor makes the flexural S‐N data equivalent to the tensile results. In fact this correction applies equally well to monotonic loading, not just fatigue. Although it is concluded that tensile loading is preferred, some cases where flexural loading may be advantageous are indicated. Polym. Compos. 25:569–576, 2004. © 2004 Society of Plastics Engineers.