Relationship between the Initial Fracture Stress and Fatigue Limit—Simple Prediction Method of Tensile Fatigue Limit of Composite

Abstract

This article presents an experimental study that clarifies the relationship between the initial fracture stress and fatigue limit of glass fiber reinforced unsaturated polyester resin specimens with a laminated structure taken from a pultruded square pipe. Quasi-static bending and tension tests are performed with acoustic emission (AE) measurements to identifying the occurrence of initial fracture during testing. AE and observation results have clarified the occurrence of initial fracture was detected by maximum acoustic energy values and corresponding fiber breakage in the unidirectional (UD) bundles. Moreover, the ratio of initial fracture stress to ultimate strength is 32% in bending and 26% in tension, when comparing stress and strains on the tension side of the UD layer. These values are in good agreement with each other and with the measured tensile fatigue limit when the cyclic stress is at 25% of the tensile strength. Initial fracture stress obtained by static tests is close values to the fatigue limit which will greatly contribute to the prediction of the fatigue limit.