Effects of Interfacial Strength on Tensile and Fatigue Properties of Long Glass Fibre-Reinforced Polypropylene

This paper describes the tensile and fatigue properties and the fracture mechanism of long glass fibre-reinforced polypropylene at ambient temperature in laboratory air. Tensile tests and pulsating-tension fatigue tests have been carried out using the smooth specimens of two grades of the material, with and without an acid modified polypropylene. Tensile and fatigue strengths were evaluated and the effects of interfacial strength on the mechanical properties were discussed on the basis of macroscopic and microscopic observations of fractured specimens and fracture surfaces. Interfacial strength showed a great influence on the tensile and fatigue strengths. By the addition of the coupling agent, both strengths were considerably improved and the predominant fracture mode changed from fibre pull-out to fibre breakage, suggesting effective load transfer from the matrix to the glass fibres due to improved interfacial strength. Close examination of the specimen surfaces during fatigue test revealed that a fatigue crack of approximately 500μm in length was generated at 70 to 80 percent of fatigue life in both materials. Before such macroscopic fatigue crack initiation, the percent of pulled-out and broken fibres was gradually increased and then saturated. Fatigue cracks were found to be initiated along fibre bundles at the corner of the specimens which made a large angle to the loading axis, and then grew with breaking fibres and with decohesion and fibre pull-out, respectively, in the materials with and without the coupling agent.