MATERIAL IMPROVEMENT AND QUALITY ASSURANCE FOR COMPOSITE LEAF SPRINGS BASED ON MICROFRACTURE MECHANISM

The understanding of the microfracture mechanism is essential for the improvement of the performance of the materials and the establishment of the quality assurance procedures. In this study, the microfracture mechanism of composite leaf springs produced from unidirectional glass fiber reinforced epoxy resin was studied using acoustic emission technique in combination with in situ failure analysis using SEM. It was found that the fracture mechanism consists of the following two steps: the breakage of individual fibers followed by matrix microcracking from the broken fiber tips, and the simultaneous breakage of some tens of fibers accompanied by small delamination failure from the broken tip, leading to final failure. It was found that the mechanical property of the matrix was greatly improved by modifying the epoxy resin, which results in a retardation of matrix microcracking from the broken fiber tips. This retardation, confirmed by the acoustic emission measurement and in situ failure analysis, can greatly improve the durability of the material. The microfracture mechanism of a prototype composite spring was characterized under flexural loading using the acoustic emission measurement. Based on the result, an optimum geometrical design of the spring to reduce the microfailure was obtained. During this study, the employment of the acoustic emission technique as a quality assurance test of the spring was assessed. It was found that improperly manufactured springs generate a lot of the emission by loading to a maximum design toad, while properly manufactured ones hardly generate the emission. It can be concluded that the acoustic emission technique is effective as a quality assurance test for the composite springs.