Mode II Interlaminar Fracture under Static and Fatigue Loadings of Alumina Fiber/Epoxy Laminates in Liquid Nitrogen

Amoung advanced composite materials, alumina fiber (ALF)/epoxy composites are selected for the structural components of superconducting magnets because of their low thermal conductivity in addition to their higher specific strength and modulus. Since laminate structures are used as the load support system for superconducting magnets, the evaluation of interlaminar strength both under static and fatigue loadings at cryogenic temperature is essential. In the present study, mode II interlaminar fracture toughness and delamination fatigue crack growth behavior in liquid nitorogen (77K) were investigated with unidirectional ALF/epoxy laminates. Tests were carried out using end notched flexure specimens with special loading apparatus. The fracture toughness values at 77K were higher than those in laboratory air at room temperature (RT). The fracture mechanism under fatigue loading at 77K was controlled by the maximum stress, and was completely different from that at RT. Then, the increase of the fatigue crack growth resistance at 77 K from that at RT was observed only under stress ratio, R=0.1. The fractographic observation showed that the resin fracture was dominant for the fracture surfaces of fatigue fracture, suggesting that the microscopic mechanism was different from that at RT where the interfacial fracture was dominant.