Study on the Notch Sensitivity of Unidirectional Composites
Shun-ichi BANDOH and Masaru ZAKO
Abstract:The aircraft structural integrity should be demonstrated by considering every factors which degrade its strength. Recent helicopter structural components such as rotor hub and blades are manufactured mainly by uni-directional composite materials. The reasons of the usage of the composite material are the low notch sensitivity and high damage tolerance. Aramid composite has extremely high fatigue strength and high toughness. Then the fatigue strength characteristics of a uni-directional glass composite and aramid composite has been studied by high cycle fatigue tests with open hole. The test result of aramid composite showed the high notch sensitivity at high cycle region. In case of the aramid composite, the crack expanded laterally to the load direction from the hole, on the other hand, in case of glass composite the crack expand parallel to the load direction from the hole. Some coupon tests about those materials and FEM analysis have been carried out. From this study, following results were obtained. 1) In case of aramid composite, high stress concentration occurs, because of low shear modulus of the fiber that is almost same as the epoxy matrix. Then, no matrix crack occurs around a hole, so fiber breakage occurs and propagates. 2) In case of glass composite, the stress concentration is relieved by small cracks around a open hole because of shear modulus difference of the fiber and matrix. Then, no fiber breakage occurs. The conclusion of this study is that the appropriate resin system design should be placed on the shear modulus difference between fibers and matrix. Key Words:Helicopter structural integrity, Notch sensitivity, Unidirectional composites, Aramid composites, Glass composites, Shear modulus, Crack propagation, FEM analysis