In order to investigate very high cycle fatigue characteristics of the Zr₅₅Al₁₀Ni₅Cu₃₀ (in at.%) bulk amorphous alloy, fatigue tests were performed in rotating bending at a frequency of 52.5 Hz with a stress ratio of R=-1. The test environment was in room atmosphere without any control of temperature nor humidity. The fatigue endurance limit σ_w of the bulk amorphous alloy was about 700 MPa and the fatigue strength ratio σ_w/σ_B (fatigue endurance limit divided by the ultimate tensile strength) was found to be 0.38. Fracture surfaces of failed specimens were observed by scanning electron microscope (SEM). The fracture surface was separated into three typical regions; (1) multi-facet region, (2) stable crack propagation region and (3) instantaneous fracture region. The cracks took place on the specimen surface and fatigue striations were clearly formed in the stable crack propagation region on the fracture surface. A distinct boundary was observed between the stable crack propagation region and instantaneous fracture region. The instantaneous fracture region was covered by the dimple-type morphology. Vein-like patterns peculiar to the static fracture surface of amorphous alloys were also found in the edge region of the fracture surface. The fatigue fracture toughness of this bulk amorphous alloy was in the range of 20-29 MPa·m^1/2.

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In order to evaluate a casting material which had a variable tensile strength, a strength evaluation method was proposed. Tensile tests were carried out on a specimen with a particular artificial defect as the fracture origin. The fracture origin size could be controlled using the proposed evaluation method. Therefore, the tensile test results, which had slight scatter, were obtained. However, the shape of the artificial defect was different from the casting defect, thus the effects of each defect on the tensile strength might be different. Both the critical stress intensity factors at the beginning of the unstable fracture were then compared, and it was clarified that the artificial defect and the casting defect had the same effect on the tensile strength. By this method, a non-combustible Mg alloy with added-Si, which contained large casting defects, could be evaluated. It was clarified that there was no difference of tensile strength between the noncombustible Mg alloys with 1% and 2% Si, and the relationship between the tensile strength and temperature was obtained.

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In resin-metal composite structures, such as electronic products with an epoxy resin molding insulator, the thermal fatigue of the epoxy resin during thermal cycling is a critical issue. Because the thermal stress in the epoxy resin and its strength simultaneously change during the thermal cycling, an estimation method for the thermal fatigue reliability in consideration of the temperature dependence of the fatigue strength is required. In this study, the technique of predicting the thermal fatigue crack growth rate in epoxy resin is studied and we proposed an estimation method using the strain intensity factor range ΔK/E as an evaluation parameter. In order to evaluate the proposed method, both the crack growth test by thermal cycling and the thermal stress evaluation by the visco-elastic FEM analysis were carried out. As a result, it was found that the proposed method estimated the crack growth rate under thermal fatigue test less than that of the iso-thermal fatigue test.

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