Study on the material properties for improving the life estimation of fatigue crack

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Various fracture phenomena are evaluated according to a comparison of mechanical environment and the limit values of materials. However, fatigue strength is evaluated using the S-N relation and propagation of fatigue cracks is evaluated using the da/dN-ΔK relation, both being evaluated using only the mechanical environment. It is known that in propagation of fatigue cracks, not only stress range and crack length, but also loading history and load size etc., also significantly influence. It is also well known that when an spike loading acts on constant amplitude loading, even after constant amplitude loading returns, for a while the deceleration propagation of a fatigue crack occurs. It is also known that this phenomenon cannot be evaluated using the da/dN-ΔK relation that is currently used. In recent years, the phenomenon of a fatigue crack opening/closing repeatedly at the crack tip during propagation has been revealed. We have developed a system that measures the hysteresis loop of the crack tip that arises due to the opening/closing phenomenon of the fatigue crack tip. Using this system we have revealed good matching of the shapes of hysteresis loops and the propagation of fatigue cracks.In this research, we have proposed a method whereby it is possible to also evaluate the deceleration in the propagation rate of a fatigue crack after spike loading, through a comparison of the mechanical environment and characteristics of a material. Also, in the characteristics of a material used in the limit of the propagation of a fatigue crack, we utilized the elongation that follows compressive plastic deformation.