低サイクル片振り引張疲れにおける鋼材の塑性ひずみ挙動に及ぼす切欠き効果

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Low cycle fatigue tests were carried out for specimens with circular hole and quasi-elliptical hole under constant load pulsating tension. The materials used are mild steel (SM 41) and quenched and tempered high tensile strength steel (HT 60).The development of plastic strain around a circular hole and a quasi-elliptical hole was experimentally analyzed by the Moire method at successive stages of fatigue, and the following characteristics were clarified ware clarified concerning the plastic behavior in the specimens under low cycle fatigue.The following three types of fracture were confirmed in the low cycle fatigue test for two kinds of steels.Type-A : A ductile fracture occurs after the necking of the whole part of the net section.Type-B : A fatigue crack initiates after the neking solely at the notch root and strain at the tip of crack increases as it extends.Type-C : A fatigue crack initiates and propagates without a little strain increase at the tip of the crack.In the case of type-A fracture the maximum equivalent strain at the fracture position becomes from 50 to 60 percent, while in type-C fracture it becomes below 20 percent at the stage of crack initiation.In the case of type-B fracture the maximum equivalent strain at the fracture position lies between type-A and type-C values. These strain values are irrespective of materials, size and shape of specimens, and the number of cycles to cause fracture of type-A or crack initiation of type-B and C.The following relation exists, in the case of type-A fracture, without concern to materials, size and shape of specimens, Δε^^-_m・N_f=ε^^-_0/2 where Δε^- _m is the steady state creep rate, N_f is the number of cycles to cause fracture of type-A or crack initiation of type-B and C, ε^^-_0 is the equivalent plastic strain at fracture in simple tension.In the case of type-C fracture, however, the formula no longer holds good and the critical number of cycles to cause fracture, which indicates that the relation between Δε-^m and N_f deviates from the formula, is larger in SM 41 than in HT60.
社団法人日本船舶海洋工学会の論文

低サイクル片振り引張疲れにおける鋼材の塑性ひずみ挙動に及ぼす切欠き効果

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