多軸応力下における改良9Cr-1Mo鋼切欠き材の低サイクル疲労寿命評価法

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This paper presents the results of a study of the notch effect in low cycle fatigue of Mod.9Cr-1Mo steel under multiaxial loading. Tension-torsion multiaxial low cycle fatigue tests were carried out at room temperature using three types of circumferential notched specimens and crack initiation, propagation and failure lives were experimentally obtained. Two proportional and nine nonproportional strain paths were used in low cycle fatigue tests. The crack initiation and failure lives decreased with the elastic stress concentration factor but the crack propagation lives slightly decreased with the elastic stress concentration factor in push-pull low cycle fatigue. The superposition of torsion straining on push-pull loading reduced the crack initiation, propagation and failure lives but they little decreased with elastic stress concentration factor. The crack initiation, propagation and failure lives in nonproportional loading were smaller than those in proportional loading but showed smaller reduction with the elastic stress concentration factor. Finite element analysis was carried out for evaluating the local strain at the notch root using elastic linear plastic model. Von Mises equivalent strain, path strain and nonproportional strain ranges conservatively estimated the crack initiation and failure lives of the notched specimens. The most suitable parameter for evaluating the crack initiation and failure lives was von Mises equivalent strain range.