SM490鋼の弾塑性破壊じん性に及ぼす塑性ひずみの影響

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Although the plastic strain induced in materials increases the mechanical strength, it may reduce the fracture toughness. The change in the fracture toughness is brought about by four factors; reduction in critical plastic strain due to pre-strain, localization of deformation, increase of material strength by work hardening, and change in material resistance for fracture. In this study, correlation between change in fracture toughness by pre-strain and the four factors was investigated. Firstly, the change in the tensile properties and fracture toughness were examined for pre-strained SM490 carbon steel. The specimens with blunt notch of 0.2 mm and 0.4 mm radius were used in addition to those with conventional fatigue pre-cracking. The degree of applied plastic strain was 5%, 10% and 20%. The fracture toughness was largest when the induced plastic strain was 5%, although it decreased in the cases for the plastic strain of 10% and 20%. Secondly, the stress and strain field near notch tip was evaluated by simulating the experiment using elastic-plastic finite element analyses. It was concluded that the change in fracture toughness was mainly brought about by the change in material resistance caused by the plastic strain. Under the plastic strain of 10% and 20%, the critical plastic strain for crack initiation decreased due to change in material resistance in addition to apparent reduction by pre-straining. The localized deformation and work hardening had minor effect on the change in fracture toughness.

SM490鋼の弾塑性破壊じん性に及ぼす塑性ひずみの影響

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