単一過大荷重を負荷した厚鋼板の疲労き裂進展寿命に及ぼす応力比の影響

抄録

The fatigue crack propagation behavior of newly developed SM490-grade steel plates under high stress ratios with a single overload was investigated in comparison with a conventional steel of the same grade. Under constant-amplitude loading, the developed steel exhibited approximately twice the crack propagation life at a stress ratio of R = 0.1, whereas the improvement was marginal at higher stress ratios of R = 0.3 and 0.5. In contrast, under single overload conditions, the developed steel showed a significant improvement in crack propagation life by up to 1.5 times even at R = 0.3. The crack growth rate after overload was lower in the developed steel, indicating a reduction in the effective crack driving force, as evaluated using the RPG (Re-tensile Plastic zone's Generating) load. Elasto-plastic FEM analysis was conducted to investigate the crack opening displacement behind the crack tip and the wedge effect associated with the overload-induced plastic zone. The results indicate that the wedge effect temporarily promotes crack opening immediately after overload, followed by enhanced crack closure during subsequent crack growth within the plastic zone. The mechanism of crack propagation life improvement considering combined effects of plasticity-induced crack closure and roughness-induced crack closure activated by the overload was discussed.