高湿度中における高強度Al合金の疲労特性に及ぼすショットピーニングの影響

Abstract

Fatigue properties of shot-peened high strength Al-Cu-Mg alloys, extruded and drawn 2017-T4 alloys, and effect of humidity on the properties were investigated in relative humidity environments of 25% and 85% under rotating bending. Fatigue strength was increased by shot-peening in each humidity. Although fatigue strength was largely decreased by high humidity in the electro-polished specimen, the decrease in fatigue strength was very small in the shot-peened one. In the electro-polished extruded alloy, the crack propagation changed from a shear mode to a tensile one with decreasing in stress level in high humidity, while the propagation in the shot-peened one was in a tensile mode only in both humidity environments similar to the ones in both of the electro-polished and the shot-peened specimens in low humidity. The propagation mode in the drawn alloy was a tensile one irrespective of stress level and humidity. Most of fracture surface in low humidity was occupied by striations in both of the electro-polished and the shot-peened specimens in both alloys. On the other hand, those in high humidity were covered with slip planes in the shear mode crack in the electro-polished extruded alloy, while a few granular facets in the extruded alloy or brittle striations in the drawn one were observed at the fracture surface in high humidity in addition to striations in the tensile one irrespective of stress level and shot peening.