高湿度下における時効硬化Al合金押出し材の疲労強度: 回転曲げ疲労と超音波疲労

摘要

環境負荷低減が緊急の課題である現状では，機器のrn軽量化やリサイクル化を図ることに加え，長期間使用rnすることも重要である．その際，機器の長期安全性をrn確保する上で，長寿命域の疲労特性の把握は必須の条rn件である．%In order to investigate the availability of ultrasonic fatigue test for the evaluation of fatigue properties under conventional loading frequency, fatigue tests under ultrasonic frequency and rotating bending were carried out using plain specimens of an age-hardened and extruded Al alloy 7075-T 6 in 7 kinds of environments of controlled humidity of 25, 50, 70 and 85%, distilled water oxygen gas and nitrogen gas. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60%～70% and fatigue strength was largely decreased above that humidity under both tests. However, the main reason for the decrease in fatigue strength by high humidity was different between rotating bending fatigue and ultrasonic fatigue. That is, the decrease in fatigue strength was mainly the acceleration of crack growth caused by brittle fracture under rotating bending.and the transition to shear mode crack accompanied with glide plane decohesion and void formation under ultrasonic loading, respectively.