Fatigue Strength of Age-Hardened & Extruded Al Alloy under High Humidity : Rotating Bending and Ultrasonic Loading

Abstract

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-T6 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.