13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of Humidity on Fracture Mechanism of Age-hardened Al Alloys under Ultrasonic Loading Norio Kawagoishi1,*, Kohji Kariya1, Hironori Matsusako2 Yuzo Nakamura3, Xishu Wang4, Qingyuan Wang5 1 Department of Mechanical System Engineering, Daiichi Institute of Technology, Kirishima 899-4395, Japan 2 All Nippon Airways Co., Tokyo 105-7133, Japan 3 Department of Mechanical Engineering, Kagoshima University, Kagoshima 890-0065, Japan 4 Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China 5 Department of Civil Engineering & Mechanics, Sichuan University, Chengdu 610065, China * Corresponding author: n-kawagoishi@daiichi-koudai.ac.jp Abstract Effect of humidity on the propagation mechanism of a fatigue crack in age-hardened Al alloy 2017-T4 was investigated under ultrasonic loading in relative humidity of 25% and 85%, respectively. Plain specimen of both extruded and drawn Al alloys were tested. In the extruded alloy, fatigue cracks that propagated macroscopically in tensile mode in low humidity changed to extend in shear mode to failure, comparing to that fatigue cracks always grew in shear mode in high humidity. On the other hand, in the drawn alloy, fatigue cracks propagated in tensile mode to final fracture in low humidity, and in a combined mode of tensile and shear in high humidity. In the both alloys, the growth of cracks shorter than a specific length, e.g. ~1 mm, which is stress level dependent, was accelerated in high humidity, but it was not or little influenced by humidity beyond that specific length. The difference in fatigue crack propagation behavior between the two alloys and the effect of humidity on the fracture mechanism were discussed based on fractographic and crystallographic analyses. Keywords Fatigue, Ultrasonic loading, Age-hardened Al alloy, Humidity, Texture 1. Introduction High strength Al alloys have excellent properties such as high specific strength, corrosion resistance and easiness to recycle, from viewpoint of reduction of environment load. However, Al alloys have no definite fatigue limit, meaning that once a crack initiates it will continue to grow and lead to failure. Therefore, the fatigue property of Al alloys in long life region is important, though the examination of fatigue behavior in long life region is a time consuming task. Recently, ultrasonic fatigue test has attracted much attention because it is a time-saving technology for the evaluation of fatigue properties in long life region in comparison with the conventional low frequency fatigue method [1]. However, the effect of high frequency on fatigue properties has not been fully understood. Especially, the effect of environment such as corrosion is one of important factors to be clarified, because corrosion is a time-dependent damage and high strength metals are highly sensitive to corrosive environment [2-4]. In the present study, fatigue tests at ultrasonic frequency (20kHz) were carried out in relative humidity of 25% and 85% respectively, for two kinds of age-hardened Al alloys 2017-T4 (Al-Cu-Mg alloy), i.e. the extruded and the drawn alloys, which have nearly the same static strengths but different microstructure to investigate the effects of microstructure and atmospheric moisture on fatigue crack propagation behavior under ultrasonic loading. 2. Material and Experimental Procedure Materials used were an extruded and a drawn bar of age-hardened Al alloy 2017-T4. The chemical
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