13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Fretting fatigue properties under the effect of hydrogen and the mechanisms that cause the reduction in fretting fatigue strength Jader Furtado1,*, Ryosuke Komoda2, Masanobu Kubota3 1 Air Liquide R&D, Centre de Recherche Claude Delorme Paris-Saclay, 1 Chemin de la Porte de Logas, Les Loges-en-Josas, 78354, France 2 Graduate School of Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan 3 Kyushu University, Air Liquide Industrial Chair on Hydrogen Structural Materials and Fracture, WPI-I2CNER and AIST, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan * kubota.masanobu.304@m.kyushu-u.ac.jp Abstract Fretting fatigue, which is a composite phenomenon of metal fatigue and friction, is one of the major factors in the design of mechanical components as it significantly reduces fatigue strength. Since hydrogen can influence both fatigue and friction, fretting fatigue is one of the important concerns in designing hydrogen equipment. The authors carried out the fretting fatigue tests on austenitic stainless steels in order to characterize the effect of hydrogen and to explain the mechanism responsible for hydrogen embrittlement. In this study, the significant reduction in fretting fatigue strength due to hydrogen is shown including other factors influencing the fretting fatigue strength such as surface roughness, hydrogen content and the addition of oxygen. The cause of the reduction in the fretting fatigue strength in hydrogen is local adhesion between the contacting surfaces and subsequent formation of many small cracks. Furthermore, hydrogen enhances crack initiation under fretting fatigue conditions. Transformation of the microstructure from austenite to martensite is another possible reason. A hydrogen charge also reduces the fretting fatigue strength. The cause is the reduction in the crack growth threshold, ΔKth, due to hydrogen. Keywords Hydrogen, Fretting, Fatigue, Austenitic stainless steels, Adhesion 1. Introduction Hydrogen embrittlement is a classic problem, but recent studies on hydrogen embrittlement have clarified that there are many technical challenges to achieving a balance between reducing the cost of hydrogen equipment and ensuring their safety. In this study, fretting fatigue in hydrogen is the focus, since fretting fatigue is one of the major factors in the design of mechanical components due to the significant reduction in fatigue strength. It has been reported that fretting can cause a reduction in fatigue strength by half to one-thirtieth of that of a smooth specimen [1]. Fretting fatigue occurs at the position where metal fatigue and fretting occur simultaneously. Fretting is the cyclic relative slip motion involving wear between the contacting surfaces of components mechanically fastened. In hydrogen equipment, fretting fatigue is definitely an important issue, since hydrogen can have an influence on both fatigue and friction phenomena. A significant reduction in the fretting fatigue strength due to hydrogen and the mechanisms causing the reduction are described. Several factors influencing the fretting fatigue properties in hydrogen are also investigated. 2. Procedures 2.1 Fretting fatigue test method Figure 1 shows the fretting fatigue test method under tension and compression loading. In addition, a bending fretting fatigue test was also carried out. Detailed configurations of the specimen and contact pad are shown in refs. [2, 3]. In either case, two contact pads were pressed onto the front and back side surfaces of the fatigue test specimen. When a fatigue load is applied to the specimen, fretting is induced between the contacting surfaces due to the difference in the
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