numero25

G. Qian et alii, Frattura ed Integrità Strutturale, 25 (2013) 7-14; DOI: 10.3221/IGF-ESIS.25.02 7 Special Issue: Characterization of Crack Tip Stress Field Crack propagation mechanism and life prediction for very-high- cycle fatigue of a structural steel in different environmental medias Guian Qian, Chengen Zhou, Youshi Hong * State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China * Corresponding author. Tel: +86 10 82543966, hongys@imech.ac.cn A BSTRACT . The influence of environmental medias on crack propagation of a structural steel at high and very- high-cycle fatigue (VHCF) regimes is investigated based on the fatigue tests performed in air, water and 3.5% NaCl aqueous solution. Crack propagation mechanisms due to different crack driving forces are investigated in terms of fracture mechanics. A model is proposed to study the relationship between fatigue life, applied stress and material property in different environmental medias, which reflects the variation of fatigue life with the applied stress, grain size, inclusion size and material yield stress in high cycle and VHCF regimes. The model prediction is in good agreement with experimental observations. K EYWORDS . Very-high-cycle fatigue; Aqueous environment; Stress intensity factor; Plastic zone. I NTRODUCTION ery-high-cycle fatigue (VHCF) [1-16] of metallic materials is regarded as fatigue failure at stress levels below the conventional fatigue limit and the corresponding fatigue life beyond 10 7 loading cycles. Lots of modern engineering structures and components, such as airplanes, turbines, automobiles and high speed trains are expected to endure the safe performance in the range of 10 7 - 10 10 load cycles. One typical feature of VHCF for high strength steels is that the S-N curve consists of two parts corresponding to subsurface and surface crack initiations, resulting in a stepwise or duplex shape of the curve [1-16]. Generally, the crack initiation in VHCF regime is observed as a fisheye pattern on the fracture surface, which is located at the specimen subsurface region and originated from a nonmetallic inclusion for high strength steels [4-11]. Since the pioneering work by Naito et al. [17, 18], there have been a variety of studies on the VHCF behavior for different materials [1-16]. Among these studies, the crack initiation mechanism in VHCF attracted most of the attention. However, the crack initiation and propagation process of high strength steels in environmental medias in VHCF is still not clear. In addition to experimental investigations, theoretical models for fatigue strength and life prediction in VHCF regime are of significant importance for both scientific and engineering applications. However, models to predict S-N curves in VHCF regime in different environmental medias are lacking due to the complicated crack initiation mechanisms. Therefore, in this paper, the process of crack initiation and propagation for a structural steel in environmental medias in VHCF was investigated based on the experiments. The fatigue test was performed in laboratory air, fresh water and 3.5% NaCl aqueous solution. The influence of environmental medias on the variation of fatigue strength and cracking process is presented. Based on the experimental observations, a model is proposed to study the S-N curves of the material in high cycle and VHCF in different medias. V