Experimental and theoretical investigation of environmental media on very-high-cycle fatigue behavior for a structural steel

Rotary bending fatigue tests were performed in laboratory air and 3.5% NaCl aqueous solution to investigate the influence of environmental media on fatigue properties of a structural steel in high-cycle and very-high-cycle fatigue regimes. The results show that the fatigue strength of the structural steel in 3.5% NaCl solution is remarkably degraded compared with the case tested in laboratory air. The fracture surfaces of specimens tested in different environments were examined to reveal the fatigue crack initiation characteristics. It shows that for specimens tested in 3.5% NaCl solution, cracks originate via multiple surface initiation modes and cracking is intergranular; in addition, widespread secondary cracks feature on the fracture surface. Based on fatigue tests and fractography observations, a numerical model of crack initiation is proposed to describe the transition of fatigue initiation site from subsurface to surface for specimens tested in air and 3.5% NaCl solution. The model reveals the influences of load, material strength, grain size, inclusion size and surface environmental media on the crack initiation site transition. It agrees well with the experimental observations.