Issue 24

Yu. G. Matvienko, Frattura ed Integrità Strutturale, 24 (2013) 119-126; DOI: 10.3221/IGF-ESIS.24.13 119 Special Issue: Russian Fracture Mechanics School The failure criterion based on hydrogen distribution ahead of the fatigue crack tip Yu. G. Matvienko Mechanical Engineering Research Institute of the Russian Academy of Sciences, 4 M. Kharitonievsky Per., 101990 Moscow, Russia matvienko7@yahoo.com A BSTRACT . The hydrogen effect on the fracture toughness and fatigue crack growth behaviour in the martensitic high strength steel is investigated. The secondary ion mass spectrometry method has been employed to analyse the distribution of hydrogen concentration in the zone of the crack tip and at its edges. Changes in hydrogen concentration are observed in the vicinity of the propagating crack tip and at a remote site. The hydrogen peak H C is reduced and moves away from the fatigue crack tip with the increase of the maximum stress intensity factor max K . The concept of damage evolution is used to explain fatigue crack propagation in connection with the hydrogen redistribution ahead of the crack tip. The physical failure criterion based on the hydrogen peak in the vicinity of the fatigue crack tip and the maximum stress intensity factor has been proposed. The criterion reflects changes in the hydrogen peak which resulted from the hydrogen redistribution due to the increase of the maximum stress intensity factor as the crack length increases under fatigue loading. K EYWORDS . Hydrogen distribution ahead of the crack tip; Fatigue crack growth; SIMS; Local failure criterion; High strength steel. I NTRODUCTION ecently many works have been performed on the hydrogen embrittlement of high strength steels (e.g., [1-4]). The deleterious effects of hydrogen on the mechanical properties of high strength martensitic steels are known to have caused premature failures. For example, it was found that the threshold stress intensity factor in steels decreases drastically in response to increased dissolved hydrogen concentration (e.g., [2, 5, 6]). Although it has been reported that hydrogen degrades mechanical properties of metallic materials, there have been few studies on the effect of hydrogen on fatigue behaviour ([7-9]). It has been known that the fracture initiates in region of highly localized stress in which hydrogen is concentrated as a result of an augmented diffusion of hydrogen. To explain this phenomenon in the case of monotonic loading, a diffusion model was proposed by Liu [10] and developed by Kim et al. [3] introducing a fracture criterion as the critical hydrogen concentration at a critical distance ahead of the crack tip. However, until recently only a few papers [11-13] have reported on the experimental distribution of hydrogen ahead of a crack tip. Under monotonic mixed (I/II) mode loading there are two hydrogen accumulation peaks ahead of the crack tip [13], which correspond to the maximum hydrostatic stress and the maximum equivalent plastic strain, respectively. The experimental results also revealed that hydrogen distribution in the vicinity of the fatigue crack tip is related to the stress-strain fields surrounding the crack tip [12, 14]. It should be also R