Fracture mechanics studies of hydrogen embrittlement

Hydrogen damage is a technological problem that limits the useful life of the components. Traditionally the influence of hydrogen has been evaluated by tensile test and dead weight loading of electrolytically charged specimens. Hydrogen affects the yield, UTS and ductility. It is also now being realised that rather than emphasing the loss in tensile ductility alone, the whole stress-strain curve should be analysed as in the slow strain rate testing. However, since last three decades fracture mechanics is being increasingly used to assess fitness with a known size of defect. The local elastic stress at crack tip is related to stress intensity factory (K). Whenever the fracture problem involves large scale plasticity, other cracking parameters like crack tip opening displacement may be used. Hydrogen embrittlement is commonly thought to occur by subscritical crack growth often producing time delayed fractures. Hydrogen induced subcritical crack growth at different stress intensity range (Delta K) is measured using LEFM. The influence of hydrogen on fatigue crack growth rate has not received sufficient attention. It is commonly assumed not detrimental in high cycle fatigue due to the fact that sufficient time is not available for the hydrogen to accumulte at the crack tip and cause damage. This paper outlines the fracture mechanics approach and presents research results carried out in the Department to study the phenomena of hydrogen embrittlement. The results have unambiguously demonstrated that: COD is a more sensitive parameter to measure fracture initiation toughness and that hydrogen plays a damaging role even at 10Hz as there is a monotonic non zero tensile stress at crack tip. The correlation of observed trend in COD and FCG rate with fractography has indicated the mechanism by which hydrogen promotes cleavage fracture.