Some fractographic and metallographic contributions to understanding fatigue crack growth in metallic materials are reviewed, with an emphasis on environmentally assisted fatigue.
Banded ferrite pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth.
However, this material is susceptible to environmetally assisted fatigue crack propagation (EAFCP) when it is exposed to water vapour and chloride solution.
Environmentally assisted fatigue crack propagation behaviour of metal inert gas (MIG) welded AA5083 joints under stress relieving treatment has been investigated.
The design life limiting material property for vessels made from tempered martensitic steel is the environment assisted fatigue crack growth rate (FCGR) that depends on ΔK, the load ratio, loading frequency, and the H2 pressure.
The reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impeded hydrogen diffusion across the banded pearlite.
MoO2−4 effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stresseduringng full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where MoO2−4-enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film.
The evidence suggests that, in fairly benign environments, egress of dislocations around crack tips predominates, whereas environmentally assisted fatigue crack growth of many materials in liquid-metal, hydrogen, and aqueous environments involves an adsorption-induced dislocation-emission (AIDE) process at crack tips.
However, intergranular cracking modes were also observed at longer dwell times (Figure 7b) indicating that creep assisted fatigue does occur in vacuum, but this is swamped by oxidation effects in air which accelerate the crack growth rates considerably via intergranular cracking mechanism (Jiang et al. 2014).
In combination with detailed stress analysis such criteria could be implemented in a numerical model to predict crack initiation in cladding, providing an analytical tool which could greatly assist the fatigue design process and lead to a reduction in the total reliance on prototype testing of each individual system used.
