A THEORETICAL AND EXPERIMENTAL-STUDY OF ENVIRONMENTAL HYDROGEN-ASSISTED SHORT FATIGUE-CRACK GROWTH IN AN AL-LI ALLOY

The initiation and propagation of fatigue cracks in an Al-Li 8090 alloy in a vapour environment of 0.6 M NaCl solution was investigated. A severe degradation of the resistance to short crack growth was exhibited. Preliminary work carried out to establish the susceptibility of the material to hydrogen embrittlement demonstrated a close correlation between the deformation mode of this alloy and hydrogen absorption. The combination of highly localized slip and highly localized hydrogen fugacity creates a high susceptibility to hydrogen-assisted crack growth. On the basis of current micro-mechanical models, it is suggested that hydrogen trapping induces a reduction of the friction stress acting in the crack tip plastic zone. Consequently, enhanced plasticity at the crack tip due to the decrease in friction stress leads to an increase in crack growth rate. An exact solution for a surface crack in a semi-infinite plane is obtained based on a dislocation crack model. Using this solution a computer method is developed to calculate the time-dependent short crack growth rate and fatigue lifetime. Both solutions show good correspondence with the experimental results.