Acceleration of fatigue crack growth due to occasional mode II loading in 7075 aluminum alloy

• We study the effect of occasional mode II loading on mode I fatigue crack growth.
• A thin-walled 7075-T6511 aluminum alloy tube is used.
• Retardation effect by crack closure is negligible.
• Mode II growth is much faster than mode I growth in this alloy.
• The accelerated mode II growth should be considered as a predominant factor.

The effect of an occasional mode II loading on the subsequent mode I fatigue crack growth was investigated in a thin-walled 7075-T6511 aluminum alloy tube. Careful observation of crack growth behavior revealed that an occasional mode II loading has two contrasting effects on the subsequent crack growth. First is a retardation effect that is associated with a crack closure development due to the mode II loading. However, this effect was insignificant with respect to the crack growth life as a whole. Second is an acceleration effect that is associated with an accelerated crack growth rate in mode II. It was found that, in a relatively high ΔK regime, mode II crack growth was from one to two orders of magnitude faster than mode I crack growth. This study shows that mode II crack growth should be considered as a predominant factor in evaluating the effect of an occasional mode II loading for a mode I crack in 7075 aluminum alloy.