Influence of microstructure and load ratio on fatigue threshold behavior in 7075 aluminum alloy

The influence of microstructure and load ratio on fatigue crack propagation in the near-threshold regime has been investigated on the 7075-aluminum al loy. The alloy heat treatment was varied to give underaged and overaged mic rostructures with approximately the same yield strength properties. Crack g rowth measurements were performed at constant load ratios between R=-1 and +0.8 in ambient air. The influence of load ratio is discussed in terms of d eformation slip, crack branching and fracture mode. Vacuum data from the li terature are compared with the current air results. For both environmental conditions, it was observed that the crack growth and threshold properties of the underaged alloy were better than the overaged alloy. The overall fat igue behavior is discussed in terms of the two parameter concept namely Del ta K and K-max. (C) 1999 Elsevier Science Ltd. All rights reserved.