Analysis of the Joint Effect of Heat Treatments and Stress Ratio on the Growth Behavior of Fatigue Cracks in Cast Aluminum Alloys under Combined Loading (Bending-Torsion)

The behavior of microstructural short fatigue cracks (20-100 µm in length on the same scale as the grain size) in the cast aluminum alloy A07710-T6 was investigated.Two heat treatment conditions have been used, an underaged and an overaged microstructure, chosen to have very similar tensile properties, so that the effects of precipitation and hence slip distribution on fatigue behavior could be studied at the same strength level.The results of short crack propagation tests performed on smooth specimens at 25℃ and R=0.1 are compared to conventional (long) fatigue crack propagation and threshold results under the same conditions.The short crack data is also compared to long crack tests conducted at constant maximum applied load (so that the R-ratio increases to a value above 0.8 as the threshold is approached).In conventional long crack tests, better low and high threshold crack propagation resistance is associated with the underaged microstructure.This behavior is also reflected in the thresholds obtained at high R.