Influence of cyclic loads below endurance limit or threshold stress intensity on fatigue damage in cast aluminium alloy 319-T7

The aluminium alloy 319-T7 shows an endurance limit at approximately 107 cycles and has been used to study the influence of numerous stress amplitudes below the endurance limit on fatigue damage. Variable amplitude tests have been performed with short repeat two-step sequences of stresses above and below the endurance limit. If high stresses of the variable amplitude sequence are 40% or more above the endurance limit, cyclic loads below the endurance limit reduce lifetimes, and the detrimental effect is more pronounced the larger the low stress amplitudes. Lifetimes could be reasonably well predicted with Miner calculations and extrapolating the S-N curve with reduced slope below the endurance limit. Beneficial influences of numerous low load cycles were found in some specimens, when the maximum stress of the variable amplitude sequence was only slightly (15%) above the nominal endurance limit. Accelerated crack growth was found in fracture mechanics variable amplitude tests and repeat sequences with high stress intensity of 25% or more above the threshold stress intensity and large numbers of cycles below. Numerous cycles below threshold stress intensity, however, may cause an arrest in fatigue crack growth, if the high stress intensity of the variable amplitude sequence is only slightly (15%) higher than the constant amplitude threshold.