Prediction and correlation of the average crack-opening stress with service load cycles

The crack-opening stresses of a crack emanating from an edge notch in three different materials — a 2024-T351 aluminum alloy, a 1045 annealed steel, and a 1045 quenched and tempered steel specimen — were measured under three different Society of Automotive Engineers (SAE) standard service load histories having different average mean stress levels. In order to capture the actual behavior of the crack-opening stress in the material, the crack-opening stress levels were measured using a 900 x optical video microscope at frequent intervals for each set of histories scaled to two different maximum stress ranges.The crack-opening stress was modeled and the model was implemented in a fatigue notch model and the fatigue lives of notched specimens of the three different materials under the three different spectra scaled to several maximum stress levels were estimated. The average measured crack-opening stresses were within between 6 and 13 percent of the average calculated crack-opening stresses. The fatigue life predictions based on the modeled crack-opening stresses and the fatigue notch model were in good agreement with the experimentally determined fatigue data.In the interest of simplifying the use of crack-opening stress in design, the average crack-opening stress was correlated with the frequency of occurrence of the cycle reducing the crack-opening stress to the average level. The use of a crack-opening stress level corresponding to the cycle causing a reduction to a crack-opening stress reached once per 200 cycles gave a conservative estimate of fatigue life for the soft metals 2024-T351 aluminum alloy and 1045 annealed steel, while for the 1045 quenched and tempered steel the cycle causing a reduction to a crack-opening stress reached once per 10 cycles gave a conservative life estimate.