Method of analysis and prediction for variable amplitude fatigue crack growth

Abstract Fatigue crack growth behavior of structure subjected to variable amplitude loading is very complex. Both the truncation and the load sequence have been shown to have a significant influence on the test results because of the load level interaction effects. To understand these interaction effects and the possible influence they can have on the results obtained a test program was performed. Fatigue crack growth tests were conducted on the program using 7075-T6 and 2024-T3 aluminum and titanium 6A1-4V mill anneal. Using the test data, an analysis method was developed. In this analysis method the crack growth rate is evaluated for each load cycle using a modification of the fracture mechanics correlation technique. The crack growth for each cycle was evaluated as a function of the stress intensity factor excursion with a correction factor for the maximum and minimum peak stress levels in the test spectrum. The fatigue crack growth correction for the peak stresses in the spectrum is given as a growth rate correction factor r . The relationship for r , is termed the ‘fatigue crack growth rate interaction model’. For verification, the interaction model was applied to test data from spectrum loading tests. The correlation obtained for the example, indicated that the model properly predicts the interaction effects and its use could significantly improve the accuracy of crack growth life calculations for programmed spectrum tests.