INFLUENCE OF TRANSFORMATION-INDUCED CRACK CLOSURE ON SLOW FATIGUE-CRACK GROWTH UNDER VARIABLE AMPLITUDE LOADING

Fatigue crack growth under constant and random loading conditions was investigated for a metastable austenitic-bainitic steel in comparison with a ferritic chromium steel at very low crack growth rates. Experim entally determined random crack growth was compared with linear Miner calculations on the basis of constant amplitude results. It was found that the measured crack growth rates in transforming material are a fa ctor of 10 lower than the calculated values, whereas the difference is only a factor of 2 for the ferritic steel. The reason for the pronoun ced crack growth retardation in the metastable alloy is transformation of part of the austenitic phase into martensite in the stress field o f the crack tip, accompanied by a volume increase and, consequently, r esidual compressive stresses. Rare high load cycles in the random sequ ence increase the closure level, which then leads to pronounced retard ation of fatigue crack growth for the numerous successive low amplitud e cycles.