ESTIMATION METHOD OF ELASTIC-PLASTIC FATIGUE-CRACK GROWTH-RATES UNDERVARIABLE AMPLITUDE LOADINGS

Tests to determine the growth of load-controlled fatigue cracks were c arried out on two kinds of structural steels under constant-amplitude and repeated two- and three-step loadings in the postyield region. Cra ck growth increment and crack closure were measured by the computer-ai ded unloading elastic compliance method. A fracture mechanics paramete r, Delta J/(1-J(max)/C), was found to be a good parameter for expressi ng the crack growth rates under constant-amplitude loadings irrespecti ve of the stress ratios, where Delta J, J(max) and C are the J integra l range, the maximum J integral and a constant related to apparent fra cture toughness of the specimen used, respectively. One-directional cy clic deformation was observed to occur under load cycles with high mea n stresses. Moreover, it was found that crack closure and plastic defo rmation under variable amplitude loadings were affected by load variat ion patterns and load sequences. However, fatigue crack growth rates u nder varying loadings where both one-directional and cyclic plastic de formation were observed to be significant could be well estimated by t he linear summation rule of crack growth using Delta J/(1-J(max)/C), where Delta J is the J integral, range-pair taking account of the abo ve mentioned one-directional deformation.