A new method of constructing the kinetic fatigue fracture diagrams -- crack propagation equation based on energy approach


The force criterion being a local criterion is frequently applied for the description of a fatigue crack growth. The rate of a crack growth is a function of stress intensity factors. This function based on experimental data has an exponential form in relation to KI, as it has been shown in Paris’ law. The deformation criterion based on the δK – model is used in the case when the plastic zone before a crack tip is of the same order as a body dimension. The presented approach is effective in the case of a stable loading cycle without taking into account the “history” of material deformation and crack propagation. The stress intensity factors or the values of a crack opening become then the invariants of a fatigue damage. The energy approach makes possible to take into consideration the elements mentioned earlier. The energy criterion of fatigue crack propagation in the isotropic body has been formulated. Finally, a kinetic equation of fatigue fracture as the analytical relation between fatigue crack surface propagation rate and dissipation energy of plastic deformation in the precracked zone has been obtained. A new method of constructing kinetic fatigue fracture diagrams (KFFD) has been presented on the basis of measurement results of a hysteresis loop area for the isotropic body with an internal flat crack under a cyclic loading. For the experimental verification, the results of fatigue crack propagation studies for 18G2A and 40H steels have been utilized.

FME Transactions