Effect of constraint on fatigue crack propagation near threshold in medium carbon steel

The influence of constraint caused by different specimen geometry on fatigue crack growth in the threshold region is analyzed and a phenomenological model is proposed to correlate the fatigue crack propagation rate under different constraint levels. The approach is based on an application of two-parameter fracture mechanics and the corresponding calculations are performed according to the finite element method. The constraint level is quantified by the value of T-stress. The fatigue crack propagation rate is then expressed in terms of the stress intensity factor K and T-stress. The influence of K and T on the plastic zone size and crack opening near the fatigue crack tip is investigated. It is shown that assuming small scale yielding conditions, the plasticity induced crack opening and crack closure are not significantly influenced by the T-values, but the influence of T-stress on plastic zone size is significant and cannot be neglected. Plastic zone size is considered as a controlling variable for near threshold fatigue crack behavior and a simple procedure making it possible to estimate the changes of the fatigue crack propagation rate in the threshold region due to different constraint level quantitatively is formulated. The numerically obtained results are compared with experimental data. It is concluded that experimental values correspond well with those numerically predicted. The result presented makes it possible to relate experimentally measured data obtained from specimens with different geometries and thus contribute to more reliable estimates of the residual fatigue life of structures.