The effect of a free surface on fatigue crack behaviour

The effect of a free surface on fatigue crack behaviour is discussed in the paper and generalized stress intensity factor methodology is used for a reliable estimation of the fatigue crack propagation rate in the area close to a free surface, where the effect of vertex singularity is important. It is shown that a decrease in the stress singularity exponent close to the vertex point leads to a decrease in the fatigue crack propagation rate and contributes to a typical curved crack front shape. The numerical estimations of the crack shape correspond well with experimental observations taken from the literature. Further change of the stress singularity exponent for thin structures was studied on M(T) specimens with different thicknesses. The proposed methodology facilitates explanation of the experimentally observed decrease in the fatigue crack propagation rate (FCPR) for thin structures. It is shown, that decrease of FCPR depends on Poisson’s ratio and parameters of the Paris–Erdogan law for corresponding materials. These results can at least partially explain the effect of a free surface on fatigue crack behaviour and help towards a more reliable estimation of the residual fatigue lifetime of the structures considered.