Critical Assessment of a Local Strain-Based Fatigue Crack Growth Model Using Experimental Data Available for the P355NL1 Steel

Fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and strain-life damage models have been proposed in the literature. The UniGrow model fits this particular class of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are used to assess the actual crack driving force, taking into account mean stress and loading sequence effects. The performance of the UniGrow model is assessed based on available experimental constant amplitude crack propagation data, derived for the P355NL1 steel. Key issues in fatigue crack growth prediction using the UniGrow model are discussed; in particular, the assessment of the elementary material block size, the elastoplastic analysis used to estimate the residual stress distribution ahead of the crack tip and the adopted strain-life damage relation. The use of finite element analysis to estimate the residual stress field, in lieu of a simplified analysis based on the analytical multi-axial Neuber's approach, and the use of the Morrow's strain-life equation, resulted in fatigue crack propagation rates consistent with the experimental results available for P355NL1 steel, for several stress R-ratios. The use of the Smith–Watson–Topper (SWT) (=σ_max.Δɛ/2) damage parameter, which has often been proposed in the literature, over predicts the stress R-ratio effects.