Combining the theory of critical distances and the extreme value statistics to interpret the effects of surface roughness and pores on the fatigue strength of Inconel 718 specimens

The purpose of this study is to develop a model to assess the influence of surface roughness and porosity on the fatigue strength of plain and V-notched specimens made of Inconel 718, produced by laser powder bed fusion and with surface conditions both as-built and machined. By combining fractographic analyses with the Gumbel and exponential probability distributions, the diameters and distances of the pores from the external surface were modelled. Surface roughness scans were performed using an optical profilometer to capture the surface morphology. The finite element method was employed to simulate a distribution of pores generated based on the identified probability distributions, as well as the surface profiles obtained from the scans. The critical distances, calculated by combining the machined blunt and sharp V-notched specimens, were used with the Gumbel or generalized extreme value distributions to estimate the fatigue strength concentration factor for the pores and surface roughness at 99% of probability. Finally, the proposed model was used to predict the fatigue strength of the blunt V-notched specimens in the as-built condition, as well as the plain specimens in both the as-built and machined conditions, showing an appreciable level of alignment with the experimental data.