Application of critical distances to fatigue at pores

Abstract

This work applies Taylor's theory of critical distance to quantify the effect of defects on fatigue initiation in an additively manufactured metal. We focus on hollow pores that are ideal spherical, prolate, and oblate spheroids isolated in an otherwise homogeneous linear‐elastic material. These conditions support the development of exact solutions using the exterior Eshelby tensor for a pore in a remote, arbitrary stress field. For spheres, this solution process admits simple closed‐form solutions for principal stresses disturbed by the pore. For prolate and oblate spheroids, we present the solutions as graphical curves showing stress variations under uniaxial tension. This report then extends the analysis to determine the effect of defects on a parametric, power‐law stress range vs fatigue life model. By propagating the distributed stress fields through this model, this study demonstrates the effect of pore size, pore shape, stress, and parametric fatigue properties on the life reduction due to porosity. These results suggest several approaches to increasing fatigue lives in porous materials, eg, reducing the pore size, promoting spherical pores, and increasing the microstructural parameter (comparable to the El Haddad parameter). Results presented in this work may be useful to inform trends of fatigue strength and fatigue initiation lives in metallic alloys with limited porosity, eg, additively manufactured materials that have been HIP'ed.

Damage tolerant design of additively manufactured metallic components subjected to cyclic loading: State of the art and challenges

“…The size of the gas pores is of the order of < 100 m, [83]. Since their shape is approximately spherical, the stress concentration factor of an internal pore is rather small (Kt  2), see Sobotka et al [170], who provides an analytical solution for embedded pores and [26]. Note, however, that the stress concentration will increase when two adjacent pores are sufficiently close such that their stress-strain fields interact as illustrated in Fig.…”

“…Note, however, that the stress concentration will increase when two adjacent pores are sufficiently close such that their stress-strain fields interact as illustrated in Fig. 24 [171], see also [170]). When the distance s between two adjacent pores decreases below the diameter D1, Kt increases and almost doubles at a very short distance.…”

Structural integrity assessment of rail steel failure modes and mechanism

“…The von Mises stress near the inclusion was selected for calculation to account for the elastic-plastic effect. From the results, it shows that is around 2.08 for the simulated pores and a similar result has been achieved by Sobotka et al [140]. The fully bonded inclusion has a lowest stress concentration factor and the high stress is constrained in the direction parallel to the outer surface.…”