New expressions for the calculation of mode I fracture Stress Intensity Factor (SIF) for additive manufacturing defects

Abstract

The defects are the main factor influencing the fatigue and fracture strength of Additive Manufactured (AM) materials, being the Stress Intensity Factor (SIF) the parameter that characterises the material behaviour with respect to these failure mechanisms. SIF of mode I fracture (KI ) for defects in a semi-infinite body under tensile stress are currently calculated using Fitness-For-Service (FFS) codes or Murakami’s model. However, large discrepancies were found among their results due to differences in engineering judgement. This work proposes new engineering expressions to estimate the KI of irregular defects in semi-infinite bodies based on the envelopes of the values provided by the FFS codes and Murakami’s model. Consistent with these, the expressions are limited to aspect ratios in the range of 0.2≤a/c≤2, which covers most of the real defects regardless of their nature (pores, lack of fusions, shrinkages, inclusions…). The results show that the proposed expressions are in line with FFS codes and Murakami’s model, and therefore they could be used to estimate the SIF range for fracture and fatigue calculations. This work is intended to shed light on the field of validation and certification of AM components, as no standard is developed for such materials.