Uniform fatigue damage tolerance assessment for additively manufactured and cast Al-Si alloys: size and mean stress effects

Abstract

• Influence of additively manufacturing and casting on microstructure and porosity characteristics as well as quasi-static and cyclic stress-strain behavior of Al-Si alloys • Linear-elastic (LEFM: Murakami, Murakami-Schijve) and elastic-plastic fracture mechanics (EPFM: Fischer) were used for determination of stress intensity factor at failure-initiating defect and plotted in Shiozawa diagrams • LEFM and EPFM enables to consider the size effect within fatigue damage tolerance assessment • Benchmark of Woehler, Murakami, Murakami-Schijve and Fischer approaches for fatigue damage tolerance assessment regarding size effect and mean stress effect, whereby only Fischer approach enables a uniform fatigue damage tolerance assessment The near-net-shape manufacturing in additive manufactured and cast of Al-Si alloys results in a heterogeneous solidification and cooling of the parts, leading to significant gradients in microstructural and defect features as well as deformation behavior. In this paper, an elastic-plastic fracture mechanical model for uniform fatigue damage tolerance assessment of Al-Si alloys was further qualified and extended for a uniform view of different testing volumes as well as various stress ratios between R = -2…0.5 based on the fracture mechanical approaches of Murakami (√area) and Shiozawa for a reliable main crack defect-based mechanical design of fatigue-loaded structures. The linear-elastic fracture mechanical (LEFM) approaches of Murakami, Murakami-Schijve and Shiozawa were used to calculate defect-based lifetime curves, where the cyclic stress intensity factor (ΔK) at the failure-initiating defect (√area) was used to describe the local stress concentration conditions (so-called K-N curves) instead of nominal stress-based S-N curves. The LEFM-based K-N curves did not allow a unified assessment of fatigue behavior. Therefore, the cyclic stress-strain (CSS) behavior (K’, n’) was used for a plasticity-modification of the LEFM approach based on the elastic-plastic fracture mechanical (EPFM) approach of Fischer by calculating the effective cyclic J integral (ΔJ eff ) to plot J-based K-N curves, called Kj-N curves. This EPFM approach could be qualified for a uniform and reliable fatigue damage tolerance assessment of AM and sand cast Al-Si alloys for the HCF regime.