Fatigue strength estimation methodology of additively manufactured metallic bulk material

The objective of this research work is to scientifically contribute to the fatigue strength assessment of selectively laser melted metallic structures and aims to derive a methodology to safely estimate bulk material strength based on experimental data. Investigations focus on determination of the relevant post build and -processing residual stress condition and implementation in state of the art fatigue design concepts, alongside defect population and material properties. Additive manufacturing, especially powder-bed based laser melting, exposes material to immense thermal gradients that provoke residual stresses which bias complex mean stress states in components, may causing premature failure. X-ray diffractometry enables the measurement and allows assessment of present stresses by providing information about the superposition of residual stresses of different order by considering peak broadening effects. Residual stresses of first order overlay with load-induced stresses and significantly alter fatigue performance. Therefore, a mean stress correction of the experimentally determined fatigue strength is carried out by applying Smith-Watson-Topper's damaging parameter. The fully reversed fatigue strength amplitude is matched by complementing Murakami's approach by a correlation coefficient which empirically considers tensile residual stresses relative to hardness. Implementation of the introduced reduction factor to established designing concepts is found to be well applicable, considering that the developed methodology estimates fatigue strength of several test series from different materials in various post treatment conditions within a conservative region of - 3% and - 8%. The approach illustrated within this paper satisfies the demand for a holistic and conservative fatigue design approach incorporating defect population and hardness as well as the effect of the local residual stress condition.

Automated summary

The research aims to contribute to the fatigue strength assessment of selectively laser melted metallic structures by deriving a methodology based on experimental data and focusing on residual stresses and material properties. By considering the effects of thermal gradients and residual stresses, the study introduces a correction factor to established designing concepts to estimate fatigue strength within a conservative region.