Towards Deterministic Computation of Internal Stresses in Additively Manufactured Materials under Fatigue Loading: Part I.

Mustafa Awd,Mhd Fateh Labanie,Ali Fatemi,Kerstin Moehring,Frank Walther

doi:10.3390/ma13102318

Mustafa Awd, Mhd Fateh Labanie + Show 3 more

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https://doi.org/10.3390/ma13102318

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Journal: Materials	Publication Date: May 18, 2020
Citations: 10	License type: CC BY 4.0

Affiliation: TU Dortmund University, University of Memphis

Abstract

The ongoing studies of the influence of internal defects on fatigue strength of additively manufactured metals adopted an internal crack or notch-like model at which the threshold stress intensity factor is the driving mechanism of fatigue failure. The current article highlights a shortcoming of this approach and offers an alternative based on X-ray microcomputed tomography and cyclic plasticity with a hybrid formulation of Chaboche and Armstrong–Frederick material laws. The presented tessellation and geometrical transformation scheme enabled a significantly more realistic morphological representation of internal defects that yielded a cyclic strain within 2% of the experimental values. This means that cyclic plasticity models have an accurate prediction of mechanical properties without repeating a full set of experiments for additively manufactured arbitrary microstructures. The coupling with a material law that is oriented towards the treatment of cyclic hardening and softening enabled more accurate computation of internal stresses under cyclic loading than ever before owing to the maturity of tessellation and numerical tools since then. The resulting stress–strain distributions were used as input to the Fatemi–Socie damage model, based on which a successful calculation of fatigue lifetime became possible. Furthermore, acting stresses on the internal pores were shown to be more than 450% concerning the applied remote stress amplitude. The results are a pretext to a scale bridging numerical solution that accounts for the short crack formation stage based on microstructural damage.

Highlights

The free-form fabrication capability offered by additive manufacturing has left many structural and design issues inconclusively answered
Quasi-static tests were executed at a displacement rate of 1 mm/min, and force–displacement curves were recorded in addition to the application of an extensometer, which had a 10 mm gauge length
The authors question if the same degree of accuracy can be found in late highcycle The and preliminary very high-cycle fatigue (VHCF), where the convergence of theinused is not investigations for this study realized a tendency the plasticity literature model to model the precise owing to low in plastic strain increments

Summary

Introduction

The free-form fabrication capability offered by additive manufacturing has left many structural and design issues inconclusively answered. Numerous research works have been dedicated to studying the relationship between process dynamics and structural properties on a meso- and microscale [5,6]. At this early phase of additive manufacturing application to Ti alloys, there were two main highlights of the research. The second is that melt pool dynamics responsible for the formation of the keyhole and metallurgical pores under wide process windows become of significant interest of process researchers, as we see in a recent study combining experimental observation and numerical work [7]. One key highlight was that process technology is still facing challenges regarding stable energy delivery to the melt pool, especially in non-linear scan tracks, which are not unidirectional

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