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

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.

Effect of microstructural heterogeneity on fatigue strength predicted by reinforcement machine learning

“…Certain researchers investigated porosity using two‐dimensional metallographic methods 66,67 . Others used three‐dimensional methods such as X‐ray microcomputed tomography (μ‐CT) to determine flaws' shape, size, and position in three‐dimensional space 68–70 . The Varian® PaxScan 1313DX detector, based on amorphous silicon, is equipped to capture the projected pictures.…”

“…66,67 Others used three-dimensional methods such as X-ray microcomputed tomography (μ-CT) to determine flaws' shape, size, and position in three-dimensional space. [68][69][70] The Varian ® PaxScan 1313DX detector, based on amorphous silicon, is equipped to capture the projected pictures. The effective detector size is 13 Â 13 cm 2 , with a pixel count of 1024 Â 1024 pixels and a pixel pitch of 127 μm.…”

Fracture–Mechanical Assessment of the Effect of Defects on the Fatigue Lifetime and Limit in Cast and Additively Manufactured Aluminum–Silicon Alloys from HCF to VHCF Regime

“…Finally, all fatigue datasets were plotted in a normalized Kitagawa-Takahashi diagram in Figure 10b, illustrating the high accuracy for determining the fatigue limit at 10 7 for specific defect parameters as well as the independence on the process route of the aluminum alloy. In comparison to the fracture mechanics-based approach presented here, a plasticity-based approach that simulates stress build-up around the real pore morphology was published in [47].…”

“…Metals 2020, 10, x FOR PEER REVIEW 14 of 17 process route of the aluminum alloy. In comparison to the fracture mechanics-based approach presented here, a plasticity-based approach that simulates stress build-up around the real pore morphology was published in [47].…”

Significant Advancements in Numerical Simulation of Fatigue Behavior in Metal Additive Manufacturing-Review

“…The stress-strain distributions served as inputs for the Fatemi-Socie (FS) damage model [25], which made it feasible to calculate the fatigue lifespan. In addition, the acting stresses on the interior pores were found to exceed 450% of the distant stress amplitude.…”