Surface Defects Sensitivity during the Unfolding of Corrugated Struts Made by Powder‐Bed Additive Manufacturing

Abstract

Corrugated struts as part of lattice structures can lead to novel mechanical behavior by a combination of material and geometrical hardening. The unfolding behavior of such struts offers a potential of large macroscopic straining. However, their ability to be unfolded is impacted by the surface characteristics inherited from the additive manufacturing process. This study evaluates the unfolding sensitivity to these surface characteristics. Corrugated struts with varying surface roughness have been produced using a combination of Electron Beam Powder-Bed Fusion to produce corrugated samples with different nominal diameters, and chemical etching assisted by micro-CT to achieve a given final diameter. In-situ micro-CT tensile tests have been carried out to track the evolution of the struts morphology under loading. Surface defects play a significant role in the unfolding ability of such struts. They are characterized either by a global roughness or by a local notch depth. A quite broad unfolding dispersion remains for samples with the same level of roughness. A finer description of notch depth and location within the gauge length allows a more accurate prediction of the unfolding ability. A model for predicting the probability of failure during unfolding is presented.

Behavior by design made possible by additive manufacturing: The case of a whistle-blower mechanical response

“…The second requirement is related to the geometrical characteristics of the corrugation (wavelength, thickness…) and the presence of surface notch-like defects. Strategies to ensure the unfolding of the corrugated struts were preliminary studied in [3] for the selection of the geometrical characteristics, and in [8] to evaluate the effect of surface defects. The geometrical features have to be selected appropriately because as-built parts made of Ti6Al4V exhibits a moderate elongation to failure (between 5 and 8%, see [16]) and this can lead to premature failure of the corrugated struts [3].…”

“…Besides, surface defects were proven to be detrimental for the mechanical properties, in particular for the fatigue life [15] but also in term of ductility (elongation-to-failure) [16]. The population of residual surface defects along single struts after chemical etching as well as their location along the corrugated struts was monitored based on XCT scans acquired subsequently to chemical etching, see [8] for a thorough analysis. It was shown that it is mandatory to avoid the presence of severe notch-like defects in the regions sustaining high tensile stresses, i.e.…”

“…It was shown that it is mandatory to avoid the presence of severe notch-like defects in the regions sustaining high tensile stresses, i.e. internal parts of the curvatures along the corrugated struts, because this can lead to premature failure before the strut is fully unfolded [8]. The objective of the chemical treatment was thus to decrease the harmfulness of those notch-like defects to make possible the unfolding of the corrugated struts as detailed in [8].…”

“…To our knowledge, this is the first time that the use of corrugated struts in lattice structures is suggested. The characteristics of the sinusoidal corrugation (diameter, amplitude and wavelength) were chosen based on previous works [3,8] that showed that the tensile response can be tuned on demand. The degree of geometrical strain hardening can be adjusted by tuning the main features of the corrugation [3,5,6].…”

High-temperature deformation followed in situ by X-ray microtomography: a methodology to track features under large strain

“…The acquisition conditions can be optimized for each type of sample by selecting a suitable load-bearing bell. The DVC procedure has been successfully applied to a wide range of acquisitions: in situ nanotomography high-temperature tensile tests (Kumar et al, 2019), in situ micro-CT firn densification (Burr, 2017) and in situ unfolding of corrugated struts produced by additive manufacturing (Suard et al, 2020).…”