Anisotropic rate-dependent mechanical behavior of Poly(Lactic Acid) processed by Material Extrusion Additive Manufacturing

The strain-rate dependence of the yield stress for Material Extrusion Additive Manufacturing (ME-AM) polylactide samples was investigated. Apparent densities of the ME-AM processed tensile test specimens were measured and taken into account in order to study the effects of the ME-AM processing step on the material behavior. Three different printing parameters were changed to investigate their influence on mechanical properties, i.e. infill velocity, infill orientation angle, and bed temperature. Additionally, compression molded test samples were manufactured in order to determine bulk properties, which have been compared to the ME-AM sample sets. Anisotropy was detected in the strain-rate dependence of the yield stresses. ME-AM samples with an infill angle of 0° have a higher strain-rate dependence than specimens with αor = 90°. Remarkably, the strain-rate dependence manifested by the ME-AM samples is considerably lower than that displayed by compression molded test specimens. The Ree-Eyring modification of the Eyring flow rule is able to accurately describe the strain-rate dependence of the yield stresses, taking two molecular deformation processes into account to describe the yield kinetics. The results from this paper further show a change from a brittle behavior in case of compression molded samples to a semi-ductile behavior for some of the ME-AM sample sets. This change is attributed to the processing phase and stresses the importance that the temperature profile (initial fast cooling combined with successive heating cycles) and the strain profile during ME-AM processing have on the resulting mechanical properties. Both these profiles are significantly different from the thermo-mechanical history that material elements experience during conventional processing methods, e.g. injection or compression molding. This paper can be seen as initial work that can help to further develop predictive numerical tools for Material Extrusion Additive Manufacturing, as well as for the design of structural components

Extra low interstitial titanium based fully porous morphological bone scaffolds manufactured using selective laser melting

This is an accepted manuscript of an article published by Elsevier in Journal of the Mechanical Behavior of Biomedical Materials on 29/03/2019, available online: https://doi.org/10.1016/j.jmbbm.2019.03.025 The accepted version of the publication may differ from the final published version.Lattice structure based morphologically matched scaffolds is rapidly growing facilitated by developments in Additive Manufacturing. These porous structures are particularly promising due to their potential in reducing stress shielding and maladapted stress concentration. Accordingly, this study presents Extra Low Interstitial (ELI) Titanium alloy based morphological scaffolds featuring three different porous architecture. All scaffolds were additively manufactured using Selective Laser Melting from Ti6Al4V ELI with porosities of 73.85, 60.53 and 55.26% with the global geometry dictated through X-Ray Computed Tomography. The elastic and plastic performance of both the scaffold prototypes and the bone section being replaced were evaluated through uniaxial compression testing. Comparing the data, the suitability of the Maxwell criterion in evaluating the stiffness behaviour of fully porous morphological scaffolds are carried out. The outcomes show that the best performing scaffolds presented in this study have high strength (169 MPa) and low stiffness (5.09 GPa) suitable to minimise stress shielding. The matching morphology in addition to high porosity allow adequate space for flow circulation and has the potential to reduce maladapted stress concentration. Finally, the Electron Diffraction X-ray analysis revealed a small difference in the composition of aluminium between the particle and the bonding material at the scaffold surface