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

Abstract

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.