Bilinear constitutive model for the anisotropic failure prediction of PLA specimens produced by material extrusion additive manufacturing

Material Extrusion (MEX) is the predominant technique in additive manufacturing of polymers, with Polylactic Acid (PLA) being the most commonly utilized material. Alongside the long list of advantages, MEX faces a major pitfall due to the mechanical weakness of parts. Moreover, accurately modeling the anisotropic failure of MEX specimens remains a persistent challenge. This paper, first, reviews the few previously established tensile strength prediction models to predict the mechanical behavior of PLA and meticulously analyzes their advantages and disadvantages. By phenomenologically exploring failure modes—specifically, the Layer Separation Mode (LSM) and the Layer Breakage Mode (LBM)—this study proposes a novel bilinear model to describe failure in MEX parts and predict the ultimate tensile strength of PLA in the conditions studied. The proposed bilinear model offers the advantage of simplicity and eliminates the need for assumptions regarding the shear strength and other complex performance factors. Experimental investigations were conducted with varying layer thicknesses (0.1 mm, 0.2 mm, and 0.3 mm) and printing angles (i.e., 0°, 15°, 30°, 45°, 60°, 75°, and 90°) were carried out, and a thorough comparison between the existing and the proposed models is made to strengthen the understanding of the behavior of PLA. In addition, three methods of deriving the shear strength are investigated for the first time, and the dependence of the models on this parameter is comprehensively explored. It was found that the established bilinear model performs exceptionally well in predicting the tensile strength, and its performance does not depend on other parameters such as shear strength.