On strain rate and temperature dependent mechanical properties and constitutive models for additively manufactured polylactic acid (PLA) materials

摘要

Fused deposition modeling (FDM) has demonstrated its effectiveness in the additive manufacturing (AM) field thanks to its numerous merits, such as low cost and ease of implementation. Polymeric polylactic acid (PLA) has been widely used in FDM attributed to its recyclable, renewable, degradable and biocompatible characteristics. However, the material properties of polymeric materials can be largely affected by temperature and strain rate; and the mechanical behavior of polymer printed by FDM are fairly different from those manufactured by traditional technology. In this study, uniaxial tensile tests of the PLA specimens printed by FDM are performed under different strain rates and temperatures. For the first time, the digital image correlation (DIC) technique is employed to capture the surface strain of the AM specimens under loading, allowing us to quantify thermal stress concentration in the FDM printed polymer. Significant strain localization induced by the embrittlement of PLA is identified by the DIC system. The scanning electron microscope (SEM) technique is used to characterize the failure modes and failure mechanisms of the printed PLA samples. With increasing strain rate or decreasing temperature, the ductile to brittle failure modes can be observed by SEM. The modified Zener model is developed here to predict the stress-strain responses accounting for the combined effects of strain rate and temperature with proper modeling accuracy. The Cowper-Symonds material model is also employed to describe the effect of the strain rate on yield strength with a maximum 3.2 relative error between the experimental results and the theoretical model. Further, the yield strength, failure strain and Young's modulus show a strong linear correlation with temperature. This study is anticipated to provide an effective and precise material model for the mechanical analysis of composite structural components made by the additive manufacturing process.