Thermomechanical behavior of polymeric periodic structures

Abstract

Additive manufacturing (AM) enables the fabrication of periodic structures with regular repeat units and tailorable mechanical properties. Polymer-based AM processes, such as fused filament fabrication (FFF), permit rapid, low-cost fabrication of periodic structures from a wide range of materials. However, the mechanical properties of polymers are strongly affected by temperature, especially near the glass transition temperature (T g ), which has a significant impact on the design and application of the structures. Herein, we utilize a coupled experimental and computational approach to evaluate the mechanical behavior of polymeric periodic structures (PPS) with uniform and spatially varying temperatures that span T g . Individual unit cells (square, auxetic, and honeycomb) are additively manufactured from polylactic acid (PLA) (T g = 54 °C) and experimentally evaluated by dynamic mechanical analysis (DMA) using a tension test fixture and thermal chamber. The unit cells are subjected to sequential tension and compression tests, up to a maximum of +/−20% strain, which are repeated from 40 °C to 65 °C in 5 °C increments. Maximum forces vary by two orders of magnitude when temperature varies across T g , and a hysteresis loop is observed for temperatures 14 °C below T g . This indicates that viscoelasticity has a significant impact on the mechanical properties of the structure below T g and that the structures are capable of dissipating energy. Experimental force-displacement and energy dissipation results agree favorably with results from a multiphysics finite element framework, which utilizes bulk material properties as inputs. The computational framework is then applied to structures comprising tessellated unit cells to evaluate the effects of uniform temperatures and temperature gradients on mechanical properties. The results demonstrate that non-uniform temperature fields can produce spatial variation of mechanical properties, such as Poisson’s ratio, throughout the structure. Insight obtained in this investigation provides guidelines for the design, fabrication, and implementation of PPS operating across a range of temperatures that passively modifies their mechanical properties and energy dissipation characteristics. • Thermomechanical analysis of square, honeycomb, and auxetic periodic structures. • Experimental and computational analysis of 3D printed PLA across glass transition. • Thermal gradients spatially affect array mechanical properties (Poisson’s ratio). • Square unit cell and honeycomb array dissipate highest energy per unit mass. • Applicable to other polymers and structures by updating bulk finite element inputs.