Thermo-Mechanical Behavior and Strain Rate Sensitivity of 3D-Printed Polylactic Acid (PLA) below Glass Transition Temperature (Tg)

Abstract

This study investigated the thermomechanical behavior of 4D-printed polylactic acid (PLA), focusing on its response to varying temperatures and strain rates in a wide range below the glass transition temperature (T_g). The material was characterized using tension, compression, and dynamic mechanical thermal analysis (DMTA), confirming PLA's strong dependency on strain rate and temperature. The glass transition temperature of 4D-printed PLA was determined to be 65 °C using a thermal analysis (DMTA). The elastic modulus changed from 1045.7 MPa in the glassy phase to 1.2 MPa in the rubber phase, showing the great shape memory potential of 4D-printed PLA. The filament tension tests revealed that the material's yield stress strongly depended on the strain rate at room temperature, with values ranging from 56 MPa to 43 MPA as the strain rate decreased. Using a commercial FDM Ultimaker printer, cylindrical compression samples were 3D-printed and then characterized under thermo-mechanical conditions. Thermo-mechanical compression tests were conducted at strain rates ranging from 0.0001 s^-1 to 0.1 s^-1 and at temperatures below the glass transition temperature (T_g) at 25, 37, and 50 °C. The conducted experimental tests showed that the material had distinct yield stress, strain softening, and strain hardening at very large deformations. Clear strain rate dependence was observed, particularly at quasi-static rates, with the temperature and strain rate significantly influencing PLA's mechanical properties, including yield stress. Yield stress values varied from 110 MPa at room temperature with a strain rate of 0.1 s^-1 to 42 MPa at 50 °C with a strain rate of 0.0001 s^-1. This study also included thermo-mechanical adiabatic tests, which revealed that higher strain rates of 0.01 s^-1 and 0.1 s^-1 led to self-heating due to non-dissipated generated heat. This internal heating caused additional softening at higher strain rates and lower stress values. Thermal imaging revealed temperature increases of 15 °C and 18 °C for strain rates of 0.01 s^-1 and 0.1 s^-1, respectively.

Keywords: 3D printing; 4D printing; shape memory polymer; smart materials; thermo-mechanical experiments.

Figure 1

The 4D printing concept with PLA material.

Figure 2

(a) geometry of 4D-printed sample and (b) 3D printing preview of slicing patterns.

Figure 3

(a) Geometry of DMTA samples and (b) 3D printing preview of slicing patterns.

Figure 4

Uniaxial tensile testing of filament procedure: (left) equipment for uniaxial filament testing and (right) sketch of equipment.

Figure 5

DMTA results for 4D-printed PLA.

Figure 6

Stress–strain curves for PLA filament at various strain rates.

Figure 7

PLA 4D-printed cylindrical samples—as printed (left) and after compression (right).

Figure 8

Stress–strain curves in uniaxial compression for PLA at strain rates of (a) 0.0001 s⁻¹ and (b) 0.001 s⁻¹ and temperatures of 25, 37, and 50 °C.

Figure 9

Stress–strain curves in uniaxial compression for PLA at temperatures of (a) 25 °C, (b) 37 °C, and (c) 50 °C at strain rates of 0.0001 and 0.001 s⁻¹.

Figure 10

Stress–strain curves in uniaxial compression for PLA at strain rates of (a) 0.01 s⁻¹ and (b) 0.1 s⁻¹ and temperatures of 25, 37, and 50 °C.

Figure 11

Stress–strain curves for PLA at temperatures of (a) 25 °C, (b) 37 °C, and (c) 50 °C at strain rates of 0.01 and 0.1 s⁻¹.

Figure 12

Yield stress value dependence on (a) temperature and (b) strain rate.

Figure 13

Temperature evolution images in 5 characteristic points (I-V) of 4D-printed PLA at strain rates of (a) 0.01 s⁻¹ and (b) 0.1 s⁻¹.