Rate-Dependent Energy Dissipation in Additively Manufactured Thermoplastic Polyurethanes

Additively manufactured thermoplastic polyurethanes (TPU) are used in compliant polymer components, yet the rate-dependent pathways by which mechanical work is dissipated and converted into heat remain insufficiently understood. This limits the physical interpretation of their thermomechanical response to the design of an energy-dissipating system. This study investigates the mechanical properties and energy dissipation of two 3D-printed TPUs under different loading paths and strain rates. Frequency sweep dynamic mechanical analysis was used to characterise the temperature- and rate-dependent viscoelastic behaviour of the materials, providing a baseline for interpreting thermomechanical responses over a wide range of strain rates. Load-relax-reload tensile experiments at different speeds revealed the interplay between Mullins-like softening and viscoelastic relaxation, while compression experiments from 0.01 to 2750 s⁻¹ established how dissipation mechanisms evolve with strain rate. Cyclic tensile and compressive tests further captured the development of hysteresis and distinct recovery behaviours and energy retention in the two materials. Infrared thermal imaging was employed in situ to relate heat generation to mechanical work input. The harder TPU consistently exhibited higher stress levels and large temperature rises, whereas the softer TPU showed a more compliant response and narrower hysteresis. The great softening occurred during the initial loading history, after which the mechanical and thermal responses became largely stabilised. This study offers a comprehensive dataset for understanding how energy is partitioned and transformed in additively manufactured thermoplastic polyurethanes, depending on strain rate, loading path, and relaxation processes.