Final work : Identification of damage-enhanced visco-elastic-visco-plastic model parameters for polymeric lattices

Abstract

This thesis investigates the material properties and behaviour of two polymer materials: Polyamide 12 (PA12) and Thermoplastic Polyurethane (TPU). These materials were 3D printed using Selective Laser Sintering (SLS) to create lattice structures. A comprehensive set of experiments was employed, including tensile, compression, relaxation and cyclic tests at various strain rates.

To model the materials' behaviour, a constitutive model was used by integrating existing models. The model accounts for viscoelasticity, viscoplasticity and damage.

The material parameters were divided into three categories. Firstly, viscoelastic parameters which govern the material behaviour at low strains. They were estimated by fitting experimental Young's modulus curves with a generalized Maxwell model. Secondly, viscoplastic parameters which include yield properties, hardening laws and failure criteria. These values were assigned by minimizing error between numerical simulations and experimental curves. And finally, failure and softening parameters that were defined through the simulated damage and critical energy release rate. Failure surfaces were defined based on plastic strain, pressure dependency and strain rate effects. Softening was modelled using saturation laws.

To validate these parameters, a numerical simulation was conducted for PA12 and compare against the experimental results. The same simulation could have also be done for TPU but due to the lack of experimental data to compare with, it was decided to only compute the simulation for PA12. The results from the simulation exhibit a strong agreement with experimental data, affirming the accuracy of the chosen parameters.

For TPU, although the initial non-linear hyper-elasticity appearing in TPU was modelled using the hardening parameters, the inferred material parameters for TPU showed good agreement with experimental data except for the cyclic loading and the critical energy release rate. This disagreements could be caused due to the initial assumption when modelling the non-linear hyper-elastic behaviour of TPU using hardening parameters.

Subsequently, simulations were carried out on single-cell lattice structures to investigate their specific energy absorption capabilities. In this objective, elastic strain, plastic dissipation, and energy dissipated by the damaging process were computed for various single-cell types and volume fractions. Unlike previous studies that employed a linear elastic model, this thesis adopts a visco-elastic-visco-plastic model with failure, enabling more precise numerical outcomes. The findings showed a better capacity of the specific energy absorption as the volume fraction increased.