Tests and numerical simulations of polymer components

Abstract

This thesis presents a material model for thermoplastics in LS-Dyna. As SIMLab at NTNU is involved in developing the improved model, they have initiated this thesis. The focus is validating the constitutive model for high-density polyethylene and polyvinyl-chloride.

First, the constitutive model requires 12 parameters to capture the main trends of thermoplastics. The coefficients are found by evaluating the true stress -strain responses from uniaxial tension and compression tests. The specimens are applied to different strain rates to capture the rate dependency. Note that polymers absorb a significant amount of energy after necking. Therefore, a traditional extensometer is replaced by anoptical technique to calculate the strains.

Second, the results from the experimental tests calibrate the constitutive model. The material response consists of part A and B, where part A represents the weak van der Waals bonds and part B covers thestraightening of the chains. The parameters capture this response by describing rate dependency, hardening, and yield criterion, among others.

After the calibration, the next step is to validate the coefficients. Therefore, a biaxial test was carried out in the laboratory as the stress-state differs from the material tests. The specimens were simulated with theconstitutive model in LS-Dyna and compared to the experimental tests.

All in all, the numerical model covers most of the characteristics of thermoplastics despite some limitations. It also adapts well to the applied velocity as the maximum forces increases with the strain rate. Last, the modelis user-friendly and can potentially be used for industrial applications.