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Osa, Aitor Arriaga (2012) Plasticity and impact test studies on thermoplastic materials, finite element analysis and experimental correlation. Doctoral thesis, London Metropolitan University.
The primary aim of the study was to evaluate the validity of the elasto-plastic constitutive models implemented in the Finite Element Analysis (FEA) software ANSYS~. This was carried out comparing experimental tensile, bending and puncture or plate penetration test results with the corresponding FEA calculations. The studies were carried out in concern to the slow rate testing and finite element analysis of two thermoplastic materials. The selected materials were a blend of Polycarbonate (PC) and Acrylonitrile-Butadiene-Styrene (ABS), Bavblend" T45 from Bayer Material Science, as an amorphous polymer and a Polypropylene (PP) copolymer, BE677AI from Borealis, as a semi-crystalline polymer. The uniaxial tensile behavior of both materials was used to generate material data input for the simulation software. Due to post yield uncertainties when measuring true values of stress and strain with conventional extensometric devices, an iterative approach was presented here as a solution for measuring large local plastic strains in tensile specimens. The experimental three point bend test and the plate perforation test with hemispherical darts, were used as correlation tests with simulations in order to validate not only the used constitutive elasto-plastic models based on the Von Mises yield criteria, but also to check the influence of different simulation variables. The parameters studied were the element type, mesh density, friction and contact conditions between different parts. Additionally, another yielding criteria, the hydrostatic pressure sensitive Drucker-Prager model, was analysed. It was observed that for both materials, the use of a classical approach for obtaining true values of stress and strain in conjunction with a Von Mises yielding criteria gave acceptable results for the studied testing modes. However, the input curves did not closely correlate with the tensile test results. This was overcome to using an iterative method to correct the input curves to match then to the experimental tensile tests. Being a precise procedure for uniaxial cases, it was observed that for bending or puncture problems this method offered stiffer responses than using the classical approach of stress-strain conversions. Finally, the Drucker-Prager criterion was able to capture in a more precise way bending dominated problems than the usual Von Mises yielding criteria.
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