Impact behavior and modeling of engineering polymers

Due to many unique and desirable properties, engineering polymers have increasingly been used in applications, such as electronic and electrical enclosures, lenses, goggles, and windows, where impact behavior is of primary concern. Currently, the evaluation of impact design failure of polymeric structures has to be experimentally performed on molding prototypes. The experimental trial and error method significantly retards design progress and optimization, wastes a lot of time, money and effort. To decrease these disadvantages, computer aided design method based on finite element analysis is proposed to simulate the mechanical behavior of polymeric structures under impact loading.; In this thesis, the impact behavior of a glassy polymer acrylonitrile-butadiene-styrene (ABS) and a semicrystalline polymer alloy polycarbonate and polybutylene-terephthalates (PBT) are obtained as a function of impact velocity and temperature from the standard ASTM D3763 multiaxial impact test. As computer simulation of destructive impact events requires two material models: a constitutive model and a failure model, uniaxial mechanical tests of the two polymers are carried out to obtain true stress vs. true strain curves at various temperatures and strain rates. A new phenomenological constitutive model (named DSGZ model) is proposed on the basis of four previous models: Johnson-Cook, G'Sell-Jonas, Matsuoka, and Brooks. The proposed constitutive model has an explicit expression of stress dependence on strain, strain rate, and temperature. It is capable of uniformly describing the entire range of deformation behavior of glassy and semicrystalline polymers under uniaxial compressive loading. Using hydrostatic pressure effect, the DSGZ model is generalized for any monotonic loading modes. The thermomechanical coupling phenomenon of polymers during high strain rate plastic deformation are considered and modeled. A failure criterion based on maximum plastic strain is proposed. Finally, the generalized DSGZ model, the thermomechanical coupling model and the failure criterion are integrated into the commercial finite element analysis package ABAQUS/Explicit through a user material subroutine to simulate the multiaxial impact behavior of the two polymers ABS and PBT. Impact load vs. striker displacement curves and impact energy vs. striker displacement curves from computer simulation are compared with multiaxial impact test data and were found to be in excellent agreement.