Experimental and numerical modeling of the fracture behavior of semicrystalline polymers

Semicrystalline polymers are used extensively in industry for their unique mechanical properties and their low-cost opportunity in comparison to metals. Although there has been substantial work to investigate the deformation mechanisms of these polymers from both the micro and macro mechanics level, the subject is still not well understood. The objective of this work is to capture the elastic-viscoplastic response of a semicrystalline polymer using a thermodynamically consistent continuum theory and offer a better predictive model that can be used for practical application within a FE framework. A numerical model is developed for isotactic polypropylene to simulate both the deformation and fracture response of the material. The theory is comprised of two parts. The first part is based on a continuum theory to model the shear yielding response of the polymer when the maximum principal stress is negative and demonstrates extensive deformation by a shear yielding mechanism [1]. In contrast, under stress states where the maximum principal stress is positive, semicrystalline polymers will tend to develop cavitations that will multiply and coalesce in an orientation perpendicular to the maximum principal stress. To capture this phenomenon, a second part considers a continuum model that uses activation energy to simulate the cavitation that develops within tensile fields [2]. To incorporate fracture, a damage model is employed that considers a peak tensile cavitation strain for initiation. A specialized form of the constitutive equations is then developed to model the competition between the shear-yielding and cavitation phenomenon. The model is then implemented within ABAQUS Explicit using a custom user VUMAT subroutine. Experimental results are compared for specimens fabricated out of isotactic polypropylene under tension, compression, SENB, and plate with hole under tension.