Experimental and numerical study of dilation in mineral filled PVC
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This thesis addresses the link between applied load and dilation caused by void growth in mineral-filled PVC employing both experimental and numerical techniques. The influence of stress triaxiality and strain rate on the dilation of the material was investigated by tensile testing using round notched specimens while measuring surface deformations using digital image correlation. The material was found to be pressure-sensitive, strain rate sensitive and non-isochoric. However, as the degree of dilation was estimated from surface deformations, no insight on the internal distribution of voids was obtained. The experimental technique was extended by introducing ex situ X-ray computed tomography, allowing the spatial relative density distribution within pre-deformed tensile specimens to be measured. The relative density fields were measured at three stages of deformation for three notch radii, showing a non-homogeneous density distribution that changed shape for increasing deformation. In order to improve the temporal resolution of the relative density measurements, an in situ X-ray computed tomography technique was proposed and verified, exploiting the axisymmetric nature of the density fields. The in situ relative density measurements were combined with surface deformations measured by DIC, making it evident that the dilation of the material is confined to a domain of material points, and does not spread to new material after its initiation. An abrupt decrease in relative density was observed at the peak force, suggesting that particle decohesion is initiated at this stage of the deformation process. The evolution of void growth and the influence of specimen geometry on void growth have been observed in the experiments, but little knowledge was gained on the governing mechanisms causing this mechanical response. In order to further understand the behaviour of voids at large volume fractions and large deformations, finite element analyses using unit cells were performed with a simplified matrix material behaviour, mimicking characteristics of thermoplastic materials. The influence of stress triaxiality ratio, Lode parameter and initial void volume fraction, as well as the hardening, strain rate sensitivity and locking stretch of the matrix material, was investigated. From the unit cell simulations, it was observed that the locking behaviour of the material is critical for allowing large void volume fractions to be sustained before failure. Further, the stress triaxiality ratio was observed to have a large influence on the degree of void growth.