Doctoral Dissertations

Date of Award

12-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Paul J. Phillips

Committee Members

John D. Landes, Donald C. Bogue, Roberto S. Benson

Abstract

An experimental method, the optical grid method, was developed to adapt the J-integral, an elastic-plastic fracture criterion, to fracture characterization of thermoplastics. The method is based on combining tensile test results with crack growth analysis, using specially designed equipment, to determine the fracture toughness at any moment of the deformation process. The values of fracture toughness of polypropylene (PP) at the initiation of crack growth were found to be near 3 KJ/m2 whereas those at the fracture moment could go as high as 15 KJ/m2. It was shown that the fracture toughness decreases with increasing strain rate over several decades for the different morphologies considered. The structural and morphological characterization of the different types of materials revealed that the effect of nucleation was significant. This effect served as a basis for the interpretation of the mechanical behavior of materials with different morphologies. It was demonstrated that an increase in percent crystallinity, at a constant spherulite size, caused the fracture toughness to drop. The effect of spherulite size was considered as a part of the whole morphological investigation, unlike other studies where certain type of mechanical behavior was attributed to the change in spherulite size while other parameters were not accounted for. It was demonstrated that at constant crystallinity, an increase in spherulite size within the same type of material resulted in more brittleness. A significant reduction in spherulite size, by the use of a nucleating agent, also resulted in more brittleness. The effect of spherulite size is then believed to be just a reflection of the "inside of the spherulite" changes. This explanation is better understood in terms of the nucleation and crystallization processes. DSC studies showed that the effective crystallization temperature of the polymer is increased by about 18°C when the nucleating agent is added. This increase in crystallization temperature results in an intraspherulitic morphology and a difference in tie molecule density that are responsible for the lower toughness despite the spherulite size being very small. Crack growth studies showed that the use of a nucleating agent slightly retards initiation of crack propagation. But once a crack is initiated it grows faster in the nucleated material, forcing it to fail at an earlier stage and lower values of toughness than the non-nucleated material. This result is not in total agreement with previously reported results which were not based on any substantial morphological studies.

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