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Experimental and theoretical studies of deformation in polyethylene films under biaxial loading conditions
This dissertation describes the characterization of post-yield deformation processes in thin polyethylene films under biaxial stress states. In particular, research focused on the development of test methodologies and analyses associated with post-yield deformation is described. Considerable emphasis is placed on the initiation and evolution of a unique heterogeneous, post-yield deformation process observed in thin polyethylene films under biaxial stress states. This process closely resembles the necking phenomenon noted in uniaxial tests, and hence initial efforts have primarily involved characterizing the new phenomenon and comparing it to uniaxial necking. The effects of initial molecular orientation and stress state on the formation of either heterogeneous or homogeneous post-yield deformation are investigated and discussed. The evolution of the heterogeneous deformation zones, which are referred to as dilatational bands (DB), is modeled within a thermodynamic framework. The results from the phenomenological characterization are used to determine how the thermodynamic model is to be applied. A kinematic analysis of DB's on the local and global level is also presented. The global kinematics are obtained by measuring changes in the overall dimensions of the DB's as they evolve, while local kinematics are obtained by measuring the principal draw ratios. Results from these studies dictate how the thermodynamic model is to be applied. The thermodynamic model accounts for the energy associated with DB evolution. Based on conclusions derived from phenomenological and kinematic observations, the model is reduced to a single parameter model manifesting expansion of the DB. Correspondingly, the M-integral is the energy release rate describing DB evolution. A material property known as the specific enthalpy of transformation is obtained for several polyethylene films under various biaxial stress states. This property is a measure of the material's resistance to undergoing a post-yield drawing from an initial state to the final, drawn state. A practical outcome for the test methodology might be a scheme to optimize operating conditions for solid-state drawing in a film processing line. The final part of this dissertation presents the development and application of an in-situ birefringence technique able to monitor the molecular orientation direction. The technique is employed to determine the molecular orientation direction field in the process zone of a crack tip for a polyethylene film subjected to a Mode I loading condition. The orientation direction is determined from an analysis of digital images obtained in-situ, in which the tested sample is placed between a pair of constantly rotating crossed polarizing films.
Sabbagh, Amiel Bassam, "Experimental and theoretical studies of deformation in polyethylene films under biaxial loading conditions" (2000). Doctoral Dissertations Available from Proquest. AAI9988840.