Mechanical & Materials Engineering, Department of

 

Date of this Version

Fall 12-14-2012

Comments

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering, Under the Supervision of Professor Kevin David Cole. Lincoln, Nebraska: December, 2012

Copyright (c) 2012 Sompong Prachumchon

Abstract

The permeation parameters of hydrogen gas in high density polyethylene (HDPE) system are sought by comparison with a diffusion model. The method of Green’s functions is used to obtain solutions for the diffusion model. Permeation parameters are found from transient experimented data during two processes; pressurization followed by depressurization. The mechanical compression of HDPE during the pressurization process resulted in lower diffusivity coefficient values and higher solubility values. The results show that the diffusivity coefficient value in the pressurization process is 37% of that during the depressurization process. At the start of the depressurization process, a short-duration fast flow rate of the hydrogen gas that is observed experimentally is explained by the addition of a contact conductance to the diffusion model.

Study of the behavior of a pre-existing flaw under depressurization process of the HDPE and hydrogen gas system is included. Under quasi-static assumption, a prediction of internal pressure inside the flaw caused by diffusion using a constant volume model and a varying volume model is given. The results from these models are verified by FEM software COMSOL with a good agreement. Determination guild line of the safety of an HDPE with a pre-existing flaw with various sizes from failure by yielding, and the critical energy release rate , and the critical stress intensity factor is provided. The flaw with radius of 4 micron, 20 micron, 40 micron, 80 micron, and 100 micron located at various positions inside the Sample is used in the simulation. The results of calculation show that the Sample embedded with spherical flaw is safe from yielding. However, the Sample embedded with circular shaped flaw is fail to crack propagation when the flaw has radius greater than and equal to 20 micron while the flaw with radius of 4 micron is safe from failure. The maximum internal pressure depends on the location of the flaw in the HDPE. The time to reach the maximum internal pressure depends on the size of the flaw.

Study of 1D model and 2D model during pressurization process in COMSOL shows relative percentage difference of the diffusive flux magnitude of 16.23 percent.

Advisor: Kevin D. Cole