University of Minnesota M.S. thesis. September 2012. Major: Mechanical Engineering. Advisors:Dr. Susan Mantell, Dr. Jane Davidson. 1 computer file (PDF); viii, 70 pages, appendices A-C.Polyethylene (PE) pipes, are used for water delivery, are susceptible to oxidation. As a result of oxidation PE becomes brittle and brittle pipes/tubes crack under the influence of tensile loads. These cracks initially propagate slowly and later on grow quickly becoming unstable. The focus of this study is slow crack growth in high density polyethylene (HDPE). Crack propagation experiments were conducted to determine the dependence of crack growth on degradation and stress levels.
HDPE samples, with 0.3mm thickness, were exposed to 80°C chlorinated water (5-8 ppm) for up to 65 days. Thin samples were selected to ensure uniform degradation through the thickness. Although the brittleness of the polymer can be evaluated using strain-at-failure, the drawback of this method is that it destroys the sample. The Carbonyl Index (CI) obtained by Fourier Transform Infrared (FTIR) spectroscopy was established as a nondestructive measure of the degradation level. CI ranged from 35 to 93. A higher value of carbonyl index represents a greater extent of degradation. The relationship between CI and loss of mechanical performance was validated by strain-at-failure. Crack propagation tests were conducted were conducted on degraded polymer samples at constant load. The load (stress level) ranged from 5.1 to 9.2 MPa. In all 5 samples were tested. It was found that the crack propagation rate ranged from 6.31 x 10-10 to 1.26 x 10-2 m/s while the stress intensity factor ranged from 0.91 to 4.07 MPa√m. For a single degradation level, regardless of stress, the data when converted to log scale, and fit with the linear elastic fracture mechanics (LEFM) relationship = CKn. As the degradation increased the crack propagation rate increased such that all data were fit by the relationship = C(CI)Kn such that the exponential parameter ‘n’ was a constant for all the samples regardless of the level of degradation. The LEFM model fit to the data was best for moderate and high levels of degradation corresponding to CI of 55 and 90. Scanning Electron Microscopy (SEM) images show minimal deformation in the region around the crack tip, and ductile fibril stretching in the process zone. While the polymer had become brittle upon oxidation, there is local ductility in the process zone. An LEFM approach is typically applied to brittle materials, while the SEM results show that crack propagation is a combination of brittle and ductile behavior. Future studies should consider other modeling approaches that allow for ductile behavior in the process zone
