Fracture mechanics analysis of slow crack growth in polyethylene

Abstract

Slow crack growth in polyethylene is often the limiting factor in long-term service of plastic pipe or other structural applications. A new test method and analysis method was developed to study slow crack growth in polyethylene. Two high density polyethylenes were used to evaluate the new test and analysis methods. Static loading of deeply notched three-point bend specimens was conducted at 26.7 'C, 40 'C, and 70 'C on 1.27 cm , 0.953 cm, and 0.635 cm material. The crack length as a function of time was determined indirectly from changes in the compliance of the specimen coupled with a fracture mechanics analysis. The new test method was able to distinguish between the two materials and was sensitive to changes in material processing. A plane strain stress state was obtained in all three thicknesses of the lower toughness material at 26.7 'C. However, a plane strain stress state was not achieved in any thickness at the elevated temperatures. The new methods predicted the actual crack length within fifteen percent error. Correlation with theoretical models was possible only when the specimens were in a plane strain stress state.