Application of the essential work of fracture concept to high temperature deformation in polyoxymethylene

Essential Work of Fracture (EWF) tests have been carried out on double edge notched samples machined from injection molded sheets of commercial grades of polyoxymethylene homopolymer with different molecular weight averages. Most of the measurements were made at 100 degrees C and over a range of tes t speeds in which polyoxymethylene is anticipated to undergo a macroscopic ductile-brittle transition with decreasing strain rate. The results reflect both the existence and the molecular weight dependence of this transition, and are argued to be valid in terms of the European Structural integrity S ociety's EWF draft test protocol under certain test conditions. However, it is shown that the applicability of the test method used here becomes highl y questionable for test speeds in the immediate vicinity of the transition, owing to the influence of the initial ligament length on the crack tip def ormation mechanisms.