Fracture toughness evaluation of ductile polymeric films

Abstract

The energy for complete fracture in double edge-notched tension test specimens has been measured for a wide range of polymer films. Results indicated that the variation of the total specific work of fracture, wT, with ligament length, L, can be described by two straight lines, both of the form wT = we + β wpL, thus giving upper and lower intercept values at zero ligament length (i.e. we) for each film. The first term, we, is the energy absorbed per unit area of fracture, whereas the second term, wp, is the energy absorbed per unit volume of plastic deformation remote from the fracture surface. The lower we value was obtained from the extrapolation of the data within the mixed mode region (plane-stress/plane-strain) where the maximum net-section stress exceeded 1.15 times that of the tensile yield stress, σy, of the material, and the upper value was ascertained by extrapolating the data within the plane stress region where the net-section stress was 1.15 σy. It appears that the transition from plane stress to plane strain mode of fracture in thin films occurs at a ligament length much greater than 5B, where B is the specimen thickness. Moreover, it was found that the linearity of the data within the plane-stress region was not affected when ligament length values exceeded the plastic zone size. Moreover, variation of the extension to break with ligament length, for both pure plane stress and the mixed mode regions, was also linear; and the extrapolation values at zero ligament length were identified as crack opening displacements. Essential work estimated from the crack opening displacement agreed reasonably well with the extrapolated values.