THE WORK OF FRACTURE IN SEMIDUCTILE POLYMERS

The work to fracture in tension double-notched samples of some semiductile polymeric materials (rigid PVC, Orgalloy and Ultranyl) has been measured as a function of the ligament length. It was established that the work of fracture was proportional to the extent of the plastic zone that developed in the ligament area during crack propagation. It is proposed that the total energy density is made up of two terms, one distributed all over the plastic zone and the other localized in the vicinity of the fracture path. It is then shown that a linear relationship exists between the specific work of fracture and the ligament size, provided the height of the plastic zone linearly depends on the ligament length. The linear extrapolation of the specific work of fracture to nil ligament, yields a value that coincides with J(Ic) and therefore can be treated as a critical parameter. It is also shown that, at large ligaments, the dependence of the specific work of fracture on the ligament length reflects the post-yield behaviour of the material and it is influenced by the tendency of the height of the plastic zone to level off. Consequently, no specific meaning can be given to quantities obtained, according to the essential work of fracture theory, in the large ligament region, e.g. ligaments larger than three to five times the sample thickness.