Brittle crack initiation at the elastic-plastic interface

Abstract

To study fracture initiation near notches experiments were conducted on polycarbonate at room temperature. It could be demonstrated conclusively that cracks are nucleated under the influence of a critical normal stress at the elastic-plastic boundary. Slip line field theory allows the determination of the critical normal fracture stress from the knowledge of the notch root radius and the location of the crack nucleation. For 0.5 in. thick specimens with root radii between 0.001 and 0.01 in. the calculated critical fracture stress of polycarbonate is nearly constant, 21.2 ±0.6 ksi. An analysis of the data in accordance with the theory developed by R. Beeuwkes, Jr. for parabolic notches resulted in critical fracture stress values of approximately 21.5 ksi. Specimens with central holes between 0.04 and 0.05 in. diam also showed crack nucleation below the surface of the hole but the calculated critical fracture stress values were considerably lower, approximately 11.2 ksi. This may be attributed to the loss of plane strain conditions in these 0.5 in. thick specimens. The experimentally observed slip lines in polycarbonate are not orthogonal to each other but intersect at a lower (~80 deg) angle. This may be due to the relief of stresses on unloading and sectioning, and it may indicate that plane strain conditions were not present. Because of its flow characteristics, which are similar to those of metals, and because of its transparency, polycarbonate appears to be a good model material for the study of plastic flow initiated brittle fracture. It could be used to check the theoretical slip line field solutions proposed for a variety of loading conditions.