Mechanisms of fast fracture and arrest in steels

Abstract

Studies of the unstable propagation and arrest of brittle fractures were conducted on four steels: plain carbon steel, 3 pct Si steel, A-517, and 4340. Unstable fractures were initiated in double-cantilever-beam test specimens by forcing a wedge between the two beams under compression. These fractures propagate at essentially constant wedge opening displacement and can be made to arrest within the confines of the specimen. The strain energy stored in the specimen at the onset of propagation was varied systematically by changing the root radius of the starting slot. The experiments show that Ka, the stress intensity at arrest, is not a materials constant but depends on the strain energy stored in the specimen. Values of άrcR, the average energy dissipation rate during propagation, calculated for the four steels, are in the range23- GIc ≲ άcrR ≲ G{Ic}. Detailed metallographic examinations show that brittle fractures appear highly segmented on interior sections, but that the individual segments are interconnected. This morphology is attributed to isolated, difficult-to-cleave regions, comparable in size to the grains, which are bypassed and remain unbroken at relatively large distances behind the crack front. Etching studies conducted on a silicon steel reveal that the plastic deformation attending crack propagation is largely confined to the plastic stretching of the ligaments behind the crack front. Increases in the size, number, and toughness of the ligaments with temperature coincide with the brittle-to-ductile transition. An analytical model consisting of an elastic crack with a regular array of tractions representing the ligaments supports the view that the ligaments are the principal source of brittle crack propagation resistance in the steels.