Dynamic crack initiation in ductile steels

The goal of the work presented here is to study dynamic crack initiation in ductile steels (Ni-Cr steel and 304 stainless steer) at different loading rates and to establish appropriate dynamic failure criteria. A variety of infrared and visible optical methods and high-speed photography are used in this study. Precracked steel specimens are subjected to dynamic three-point bend loading by impacting them in a drop weight lower. During the dynamic deformation and fracture initiation process the time history of the transient temperature in the vicinity of the crack tip is recorded experimentally using a high-speed infrared detector. The dynamic temperature trace in conjunction with the HRR solution is used to determine the time history of the dynamic J-integral J(d)(t), and to establish the dynamic fracture initiation toughness, J(c)(d). The measurements made using high-speed thermography are validated through comparison with determination of S'(t) by dynamic optical measurements of the crack tip opening displacement (CTOD). Finally, the micromechanisms of fracture initiation are investigated by studying the fracture surface using scanning electron microscopy. (C) 1998 Elsevier Science Ltd. All rights reserved.