Hydrogen-assisted cracking studies of 4340 steel by using the optical method of caustics

Document Type


Publication Date



The optical method of caustics is used to measure the effect of hydrogen on the localized deformation within the fracture process zone (PZ). In particular, the role of the PZ in hydrogen-assisted cracking (HAC) of 4340 steel in hydrogen gas at 1 atm is examined. No change in the caustic diameter was detected prior to crack initiation but an "anomalous" enlargement of the caustic diameter as a function of crack growth was observed as a result of crack tunneling. The "center" of the caustic follows the position of the internal crack front, thus providing a simple means to monitor the growth of internal cracks. It is demonstrated that the technique provides a sensitive means for detecting HAC initiation (the smallest crack depth change detected is 35 μm) and insight into the three-dimensional character of the PZ. © 1989.

Publication Title

Engineering Fracture Mechanics