Plastic zone formation around an arresting crack

Abstract

We have followed the evolution of strain near the tip of an arresting cleavage crack in a pressure vessel steel using a strain gage technique. The measurements were made using a linear array of strain gages located along the intended crack path and 0.65B (B is the specimen thickness) above this plane. The gages were read using instrumentation capable of resolving strain in time intervals of between 2 and 4µs. The results were obtained in a thick plate, large enough that reflected elastic waves did not contribute significantly to the strain records or crack motion during the time interval of interest.

To within the resolution of this technique, the gages did not detect plasticity near the rapidly propagating cleavage crack tip. At the instant of arrest, however, the strain gages detected a plastic zone which increased in intensity over a period of several milliseconds. We have interpreted this increasing intensity as an elastic-plastic boundary emanating from the arresting crack tip and eventually reaching an equilibrium size, i.e., that predicted from the applied stress intensity factor and the static plastic properties. A simple analysis is presented to estimate the velocity of this elastic-plastic boundary from the strain data and results are given for several experiments.