UNDERSTANDING RAPID FRACTURE PHENOMENON IN HIGH TEMPERATURE WATER

摘要

Rapid fracture (or sudden fracture) of various wrought and weld metals in high temperature water has been observed during compact tension (CT) testing in different laboratories. K values at fracture were all lower than the fracture toughness of those materials. The mechanism behind these observations was largely unknown. Air tests had been performed using fatigue loading, and the K_(max) values 2 - 3X higher did not produce rapid fracture. Two mechanisms to explain such a sudden or very rapid crack advance seemed most plausible: the presence of hydrogen distributed throughout the metal, and plasticity instability. This paper summarizes the findings of inadvertent rapid fracture events at GE. A testing methodology was developed to evaluate rapid fracture in intentional tests. The controlled experiments were designed to understand rapid fracture in cold-worked stainless steel and Alloy 82/182 weld metal (UNS W86182/UNS N06082). SEM characterization confirmed that the controlled experiment reproduced the inadvertent observation. The effects of dissolved H_2 and/or O_2 concentration, stress intensity factor (K), and the change in K on the occurrence of rapid fracture were also investigated using CT specimens in high temperature water. Formulations for limit load analyses were identified, and applied to determine if plastic instability was consistent with those experimental observations. The results suggest that plastic instability plays a very important role on the occurrence of rapid fracture. The role of dissolved hydrogen, at least under the condition that has been tested, is not a big contributor to the phenomenon. However, some observations in water remain below those observed in air, and are in any event lower than for other nominally similar materials. Also, a broad cross-section of structural materials was not evaluated, nor a broad range of material conditions, water chemistries and temperatures, so that some role of environment on rapid fracture cannot yet be excluded.