This paper reviews the phenomenon of dynamic strain aging in carbon steels and considers its effects on the fracture behavior of carbon-steel pipes and pressure vessels in lightwater reactors operating at elevated temperatures near 290°C (550°F). Dynamic strain aging is a phenomenon in which aging occurs simultaneously with plastic straining. It occurs over a range of temperatures that depends on strain rate. In tensile tests, it is manifested by increased tensile strength, increased strain-hardening rate, serrated stress-strain curves, and decreased ductility. Evidence is presented to show that the occurrence of dynamic strain aging can significantly lower the fracture resistance of carbon steels. This lowering of fracture resistance may be manifested in several ways: (1) JIc is lower at light-water reactor (LWR) temperatures than at room temperature, (2) the tearing modulus is lower at LWR temperatures than at room temperature, and (3) stable ductile crack growth may be interrupted by unstable ductile fracture at LWR temperatures but not at room temperature. The paper examines probable causes of dynamic strain aging and describes methods for identifying which steels are susceptible to it.

The direct-current electric potential method is receiving increasing attention for monitoring crack extension in J-resistance curve testing. Among its advantages over the unloading-compliance method are: (1) no time-consuming unloadings are required, (2) a continuous record of crack extension versus displacement can be obtained, and (3) the method can be used at higher strain rates where unloading compliance cannot be used. Despite the advantages of the direct-current electric potential method, questions persist regarding its ability to monitor large amounts of crack growth in highly ductile materials where large displacements and large amounts of plastic strain occur. This paper presents details of an experiment conducted on a 3T planform-size compact specimen of 25.4 mm thickness to assess the ability of the direct-current electric potential method to accurately measure crack extension in a highly ductile material. The material selected was Type 304 austenitic stainless steel. It was found that the Johnson expression, often used to calculate crack extension from direct-current electric potential data, significantly underestimated the actual amount of crack extension. However, a simple modification of the Johnson expression resulted in excellent agreement between calculated and measured crack extensions.

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