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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.

The ASTM Crack-Arrest Test, which is under development, is generally limited to providing data up to 20 to 40°C above the ductile-to-brittle transition temperature (expressed as RTNDT in the ASME Boiler and Pressure Vessel Code). For this test, the geometry is a wedgeloaded compact specimen containing a brittle weld starting notch, which appears incapable of reliably generating a rapid crack above the stated temperature range. Two alternatives have been used for obtaining data at higher temperatures: very large specimens combined with soft loading systems and modified compact specimens. This paper describes compact-specimen modifications which have extended the successful temperature range to the onset of the Charpy upper shelf, the highest relative temperature reached being RTNDT + 100°C. Specific modifications utilized are: a high-strength steel crack-starter section (duplex specimen), a low-friction loading arrangement (inverted split pins), and contoured side grooves. The high-temperature data generated using these modifications compare favorably with largespecimen results.

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