Slow strain rate (SSR) tests are widely used to evaluate materials for service in the nuclear power industry. The slow strain rate method is used to select new materials and to qualify heats of existing materials for use in components exposed to reactor coolant.
Several different modifications of the SSR test have been developed over the years to meet the demands of specific reactor applications. For testing the nickel-base superalloys used in reactor core internals, tensile bars may be notched or fatigue-precracked to accelerate crack initiation. A precracked fracture mechanics specimen may be substituted for a tensile bar. Three-point-bend loading, rather than tensile loading, is used for evaluating material resistance to cracking during the intermediate temperatures associated with reactor heat-up and cool-down transients.
for testing thin-walled steam generator tubing, tube tensile specimens or single-ligament tensile bars are used. Rather than testing in a typical steam generator environment, a high-temperature caustic environment may be chosen, with an anodic potential applied to the specimen to accelerate cracking.
The SSR test method is also applied to reactor pressure vessel steels and weldments. Tests conducted in reactor coolant environment have established the importance of maintaining low oxygen levels in preventing stress corrosion cracking (SCC).