Review of Current International Stress Relaxation Testing Standards

Abstract

In modern power plant however, longer overhaul periods are demanded with the result that accurate and reliable uniaxial stress relaxation data extending to at least 50 000 h duration are now required. Careful attention to the design and manufacture of stress relaxation test pieces are important factors affecting the generation of accurate and reliable long term data, as is the means of attaching the extensometer. For the generation of long term, low strain stress relaxation data (> 30 000 h) circular cross section test pieces having gauge lengths of 100 mm or greater are generally preferred. Careful machining and close control of machining tolerances are also required, particularly with respect to parallelism of the gauge length and co-axiallity of gripping ends to minimise bending errors during testing. In terms of loading the stress relaxation test, all of the national standards require that the load should be applied smoothly without shock or vibration and that load/extension plot be recorded for each test.