Evolution of a modern mechanical testing and design standard for high temperature materials

The basis for high-temperature testing and analysis was established a century ago using test equipment that was available at the time. This fostered a tradition in research and engineering applications that has not changed fundamentally. The limitations of this approach are described, with emphasis on problems associated with predicting the remaining life of operating components. A new testing method and analysis philosophy is proposed based on measurement of current performance, with end of life unambiguously identified with limiting acceptable values of these current performance metrics. Instead of a single test to measure creep strength and fracture resistance, the new approach uses separate tests. For creep strength, the stress vs. creep rate curve determined at nearly constant state and covering five decades in creep rate in a one-day test is obtained from a high-precision stress relaxation test (SRT). The results for many alloys and materials often show a sigmoidal curve. Conventional long-time creep tests of steels have shown a similar effect. However, rather than recognising this as a fundamental rate-dependent phenomenon, it has been typically explained in terms of microstructural coarsening. For fracture resistance, a constant displacement rate test across a notch is used at the temperature of greatest sensitivity to high-temperature fracture. This latter test has been very successful in detecting embrittlement in superalloys. For more ductile alloys, a non-destructive measure of ductility based on the strain rate sensitivity determined from the SRT data is shown to have great potential. This introduces the concept of intrinsic ductility that can be determined without ambiguity over a wide range of stress and temperature for both new and serviced components.