Direct and indirect strain measurement during low cycle fatigue of metals at elevated temperature

Current testing practices in low cycle fatigue (LCF) determinations at elevated temperature recommend the use of side-contacting extensometers or other direct methods for the measurement of strain. In many cases (e.g., when an environmental chamber surrounds the test specimen) this is not possible and resort must be had to indirect methods, which require a correction factor depending on the cyclic stress strain response of the material. By mounting direct and indirect extensometers on the specimen and grip assembly of the testing machine, direct comparisons were made on the output strain signals during LCF tests on alloys 316, Nimonic 90 and IN718 at temperatures in the range 600-900 degrees C. Agreement for plastic strain range was in general good, the remote method however requiring an independent value of the elastic modulus for determination of total strain range. Typical uncertainties in these values are discussed. All comparisons were made under continuous cycling conditions and a correction factor for use under creep-fatigue conditions is suggested.