THE DETERMINATION OF HYSTERESIS LOOPS IN THERMO‐MECHANICAL FATIGUE USING ISOTHERMAL STRESS‐STRAIN DATA

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Abstract
Abstract—Thermo‐mechanical fatigue stress‐strain data on ferritic/austenitic steels and superalloys from a variety of sources are analysed with regard to hysteresis loop stress asymmetry. This arises from a decoupling of the thermal and mechanical strain signals in the test technique so that many tension‐compression load combinations are possible. Data from simplified isothermal and bithermal tests are also examined. Taking a typical example of an “out‐of‐phase” thermo‐mechanical loop on a 1/2 CrMoV steel cycled between 200 and 550°C, isothermal stress‐strain data were generated at 50°C intervals on material from the same cast and, used in conjunction with the elastic characteristics of the apparatus, an attempt was made to re‐create this loop. The methods employed were (i) a graphical construction between appropriate isothermal yield contours (ii) a tangent modulus calculation (iii) a secant modulus calculation. Method (i) appeared to give the closest agreement in the present case.

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