Thermal Fatigue under Multiaxial Thermal Stresses

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Experimental investigation on multiaxial thermal fatigue was performed by using solid spherical and cylindrical specimens of low carbon steel. The specimens were subjected to cyclic multiaxial thermal stresses introduced by repeated heating and cooling processes. The uniaxial thermal cycling tests and mechanical strain cycling tests at elevated temperature were also conducted on the same material, and the relationship between the strength of material under three types of cycling were studied.The thermal strain range containing plastic strain of multiaxial thermal fatigue has been analyzed by means of successive approximation. Taking account of the fact that the cracks nucleate and propagate on the surface of the specimen, Mises' equivalent total strain on the surface is considered to be the strain range that determines the lifetime of the material subjected to the multiaxial thermal stress cyclings. For the purpose of elucidating the relationship between these three types of strength, the basis for the criterion of lifetime in terms of the number of cycles to crack initiation, constraint of thermal strain and equivalent steady temperature has been established as variables. The comparison between the three types of test results are made by using these common bases.From the fact that the results of multiaxial thermal fatigue tests by the use of both solid spherical and cylindrical specimens are in good correlation with the uniaxial thermal and mechanical fatigue test results, the following conclusion has been made. The strength of the material subjected to multiaxial thermal stress cycling can be predicted from the information of conventional tests, namely, the uniaxial thermal fatigue test under the same constraint of thermal strain as that of the multiaxial thermal cycling, or the uniaxial mechanical fatigue test at equivalent steady temperature.