Thermal Fatigue Combined with Mechanical Stress

Abstract

The strength characteristics of metallic materials which are to be subjected to thermal stress cycling superimposed on mechanical stress is of interest from the practical point of view in design purpose. The authors have pointed out in the previous papers an importance of the consideration of the problem in relation with the strength characteristics of materials in cyclic mechanical strain fatigue and creep rupture at elevated temperature, and have presented some analytical and experimental works in this line.In the present paper, the thermal fatigue combined with steady mechanical stress is treated. The elongation of material is not prohibited and the mechanical mean stress does not decrease with number of cycles of thermal stress. There is another mode of combination of mechanical stress and thermal stress cycling, that is, both of mechanical and thermal stress are cycling. This category of problem has been discussed in another paper of the authors. The interest of the present study is placed on the former problem, and the fracture life and deformation of austenitic stainless steel subjected to cyclic thermal and steady mean stress is discussed in connection with both strength characteristics of fully reversed thermal stress cycling and simple creep rupture.Test condition of combination of cyclic thermal stress amplitude Δσ and steady mechanical stress σm is characterized by the stress ratio Δσ/σm. Thermal stress in hollow cylindrical specimens arose by prohibiting free longitudinal expansion and contraction of the specimen subjected to temperature cycling. The mechanical mean stress in tension is applied by pull head mechanism. In the test of small stress ratio, specimen elongates and it causes relaxation of the mean stress. In order to realize the test condition of thermal fatigue under mean steady stress, the relaxation of mean stress must be compensated. This function is realized by a relay mechanism, in which a pulling head worked by the signal sent from the electric circuit of wire strain gage mounted on a weighing bar that is installed in series with specimen. The lower stress level in the cycling of combined stresses is taken as the basis of the automatic control of combined stresses, and the control unit is set to maintain the level of the minimum stress constant.From the present study the followings are concluded:(1) Fracture life of materials subjected to thermal fatigue combined with tensile mean stress can be predicted approximately from both test data of completely reversed thermal stress cycling and simple creep rupture, by employing the criterions of fatigue damage and life consumption in creep rupture for the material subjected to simultaneous cyclic variation of stress and temperature. The analytical results are in good agreement with experimental results on 18-8 Cb stainless steel where Δσ/σm is chosen as 0.5, 1 and 2.(2) Tensile deformation of materials subjected to combined thermal cycling and tensile mean stress, where tensile mean stress is larger compared with thermal stress amplitude, is estimated from the test data of simple tensile creep, by using the principle of strain hardening for the material subjected to simultaneous cyclic variation of stress and temperature. The analytical results are approximately in agreement with experimental results where Δσ/σm is smaller than unity.