Torsional low-cycle fatigue behavior of C250 steel using funnel specimens at elevated temperature

Torsional fatigue rupture is one of C250 steel's main failure forms. In order to obtain torsional fatigue characteristics of C250 steel and eliminate the disadvantages of straight rod specimen, a series of torsional fatigue tests on C250 steel at three different elevated temperatures, 150, 200, 350℃ using funnel specimens has been carried out, and the curves of nominal torque-nominal torsional angle for C250 steel at each temperature are given. Based on the test results and finite-element-analysis aided testing (FAT) method, the effective cyclic constitutive relationships of C250 steel at those temperatures have been obtained. By using these effective cyclic constitutive relationships, 3D finite element analyses of funnel specimen under torsion are used to reveal the transforming relationship between the torque and shear stress at the root of funnel specimen, as well as the relationship between the nominal torsional angle and shear strain at the root of funnel specimen. On the basis of those test and analyzing methods, the curves of shear stress-times cycles,shear strain amplitude-cyclic fraction and stable hysteresis loop of shear stress-shear strain have been obtained. The material shows cyclic softening by the analysis of the low-cycle fatigue behavior, and the typical Manson-Coffin model is employed to predict the torsional fatigue life of C250 steel at three different elevated temperatures. © 2016, Press of Chinese Journal of Aeronautics. All right reserved.