On the Fatigue Life Law for Smooth Specimens at Elevated Temperatures Derived from Fracture Mechanics Law of Crack Propagation

Abstract

The authors have reported that the fatigue crack propagation behavior at elevated temperatures can be classified into cycle dependent- and time dependent-types and that there exists an independent fracture mechanics law between the crack propagation rate and the J-integrals for each type. In this study, the fatigue life law for smooth specimens was derived by integrating the crack propagation law. They are formularized as follows;
(1) cycle dependent fatigue, ΔW_f^mfN_f=D_f
(2) time dependent fatigue, ΔW_c^mcN_f=D_ct_o^1-mc where ΔW_f and ΔW_c are strain energy parameters, N_f; number of cycles to failure, m_f, m_c and D_f, D_c; constants, t_o; tension time (see Fig. 1). Here, it should be emphasized that ΔW_f and ΔW_c are functions of not only the plastic or the creep strain range but also the stress range, the maximum stress and the elastic strain range.
The validity of the above fatigue life law was discussed using the elevated temperature fatigue test data of three alloys; 2¼Cr-1Mo steel in which the effect of strain waveform on the fatigue life is known to be small, 304 stainless steel which shows the life dependence on dynamic aging, and Inconel 718 which is one of the low ductility superalloys. As a result, the fatigue life was correlated well with ΔW_f for the cycle dependent fatigue and with ΔW_c for the time dependent fatigue for all the materials.