An Energy-Based Axial Isothermal-Mechanical Lifing Procedure

Abstract

An energy-based fatigue lifing procedure for the determination of fatigue life and critical life of in-service structures subjected to axial isothermal-mechanical fatigue (IMF) has been developed. The foundation of this procedure is the energy-based axial room-temperature fatigue model, which states: the total strain energy density accumulated during both a monotonic fracture event and a fatigue process is the same material property. The energy-based axial IMF lifing framework is composed of the following entities: (1) the development of an axial IMF testing capability; (2) the creation of a testing procedure capable of assessing the strain energy accrued during both a monotonic fracture process and a fatigue process at various elevated temperatures; and (3), the incorporation of the effect of temperature into the axial fatigue lifing model. Both an axial IMF capability and a detailed testing procedure were created. The axial IMF capability was employed in conjunction with the monotonic fracture curve testing procedure to produce eight fracture curves at three operating temperatures. The strain energy densities for these fracture curves were compared, leading to the assumption of constant monotonic fracture energy at operating temperatures below the creep activation temperature.

An Energy-Based Axial Isothermal-Mechanical Fatigue Lifing Method

“…(2) and (3)) and a dynamic process (Eq. [11,19]; (b) expérimentai iiysteresis ioop the lower tail is removed, more closely matching the geometry of (2) a typical experimental hysteresis loop (see Fig. 3…”

“…Cooling was provided by an Electro Impulse RU-300 chiller. A Chino DPIOOOC was used to control the temperature [19].…”

“…The temperature was measured using a thermocouple positioned outside of the gauge section of the specimen. The temperature at this location was correlated to the temperature at the center of the gauge section via a temperature correlation study [19].…”

“…The axial IMF model is a direct evolution of the axial roomtemperature fatigue lifing method [12,19]. This method hinges on the equality between the strain energy dissipated during both a quasi-static process and a dynamic process (see Eq.…”

“…(8) is the lack of the ßthermai term, the absence of which is a product of the boundary conditions employed during testing [19]. The critical-life is found through the critical-life coefficient as ascertained through experimentation (see Eq.…”