Mechanical loading and cyclic heating phase shift effect on fatigue crack propagation rate. Part 2. Fatigue crack propagation rate prediction

Abstract

539.4 Using the relation between plastic zone size in the crack tip vicinity, loading cycle parameters, and material properties, which has been derived in Part 1, we have elaborated fatigue crack propagation rate prediction for low-cycle mechanical loading combined with cyclic heating with account of the phase shift effect. The data for governing equation were obtained from simple tests. Application of the proposed method yields a good fit of calculated and experimental results.Introduction. Fatigue crack initiation and propagation from damaged zones of materials operating under low-cycle isothermal loading conditions have been studied by numerous investigators with elaboration of the respective criteria of strength assessment and life prediction under such conditions [1]. However, such criteria for complex low-cycle nonisothermal loading, especially with phase shift between low-cycle mechanical loading (ML) and cyclic heating (CH), have not yet been developed. Application of criteria developed for isothermal loading conditions to the nonisothermal loading provides incorrect results, due to sufficient differences between deformation and fracture processes in materials under isothermal and nonisothermal conditions [2,3]. Available approaches to fatigue crack propagation rate (FCPR) assessment for thermomechanical loading (TML) based on summation of cyclic loading and loading time inputs [4] are quite cumbersome and sophisticated for engineering practice.In Part 1 [5], the ML-CH phase shift effect on fatigue life and crack resistance of structural materials has been briefly discussed. The influence of ML modes in isothermal and nonisothermal cases (with the ML-CH phase shift) on variation of plastic zone size in the crack tip has been demonstrated.Taking advantage of the relation between FCPR and plastic zone size established in [6], present study is aimed at development within the linear fracture mechanics framework of FCPR assessment technique for nonisothemal loading with the ML-CH phase shift based on simple test results.Elaboration of Calculation Formula. Mode I stress intensity factor (SIF), which depends on the solid body geometry and crack size, can be treated as a parameter controlling stress redistribution in the crack tip after crack formation. Fracture processes after fatigue crack formation are localized in the crack tip, whereas SIF describes the stress-strain state in this particular zone. Under cyclic loading conditions, crack propagation behavior can be described via SIF using the so-called fatigue fracture kinetic diagram (FFKD). In compliance with [1], such diagram is a reverse curve, which is conventionally subdivided into three curve pieces (each characterized by intrinsic fracture mechanism, changeover of which determines transition from one curve piece to others).For steady-state crack propagation curve piece the relation between SIF and FCP is described by the Paris equation