An investigation of the features of loading in thermal fatigue

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P ecu liarities o f th e M a te ria l D am ag e u n d e r Iso th e rm a l a n d N on Is o th e rm a l C o n d itio n s. The life o f m aterials under therm al fatigue is generally determ ined from the calculation relationships that correspond to a fatigue or quasi-static fracture m ode under low -cycle m echanical loading under isotherm al conditions. A ttem pts to use the sam e calculation relationships for therm al fatigue conditions yield an incorrect result despite the sim ilarity o f m any processes under low -cycle m echanical fatigue in isotherm al conditions and under therm al fatigue, such as the developm ent o f the local plastic strains and form ation o f cracks in the m ost heavily stressed zones (with the accum ulation o f the num ber o f cycles); an increase in the stress and strain am plitudes leads to a decrease in the num ber o f cycles to fracture; a sim ilar effect o f the stress and strain concentration, stress state m ode, and m any other factors is observed.A t the sam e time, there is a fundam ental difference betw een the low-cycle m echanical fatigue and therm al fatigue. U nder conditions o f low-cycle mechanical loading at constant tem perature, the sam e zones o f the m aterial are in turn subject to plastic tensile and com pressive strains. In this case, the alternating cyclic elastic-plastic deform ation is caused by a change in the resistance to the elastic and inelastic strains (the B auschinger effect). Therm al stresses occur due to the therm al strain constraint, and therefore, they generally decrease rapidly w ith increasing tem perature due to an increase in the plastic strain that has a tendency © A. P. GOPKALO, 2009 ISSN 0556-171X. Проблемы прочности, 2009

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confidence: 92%

“…V r c r ( 1) and in the case o f fatigue fracture [ 12] (2) Prediction o f a possible lim it state can also be m ade using the values o f the steady-state creep rate. I f the steady-state creep rate exceeds the threshold value, one should expect a quasi-static fracture (a change in the shape).…”

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confidence: 99%

Evaluation of the limit state and cyclic life of structural materials under thermal fatigue

Gopkalo

2009

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“…3). 3. Determine coefficients C and n from the above diagrams for the preset temperatures (Table 1).…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

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

Gopkalo¹,

Novikov²

2011

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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

Mechanical loading and cyclic heating phase shift effect on fatigue crack propagation rate.

Gopkalo¹

2011

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A. P. Gopkalo UDC 539.4 Calculations of the stress-strain state in the crack tip during mechanical loading and cyclic heating with account of the kinetics of loading parameters and material properties per cycle have shown that the maximal size of a plastic zone in cases of in-phase and antiphase mechanical loading and cyclic heating differ by 2.77 times. In the in-phase case, fatigue crack propagation rates are three times higher than the antiphase ones at the same stress intensity factor values.Introduction. Analysis of fracture of critical structural components operating under heavy-duty conditions of mechanical loading and cyclic heating shows the main fracture sources are technological or operational cracklike defects. Practical experience shows that fracture process proceeds at a certain rate and can reach 90% of the total life of a component. Crack propagation process in structural steels subjected to variable temperatures and mechanical loads is significantly affected by the minimal and maximal levels of temperature per cycle, cycle shape and length, load ratio, temperature variation range, phase shift between mechanical loading and heating cycles, loading frequency, heating and cooling rates, load magnitude and loading rate, loading modes: stress-, strain-, or displacementcontrolled ones, where the minimal and maximal values of the respective parameters are kept constant from cycle to cycle and during the whole period of testing. Moreover, under conditions of combined low-cycle mechanical loading and cyclic heating, the fracture pattern is complicated by variation of the material mechanical and physical parameters during each loading cycle, insofar as some of these depend on temperature [1][2][3]. Under such complex loading conditions many of the above-mentioned factors can significantly affect the crack propagation rate (CPR).Thermomechanical loading implies a wide range of possible combinations of mechanical and thermal loading constituents. Depending on particular variation of quantitative ratios of thermal and mechanical constituents, one can distinguish cases of thermal, low-cycle and high-cycle fatigue. In case of thermal fatigue, temperature variation induces thermal stresses. In case of low-cycle fatigue, fatigue processes can interact with creep. Nonisothermal loading conditions conventionally imply simultaneous cyclic variation of temperature and mechanical load in time, so that temperature variation induces no thermal stresses.It is known that reversal cycling under isothermal conditions results in successive plastic tensile and compressive deformation of the same parts of the material. Under nonisothermal conditions the deformation pattern becomes much more complicated. In thermal fatigue (a particular case of thermomechanical loading with no mechanical constituent), a material is subjected to plastic deformation in cooling half-cycles and to tension at the minimal temperature per cycle, while the material strength is much higher than that at the maximal temperature. Then, in heating half-...