A B S T R A C T PVD coatings applied to components form hard, stronger layers and generate high residual compressive stresses that limit the plastic deformation in surface layers of the base metal thus increasing its tensile strength and resistance to fatigue loading. The purpose of this paper is to experimentally determine the influence of the deposition of 2 to 16.5-μm-thick PVD coatings of TiN , Cr, (Cr+TiN), (TiC)N, (TiAl)N onto specimens of stainless steel 321 and titanium alloys of types MILT-81556A and (10-2-3; 4966) on their tensile strength and low-cycle fatigue resistance when the development of large elastic-plastic strains takes place. The tensile and low-cycle fatigue tests were conducted under conditions of axial zero-to-tension cycle of the stress-controlled loading on flat 1-to 1.5-mm-thick specimens in the initial state (uncoated specimens) and after application of a PVD coating, including those after pretensioning or after cyclic prestraining in the low-cycle fatigue range. The deposition of PVD coatings is found to enhance the characteristics of tensile strength and low-cycle fatigue resistance in the quasi-static fracture range. The deposition of PVD coatings on specimens cyclically prestrained to the values of 53-86% of the number of cycles to fracture, changes the cyclic properties of the material and predetermines the fatigue fracture mode only.

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-...

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