620.178.15/179.119 and T. N. Mozharovskaya The investigation results are presented for the kinetics of damage accumulation and the process of deformation in a heat-resistant steel 10GN2MFA for different loading conditions under complex stress state.Introduction. At present, investigations on the assessment of the load-carrying capacity and life of structures performed with consideration for the current state of structural materials are based on a rather large volume of theoretical developments whose foundation is formed by hypothetical prerequisites concerning the properties of materials with no consideration for their current state and actual operating conditions. The development of the methods for assessing the residual life of machines and structures, the majority of which operates under cyclic loading, is the urgent problem. The main reason for the premature loss of the load-carrying capacity for that kind of objects is the out-of-control damage accumulation and crack formation processes in structural materials.Testing Procedure. In this paper, the results of experimental investigations are presented for the development of the process of deformation and kinetics of damage accumulation in heat-resistant steel 10GN2NMFA at room temperature under conditions of a single load application, stepwise cyclic and continuous cyclic loadings at complex stress state. The tests were performed on thin-walled tubular specimens (D out d = 50, where D out is the outer diameter of the specimen and d is the specimen wall thickness) loaded by the axial force and internal pressure in different combinations using a test rig type SNT-8U [1]. The experiments involved a uniaxial (K
Investigation results for VNS-25 high-strength steel in a multiaxial stress state under stepwise cyclic loading were presented. The applicability of the viscoelastoplastic model to studying structural materials was substantiated experimentally.Keywords: high-strength steel, viscoplastic properties, cyclic loading, multiaxial stress state.The results of studies on the behavior of materials determined by the effect of major operational factors (stress state, loading mode, temperature) on their mechanical properties can provide the basis for the design of modern heavy-duty structural elements and for the development of new deformation and fracture models as well as life calculation methods of high accuracy. In particular, experimental investigations in different stress states with the account of time factors allow assessment of loading conditions, whereby the time factor may be neglected, and analytical description of the detected effects.Here the results of studies on the viscoplastic properties of VNS-25 steel in a multiaxial stress state are presented. The investigations were performed on thin-walled tubular specimens under stepwise trapezoidal cyclic loading on an SNT-8U unit by an earlier developed procedure [1,2], with the specimens being kept for 6 s at maximum stress intensities and 20°C and with K z = =∞ σ σ θ , 1, and 0.5 being observed. The strain rate in terms of intensity & . ε i = ⋅ − − 2 5 10 3 1 s was ensured by the choice of a certain loading frequency at each stage. Deformation diagrams for VNS-25 steel under stepwise cyclic loading are presented in Fig. 1. The experimental results show that the viscoelastoplastic model [3, 4] offers a means of describing deformation of VNS-25 steel under stepwise cyclic loading and at different ratios of principal stresses. For several materials [5], including VNS-25 steel, stress and strain intensities may be employed as strength and deformation characteristics [4, 6].With the deformation diagrams ( Fig. 1), cyclic creep curves [7] corresponding to the primary (transient) creep stage can be plotted. Such creep curves for VNS-25 steel at several ratios of principal stresses are shown in Fig. 2.To determine initial creep rates at different ratios of principal stresses, the results were presented in the log creep rate-creep strain coordinates with extrapolation of the relation to zero creep strain (Fig. 3). Initial creep rates were established to be lower than active prestrain rates, i.e., at an instantaneous termination of loading strain rates vary stepwise. This fact is the experimental corroboration of the applicability of the viscoelastoplastic model to the investigated material. This effect for VNS-25 steel is most pronounced in uniform biaxial tension, it is least pronounced in uniaxial tension.Initial creep rates as a function of prestrains determined with creep curves at different ratios of principal stresses is illustrated by Fig. 4. Under active deformation of VNS-25 steel, with an increase in irreversible strain the contribution from the viscous component...
We study the regularities of deformation of 1Kh18N9T steel under long-term static loading along different paths of radial loading with regard for the influence of the type of stressed state. The viscoplastic properties of 08Kh18N10T-VD steel subjected to stepwise static loading under conditions of complex stressed state are analyzed.Keywords: heat-resistant steels, long-term and stepwise static loading, complex stressed state, degree of damage, viscoplastic properties of steel.Introduction. The major part of actual structural elements suffer the action of complex thermomechanical loading in the process of operation, which substantially affects their load-carrying capacity. From the viewpoint of improvement of the numerical methods used for the analyses of structural elements, it is necessary to perform comprehensive investigations of the properties of various metals and alloys in the process of creep under conditions of complex stressed state.The analysis of the well-known data obtained as a result of long-term testing of structural materials operating under conditions of complex stressed state shows that the processes of plastic deformation of materials in creep strongly depend on the type of stressed state [1-7 and others].The results of numerical analyses of the structural elements operating under conditions of creep are, as a rule, based on the data obtained in the case of uniaxial tension. At the same time, the attempts to generalize the equations of creep deduced and experimentally substantiated for the uniaxial stressed state to the case of arbitrary systems of stresses by introducing certain generalized stresses without taking into account the influence of the first invariant of the stress tensor and the third invariant of the stress deviator on the mechanical properties of materials are not always correct.In the present work, we consider the regularities of deformation of heat-resistant steels subjected to long-term static loading along different paths of radial loading (1Kh18N9T, T =°600 C) and stepwise static loading (08Kh18N10T-VD, T =°320 C) under conditions of complex stressed state.Influence of the Form of Stressed State on the Minimum Intensity of Creep Strain Rates. In Fig. 1, we present the experimental dependences of the minimum intensity of creep rates x ic min on the intensity of stresses s i constructed for 1Kh18N9T steel at 600°C according to the results from [8]. It is easy to see that these plots depend on the form of the stressed state. This is explained by the fact that the same value of s i obtained for different paths of radial loading (Fig. 2) [9] leads to the accumulation of different degrees of damage w j to the material for different types of the stressed state.
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