Evaluation of viscoplastic properties of VNS-25 high-strength steel in a multiaxial stress state

Abstract

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...

Life estimation for high-strength materials under cyclic loading in the multiaxial stress state

“…The results of studies on viscoplastic properties of titanium alloys and steels under cyclic loading show [1,2] that their life estimation should be performed with the account of directed plastic deformation kinetics. One of the optimum versions of life calculations can be the calculation with the deformation-kinetic criterion developed at our Institute [3]: where ¢ C i is the parameter of deformation properties of the material and a stress state, determined according to [4] from the condition of plastic deformation stability loss under stepwise cyclic loading, s i max is the maximum stress intensity in the cycle, and s itr is the stress of fatigue-to-quasistatic fracture transition, determined from the condition of plastic deformation stability.…”