Strain instability in metals at cryogenic temperatures and potentials of practical application

Abstract

Force, deformation, velocity, energy, and other criteria of limiting states at low-temperature serrated yield of metals are examined. The feasibility of their use for setting norms and standards of strength and mechanical tests is demonstrated.Introduction. Low-temperature strength is related directly to optimum account of strain stability losses in metals, i.e., the serrated yield effect, which is observed at temperatures below 30 K [1,2]. This effect is greatly dependent on loading conditions and strain rates, the rigidity of a loading system, on the influence of electromagnetic and other physical fields as well as on design and technology factors, viz prestrains and stress concentrations, object shapes and dimensions [2-9], etc. An abrupt transfer from elastic deformation to high-rate localized strain typical of high-strength structural alloys is considered as a stepwise reduction of loading resistance or instantaneous carrying capacity losses in a structural element due to overstrain or fracture [10]. It should be noted that current standard strength tests for the structures operating at temperatures below 30 K [11][12][13][14] as well as methods for determining the mechanical characteristics of materials [15, 16] do not take account of a low-temperature serrated yield effect. As would be shown below, its account should be based on the criterion estimation of limiting states of metallic materials arising under discontinuous strain. The criteria can be classified, considering anisothermic, adiabatic, and inhomogeneous deformation characteristics as well as the multiple-factor and multiphase nature of this process [17].Limiting States and Criteria. The authors have already presented the results of complex studying the effect of elastic, inertial, kinetic characteristics of a loading system and design-technology factors on the plastic strain, strength, and fracture behavior of alloy specimens in static and quasi-static tension in liquid helium, accounting for the stepwise strain accumulation nature. The major stages are represented by subcritical macrohomogeneous deformation of the material, deformation stability losses, viz strain discontinuity, nonadiabatic deformation just after the discontinuity termination.The investigation offers the answer to several critical challenges: the effect of serrated yield under known loading conditions, overstrain or fracture in the discontinuity or after its termination, changes in serrated yield parameters and standard mechanical characteristics of the material under the influence of loading-or designtechnology-related factors.The solution of those problems is directly relevant to the criterion estimation of the examined effect. It suggests revealing the conditions of the onset of limiting states, including plastic strain stability losses, if serrated yield starts after the stage of monotonous homogeneous plastic strain or the strain discontinuity occurs just after the stage of elastic deformation (Fig. 1). In both cases the basic criterion should be observed, viz thermal in...