Procedure for the evaluation of the accumulation of defects in metallic structural materials under complex elastoplastic loading

Abstract

Within framework of the continual fracture mechanics, we describe the engineering approach to the assessment of scattered microdamage accumulation kinetics in metallic materials under elastoplastic loading conditions in case of plane stressed state. Automatized experimental stand and the respective investigation technique are discussed. The stand has been developed based on modification of the UMÉ-10T electromechanic test machine. State-of-the-art computer technologies and microprocessing hardware are incorporated in the stand automation. We present the technique of experimental assessment of damage accumulation kinetics in metallic structural materials under complex elastoplastic loading conditions with account of two different fracture (cleavage and shear) processes, which technique is based on measuring the specific electric resistance of the specimen.Keywords: accumulation of defects, complex elastoplastic loading, resistivity of the specimen, testing installation.Introduction. Under conditions of intense elastoplastic deformation and in the processes of creep and low-cycle fatigue of structural elements, microdefects (micropores and microcracks) appear and accumulate in the material in the zones of elevated loading [1][2][3][4][5][6]. The improvement of the accuracy of prediction of the service life of structural elements of this sort in the stage of engineering design and the accuracy of evaluation of the residual service life in the stage of operation mainly depend on the reliability of the system of determining equations which must include the parameters of damageability [4,5]. The damageability of the material characterizes the dependence of the density of microdefects in a certain plane section of an element of the material on the action of external thermal and force factors. At present, there is no common point of view on the physical meaning of the parameter of damageability [7][8][9][10][11][12][13][14].Continual fracture mechanics (CFM) successfully develops for the last 40 years as a promising scientific trend in the mechanics of deformable bodies. The formation of this trend was initiated by Kachanov [4] and Rabotnov [5] who used the concept of internal variables [3, 9, 12] with a system of the corresponding kinetic equations and fracture criteria in the stage of initiation of a microcrack.For the purposes of engineering calculations, the kinetics of accumulation of microdefects is especially conveniently analyzed in terms of macrocharacteristics of the mechanical properties of structural materials in the form of force, deformation, and energy approaches [12,[15][16][17]. In this case, the application of CFM is, as a rule, based on the hypothesis on averaging of microdefects over the volume of the material and the phenomenological parameter of damageability is represented either as a scalar quantity [16], or as a vector [17], or in the tensor form [18]. The analysis of the data available from the literature demonstrates [17,19,20] that, under conditions of complex stressed state, both the general...

Damage of structural materials under complex low-cycle loading

“…The principal concepts of the CMD (or the continuum fracture mechanics in the stage of crack initiation) were formulated by Kachanov and Rabotnov as applied to the processes of creep [3,4]. At present, there are no generally accepted reliable procedures for the determination of regularities of the processes of initiation and accumulation of SD [16][17][18]. In the first approximation, as a parameter of damage, it is customary to use a scalar quantity and the regularities of its variation depending on thermal and force loading conditions.…”

“…In the first approximation, as a parameter of damage, it is customary to use a scalar quantity and the regularities of its variation depending on thermal and force loading conditions. In the general case, the model of damage is included in the system of determining equations whose reliability specifies the accuracy of evaluation of the service life of structural elements under conditions of low-cycle fatigue.The results of investigation of cyclic strength for a large class of metallic materials and structural elements in the uniaxial stressed state by applying a broad variety of physical methods show that the principal role in the processes of accumulation of SD is played by the structural microinhomogeneities of the material and additional stresses induced by these inhomogeneities (in the literature, the are called microstresses) and manifested on the macrolevel in the form of the Bauschinger effect [8,14,[17][18][19]. In [5], the CMD was additionally substantiated and developed on the basis of the concepts of the thermodynamics of irreversible processes.…”

“…The results of investigation of cyclic strength for a large class of metallic materials and structural elements in the uniaxial stressed state by applying a broad variety of physical methods show that the principal role in the processes of accumulation of SD is played by the structural microinhomogeneities of the material and additional stresses induced by these inhomogeneities (in the literature, the are called microstresses) and manifested on the macrolevel in the form of the Bauschinger effect [8,14,[17][18][19]. In [5], the CMD was additionally substantiated and developed on the basis of the concepts of the thermodynamics of irreversible processes.…”

“…The experimental verification of the energy approaches was performed by using a modified UMÉ-10TM-type testing unit in the plane stressed state (under the combined action of an axial force and a torque) for different loading paths according to the procedure detailed in [18]. The current values of the intensities of additional stresses ρ i and σ xi are given by the formulas…”

Kinetics of scattered fracture in structural metals under elastoplastic deformation

“…The static elastic modulus E was found as a slope of the σ ε ( ) curve by the least squares method [7] as per the State Standard GOST 1497-84, using experimental data obtained on a test bench [6]. Figure 1 illustrates variation of elastic modulus as a function of elastoplastic strains (up to the ultimate stress limit of material) for some metallic structural materials.…”