Influence of Machine Compliance on the Mechanical Characteristics of Materials in the Course of Static Tensile Testing

Abstract

ev UDC 539.4The influence of machine compliance on the standard mechanical characteristics of metallic materials is investigated. It is shown that with an increase of compliance percent elongation and reduction of area after fracture decrease by several times. At cryogenic temperatures ultimate strength can decrease to the level of yield strength due to the effect of jumplike deformation of metals.Many experimental data, including [1][2][3][4][5], provide evidence of the influence of compliance, or its inverse value -stiffness of loading system, on the mechanical behavior and characteristics of materials. This factor at the specified loading level determines the content of elastic energy W Ð = P 2 2, where P is the compliance of the specimen-machine system and P is the load. Potential energy transforms into the active form (energy of deformation or fracture) as soon as load starts decreasing. Using standard tensile tests of metals [6] the modulus of elasticity, limit of proportionality, yield strength (physical and offset) and ultimate strength (corresponding point within the border zone of the diagram) are determined on the ascending branch of the diagram load vs elongation. The rest standard characteristics of plastic materials such as percent elongation after fracture d and reduction of area of the cross section after fracture y are determined after passing through the stage of load fall on the descending branch of the diagram and final fracture of the specimen, i.e., they are subjected to the influence of compliance factor. Moreover, the magnitude of compliance is not regulated and for various tensile testing machines it can differ by an order and even more, since the range of their ultimate loads is very wide (0.5-1000 kN) [7]. It is required to evaluate the degree of the influence of compliance on the specified mechanical characteristics.Let us consider the diagram of static tension with control of the displacement (severe conditions) of the specimen made of plastic metal, which equation of state is described by the exponential function of the following type:where s and e are the true stress and strain, A and m are the constants, 0 1 £ £ m . It is assumed that the specimen made of uniform anisotropic material undergoes fracture upon attainment of some critical stress s c without necking (its contribution to the total elongation magnitude is insignificant). From Eq. (1) the relation for the description of the diagram "load P vs elongation Dl" is obtained under tension of the specimen with the initial cross-sectional area F 0 and initial calculated gauge length l 0 (extensometer gauge length): P AF l l l l m = + + 0 0 0 1 1 [ln( )] ( ). D D (2) 0039-2316/15/4704-0561