THE TENSILE TEST CURVE OF WIRE AFTER ITS CURVATURE CHANGING

摘要

Stress and strain state during the curvature change are characterized by great non-uniformity. There are circumferential tensile stresses in the external elongated zone and compressive ones in the internal zone. Both zones are separated by the neutral surface where circumferential stresses equal zero. Circumferential strains during curvature change can be determined from: epsilon_(theta) = (x/rho). In the neighbourhood of the neutral surface the material is in the elastic state and the circumferential stresses can be calculated on the basis on equation: sigma_(theta) = (x/rho) centre dot E. Range of the elastic zone can be determined from the condition that the circumferential stresses reach the value of yield stress. ((x_0)/rho) centre dot E = sigma_p. As a result of superimposing of the unloading stresses on the stresses appearing during the active phase of the process some stresses called residual stresses will remain after the process is finished. During tension of a wire having the residual stresses first elastic deformation is observed in the wire, and the tensile force changes linearly depending on the deformation. When sum of the stresses resulting from the external loading and residual stresses reaches the yield stress value, the material is in the plastic condition. With the increase of the stress resulting from the external loading the plastic zone is gradually expanding. In the elastic zone stresses grow proportionally to the strain while in the plastic zone rise in stresses will occur in the range from residual stresses to the yield stress value. Borders between the zones can be determined from the boundary conditions saying that sum of stresses from external loading and residual stresses reaches value of yield stress. On the basis of given equations first distribution of the residual stresses on the cross-section of the wire and then values of forces for the elastic and plastic zones were determined. Considering the peculiarity of the change of stresses in the elastic and plastic zones equations describing tensile force for elastoplastic stage of tension were derived and compared with forces observed in the experiment.