Experimental and Finite Element Study of Residual Stresses in Zircaloy-4(r) and OFHC Copper Tubes.

Abstract

Non-homogeneous plastic deformation associated with metal-forming operations results in a state of residual stress in the final product. Complete evaluation of the mechanical behavior of the product under operating condition requires a knowledge of these stresses. In order to evaluate the magnitude and distribution of residual stresses, existing experimental techniques are complemented by computer-aided simulation of the forming process using elastic-plastic finite element method (FEM). It is found the FEM can be effectively utilized to simulate metal-forming operations in order to determine the combination of process variables which optimizes the residual stress distribution in the final product. In the experimental phase of this study, residual stress distribution in Zr-4(R) and copper tubes is determined by electropolishing the outer(or inner) surface of the tubes while measuring the developed strains at the inner(or outer) surface. Material removal by electropolishing proved to be an efficient and suitable technique due to its constant mass removal rate under conditions which do not alter the pattern of residual stresses in the specimen. In the case of Zr-4(R) specimens, experiments were conducted on the as-received(stress relieved) as well as specimens annealed at 500 C for one hour. The effect of various degrees of cold working on the residual stress patterns of drawn copper tubing is determined by performing similar experiments on soft, 1/4-hard, 1/2-hard, and hard temper OFHC copper specimens. In the second phase of this investigation, an elasticplastic finite-element code (ABAQUS) is employed in a parametric study to determine the optimum processing conditions for drawing copper tubes. Various die/plug angles along with different area reductions are considered to simulate the drawing process. Comparison of the simulation results yields that for a given area reduction, there exists a specific combination of die and plug angles which minimizes the drawing stress. The results demonstrate that the magnitude and distribution of residual stresses are greatly influenced by the die and plug angles. It is concluded that proper selection of theses parameters increases the efficiency of the drawing process, enhances the integrity of the final product; all resulting in greater productivity and reduction in the operating costs.