ANALYSIS OF DEFORMATION-INDUCED HEATING IN TENSILE TESTING USING A FINITE ELEMENT METHOD.

摘要

A numerical method for analyzing non-isothermal viscoplastic deformation problems has been developed. The physical problem is cumbersome because the thermal and deformation effects are coupled both ways, i.e. plastic deformation generates heat and the temperature rise affects material flow behavior. As an application of this method, sheet tensile tests conducted in air have been analyzed using a two-dimensional finite element formation. A modified Bishop's method is used to solve the thermoplasticity problem in decoupled form at each time step. The analysis consists of two main parts: a rigid-viscoplastic finite element method to analyze the deformation, and a transient heat transfer finite element method. Each part is assumed to occur during sufficiently small, consecutive time steps. Using the present method, the various factors affecting the nonisothermal ductility of material and flow characteristics can be investigated. The accuracy of the analysis is confirmed by comparison with experimental tensile test data for several engineering materials. Loss of total elongation by adiabatic deformation reaches 6.2% and 35% for I.F. steel and 304 stainless steel, respectively, illustrating the importance of deformation heating. For the intermediate case in air, both uniform and total elongations decrease with testing speed as a result of a drop in heat transfer to the environment. The competing effect of deformation heating and strain-rate sensitivity of AK steel is also examined and the FEM results showed the "near-invariance" of non-isothermal tensile ductility of this alloy. It is observed that the effect of deformation heating becomes more pronounced as necking develops and at higher testing speeds. The development of a temperature gradient is found to have a detrimental effect on ductility as opposed to the stabilizing effect of rate-sensitivity. Consequently, better formability can be achieved by controlling heat transfer conditions during forming. In addition, several numerical techniques, which often arise in finite element analysis of large deformation problems with high strain localization, are examined and an automatic remeshing technique has been developed.