Effects of Strain Path Changes on the Kinematics and the Intrinsic Dissipation Accompanying PLC Bands in Al-Mg Alloys

Abstract

Plastic instabilities, such as the Portevin-Le Chatelier (PLC) effect, reduce material ductility and induce surface roughness during sheet metal forming. The formation and propagation of PLC bands have been extensively studied by using the uniaxial tension test. However, this strain state differs from the complex strain paths encountered in most metal forming operations. In this work, the linear and non-linear strain path effects on the kinematics of PLC instabilities are investigated in an AA5086-H111 Al-Mg alloy, at strain rates between 0.1 and 0.5 s− 1. This is the first study on the spatio-temporal distribution analysis of heat produced by PLC bands during non-linear loadings. Non-linear strain paths are generated with an innovative one-step procedure, without unloading. The strain path changes are controlled by the displacements along the two perpendicular directions of a cruciform specimen loaded with a planar biaxial tensile device. For a given linear or non-linear strain path, full kinematic and thermal fields on the specimen surface were characterized by using Digital Image Correlation (DIC) and infrared thermography (IRT). Heat source fields were reconstructed from the temperature fields and the heat diffusion equation. The calorimetric response, which mainly corresponds to the intrinsic dissipation produced by the material, permits the kinematics of PLC bands to be investigated. It is shown that the strain state (uniaxial, plane strain or equibiaxial) strongly affects the formation and propagation of such plastic instabilities. For each strain path, a band typology is clearly identified. For two-step non-linear strain paths, the change in the strain state induces an instantaneous modification of the kinematics. The band kinematics is directly linked to the current strain path and the plastic deformation history does not appear to influence the typology of bands.