| Modeling sheet metal forming operations requires understanding of the plastic behavior of sheet alloys along complex strain paths. In most materials, plastic deformation in one direction will affect subsequent deformation in another direction. For one-dimensional deformation, this phenomenon is known as the Bauschinger effect. The Bauschinger effect in heat-treatable aluminum alloys is heavily dependent on the presence of hardening precipitates.; A new method was developed to test sheet materials under uniaxial reversed loading to compressive strains greater than 0.20. Studies of commercial aluminum alloys 2524 and 6013, show a larger Bauschinger effect for materials that have been artificially aged past peak strength, where the precipitates are semi-coherent or incoherent. Not only is there a substantial reduction of the reverse yield stress, the period of the transient behavior following the load reversal is also lengthened. This effect is seen in over-aged materials after prestrains as small as 0.4%. Materials with less aging had shorter transient periods but showed some permanent softening after the reversal, which was a function of the prestrain.; A constitutive model was developed, based upon the nonlinear kinematic hardening model, which is capable of describing the reduction in the reverse yield stress and the hardening transient observed after the reversal. The ability to model the permanent offset was introduced by the addition of a new term into the formulation, which can be defined as a function of plastic strain. This model has been implemented in a finite element program that successfully reproduces the main features of the experimental results for both uniaxial and more general strain paths. Simulations of draw-bead forces showed significant reductions, up to 25%, because of the Bauschinger effect. Simulations of the springback angle using the new model were within 1.5° of the experimental results for the 2524 over-aged and peak-aged tempers that show a large Bauschinger effect. |
