Experimental studies of yield surfaces of aluminium alloy and low carbon steel under complex biaxial loadings

摘要

In order to develop an efficient computer code for the numerical prediction and simulation of forming processes, a constitutive law which adequately accounts for induced anisotropy is required. Cordebois and Boucher propose [Boucher, 1991], [Boucher, 1994] a new method of determining the plastic behaviour using a model which differs completely from classical ones. The model formulation consists of describing the yield surface evolution in terms of velocity. The yield function is not directly known but is obtained through the integration of a differential law starting from an initial locus. The incremental evolution law uses three strain hardening functions. The development of this theory is based upon a hypothesis connected with experimental observations and multiaxial tests are required in order to obtain a multidimensional representation of the yield surface. Then, in order to observe the phenomena during more complex loading paths, to identify the three hardening functions and to validate the model, we realize bidimensional tests of tension-compression-torsion on thin-walled tubes in aluminium alloy and low carbon steel. The aim is to investigate the evolution of the yield surface for large plastic strain. The yield surfaces are obtained using an automatic procedure in which several yield points can be determined from a single specimen. The test sequence begins with a prescribed prestress or prestrain. Then, points on the yield surface are obtained on radial stress paths starting from an elastic loading state. Plastic yielding is detected from an offset equal to 4.10~(-5). The tensile load and twisting torque are applied using a MTS servo-controlled hydraulic machine piloted by a DN 3500 Apollo workstation. Strains are measured by mean of three strain gages attached to the specimen. Our numerical simulations compute a stress loading incrementally. Some numerical results show yield after proportional loading, monotonic plastic loading and intermittent plastic loading [Cayla, 1990]. We shall make the assumption that a monotonic plastic loading is prevalent if, at any instant, the plastic strain velocity is not nul. This means that no unloading process, in terms of plasticity, is occurring. When unloading does occur, we shall speak of intermittent plastic loading.