Identification of the stress-strain behaviour of a tensile specimen after necking

When performing a basic tensile test on a plate specimen, one can obtain the true plastic stress-strain relation of the material prior to necking. With the aid of finite element analysis (FEA) it should also be possible to learn more about the post-necking behaviour of the material. Basically, one starts from an initial material model obtained from the basic tensile test (up to the ultimate tensile strength); the plastic stress-strain curve is then extrapolated beyond the ultimate tensile strength. Next, the tensile test is simulated with this theoretical material input and from the results a conventional stress-strain diagram is constructed. This theoretical stress-strain diagram is then compared to the experimental diagram. Based on the differences between the theoretical and the experimental stress-strain behaviour, the material model is adjusted and the above steps are repeated until a satisfactory convergence is reached. The main difficulty of this procedure lies in the correction of the initially proposed material model. We will present a number of options to achieve this. However, although the initial results from such an approach might look reasonably well, it will also be shown that this commonly adopted procedure has it flaws which are not easily overcome.