Analysis of necking in high speed experiments by stereocorrelation

Tensile tests with Hopkinson bars are monitored with high speed cameras to extract 3D displacement fields and the corresponding strain fields by resorting to stereocorrelation. The aim of the experiments is to analyze the onset and development of necking for various strain rates and samples. Various challenges have to be dealt with, namely small definition of the pictures, large displacements that occur during the experiment, and the need for good spatial resolutions to capture the strain levels within the neck. In order to address all these issues, pre-correction techniques are used to allow for the required stability and accuracy of the analysis. Eight experiments on cylinders and parallelepipeds at different loading rates are analyzed. Either single or multiple neck inceptions were observed, irrespective of the mean strain rate, supporting a random nucleation picture rather than a deterministic result of the constitutive law. Local strains up to about 600% were measured. In the case of multiple necking, a selection leading to the growth of a dominant one is observed, in agreement with Mottʹs theory of an “unloading wave” emanating from neck inception. A propagation speed of this wave can be estimated from the analysis and is shown to agree with the theoretical prediction.