Strength Model Evaluation Based on Experimental Measurements of Necking Profile in Ductile Metals

Abstract

The reliability of the numerical results strongly depends upon the choice of the constitutive models, which sometimes requires an ad-hoc calibration of the parameters starting from experimental results. Often, the tension test is chosen since it allows obtaining a big amount of data, but in case of ductile materials, in which the specimen could be subjected to high value of plastic strain before the fracture, it is necessary to properly manage the experimental results. For low values of deformation, the deformation of the gage length of the specimen is uniform and the true stress can be easily derived from the longitudinal one and considered as the equivalent stress. At the onset of necking, geometrical instability and concentration of deformation are developed. In the scientific literature several different approaches have been proposed in order to estimate the equivalent stress distribution in the post-necking regime. In this work, a new approach was proposed by the combination of digital image analysis and numerical inverse method. The evolution of the profile deformation of the specimen observed during the test is directly used to identify the constitutive relation of the material in case of dynamic tensile tests.

A Methodology for Post-Necking Analysis in Isotropic Metals

“…These approaches are known as FE-based inverse methods and consist of a recursive comparison between measured quantities and the corresponding predicted ones. Such quantities may be: the total axial force at given displacement increments beyond the maximum force [20][21][22][23][24], the specimen necking shape after the maximum force [25,26], and the shape along with the force time history [27,28]. Since an optimization is required, such approaches are more computationally expensive than analytical corrections.…”