Estimating the strain-rate-dependent parameters of the Johnson-Cook material model using optimisation algorithms combined with a response surface

Under conditions where a product is subjected to extreme mechanical loading over a very short time period, the strain rate has considerable influence on the behaviour of the product’s material. To simulate the behaviour of the material accurately under these loading conditions, the appropriate strain-rate parameters for the selected material model should be used. The aim of this paper is to present a quick method for easily determining the appropriate strain-rate-dependent parameter values of the selected material model. The optimisation procedure described in the article combines the design-of-experiment (DoE) technique, finite-element simulations, modelling a response surface and an evolutionary algorithm. First, a non-standard dynamic experiment was designed to study the behaviour of thin, flat, metal sheets during an impact. The experimental data from this dynamic and the conventional tensile experiments for mild steel were the basis for the determination of the Johnson-Cook material model parameters. The paper provides a comparison of two optimisation processes with different DoE techniques and with three different optimisation algorithms (one traditional and two metaheuristic). The performances of the presented method are compared, and the engineering applicability of the results is discussed. The identified parameter values, which were estimated with the presented procedure, are very similar to those from the literature. The paper shows how the application of a properly designed plan of simulations can significantly reduce the simulation time, with only a minor influence on the estimated parameters. Furthermore, it can be concluded that in some cases the traditional optimisation method is as good as the two metaheuristic methods. Finally, it was proven that experiments with different strain rates must be carried out when estimating the corresponding material parameters

Design, Testing, and Sensitivity Analysis of a Torsional Cyclic Test Adapter

In this article, a torsional adapter is designed and evaluated through the comparison of analytical, numerical, and experimental tools. The adapter converts a conventional tension–compression test machine for cyclic loading to a modified application of both force-controlled and displacement-controlled torsional loading. The mechanism ensures a uniform distribution of loading application on both sides of the specimen. The determination of the durability curve can therefore be consistently carried out by acknowledging the geometric relation between the displacement of the test rig and the strain on the specimen. However, friction and clearance in the mechanism joints can cause energy dissipation; therefore, a detailed evaluation of this effect is mandatory before the use of the adapter. Here, it is shown that, using the current version of the adapter, the energy dissipation during torsional testing can be measured and later successfully considered during the determination of the torsional cyclic curve. Future improvements of the adapter will involve the reduction of the friction between the components of the mechanism