Optimizing the Post Sandvik Nanoflex material model using inverse optimization and the finite element method

This article describes an inverse optimization method for the Sandvik Nanoflex steel in cold forming processes. The optimization revolves around measured samples and calculations using the Finite Element Method. Sandvik Nanoflex is part of the group of meta-stable stainless steels. These materials are characterized by a good corrosion resistance, high strength, good formability and crack resistance. In addition, Sandvik Nanoflex has a strain-induced transformation and, depending on austenising conditions and chemical composition, a stress-assisted transformation can occur. The martensite phase of this material shows a substantial aging response. The inverse optimization is a sub-category of the optimization techniques. The inverse optimization method uses a top down approach, as the name implies. The starting point is a prototype state where the current state is to converge on. In our experiment the test specimen is used as prototype and a calculation result as current state. The calculation is then adapted so that the result converges towards the test example. An iterative numerical optimization algorithm controls the adaptation. For the inverse optimization method two parameters are defined: shape of the product and martensite profile. These parameters are extracted from both calculation and test specimen, using Fourier analysis and integrals. An optimization parameter is then formulated from the extracted parameters. The method uses this optimization parameter to increase the accuracy of ”The Post” material model for Sandvik Nanoflex. [1] The article will describe a method to optimize material models, using a combination practical experiments, Finite Element Method and parameter extraction.