Material Characterization for Modelling of Sheet Metal Deformation and Failure

Weight reduction is one possibility to reduce fuel consumption and emission of transportation vehicles. Sheet metals are often used in automotive and aerospace applications and therefore the weight reduction achieved by reducing the sheet metals thickness is an important contribution to weight reduction. Increasing the strength of sheet metal materials gives the opportunity to reduce the total weight while maintaining safety. To prove a maintained safety for parts with a decreased weight Finite Element (FE) simulations are commonly used. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the post-necking behaviour of materials is needed. Improved FE simulations are reducing time and costs during the development processes. One application to improve the strength of sheet metals in the automotive industry is the usage of ultra high strength steels, which has constantly increased in usage during the last decades. The development of the press hardening process, where sheet metal blanks are formed and quenched simultaneously, brings new design opportunities. Using press hardening tools with zones that uses different cooling rates sheet metal parts can be produced with tailored properties, to improve their performance. Simulating materials based on the microstructure demands high precision on the plasticity modelling for high strain values. In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and computational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture modelling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. The results shows that the stepwise modelling method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be characterized in future work