Circumferential crack in a real impact-extruded cup (left) and in a simulation (right).
Fraunhofer IWM
Car and machine components are often made in several cold-forming steps, which can stress the materials up to their deformation limit and thereby cause invisible damage. Previous computer simulations commonly used to design components usually could not calculate the damage progression precisely enough to predict the time and location of material failure. The Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, Germany, therefore developed a simulation model that maps the actual damage and failure mechanisms in metallic materials. This simulation tool allows companies performing sold-forming work to reduce costs when designing processes and shorten development times. Furthermore, the new simulation can implement even complex component shapes that would be very difficult to achieve otherwise.
The model describes the preconditions under which pores form during plastic deformation of the material, as well as the conditions under which they grow or under which several pores merge. These merged pores can result in macro-pores, followed by micro-cracks and ultimately a critical macro-crack. Until now, the standard simulations merely assisted in the decision process for component design. So-called mechanism-based damage models provide more accurate predictions. "As part of a joint industrial research project, we extended the familiar mechanism-based material model according to Gurson, Tvergaard and Needleman (GTN model) for processes during cold solid forming so that it maps the observed ductile damage mechanisms for the relevant materials more precisely than before", explains Dr. Maksim Zapara, head of the Solid Forming team in the Forming Processes group at Fraunhofer IWM.
The physical causes of the material behaviour in cold solid-forming processes were studied as a basis for the new model. In comprehensive microstructure examinations, researchers learned that pores tend to form particularly at non-metallic inclusions in the material during forming, for example. The inclusion either breaks or it separates from the surrounding material. The damage model can be used for many materials thanks to its modularity.