FE Fracture Prediction on Edges of Punched Holes

Many industry sources identified fracture from holes and edges as a key knowledge gap for fracture prediction of AHSS and UHSS grades. Ford Aachen with MATFEM cooperation [1] has shown an importance of including fracture failure models into their finite element (FE) simulations especially when designing with UHSS such as Boron Steel. Hole expansion coefficient (HEC) is one of the ways to characterise the edge cracking of metals. Many studies investigated the best way of predicting the fracture using HEC in their experimental work [2], [3] combined with FE simulation [4]. In the later one a simplification for an FE simulation is done such that only 2D model is utilized. Since the study examines the area around the hole in quite a detail, the mesh density is too high for general use in crash FE model of a full vehicle. In forming simulations HEC and forming limit curve (FLC) are often the criteria for inspecting the possibility of material failure. In crash simulations neither HEC nor FLC are generally used. More suited are the phenomenological fracture models which are more applicable for a wider range of engineering problems. There are now a number of these fracture models available. Examples of some of these fracture models are from Gurson [5] and various extensions to Gurson, Dell and Gese [6], Wierzbicki [7] and Wilkins [8]. Dell and Gese use CrachFEM material model developed by MATFEM [9]. Tata Steel adopted the use of CrachFEM material in past few years. Horn [10] and his colleagues at Tata Steel Research and Development have investigated several methods for measuring and analysing the data needed as input variables in CrachFEM material cards. CrachFEM material data are now available for the customers in AURORA material database. Norman and Buckley [11] have successfully used this data in their automotive virtual development process. Their test-FE correlation gave very promising results. CrachFEM material model has therefore become a winning tool for developing a simple method of fracture prediction in a crash event. A simple rectangular test specimen with a hole in the middle was designed such that it was possible to use the optical strain measurement system - ARAMIS. This gave an opportunity to correlate not only force-displacement curves between the test and FE simulations but also strain development around the hole.