Improvement of fracture characterisation of high-strength steel sheet metals by means of coupling of optical strain analysis systems with acoustic emission techniques

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High-strength steels play an important role in the automotive industry. Simulation-based design of forming processes can help utilise their high potential. This requires an accurate description of the material formability. For that, forming limit curves are usually used. However, the prediction quality of forming limit curves for high-strength steels not always suffice. Alternatively, fracture initiation models, which consider the stress state, can be used for fracture prediction in the FEA of sheet metal forming. With the help of an optical measurement system for determination of local deformations and numerical simulations for identification of the stress state at the location of crack initiation, it is possible to obtain fracture strains via testing of specimens of different geometries. Resent results show, however, that many specimens which are currently used for characterisation of the fracture behaviour of high-strength sheet materials, exhibit critical damage and crack initiation first in the specimen interior. Depending on the further strain localisation, damage accumulation rate, and fracture resistance of the material, the material failure reaches the specimen surface more or less delayed, where it can be detected by the optical measurement system.The main objective of the project is to develop a method, which enables in-time detection of critical damage accumulation rate and crack initiation for the fracture characterisation of high-strength steel sheet metals by means of acoustic emission technology coupled with imaging techniques. With the help of the coupled measuring systems, critical strains, which lead to a critical damage accumulation rate or crack initiation in the specimen interior and, as a results, go along with a release of energy stored in elastic deformation, are to be accurately detected and recorded. For many specimens and test arrangements, it should be possible to detect material fracture earlier than with an optical measurement system only, which can only detect cracks of a sufficient size on the specimen surface. Via a more accurate determination of the time point of the critical damage accumulation rate or fracture inside the specimen, which is invisible for optical measurement systems, the accuracy of the material failure characterisation should be improved. This should lead to an increase of the quality of the simulation-based design of forming processes with high-strength steels and allow a better material utilisation and higher manufacturing process stability. Moreover, new insights regarding material damage on the microstructure level are expected to be gained during the project, which can provide a basis for a further development or optimisation of high-strength sheet steels.