Lehrstuhl für Fertigungstechnologie, Universität Erlangen-Nürnberg

Report on the surface fatigue of metal forming tools



Datum: 19.03.1993


Author


Reporter

  • Prof. Dr.-Ing. M. Geiger
  • Prof. Dr.-Ing. habil. G. Kuhn
  • Prof. Dr.-Ing. Dr. h.c. K. Lange

Tool problems in metal forming are characterized by a too small service life of the tools, inadequate process reliability and insufficient machine availability because of unexpected tool failure as well as excessively long periods of development concerning tool construction. Only the consequent use of Computers for construction and process development offers a decisive improvement of the situation for the future. An important step towards the mentioned improvement is the introduction of the FE-process simulation and of the simulation of tool failure based on that. One of the important tasks of process simulation in metal forming is to take the tool behavior also into account, which is then calculable for the developing engineers. As a result, the possibility of shaping the tool to a process-optimized extent already in the development phase of production, i.e. in the period of designing and construction, is opened up. By means of simulation of the loading during operation which is accompanied by tool damage weak points of the selected tool concept can be recognized and possible optimizing measures can be tested and assessed with the Computer model. In the field of cold forging, especially the tool failure by fatigue fracture is interesting. In order to obtain a process-optimized tool concept it is above all important to improve decisively the resistance against fatigue crack initiation of the tools which determines to a considerable extent the further crack behavior. An essential aspect is the period of fatigue fracture formation, i.e. of the fracture initiation, at the surface of the tool. The provision of the necessary theoretical basis for the simulation of the surface fatigue or fracture initiation within the context of the failure simulation and the following optimization of the tools was the task and subject of this research. A first comparison of the approaches describing surface fatigue known by literature shows that the present solutions for the application of metal forming tools do not seem to be suitable because of the predominant complex loading conditions. A further contribution presents in addition a further developed failure criteria in relation to energy density for the simulation of fatigue crack initiation. The direct comparison of numerous experimental and numerical fatigue studies using the example of three point notched bending specimen as well as extrusion dies demonstrates the usefulness and advantage of this approach. The concluding application examples summarize the future possibilities of application for failure simulation in order to optimize tools.