For the process design of hydroforming in the ''hot'' temperature range, reliable data are necessary to describe the material behaviour at elevated temperatures under the occurring loads of hot hydroforming processes. State-of-the-art technologies for the investigation of material behaviour, like uniaxial tensile tests or hydraulic bulge tests, do not provide enough similarity with the process of hot hydroforming. This paper describes a new testing technique, capable of realizing high process temperatures and constant strain rates. It represents a further development of the established technology of tube bulge tests. The hardware is described, its functionality is proven and mathematical approaches for the calculation of stress/ strain-curves from experimental data are presented.
For the purpose of numerically simulating metal forming processes, material data are necessary, determined by testing procedures similar to the particular process. The new technology of hot tube bulge tests has been introduced recently, fulfilling the requirements of material data determination for hot hydroforming. Based on measurement data gained by this technology, selected constitutive relations for approximating the flow stress depending on temperature, strain rate and logarithmic strain were parameterized applying linear regression analysis. Using the material law with the best approximation quality among the regarded equations, a numerical simulation of an exemplary forming process was accomplished. A comparison between the experimentally obtained geometry after a hot hydroforming process and the prediction by numerical analysis is used for evaluating the quality and applicability of the determined material data for this kind of process. Additionally, a process simulation, using extrapolated material data from compression tests is presented to visualize the influence of the testing procedure on the resulting part geometry prediction.
Hydroforming tools must be designed fatigue resistant because in case of tool rupture high costs for repair and production downtime occur. Available guidelines for lifetime estimation of cyclically loaded parts usually are of general nature. In order to examine their applicability to the special concerns of hydroforming tools, stress states occuring during operation were investigated by means of numerical simulation. Basing on the results, specimen geometries were developed, representing a typical stress state. The estimated durability was calculated using the "FKM-guideline". The real durability was determined experimentally in form of Wöhler-graphs. Eventually calculation and experiments were compared. Hence, the qualtity of the lifetime prediction was evaluated and correction factors were proposed.Keywords
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