Development of a gas based bulge test for glowing metal sheets

Schematic structure of the Bulgetest

Press hardened components are increasingly being used to reduce weight, but these require a complex process design.For the necessary material characterization of red-hot sheet metal materials,test rigs for multiaxial loading of test specimens have not been developed at present and measurement under these conditions has not been possible so far, so that extrapolated material data must be used. The aim of the project is the development of a digital-pneumatic high-pressure system, which allows a controlled material forming and therefore enables the measurement of the visco-elastic material behaviour.

Benefit Procedure

Development of a multiaxial measuring method for red-hot sheet samples

Development and construction of the test bench for sheet metal forming

Measurement of the yield stress of red-hot sheet samples

Development of the valve battery and control

Proof of feasibility digital-pneumatic high-pressure system from series components

Development of the valve battery and control

Development of process control

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Development of a test bench for forming red-hot metal samples

Hot Gas Bulgetest Test Bench Hot Gas Bulgetest Test Bench

Press hardening of steel enables the automotive industry to produce high-strength components with comparatively complex geometries. Crash-relevant components in particular are often press hardened. Tanks to forming simulations, press hardening processes can be designed quickly and cost-effectively if precise material data are available. For this reason, a bulge test is being developed within the context of the project, which enables material testing under the conditions of press hardening. In the bulgetest, a sheet metal sample is clamped between two round cavitys, one side is then subjected to pressure. Under the load a bulge forms, which is rotationally symmetrical. Due to the geometrical conditions, the existing yield stress at the top of the bulge can be calculated with membrane equation. The forming process must be as contactless as possible in order to avoid falsification of the results by friction in contact. The test bench allows contactless forming of sheet metal at temperatures of 1000 C. Nitrogen is used as the pressure medium because it absorbs heat radiation only to a small extent. The use of classical media such as water or oil is excluded due to the high temperatures. The sheet temperature can be varied and controlled before and during the forming process. A cooling device is integrated into the tool, which directs compressed air onto the sheet metal. Resistance heating enables the sheet metal to be heated up.

The case study examines different aspects of the Industrial Internet by investigating the special case of an automated commissioning process of a machine. The goal is to propose a need of action for whole sector of fluid power and all related parties to take part at its benefits and stay involved in a highly advanced future, achieved by the 4th Industrial Revolution.

Development of a digital-pneumatic drive system

Digital pneumatic drive system of the Bulge Test Digital pneumatic drive system of the Bulge Test

The digital hydraulics, which aims to transfer principles proven in electrical engineering to fluid power, has received increasing attention in the last two decades. This discipline primarily tries to reduce the principle-related losses of resistance control, for example by replacing continuous with binary switching actuators. This also breaks up the concept of component series, which is widely used in industry, since virtually any size of actuator can be approximated, provided the actuators have a sufficiently small resolution. A frequently neglected aspect is that it is also possible to provide actuators for applications which are currently not available on the market. For fluid power applications, where the use of oil as a medium is excluded, only very limited components are available. Especially proportional actuators with high bandwidth are hardly or not at all available. This deficiency can be avoided in principle by applying the principles of digital hydraulics, but requires a deviation from control concepts widely known in fluid technology. The pneumatic system of the Bulgetest uses nitrogen as medium and requires system pressures of up to 300 bar. Since there are no proportional valves of sufficient speed available on the market, a cascade of switching valves with binary graded conductance values was developed, which is controlled by means of pulse code modulation.

Regulation of the conversion rates during the test Regulation of the conversion rates during the test

The pneumatic system allows the speed of the forming process to be regulated until the process reaches critical elongation and becomes unstable.

Acknowledgement

The IGF project 19229 N 1 of the Forschungsvereinigung Stahlanwednungen e.V. was funded by the Federal Ministry of Economics and Energy via the AiF as part of the programme for the promotion of community industrial research and development (IGF).