Mikromechanische Simulation

Micro-mechanical simulation of boundary and mixed friction phenomena in sheet metal forming

 

Issue

During sheet metal forming processes the large-area contact and the relative movement between workpiece and tool have an influence on the process. Therefore the knowledge of tribological parameters is necessary for the process design. The simulation of friction using coulomb law with the Finite-Element-Method is possible with suitable results. But the calculation of local shear stresses in sheet metal couldn’t be achieved till now.

During deformation hydrostatic und hydrodynamic effects in the interface between workpiece and tool arise because of roughening and flattening the surface area.

The interaction in a mixed lubrication regime is basically unknown. Therefore this project deals with the challenge to get more knowledge of the physical procedures in this interface. Experimental, analytical and numerical techniques will be used to reach this goal.

Objective

The analysis and modelling of boundary and mixed friction between workpiece and tool are topic of this project.

Approach

Experimental investigations on the hydroforming test bench are planned. They allow for the in situ observation of the changes of tribologic conditions, such as the lubricant’s thickness and pressure and the surface topography during the process.

Figure 1: schematic representation of the test bench for the in situ observation during hydroforming of tubes; above: before forming; below: after forming
Figure 1: schematic representation of the test bench for the in situ observation during hydroforming of tubes; above: before forming; below: after forming

The tube is placed in the die, sealed by plungers and filled with a pressure medium, mostly hydraulic fluid. The internal pressure increases until the tube is moulded completely in the designated shape. The normal stresses, caused by the pressure, change the tribologic conditions between tool, lubricant and surface. To provide for the direct observation of the procedures in the tool-workpiece-interface, there is a gauge-glass worked into the die. Like that it is possible to take a look inside.

Several lubricant pockets because of surface flattening and roughening are generated during forming processes [Figure 2]. Their influence to the stress distribution in the sheet metal will be discussed using numerical methods.

Figure 2: Simplified surface topography with hydrostatical and hydrodynamical lubricant pockets
Figure 2: Simplified surface topography with hydrostatical and hydrodynamical lubricant pockets

A Fluid-structure-interaction is used to analyse the influence of the liquid lubricant to the sheet metal. Two solvers (one for the fluid mechanics and another for the structural mechanics) are connected over a coupling-algorithm [Figure 3].

Figure 3: Schematical construction of a fluid-structure-interaction (FSI)
Figure 3: Schematical construction of a fluid-structure-interaction (FSI)