Under surface pressure sensing technique for the evaluation of contact stresses

Abstract

In metal forming operations, the intensity and distribution of stresses at the tool/material interface have great influence on the pattern and mode of deformation. This is due to the effect of frictional forces, being resistive to material movements and hence affects material flow and the directionality of volume distribution of material inside die cavities. This paper explores the use of the pressure pin technique for the measurement of tool stresses at the interface between the tool and the deforming workpiece material. It also introduces a new experimental methodology for the measurement of stresses where the measuring device is concealed inside the tool and not subjected to the severe conditions at the interface surface. Such an arrangement prohibits the severe conditions at the interface from influencing or distorting the experimental readings and facilitates repeatability of results. Each tool containing measuring devices is split into two parts, one that includes the shape to be formed and house the pin heads (industrial die), while the second part includes the rest of the measuring column including the load cells. Using thermal insulation the second part can be protected from any thermal conditions at the interface when the tools are used in hot forming operations. Theoretical investigation of tool stresses is carried out using stress analysis. Experimental measurements of axial and radial stresses at different positions along the radius of an axi-symmetric billet are presented and compared with the theoretical results. Friction resistance and friction coefficient are also evaluated across billet surface and at different height reductions. Such an investigation gives better understanding of the interface conditions and parameters, which are among the essential input to the sophisticated modelling technology applied to metal forming processes.