Identification of the work of plastic deformation and the friction shear stress in wire drawing

This paper demonstrates a new method to extract the friction stress τ from measurements in wire drawing. It is based on simultaneous measurement of drawing force and die temperature; an analytical thermo-mechanical model of the wire drawing operation is used to separate the plastic work Wpl from the frictional work. Thanks to the exploitation of the temperature measurement, the necessity to know the stress–strain curve of the drawn metal beforehand is avoided. This new approach is compared with the more traditional one, in which only the force is measured and friction is extracted by inverse modelling using an experimentally determined stress–strain curve. Drawing experiments have been performed on stainless steel wire, using dies in which thermocouples have been inserted. The linearity of the measured drawing stress–measured die temperature relation is a first confirmation of the model. On the other hand, stress–strain curves have been measured at both low and high strain rates in the [20–150 °C] range, and used to compare the identified values of Wpl both with analytical estimations and numerical simulations of wire drawing; again, the thermo-mechanical model has been confirmed. This has allowed the friction stress to be measured for various metal soap lubricants. The experimental uncertainties and their impact on the identified friction stress have been analysed.