Yilong Bai

A thermo-plastic shear instability criterion applied to surface cracking in upsetting and related processes

Abstract The workability of wire and rod in upsetting and related processes is dependent on the ductility exhibited by the expanding free surface. From previous measurements of the equatorial strain histories during incremental upsetting of cylinders, linear fracture conditions for a large number of materials of engineering importance have been derived. A simple theoretical explanation of the experimentally observed fracture condition is given. This is done using the concept of a thermo-plastic shear instability criterion. It is shown that one mode of shear instability coincides with the observed straight-line fracture condition.

A shear instability criterion applied to surface cracking in upsetting

Abstract The workability of a ductile material in an upsetting operation is limited by the onset of cracking on the expanding free surface of the workpiece. This free surface ductility is primarily dependent upon the die—workpiece interfacial friction, the dimensions of the workpiece and the ductility of the work material. A fracture condition may be determined for a material by upsetting cylinders with different aspect ratios and lubricants; the locus of the equatorial axial and circumferential fracture strains provides a linear fracture condition. The linear fracture condition is valuable to industry as it gives a measurement of material ductility and can be used in the design of forged components. In this paper, a thermo-plastic shear instability criterion is applied to the onset of surface cracking in upsetting. It is shown that this criterion results in the same linear fracture condition as that observed experimentally. The application of this criterion provides a theoretical basis for previous experimental observations. It is suggested that a simpler and more rapid method of deducing the linear fracture condition is to measure the instability strain in a torsion on a thin-walled tube.