Prediction of fracture in cold forging with modified Cockcroft-Latham criterion

Abstract

Cold forging technology provides production of net shape parts that required very little or no further machining. Meanwhile one of the technological problems of the cold forging is fracture of the workpiece during production. This is especially important when using less ductile high strength materials or technological schemes that require significant deformation. The use of reliable fracture criteria implemented in metal forming software can help predict possible cracks and find ways to avoid them. There are many criteria that can be used for this task, for example, Gurson and its modifications, Johnson-Cook, Wierzbicki and Cockcroft-Latham. The latter one is the most widely used due to its simplicity since only the first principal component of stress is required for calculation. On the other hand, in this simple form, it does not take into consideration the influence of the stress state and the path of plastic deformation. In the presented work, a comparative analysis of different criteria has been done to find out the relations among them and a modified Cockcroft-Latham criterion of fracture has been proposed. It considers both the influence of the normalized first principal stress and the influence of the stress state scheme. A new approximation of the surface of critical plasticity has also been proposed that considers the limit values of the normalized average stress. The modified Cockcroft-Latham criterion was programmed as a subroutine in QForm metal forming simulation software and it has shown good accuracy in tests for different practical cold forming processes.