May 2021 Volume 3

FORGING RESEARCH

elastic analysis was done to display its stress distribution. Thus, the influence of the threaded hole and the bottom radius on the stress distributions in the punches could be evaluated.

6.3 Finite element analysis-assisted study of punch fracture in a punching process In a CV joint cage window perforating process, the punches failed prematurely after approximately 30,000 strikes. The failure mode was clearly a brittle fatigue fracture that occurred after repeated strikes created a cumulative effect of stress concentration and initiated crack propagation from the bottom to the top, as shown in Figure 26. As can be seen in the schematic in Figure 27, the lasting force imbalance between the flat bottom area (Area 1) and the sphere bottom area (Area 2) in the entire stroke generated tensile stresses at the bottom of the punch and compressive stresses at the top. While the tool steel used in the punch can resist a very high compressive force, it has low load capacity in tension. This may explain why the fracture initiated from the bottom of the punch.

Figure 28. Existing and modified punch configurations The simulation showed that when the bottom surface of punch configuration 3 had a zero radius where the sphere and flat surfaces met, the tensile stress concentration occurred along the sharp transition line, as shown in Figure 29. Therefore, configuration 3 was removed from the study.

Figure 29. Tensile stress distribution on bottom surface of punches

Figure 26. Fractured punch

Figure 30. Locations where stresses were tracked

a b Figure 27. Cross-sectional side (a) and front (b) views of the 3Dmodel setups This investigation was expedited by finite element modeling. Instead of doing fatigue analysis, which was limited by the software’s capabilities, its goal was to identify the high-tensile stress areas and eliminate any potential stress riser by lowering the stress level through punch geometric modifications. In this investigation, in addition to the existing punch (1 in Figure 28), three other punch configurations (2, 3 and 4 in Figure 28) were studied. Punches 2 and 4 had the same punch geometry as that of the existing punch 1, but with a different thread hole depth. Punch 3 had the same through thread hole as punch 1, but the radius where the sphere and the flat faces met was set to zero. Each simulation consisted of two stages. In the first stage of the simulation (a plastic analysis), the workpiece was assumed to behave as a rigid-plastic material, and the bottom die and punch were assumed to be rigid. The first stage ended when the simulation reached the maximum load in the punch, which defined the starting point of the second stage of simulation. In the second stage (an elastic analysis), the punch was assumed to be elastic and one-step

To study the influence of the thread hole on the punch’s stress distribution, stress data were extracted from three locations (1, 2 and 3 in Figure 30) around the edge of the hole in the punch bottom and one (4 in Figure 30) at the edge of the hole on the punch top. The stress values are listed in Table 4. The x direction was in the short span of the punch, the y direction was in the long span, and the z direction was along the height, as illustrated in Figure 30. The stress data showed that in the x direction, the bottom of the punch had tensile stress and the top of the punch had compressive stress in all three configurations, which confirmed the assumption described previously. In punch configurations 2 (blind hole) and 4 (without hole), the stresses in both the x and y directions were considerably smaller and more uniform than in punch 1. These

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FIA MAGAZINE | MAY 2021