The fracture morphology of the failed nodular cast iron crankshaft shows the fracture process and mechanism of the nodular cast iron crankshaft under the comprehensive action of three factors: material, operating environment and load. Fracture analysis is to judge the load and environment borne by the nodular cast iron crankshaft during operation according to various characteristics of the failed nodular cast iron crankshaft fracture, Combined with the characteristics of nodular cast iron crankshaft material, the fracture mechanism of the material is revealed, and finally the failure reason of nodular cast iron crankshaft is found out.
According to Fig. 2 and Fig. 3, the crack originates from the lower part of the junction between the lower dead center neck of the coupling and the edge of the rolling groove. The crack basically expands in the direction of an included angle of 45 ° with the axial direction of the nodular cast iron crankshaft. Finally, an instantaneous fracture occurs at the edge of the fillet rolling groove on the other side.
The fracture surface was observed. The characteristic distribution of crack source area, propagation area and instantaneous fracture area at the fracture surface was obvious, and there was no obvious collision wear trace in the crack source area, indicating that the nodular cast iron crankshaft broke rapidly under stress. See Fig. 3 for the fracture photo. When the crack source area is magnified by 10 times, it is found that the crack source area is located below the junction of the connecting rod journal and the edge of the rolling groove, about 4.4 mm from the surface of the connecting rod journal. The crack source is in a radial pattern structure, with obvious edges extending around, neat and without obvious plastic deformation. The convergence of the radial stripes is the crack source, and the enlarged photo of the crack source area is shown in Fig. 4.
In Figure 1, the color of the second connecting rod journal of the failed nodular cast iron crankshaft is blue black, and there are a lot of strain marks on the journal, and there are many adhesive bearing bush alloy metals on the surface of the connecting rod journal. In Figure 3, there is high-temperature oxidation discoloration in more than 1 / 3 of the fracture area. In addition, at the position indicated by the red arrow in Figure 2, there is obvious burning deformation at the junction of the connecting rod journal and the edge of the rolling groove, indicating that the second connecting rod journal has the phenomenon of holding and pulling the Bush due to unknown reasons during the full speed and full load bench test, and the connecting rod shaft diameter and the bearing bush are not suddenly locked, but gradually locked, accompanied by the process of pulling and holding the bush, A large amount of heat is generated, resulting in high-temperature oxidation of the connecting rod journal towards the inner side of the fracture, accounting for more than 1 / 3 of the fracture, and finally sintering and melting of the bearing bush and journal at the junction of the connecting rod journal and the rolling groove.
The general fracture process of nodular cast iron crankshaft can be obtained by low magnification analysis. Firstly, the connecting rod is pulled due to unknown reasons, secondly, the friction produces high temperature, and then the bearing bush and journal at the junction of the connecting rod journal and the rolling groove are sintered, melted and locked. However, the nodular cast iron crankshaft continues to rotate due to inertia, resulting in excessive bending and torsional stress near the fillet of the journal, and the stress is concentrated at the crack source position shown in Fig. 4, Finally, the instantaneous fracture exceeds the fatigue limit at the crack source.