PSI - Issue 58

Włodzimierz Dudziński et al. / Procedia Structural Integrity 58 (2024) 54 – 60 D. W ł odzimierz et al. / Structural Integrity Procedia 00 (2019) 000–000

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2. Failure Description The damage discussed in the article concerns the suspension system of a bucket wheel excavator. An illustrative drawing of such a machine is presented in the figure below. As mentioned in the Introduction, machines dedicated to material handling and open-cast mines take various structural forms. This machine is dedicated to work with excavated hard material. Therefore, it is characterized by a compact design. It consists of the following main components: bucket wheel boom (1), pylon and mast (2), counterweight boom (3), slew deck (4), discharge boom (5) and undercarriage (6). In this case, the vertical axle of the discharge boom deception point broke. The area is marked in Fig. 3 as position 7. The consequence of damage to the axle is the uncontrolled movement of the discharge boom according to the direction of gravity. That was the case here. During normal operation, the range in the vertical direction is 11.3 m, and it cooperates with a belt conveyor or other loading machine. For this reason, damage to the bolt point axle of the discharge boom also damages elements that cooperate with the excavator. Fig. 4 shows a photo of axle disassembled after failure. In this picture, there are all the typical features of the fatigue fracture.

Fig. 4. Post-failure view of fatigue fracture of a damaged discharge boom vertical axle.

3. Fracture and Material Microstructure Analysis Researches aimed at finding cause of the failure, firstly, were focused on metallographic analysis of the broken axle, made from steel grade 40HM. Material testing was conducted in order to evaluate material properties, type of heat treatment, and type of damage. In the first step, macroscopic and fractography analysis was performed with use of stereoscope and scanning electron microscope (Liu, 2005; Macek et al., 2022). Analysis showed that there is fatigue fracture with slip bands and origin point next to the surface, well visible in Fig. 5. Final rupture zone has features of a ductile and brittle fracture. This zone is only 20% of the fracture, which indicates that the safety factor was about 5. The fracture mode indicates that it was the result of a unidirectional bending. The initiation of the fatigue fracture took place in the notch. It is shown in Fig. 6 as a cross section change with radius 0,84 mm, which is in accordance with technical documentation. Further analysis was aimed at microscope research of the material microstructure (Mueller et al., 2012; Brooks et al., 1994; Strnadel, 2018). It was realized with magnification in range of 50x to 1000x after micro-etching of the selected samples. The results showed that microstructure is a nonhomogeneous ferritic-pearlitic structure, coarse-grained with bainite bands and a ferrite acicular structure having features of the Widmanstätten