Engineering Failure Analysis 16 (2009) 740–750

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Engineering Failure Analysis

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Failure analysis of the end eye connection of the bucket wheelexcavator portal tie-rod support

Srdjan Bošnjak a, Nenad Zrnic a*, Aleksandar Simonovic a, Dejan Momcilovic b

a University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11000 Belgrade, Serbiab Institute for Testing of Materials IMS, Bulevar Vojvode Mišica 43, 11000 Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 10 June 2008Accepted 11 June 2008Available online 20 June 2008

Keywords:Accident investigationsBucket wheel excavatorFailure analysisFinite element analysis

1350-6307/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.engfailanal.2008.06.006

* Corresponding author. Tel.: +381 11 3302454; fE-mail address: nzrnic@mas.bg.ac.yu (N. Zrnic).

a b s t r a c t

This paper describes the failure analysis results of the bucket wheel excavator (BWE) portaltie-rod support. The results of finite elements analysis (FEA) point out the occurrence ofpronounced stress concentration, whereby multiple stresses in the failure zone exceedthe yield stress of the basic material. Experimental investigation on fracture surfaces byscanning electron microscope (SEM) and light microscope (LM) revealed the existence ofporosities and inclusions in the weld metal. Moreover, various types of fracture (fatigue,transition, brittle) and their uneven fraction observed on fracture surfaces are inducedby structural characteristics of the base metal, welding technology and the constructionitself. Based on the presented results it is conclusive that the failure of the support wasthe consequence of superposition of the negative effects caused by inadequate shapingand dimensioning of the support assembly for given load conditions, as well as influencesof mentioned defects of the metal weld structure.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The bucket wheel excavator (BWE) Sch Rs 1760/5 � 32, Fig. 1a, was in use for overburden digging and deposit in open pitmine ‘‘Kolubara’’ – Serbia. At the end of 2007 the heavy accident caused the total collapse of the machine, Fig. 1b.

The basic structural elements of the considered BWE, Fig. 2, are lower structure with mechanism of transport crawler(vehicular base with caterpillar track) (1), pylon (2) with counterweight arm (3), portal (4), bucket wheel boom – BWB –(5), bucket wheel (6), mechanism comprising rope system for BWB hanging (7), portal tie-rods (8) and supports of portaltie-rods (9).

The portal tie-rods supports, Fig. 3, are the vital parts of BWE structure. By means of tie-rods (rope diameter 110 mm),they accept a part of the BWB and portal loads and transmit it onto the counterweight arm. At the same time they are sup-ports of the rope system drum for BWB hanging. The straightforward cause of BWE collapse is the failure of the end eye con-nection of the support of the right portal tie-rod (Figs. 3 and 4).

In order to detect the reason of the end eye connection failure it was necessary to do the following:

� Calculation of the stress state of the eye assembly;� Visual, metallographic and SEM inspection of the crack surface;� Testing of chemical composition and mechanical properties of the end eye connection plate.

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Fig. 1. BWE Sch Rs 1760/5 � 32: (a) before collapse and (b) after collapse.

Fig. 2. Structural scheme of BWE Sch Rs 1760/5 � 32 with mobile conveyor.

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The paper presents the results of the research done by the Faculty of Mechanical Engineering (University of Belgrade) andthe Institute for Testing of Materials IMS – Belgrade.

2. Calculation of the stress state

The identification of the portal tie-rod support stress state is done by applying linear finite element method (FEM).

2.1. FEM model

By the synthesis of 3D model of all structural parts, the 3D model of the support of the portal tie-rod is set up and pre-sented in Fig. 3. The model presents the continuum discretized by the 10-node parabolic tetrahedron elements [1] and [2] inorder to create FEM model (1,99,639 nodes, 1,14,504 elements), Figs. 5 and 6. In addition, to model the mechanical interfaceof adjacent structural components of BWE, the nodes of the portal tie-rod lowest surface are fixed by restraining all threedegrees of freedom.

Fig. 4. Broken-down support of the right portal tie-rod: (a) element on the counterweight arm and (b) broken-away part.

Fig. 3. 3D model of the support of the right portal tie-rod.

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2.2. External load

The load analysis of the support of the portal tie-rod is carried out according to the rules given in the German code [3],based on which the considered BWE was designed. The intensities of forces in the portal tie-rods (FTR) are defined for fourcharacteristic load cases that are: H1 (FTR = 2980 kN), H2 (FTR = 3755 kN), HZ1 (FTR = 4040 kN) and HZS1 (FTR = 4260 kN).

The external load has affected the structure of the portal tie-bar support by means of a pin which connects the tie-rodyoke and the eye. The sinusoid function of load distribution (bearing load) was adopted, Fig. 7.

2.3. Stress state of the structure

The uniaxial stress field obtained according to the Huber–Hencky–von Mises hypothesis, for load case HZS1, is shown inFig. 8. Maximum value of uniaxial stress is obtained in the left eye, in the zone of its connection with the lengthwise sup-porting plate, Fig. 9.

Fig. 6. Detail of the mesh in the failure zone.

Fig. 5. FEM model.

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2.4. Discussion of FEM analysis results

Based on the FEA results for the portal tie-rod support structure, it is conclusive that:

� The stress state of the right eye is more convenient than the stress state of the left eye, Fig. 10, due to the influence of thefront support;

� Due to the prompt incursion of the lengthwise supporting plate into the eye structure and proximity of the location whenthe load is applied, the pronounced stress concentration occurs in the failure zone of the eye, Figs. 10 and 11;

� The stress state level in the failure zone of the eye is very high; The values of uniaxial stresses are exceeding the yield stress;� The size of the high stress state zone is expanding with the increase of load intensity, Fig. 11.

3. Experimental procedure

The examination of the specimens sampled from the undeformed part of the plate revealed that the chemical compositionand mechanical properties match the quality class of the steel St 52.3. (DIN designation). The amount of the fracture energy

Fig. 8. Uniaxial stress field of the portal tie-rod support structure: (a) right side view and (b) left side view.

Fig. 9. Distribution of the left eye uniaxial stress: (a) internal surface, (b) external surface and (c) cross section in the fracture zone.

Fig. 7. External load of the eye.

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Fig. 10. Stress state of eyes: (a) left and (b) right.

Fig. 11. The size of the high stress state zone: (a) Load case H1 and (b) Load case HZS1.

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obtained on Charpy specimens with V notch (KV300) shows that basic material has not been normalized. Hardness in theweld metal and in the base material is appropriate, according to the standard requirements. Measured hardness in heat af-fected zones (HAZ) exhibits a significant scattering. The largest measured hardness values in HAZ are 10% larger than pre-scribed for tested material.

The characterization of the fracture of the right portal support tie-rod eyes is accomplished based on the experimentalresearches. Visual examination was carried out by means of unaided eye and stereoscopy. Metallographic examinationwas made using the LM Karl Zeiss Metaval H. Fractographic examinations were made using a JEOL JSM – 6460 SEM.

3.1. Results and discussion

The look-out of the crack surface of the right eye, Fig. 12a, whereupon one can see the characteristic chevron marks [4],Fig. 12b, indicates the static – brittle fracture. The initiation of the brittle fracture is in the T-joint area, Fig. 12c.

Fig. 12. Crack surface of the right eye: (a) general look, (b) detail and (c) site of the brittle fracture initiation.

Fig. 13. Crack surface of the left eye: (a) general look, (b) detail and (c) place of fatigue crack initiation.

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On the crack surface of the left eye, Fig. 13a, a typical zone of fatigue fracture is visible, Fig. 13b. In the area of T-joint, one can see the places of fatigue cracks initiation and ridge, framed in Fig. 13c, where several fatigue crack frontsmeet.

The transition from fatigue fracture to brittle fracture of the left eye is shown in Fig 14a, detail A, while the crack arrestzone [4,5] is clearly visible in Fig. 14b.

The structure of the base material for eyes is strip-type, ferrite-pearlit, Fig. 15a, while the structure of the weld metal isferrite-pearlit-dendrite, Fig. 15b. During investigations on T-joints, the following is observed: rough porosity in the weld me-tal, the root is not welded through and contains initial cracks and flaws, Fig. 16.

SEM results obtained in the left eye in the ridge zone, Fig. 13c – framed detail, are shown in Fig. 17. Detail A in Fig. 14a,presents the transition from fatigue fracture to brittle fracture and is shown in Fig. 18. In the edge area, Fig. 14a, detail B, theshear fracture is dominant, Fig. 19. Crack arrest zone, framed detail in Fig. 14b, is shown in Fig. 20 [6].

Fig. 15. The structure of the base material (a) and the weld metal (b).

Fig. 14. Left eye: (a) transition from fatigue to brittle fracture and (b) crack arrest zone of the brittle structure.

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Fig. 17. SEM micrographs of the left eye: (a) ridge and (b) detail with striations.

Fig. 16. Corner welding seam: (a) right eye and (b) left eye.

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Framed detail in Fig 12c is shown in Fig. 21a. The transition line, marked with the arrow in Fig. 21a, is shown in Fig. 21b.The right part of Fig. 21b looks similar to the fatigue lines, Fig. 17, but it is actually microstructure with ferrite bands from Fig.15a [5]. Carbide structure with deformed matrix can clearly be seen in Fig. 21c.

Fig. 19. Predominantly shear fracture in the edge area of the left eye.

Fig. 18. Transition from fatigue to brittle fracture of the left eye.

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4. Conclusion

Based on the presented results it is clear that the failure of BWE araised as the cumulative influence of factors that haveaffected the local distribution of stresses such as:

� Influence of the prompt incursion of the lengthwise supporting plate in the eye structure;� Influence of the proximity of the cross section affected by the load;� Influence of the plate thickness reduction from 40 mm down to 20 mm;� Influence of the hole of the eye plate;� Negative influence of the welding seam perpendicular to the direction of force action.

The high stress state of the designed support structure of portal tie-rods, conjugated with the detrimental effects of thewelding seam (not welded through root, porosity, inclusions) perpendicular to the force direction and dynamic character ofloads, is the principal reason of the end eye connection failure and BWE collapse.

Fig. 21. SEM micrographs of the right eye: (a) detail of the brittle fracture initiation, (b) look-out of the transition line and (c) structure over the transitionline.

Fig. 20. Crack arrest zone.

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Acknowledgments

The research reported in this paper is a part of the project 14052 supported by the Serbian Ministry of Science.

References

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