is

sign

achapphardtionmor exl us

conditions, variable operating conditions and are often subjectto percussive loads (Fig. 1).

order to achieve loads coming from operational conditions [3,4].

Even though modern design methods are already employed,

technological errors during the design or production stage:incorrect technology, wrong fits in connection, bad welds

A precise analysis of damage occurring during exploitationallows for better understanding of circumstances and causes offaults, thus allowing for improvement of design of future objects.

Automation in Construction 17

Paper presented on the 8th International Scientific Conference, ComputerAided Engineering, Polanica Zdrj 2006, Poland.Design of mining machines requires from the constructor touse quick and accurate calculation methods. The design shouldresult in a reliable construction, withstanding the required loads,whilst also being economical [2]. This can be achieved throughthe use of modern integrated CAD/FEM systems. Otherapproaches can be also used for the designing purpose and in

quality and wrong welding technology, material faults incorrect steel grade, lamination of material in tensionedconnections,

exploitation errors overloads caused by improperexploitation or by unpredicted circumstances, exploitationwith mechanical failures.Machines used in underground mining, such as: derricks,roof bolting machines, loaders, transportation vehicles as wellas others are generally used for ore exploitation, loading andtransportation, i.e. basic mining tasks [1]. Construction designpractice, exploitation and tests prove that such machines as wellas their sub-components are subject to requirements radicallydifferent from machines operating on the surface. In generalexploitations conditions are much heavier. Considering theirspecific application, they are subject to adverse operating

we still observe damage of load bearing elements of machines.Some of the reasons for this include:

design errors lack of precise calculation methods (olderconstruction), load underestimate, simple mistakes made bydesigner, neglecting influence of some factors such asresidual stresses, mean stress, fits influence [5], which incertain circumstances can drastically change stress effort ofstructures. This situation is observed in welded structures,forged and cast parts.1. IntroductionNumerical and experimental analys

Eugeniusz Rusiski, PrzemysawWrocaw University of TechnologyInstitute of Machines De

Abstract

The main purposes of the paper are to discuss designing problems of mcracked boom of underground mine machine. Numerical and experimentalsimulation. Fractographic and microscopic evaluation, chemical analysis,achieved by numerical simulation of cracked loader boom, material evaluaThese were determined through a numerical experiment, based on a discreteanalysis for the jib boom provided information about stress distribution foinspection, microscopic evaluation as well as hardness tests of the materia 2007 Published by Elsevier B.V.

Keywords: Analysis and modelling; Cad/cam; Materials; Metallography Corresponding author. Tel./fax: +48 71 3204097.E-mail addresses: eugeniusz.rusinski@pwr.wroc.pl (E. Rusiski),

przemyslaw.moczko@pwr.wroc.pl (P. Moczko),jerzy.czmochowski@pwr.wroc.pl (J. Czmochowski).

0926-5805/$ - see front matter 2007 Published by Elsevier B.V.doi:10.1016/j.autcon.2007.05.010of a mine's loader boom crack

Moczko , Jerzy Czmochowski

and Operation, Lukasiewicza 7/9, 50-371 Wroclaw, Poland

ines used in underground mining and investigation of its reasons based onroach was considered. The finite element method was used for numericalness tests were used to perform material evaluations. The objectives ares of specimens and comparison of results achieved from both approaches.del of the jib boom and predefined boundary conditions. The finite elementtreme load conditions. The study included macroscopic and fractographiced for the jib boom. Conclusions from both approaches were drawn then.

(2008) 271277www.elsevier.com/locate/autconAmong the vast number of machines operating in under-ground mines, we would like to concentrate on loaders. Acommon fault, which is found in machines of this type is damage

to the scoop bucket and the cutting blade. There are also casesrelated to damage of the frame or the loading jib. Duringexploitation of one of such machines in an underground coppermine, the jib suffered damage as shown in Fig. 2. This consistedof a cross fracture of the jib boom, causing complete separationof the front part of the jib from the rest of the machine. The faultoccurred during unloading of the scoop.

In order to determine the causes of the jib damage, a decisionwas made to verify the design of the machine, using CAD/FEM

2. Numerical experiment

A discrete model of the loader jib boom was built based onits geometrical representation, as shown in Fig. 3.

The geometrical model of the jib boom was used to create adiscrete model. Digitization was performed using the finiteelement method [68] assuming:

modeling of sheet metal using Shell elements, modeling of connectors, actuators, axles and bolts/pins usingmodified Beam elements,

modeling of bearing nodes using RBE3 type elements, modeling of the scoop bucket and the boom using Rigidtype elements.

Fig. 1. SWB (Self-propelled roof ripping vehicles) during operation at anunderground mine.

272 E. Rusiski et al. / Automation in Construction 17 (2008) 271277numerical stress assessment of the jib boom. Furthermore,detailed material analysis was also performed, to check forpossible material and technological faults, which could also beplausible causes of this damage.Fig. 2. Damaged loader boom.The digital model of the jib boom is shown in Fig. 4.Technical parameters of the loader:boom erection is performed using two hydraulic actuators:

of the piston 160 mm of the piston rod 80 mm total extension s1=630 mm length of the actuator l1=1170+s1the scoop bucket is alsocontrolled using two hydraulic actuators:

of the piston 140 mm of the piston rod 70 mm total extension s2=470 mm length of the actuator l2=1036+s2 weight of the scoop bucket m=2450 kg capacity of the scoop bucket V=3.5 m3 average ore density s=2000 kg/m3 hydraulic fluid pressure p=155 bar

The analysis assumed four positions of the jib boom, asshown in Fig. 5. Stress analysis was performed for 18 differentcases. Each of these assumed a fixed position of the scoopbucket, with the stress load being generated by the actuators. Asimplifications was made, assuming the scoop bucket as anFig. 3. Geometrical model of the loader jib boom.

ideally rigid construction, similarly a same assumption wasmade for its rotation axis [9].

The stress calculations for the jib boom were performedusing finite element analysis using the CAD-FEM system.Sample stress calculations are presented in Fig. 6.

Fig. 4. Discrete model of the jib arm.

E. Rusiski et al. / Automation in CFig. 5. Diagrams of jib boom loads.The computations provided a 3D representation of the stresslevels as well as show the deflection of the jib boom, dependingon the load size and geometrical configuration. The mostrepresentative case occurs when the scoop bucket was loadedunevenly, causing significant torsion of the jib boom. Thehighest stress level occurs at the point where the scoop bucket ismounted to the boom, as shown by setup C (Fig. 5), when the jibboom and scoop bucket are being lifted. The maximumcombined stresses in this case are:

rMAX 413 MPa

Stress concentration is caused at a structural notch at theboom's actuator mounting point. At this point there is also achange in the rigidity of the boom's side strip caused by thebushing for the actuator mechanism's mounting bolt. This isalso the point where the boom cracking was initiated.

3. Material evaluation

Fig. 6. Contour lines representing stress levels in the boom, according to theHuber-Mises theory.

273onstruction 17 (2008) 271277The damaged jib boom as well the materials it is made ofwere evaluated using the following methods [10]:

macroscopic visual inspection as well as stereomicroscopeinspection using magnifications up to 30,

fractographic evaluation using a JEOL-JSM 5800 LVscanning electron microscope equipped with an EDXLINK-ISIS 300 microanalysis system,

chemical analysis, which included five elements: C, Si, Mn,S, P,

microscopic evaluation using a NEOPHOT 32 microscope,performed according to the Polish standard PN-EN 1321,using a Visitron Systems digital camera,

hardness tests using a Vickers-Zwick tester, according to theVickers method.

3.1. Macroscopic and fractographic evaluation

The fragment of boom supplied for testing was made of a50 mm metal sheet and had a fracture running across the entire

Fig. 9. Morphology of the fracture surface at the immediate brittle fracture zone.Fig. 7. Topography of the immediate brittle fracture zone of the jib boom. Spots

274 E. Rusiski et al. / Automation in Construction 17 (2008) 271277cross section of the boom. The fracture ran from the edge of thefillet weld (Fig. 7) used to attach an additional flat bar with alubrication groove. The fracture surface was corroded, thereforebefore fractographic analysis it was chemically cleaned usingFosol.

The fractographic analysis concluded that the fracture was animmediate brittle fracture, originating at points marked A and Bin Fig. 7, located along the S1 weld joint fusion with the boommaterial. Analyzing the surface topography of the fracturepoints A and B using a scanning electron microscope showed asmoothed surface, which is characteristic for fracture origina-tion points. The fractures at points A and B probably originatedalready during or shortly after welding, and ultimately lead toan immediate brittle fracture of the jib boom. It is also probablethat the welding fractures lead to a small fatigue zone. However,because of corrosion it is impossible to observe a clear fatigueline, which would confirm the presence of the fatigue zone.Surface morphology observed at point A is shown in Fig. 8. Theimmediate brittle fracture zone is shown in Fig. 9.

Because of corrosion it is impossible to observe a clear

A and B mark the fracture origination points, located at the S1 weld joint fusion.fatigue line, which would confirm the presence of the fatiguezone. Macroscopic evaluation of the weld joint was performed

Fig. 8. Surface morphology of the fracture at point A. Zone with smoothedmaterial grain. SEM image.at the cross microsection of the S2 fillet weld (Fig. 10). Theweld surface was etched using Adler's etching solution.

Observations proved incomplete weld penetration at the rootof the fillet weld. Both welds as well as the weld fusion line alsoexhibited numerous welding errors in the form of interruptionsas well as gas bubbles.

A macroscopic image of the weld joint showing thesewelding errors is presented in Fig. 10. The macrostructureexamination of the weld joint also revealed various structures inthe flat bar, jib boom and weld joint materials as well as withinthe HAZ (heat affected zone). The welding errors lead toweakening of the weld joint, however they are not a direct causeof the brittle fracture, which occurred. Occurrence of theseerrors is caused by improper welding technique.

3.2. Microscopic evaluation

Microscopic evaluation was performed for the crossmicrosection of the weld joint, which was earlier subject ofthe macroscopic evaluation.Fig. 10. Weld joint between the flat bar and jib boom. Clearly visible differencesin structure of the welded materials, the weld and the HAZ, along with weldingerrors in the form of incomplete weld penetration and trapped gas bubbles.Regular microscope.

Fig. 11. Microstructure of the jib boom material. Ferriticperlite structureshowing evidence of Widmannsttten structure characteristics. Etched using

Fig. 13. Microstructure of the weld joint. Non-homogenous pseudoeutectoidstructure with local occurrences of bainite, characteristic for rapidly cooledpseudoeutectoid steel. Etched using Mi1Fe, regular microscope.

275E. Rusiski et al. / Automation in Construction 17 (2008) 271277After etching with Mi1Fe it was concluded that the jib boommaterial microstructure outside of the weld joint is a ferriticperlite structure exhibiting slight characteristics of a Widmann-sttten structure. This structure results from improper heattreatment and forging (rolling of the steel), which could be aconsequence of the significant thickness of the boom element(50 mm).

This type of structure results in weakening of the mechanicalparameters and also causes problems during welding, because ofthe non-homogeneous chemical composition of the material.The microstructure of the added flat bar is ferriticperlite,characteristic of low carbon steel. The microstructure of the jibboom is shown in Fig. 11, whilst of the flat bar in Fig. 12. Theweld area has a perlite (pseudoeutectoid) structure with local

Mi1Fe, regular microscope.occurrences of bainite. The pseudoeutectoid structure of the jointsuggests that welding was performed using a medium carbonwelding rod. The weld joint microstructure is shown in Fig. 13.

Fig. 12. Microstructure of the additional flat bar material. Fine grain ferriticperlite structure. Etched using Mi1Fe, regular microscope.The heat-affected zone exhibits small plate perlite structuresas well as areas of martensite structure, where the HAZ washardened, thus leading to formation of brittle cracks. The weldjoint and HAZ have similar structure, differing only in grainorientation, which is the effect of weld joint crystallizationdirection. When comparing the jib boom material and the HAZthere is a clear difference between the ferriticperlite structureof the jib boom and the perlite structure of the HAZ and weldjoint. This rapid change of structure leads to significant changeof parameters at the connection of the welded material andweld joint. It suggests also, that the weld was performedusing an improper welding rod, having a significantly differentcomposition as compared to the welded materials. Themicrostructure of the weld joint, HAZ and the welding errors

are shown in Fig. 14. The HAZ martensite structure is shown inFig. 15.

Fig. 14. Microstructure of the welded material, HAZ and weld joint. Ferriticperlite structure of the welded material and perlite structure of the HAZ and weldjoint. Evidence of welding errors in the form of trapped gas bubbles andincomplete weld penetration. Etched using Mi1Fe, regular microscope.

investigation of its reasons based on cracked boom of miningmachine, which suffered damage as shown in Fig. 2. Numericaland experimental approaches were used in order to achievewider point of view of such accidents, which happens in thistype of machines.

Based on the performed boom material tests, evaluation ofthe fracture, weld joint between the flat bar with the lubricationgroove and the jib boom side strip as well as the MES stressanalysis, it was found that:

1. The material used for the jib boom has a chemicalcomposition required for S355JR high-strength low alloystructural steel, complying with the harmonized PN-EN10025:2002 (old name: 18G2 according to PN-86/H-84018).

2. The microscopic evaluation of fragments etched using

276 E. Rusiski et al. / Automation in Construction 17 (2008) 2712773.3. Hardness testing

Hardness was checked using the Vickers method, usingsingle impressions according to the Polish standard PN-EN1043-1, with a weight of 5 kg (49 N). The hardness tests wereperformed at the earlier evaluated (macro/microscopic evalua-tion) weld joints shown in Fig. 10. The test results are presentedin graphical form in Fig. 16. The tests showed significanthardening of the HAZ at the weld joint with the jib boommaterial. The hardness of both heat affected zones of fillet weldS1 and the weld joint parallel to it on the other side of the flat barranged from 435 to 466 HV5. This result significantly breachesthe maximum allowable value for weld joints, given as 350 HV.Such hardness in the heat affected zone along with the areas ofmartensite structure confirm local hardening of the material,which lead to occurrence of the brittle fractures.

Fig. 15. Magnification of HAZ, showing evidence of martensite structure.Etched using Mi1Fe, regular microscope.4. Conclusion

The main purposes of the paper were to discuss designingproblems of machines used in underground mining and

Fig. 16. Graphical presentation of the hardness tests of the jib boom weld joints.Mi1Fe, proved that the material of the jib boom outside ofthe weld has a ferriticperlite structure showing slightevidence of Widmannsttten structure characteristics. Thisstructure resulted from improper heat treatment and forging,meaning that the metal used for the jib boom wasinsufficiently rolled and thus has lower mechanical strength.It also makes welding of this material difficult.

3. Calculation of the carbon equivalent according to the Polishstandard PN-86/H-84018, based on the chemical composi-tion of this steel determined during testing, is 0.453% andis close to the allowable value of 0.46%. However, theevaluated jib boom steel has poor weldability, whichshould be taken into account before performing any weldingrepairs;

CE CMn=6 Cr Mo V =5 Ni Cu =15 0; 22 1; 40=6 0; 453% 0; 46%

4. The finite element analysis performed for a wide variety ofloads proved that the brittle fracture occurred at a structuralnotch, causing concentration of stress forces (Fig. 17). Themaximum combined stresses in cases where the jib boomwas subjected to torsion forces amounted to:MAX=336413 MPa.Fig. 17. Area of concentrated stress.

5. It was concluded that welding performed without informa-tion (which could only have been obtain through laboratorymaterial analysis) about:

presence of Widmannsttten structures borderline CE Carbon equivalent valuecaused introduc-tion of additional residual stress and local change inmaterial characteristics (material hardening). This in turnaccelerated the occurrence of brittle fracture.

6. The jib boom fracture was inevitable, because the material isdefective and has an improper structure. The loader jib boomthus has limited fatigue life.

References

[1] K. Pieczonka, Scoop Loaders (in Polish), Wrocaw University ofTechnology Publishing House, Wrocaw, 1988.

[2] Smolnicki T., Rusiski E., Czmochowski J., Some aspects of load caryingstructures of mining machines, Mechanical Review (Przegld Mechan-iczny), 1/2004 p. 32.

[3] G. Wszoek, Vibration analysis of the excavator model in GRAFSIMprogram on the basis of a block diagram method, Journal of MaterialsProcessing Technology 157/158 (2004) 268273.

[4] A. Buchacz, A. Machura, M. Pasek, Hypergraphs in modelling andanalysis of complex mechanical systems, Systems Analysis ModellingSimulation, Taylor & Francis, New York, 2003.

[5] A. Krukowski, J. Tutaj, Deformational connections, National Scientificpublications, 1987.

[6] E. Rusiski, J. Czmochowski, T. Smolnicki, Advanced Finite ElementMethod for Load-carrying Structures of Machines (in Polish), WrocawUniversity of Technology Publishing House, Wroclaw, 2000.

[7] E. Rusiski, Finite Element Method; System COSMOS/M (in Polish),WK, Warsaw, 1994.

[8] Zienkiewicz O.C., Taylor R.L.: The finite element method. Vol. 1, Vol. 2.McGraw-Hill Bool Company, London 1991.

[9] Rusiski E., Kanczewski K., Moczko P., Dudziski W., Lachowicz M.,Determination of causes of fracture of loader jib boom K-2NCC. ReportNo. S-019/2005, Institute of Machine Design and Operation at theWrocaw University of Technology.

[10] W. Dudziski, et al., Structural materials in machines design, WrocawUniversity of Technology Publishing House, Wrocaw, 1994.

277E. Rusiski et al. / Automation in Construction 17 (2008) 271277

Numerical and experimental analysis of a mine's loader boom crackIntroductionNumerical experimentMaterial evaluationMacroscopic and fractographic evaluationMicroscopic evaluationHardness testing

ConclusionReferences