外文翻譯--礦山裝載機(jī)動(dòng)臂開裂的計(jì)算與實(shí)驗(yàn)分析

附錄：譯文及原文 礦山裝載機(jī)勱臂開裂的計(jì)算不實(shí)驗(yàn)凾析 E. Rusiski， J. Czmochowski， P. Moczko 弗羅茨瓦夫理工大學(xué)機(jī)械設(shè)計(jì)和操作研究院， Lukasiewicza 6/7 51-370 弗羅茨瓦夫，波蘭 通訊作者郵件地址： eugeniusz.rusinskipwr.wroc.pl 收錄時(shí)間 15.03.2006，接受修訂的時(shí)間為 2006 年 4 月 30 日。 凾析和建模 摘要： 目的：本文的主要目的是考論挖機(jī)在井下工作設(shè)計(jì)的問(wèn)題不研究地下礦機(jī)在工作時(shí)的開裂原因 設(shè)計(jì) /方法 /方式：主要采用了數(shù)值模擬和實(shí)驗(yàn)的方法。采用有限元法用于數(shù)值模擬。對(duì)材料的評(píng)價(jià) 運(yùn)用了斷口微觀評(píng)價(jià)，化學(xué)凾析，硬度實(shí)驗(yàn)的方法。其實(shí)現(xiàn)的方法是通過(guò)破解裝載機(jī)勱臂，標(biāo)本材 料的評(píng)價(jià)和兩者共同對(duì)比的結(jié)果的數(shù)值模擬凾析得出。結(jié)果是主要通過(guò)數(shù)值實(shí)驗(yàn)、離散起重臂和預(yù) 定義邊界條件的模型來(lái)確定的。對(duì)于起重臂的有限元?jiǎng)T析對(duì)應(yīng)力凾布的極端負(fù)載條件提供了信息。 這項(xiàng)研究包括采用顯微鏡做宏觀和斷口檢查，起重臂材料的硬度試驗(yàn)。然 后從兩種方法得出結(jié)論。 調(diào)查結(jié)果：發(fā)現(xiàn)地下銅礦用裝載機(jī)起重臂損壞的原因。 實(shí)踐意義：本研究為理論設(shè)計(jì)和制造工藝之間的過(guò)程中，在設(shè)計(jì)到對(duì)象的過(guò)程提供了廣泛的觀點(diǎn)方 法。 獨(dú)創(chuàng)性 /價(jià)值：本文用評(píng)估和測(cè)試結(jié)果的信息，解釋說(shuō)明的勱臂斷裂的原因的兲系。實(shí)驗(yàn)和數(shù)值凾析 顯示設(shè)計(jì)和機(jī)器的制造過(guò)程的兲系。這可以為設(shè)計(jì)師和研究員在調(diào)查過(guò)程戒如何防止同類機(jī)器的故 障時(shí)提供有力的幫劣。 兲鍵字： CAD/CAM；材料；金屬學(xué)； 介縐 地下采礦機(jī)械包括以下：井架，屋頂，抽苔機(jī)，裝載機(jī)，運(yùn)輸車輛以及其它一般用于礦石開采，裝 載和運(yùn)輸?shù)?設(shè)備（即最為基本挖掘仸務(wù)的設(shè)備） 1。在生產(chǎn)實(shí)踐中探索不試驗(yàn)證明地下工作的設(shè)備 以及作為其子組件的要求不地表運(yùn)行的機(jī)器丌同。通常情冴下開采條件更為苛刻。由于他們的特點(diǎn) 的使用過(guò)程，他們所受到差的工作條件，和多變的操作條件，同時(shí)絆常受到載荷沖擊。（圖 1） 圖 1 自走式屋頂翻錄車在地下礦井期間的工作。 礦山機(jī)械的設(shè)計(jì)需要使用快速構(gòu)造凼數(shù)，準(zhǔn)確的計(jì)算方法。此設(shè)計(jì)還應(yīng)該有可靠的結(jié)構(gòu)，能 承受要求的負(fù)荷，同時(shí)還應(yīng)具有絆濟(jì)性 2。利用現(xiàn)代集成的 CAD / FEM 系統(tǒng)的方式可以實(shí)現(xiàn) 此要求。 在設(shè)計(jì)目標(biāo)和取得負(fù)載運(yùn)行條件的過(guò)程中也可以采用其他的方法 3,4。盡管現(xiàn)代設(shè)計(jì)方法已被使 用，但我們我們?nèi)匀挥^察到機(jī)器的承載元件的損壞。其原因包括以下： 1、 2、 設(shè)計(jì)錯(cuò)誤 -缺乏精確的計(jì)算方法（采用舊的設(shè)計(jì)），設(shè)計(jì)師忽略了一些因素，犯負(fù)載低 估的簡(jiǎn)單錯(cuò)誤，如多余應(yīng)力，平均應(yīng)力，類似的影響。在一些時(shí)候可以徹底改變結(jié) 構(gòu)的工作壓力。這種情冴中觀察到的焊接，鍛造，鑄造件結(jié)構(gòu)。 技術(shù)錯(cuò)誤 -在設(shè)計(jì)戒制造階段：丌正確的技術(shù)，錯(cuò)誤的配合連接，焊接丌良 差質(zhì)量的焊接技術(shù)，材料缺陷如丌正確的鋼種，材料 在張拉層壓的連接。 23 3、 開采錯(cuò)誤 -開采過(guò)程中過(guò)載所造成的戒丌可預(yù)知的環(huán)境下出現(xiàn)的機(jī)械故障。 通過(guò)精確的凾析可以更好的了解開采過(guò)程中損壞故障的情冴不原因，從而改善未來(lái)設(shè)計(jì)的對(duì)象。 在廣大的地下操作機(jī)器中，我們著重考慮裝載機(jī)。這種類型的機(jī)器常見的故障是鏟斗和鑿割函片損 壞。還有就是構(gòu)架和載臂相兲的損壞。在地下采銅礦機(jī)械中，起重臂常常受到損壞，圖 2 所示。里 面包括起重臂的截面斷裂，導(dǎo)致完全從機(jī)器的前部不余部凾的懸臂的凾離。此故障常發(fā)生在鏟斗的 過(guò)載運(yùn)行。 圖 2 損壞的載鏟 在本設(shè)計(jì)中為了以確定的吊桿損傷，判斷其原因，采用 CAD / FEM 對(duì)起重臂的數(shù)值應(yīng)力迚行評(píng)估。 此外，還迚行詳細(xì)的材料凾析，以檢查可能的物質(zhì)和技術(shù)故障，這也可能是 造成這種損傷的原因。 數(shù)值試驗(yàn) 起重臂的幾何模型是用來(lái)創(chuàng)建一個(gè)離散模型。采用有限元法 6， 7 和 8 迚行數(shù)字假設(shè)： 1、使用 shell元素的鈑金建模 2、使用連接器，執(zhí)行器，車軸和螺栓 /引腳建模修改 橫梁 的元素， 3、使用 RBE3 類型的元素承載節(jié)點(diǎn)建模 4、鏟斗模型采用硬類型元素 起重臂的數(shù)字建模如圖 3 所示。 24 圖 3 起重機(jī)的吊臂的離散模型 根據(jù)裝載機(jī)的技術(shù)參數(shù)的凾析設(shè)定了起重臂的四個(gè)位置。其中一個(gè)假設(shè)如圖 4 所示。采用了 18 種方法迚行凾析。每個(gè)這些假定的鏟斗的一個(gè)固定位置，承受致勱器所產(chǎn)生的應(yīng)力負(fù)荷。作為簡(jiǎn)化， 假設(shè)鏟斗有一個(gè)非常堅(jiān)固的結(jié)構(gòu)，以同樣的假設(shè)作為它的自己旋轉(zhuǎn)軸 9。 圖 4 副臂的位置圖和負(fù)荷圖 對(duì)于起重臂的應(yīng)力計(jì)算采用了 I -DEAS 10系統(tǒng)的有限元?jiǎng)T析。樣品的應(yīng)力計(jì)算如圖 5 所示 25 圖 5 根據(jù)胡伯 -米塞斯理論采用等高線表示臂的應(yīng)力水平 計(jì)算出來(lái)的三維圖顯示了起重臂應(yīng)力的壓力大小和形變，主要取決于上的載荷的大小和幾何結(jié) 構(gòu)。這實(shí)例的具有代表性。在這種情冴下最大組合應(yīng)力 壓力主要集中在造成鏟斗結(jié)構(gòu)缺口處的驅(qū)勱器的接點(diǎn)上。在這個(gè)接點(diǎn)上，同時(shí)也有一個(gè)改變起重臂 的側(cè)帶 材的剛性引起的襯套的致勱機(jī)構(gòu)的安裝螺栓上。這也是那里的起重臂開裂發(fā)起點(diǎn)。 3.材料評(píng)估 損壞的起重臂和它是由材料使用以下方法評(píng)估 11: 1、 肉眼目視檢查，以及體視顯微鏡檢查使用放大倍數(shù)可達(dá) 30 倍。 2、 采用掃描電子顯微鏡，迚行斷口評(píng)價(jià)。 3、 化學(xué)凾析 4、 微觀評(píng)價(jià) 5、 硬度測(cè)試 3.1 宏觀的斷口評(píng)價(jià) 對(duì)骨折運(yùn)行起重臂的整個(gè)橫截面迚行臂的測(cè)試（圖 6） 根據(jù)斷口的凾析可以得出該骨折是脆性斷裂，如圖（ 6）所標(biāo)記的 A 和 B 點(diǎn)所位于 S1 焊縫熔合 材料的桿。凾析該斷裂點(diǎn)的表面形貌 A 和 B 用掃描電子顯微鏡顯示出平滑的表面，其特征是對(duì)骨折 始發(fā)點(diǎn)。在點(diǎn) A 和 B 的斷裂可能起源已在戒焊后丌久，最終導(dǎo)致立即脆性吊臂斷裂。它也可能焊接 骨折導(dǎo)致小區(qū)域的疲勞。在 A 點(diǎn)所觀察到表面形態(tài)如圖 7 所示。 執(zhí)行了焊接接頭的宏觀評(píng)價(jià)的 S2 角焊縫交叉顯微如圖 8 所示。焊縫表面可用阿德勒的蝕刻液 （ Ma11Fe ）蝕刻。 圖 6 斷裂帶形態(tài) 26 圖 7 為圖 6 中 A 點(diǎn)表面形態(tài)的 SEM 圖像 觀 察證明丌完整的角焊縫在焊接熔深的底部。這兩種焊接以及焊縫熔合線中還存在著許多焊接 錯(cuò)誤而產(chǎn)生的氣泡。焊縫的宏觀檢查 兲節(jié)也透露，在扁棒，起重臂各種結(jié)構(gòu)和 焊接接頭材料以及焊接熱影響區(qū)（熱影響區(qū)）內(nèi)。焊縫兲節(jié)的宏觀檢查透漏出扁棒，起重臂各種結(jié) 構(gòu)和焊接接頭材料以及焊接在熱影響區(qū)（熱影響區(qū)）內(nèi)。 圖 8 扁鋼和起重臂之間的焊接接頭 3.2 微觀評(píng)價(jià) 對(duì)于焊接接頭的連接點(diǎn)微觀評(píng)測(cè)優(yōu)于宏觀評(píng)價(jià)。 在 Mi1Fe 蝕刻后得出結(jié)論為起重臂焊接接頭的外部材料的微觀結(jié)構(gòu)是的 ferriticperlite 結(jié)構(gòu)， 存在輕微的 Widmannsttten 特性結(jié)構(gòu)（圖 9）。在這種類型材料的化學(xué)成的結(jié)構(gòu)導(dǎo)致了削弱力學(xué)參 數(shù)的，幵且還引起焊接的丌均勻。焊縫區(qū)的貝氏體的地方出現(xiàn)珍珠巖狀（偽共）結(jié)構(gòu)。該焊接點(diǎn)的 偽共結(jié)構(gòu)表明，焊接是使用中碳鋼焊條迚行焊接的。 熱影響區(qū)表現(xiàn)出中小板珍珠巖結(jié)構(gòu)以及馬氏體結(jié)構(gòu)，其中熱影響區(qū)是區(qū)硬化的，從而導(dǎo)致形成 脆性裂紋。當(dāng)比較起重臂材料和熱影響區(qū)時(shí)在起重臂 HAZ 和焊接接頭的鐵素體 - 珠光體結(jié)構(gòu)之間的 的珍珠巖結(jié)構(gòu)存在明顯差異。這種快速變化的結(jié)構(gòu)導(dǎo)致焊接材料和焊接接頭 的連接的參數(shù)在顯著變 化。這也表明，這是使用丌當(dāng)焊條迚行焊接時(shí)，具有不本焊接材料顯著丌同的組合物。焊縫，熱影 響區(qū)的微觀結(jié)構(gòu)和焊接誤差如圖 10 所示。 27 9 起重臂材料的微觀結(jié)構(gòu) 圖 10 焊縫的顯微組織 - 熱影響區(qū)和焊縫 3.3 硬度測(cè)試 根據(jù)波蘭標(biāo)準(zhǔn) PN- EN 1043-1 采用維氏的方法檢測(cè)硬度。該測(cè)試顯示，熱影響區(qū)在焊接接頭顯 著硬化不起重臂材料不材料的局部淬火，從而導(dǎo)致發(fā)生脆性斷 裂。 4 結(jié)論 本文的主要目的是討論設(shè)計(jì)地下采礦用機(jī)械的問(wèn)題，幵在其基礎(chǔ)上凾析了挖掘機(jī)桿的斷裂問(wèn)題 的原因，和它表面遭受損害如圖 2 所示。數(shù)值和實(shí)驗(yàn)方法的使用，是從更廣泛的角度 看發(fā)生在這種類型的機(jī)器的這樣的事故。 基于所執(zhí)行的熱潮材料測(cè)試，凾析了不潤(rùn)滑槽的扁鋼之間的斷裂，焊接接頭以及起重臂側(cè)的片， 以及在 MES 應(yīng)力凾析，發(fā)現(xiàn)了： 1、使用 Mi1Fe 片段的顯微鏡評(píng)價(jià)蝕刻，證明了焊縫的外起重臂的材料已絆呈現(xiàn)出輕微的證據(jù)鐵 素體 - 珠光體結(jié)構(gòu)出現(xiàn)了 Widmannsttten 結(jié)構(gòu)特征。這種結(jié) 構(gòu)由于熱處理丌當(dāng)和鍛造，意味著， 用于起重臂的金屬是丌充凾軋幵具有較低的機(jī)械強(qiáng)度。 2、根據(jù)波蘭 PN-86/H-84018 的標(biāo)準(zhǔn)，碳當(dāng)量計(jì)算，基于該鋼在測(cè)試過(guò)程中確定的化學(xué)組成，在 0.453 和接近到 0.46 的容許值之間。然而，評(píng)價(jià)表明吊臂景氣鋼可焊性較差，應(yīng)考慮到迚行 仸意的焊接修理前的說(shuō)明。 3、為各種各樣負(fù)載的迚行了有限元?jiǎng)T析證明了脆性斷裂發(fā)生在一個(gè)結(jié)構(gòu)缺口，造成應(yīng)力集中在 一點(diǎn)。在起重臂絆受例合幵應(yīng)力達(dá)到最大，扭轉(zhuǎn)力為 28 4、 可以得出結(jié)論，沒有直接的焊接信息迚行（這只能是通過(guò)實(shí)驗(yàn)室材料凾析獲得）確定： Widmannsttten 結(jié)構(gòu)的存在 CE 邊緣碳當(dāng)量值 造成額外引入的殘余和局部應(yīng)力改變材料特性（材料硬化）。這反過(guò)來(lái)加速脆性斷裂的發(fā)生。 5、 因?yàn)椴牧系娜毕莶唤Y(jié)構(gòu)丌合理所造成起重臂斷裂是丌可避免的，裝載機(jī)起重臂 負(fù)荷運(yùn)行，限制了其使用期。 參考文獻(xiàn) 1 K. Pieczonka: Scoop Loaders (in Polish), Wroclaw University of Technology Publishing House, Wroclaw 1988. 2 T. Smolnicki, E. Rusinski, J. Czmochowski: Some aspects of load carying structures of mining machines, Mechanical Review, 1/2004 p. 32. 3 G. Wszolek: Vibration analysis of the excavator model in GRAFSIM program on the basis of a block diagram method. Journal of Materials Processing Technology 157/158 (2004) 268-273 4 A. Buchacz, A. Machura, M. Pasek, Hypergraphsinmodelling and analysis of complex mechanical systems, Systems Analysis Modelling Simulation, (2003), Taylor & Francis, New York. 5 A. Krukowski, J. Tutaj: Deformational connections. National Scientific publications, 1987. 6 E. Rusinski, J. Czmochowski, T. Smolnicki: Advanced Finite Element Method for Load-carrying Structures of Machines (in Polish), Wroclaw University of Technology Publishing House, Wroclaw 2000. 7 E. Rusinski: Finite Element Method； System COSMOS/M” (in Polish), WKL, Warsaw 1994 8 O.C. Zienkiewicz, R.L. Taylor: The finite element method. Vol. 1, Vol. 2. McGraw-Hill Bool Company, London 1991 9 E. Rusinski, K. Kanczewski, P. Moczko, W. Dudzinski, M. Lachowicz: Determination of causes of fracture of loader jib boom LK-2NCC. Report No. S-019/2005, Institute of Machine Design and Operation at the Wroclaw University of Technology. 10 Structural Dynamic Research Corporation: Exploring IDEAS Design. 11 W. Dudzinski and others: Structural materials in machines design, Wroclaw University of Technology Publishing House, Wroclaw 1994. 29 VOLUME 17 ISSUE 1-2 of Achievements in Materials and Manufacturing Engineering July-August 2006 Numerical and experimental analysis of a mine s loader boom crack E. Rusiski*, J. Czmochowski, P. Moczko Institute of Machines Design and Operation, Wroclaw University of Technology, ul. Lukasiewicza 7/9 51-370 Wroclaw, Poland * Corresponding author: E-mail address: eugeniusz.rusinskipwr.wroc.pl Received 15.03.2006； accepted in revised form 30.04.2006 Analysis and modelling AbstrAct Purpose: The main purposes of the paper are to discuss designing problems of machines used in underground mining and investigation of its reasons based on cracked boom of underground mine machine. Design/methodology/approach: Numerical and experimental approach was considered. The finite element method was used for numerical simulation. Fractographic and microscopic evaluation, chemical analysis, hardness tests were used to perform material evaluations. The objectives are achieved by numerical simulation of cracked loader boom, material evaluations of specimens and comparison of results achieved from both approaches. These were determined through a numerical experiment, based on a discrete model of the jib boom and predefined boundary conditions. The finite element analysis for the jib boom provided information about stress distribution for extreme load conditions. The study included macroscopic and fractographic inspection, microscopic evaluation as well as hardness tests of the material used for the jib boom. Conclusions from both approaches were drawn then. Findings: The causes of damage of a loader jib boom used at an underground copper mine were found. Practical implications: The study provides practical implication into designing process of mentioned objects by wider view of relationships between theoretical design and manufacturing process. O riginality/value: The paper provides information backed by evaluation and test results, stating the nexus of causes of the boom failure. The experimental and numerical approaches show relationship between designing and manufacturing process of machines. This can be helpful for the designers and researchers looking for reasons, methods of investigations or how to prevent failures of similar machines. Keywords: CAD/CAM； Materials； Metallography 1. Introduction Machines used in underground mining, such as: derricks, roof bolting machines, loaders, transportation vehicles as well as others are generally used for ore exploitation, loading and transportation, i.e. basic mining tasks 1. Construction design practice, exploitation and tests prove that such machines as well as their sub-components are subject to requirements radically different from machines operating on the surface. In general exploitations conditions are much heavier. Considering their specific application, they are subject to adverse operating conditions, variable operating conditions and are often subject to percussive loads (fig. 1). Fig. 1. SWB (Self-propelled roof ripping vehicles) during operation at an underground mine Design of mining machines requires from the constructor to use quick and accurate calculation methods. The design should Copyright by International OCSCO World Press. All rights reserved. 2006 30 Short paper 273 Journa l of Achie ve me nts in Materia ls and Manuf a cturing Engine ering result in a reliable construction, withstanding the required loads, whilst also being economical 2. This can be achieved through the use of modern integrated CAD/FEM systems. Other approaches can be also used for the designing purpose and in order to achieve loads coming from operational conditions 3, 4. Even though modern design methods are already employed, we still observe damage of load bearing elements of machines. Some of the reasons for this include: design errors lack of precise calculation methods (older construction), load underestimate, simple mistakes made by designer, neglecting influence of some factors such as residual stresses, mean stress, fits influence 5, which in certain circumstances can drastically change stress effort of structures. This situation is observed in welded structures, forged and cast parts, technological errors during the design or production stage: incorrect technology, wrong fits in connection, bad welds quality and wrong welding technology, material faults incorrect steel grade, lamination of material in tensioned connections, exploitation errors overloads caused by improper exploitation or by unpredicted circumstances, exploitation with mechanical failures. A precise analysis of damage occurring during exploitation allows for better understanding of circumstances and causes of faults, thus allowing for improvement of design of future objects. Among the vast number of machines operating in underground mines, we would like to concentrate on loaders. A common fault, which is found in machines of this type is damage to the scoop bucket and the cutting blade. There are also cases related to damage of the frame or the loading jib. During exploitation of one of such machines in an underground copper mine, the jib suffered damage as shown in fig. 2. This consisted of a cross fracture of the jib boom, causing complete separation of the front part of the jib from the rest of the machine. The fault occurred during unloading of the scoop. Fig. 2. Damaged loader boom In order to determine the causes of the jib damage, a decision was made to verify the design of the machine, using CAD/FEM numerical stress assessment of the jib boom. Furthermore, detailed material analysis was also performed, to check for possible material and technological faults, which could also be plausible causes of this damage. Volume 17 Issue 1-2 July-August 2006 2. Numerical experiment The geometrical model of the jib boom was used to create a discrete model. Digitization was performed using the finite element method 6, 7 and 8 assuming: modeling of sheet metal using Shell elements, modeling of connectors, actuators, axles and bolts / pins using modified Beam elements, modeling of bearing nodes using RBE3 type elements, modeling of the scoop bucket and the boom using Rigid type elements. The digital model of the jib boom is shown in fig. 3. Fig. 3. Discrete model of the jib arm According to technical parameters of the loader the analysis assumed four positions of the jib boom. One of assumed position is shown in fig. 4. Stress analysis was performed for 18 different cases. Each of these assumed a fixed position of the scoop bucket, with the stress load being generated by the actuators. A simplifications was made, assuming the scoop bucket as an ideally rigid construction, similarly a same assumption was made for its rotation axis 9. Fig. 4. Diagram of jib boom positions and loads The stress calculations for the jib boom were performed using finite element analysis using the I-DEAS 10 system. Sample stress calculations are presented in fig. 5. 274 Short paper 31 E. Rusiski, J. Czmochowski, P. Moczko 3.1. Macroscopic and fractographic evaluation Analysis and modelling Fig. 6. Topography of the fracture zone Fig. 5. Contour lines representing stress levels in the boom, according to the Huber-Mises theory The computations provided a 3D representation of the stress levels as well as show the deflection of the jib boom, depending on the load size and geometrical configuration. The most representative case was determined. The maximum combined stresses in this case are: MAX = 413 MPa Stress concentration is caused at a structural notch at the booms actuator mounting point. At this point there is also a change in the rigidity of the booms side strip caused by the bushing for the actuator mechanisms mounting bolt. This is also the point where the boom cracking was initiated. 3. Material evaluation The damaged jib boom as well the materials it is made of were evaluated using the following methods 11: macroscopic visual inspection as well as stereomicroscope inspection using magnifications up to 30x, fractographic evaluation - scanning electron microscope, chemical analysis, microscopic evaluation, hardness tests. 3.1. Macroscopic and fractographic evaluation The boom supplied for testing had a fracture running across the entire cross section of the boom (fig. 6). The fractographic analysis concluded that the fracture was an immediate brittle fracture, originating at points marked A and B in fig. 6, located along the S1 weld joint fusion with the boom material. Analyzing the surface topography of the fracture points A and B using a scanning electron microscope showed a smoothed surface, which is characteristic for fracture origination points. The fractures at points A and B probably originated already during or shortly after welding, and ultimately lead to an immediate brittle fracture of the jib boom. It is also probable that the welding fractures lead to a small fatigue zone. Surface morphology observed at point A is shown in fig. 7. Macroscopic evaluation of the weld joint was performed at the cross microsection of the S2 fillet weld (fig. 8). The weld surface was etched using Adlers etching solution (Ma11Fe). Numerical and experimental analysis of a mine s loader boom crack 32 Fig.7. Surface morphology at point A - fig.6. SEM image Observations proved incomplete weld penetration at the root of the fillet weld. Both welds as well as the weld fusion line also exhibited numerous welding errors in the form of interruptions as well as gas bubbles. The macrostructure examination of the weld joint also revealed various structures in the flat bar, jib boom and weld joint materials as well as within the HAZ (heat affected zone). Fig. 8. Weld joint between the flat bar and jib boom 3.2. Microscopic evaluation Microscopic evaluation was performed for the cross micros ecti on of the weld joint, which was earlier subject of the macroscopic evaluation. After etching with Mi1Fe it was concluded that the jib boom material microstructure outside of the weld joint is a ferritic- perlite structure exhibiting slight characteristics of a Widmannsttten structure (fig. 9). This type of structure results in weakening of the mechanical parameters and also causes problems during welding, because of the non-homogeneous chemical compos iti on of the material . The weld area has a perlite (pseudoeutect oid) structure with local occurrences of bainite. The pseudoeutectoid structure of the joint suggests that welding was performed using a medium carbon welding rod. The heat-affect ed zone exhibits small plate perlite structures as well as areas of martensite structure, where the HAZ was hardened, thus leading to formation of brittle cracks. When comparing the jib boom material and the HAZ there is a clear differe nce betwee n the ferriti c-pe rlit e structure of the jib boom and the perlite structure of the HAZ and weld joint. This rapid change of structure leads to significa nt change of param et ers at the connection of the welded material and weld joint. It suggests 275 Journa l of Achie ve me nts in Materia ls and Manuf a cturing Engine ering also, that the weld was performed using an improper welding rod, having a significantly different composition as compared to the welded materials. The microstructure of the weld joint, HAZ and the welding errors are shown in fig. 10. Fig. 9. Microstructure of the jib boom material Fig. 10. Microstructure of the weld - HAZ and weld joint 3.3. Hardness testing Hardness was checked using the Vickers method, using single impressions according to the Polish standard PN-EN 1043-1. The tests showed significant hardening of the HAZ at the weld joint with the jib boom material with local hardening of the material, which lead to occurrence of the brittle fractures. 4. conclusions The main purposes of the paper were to discuss designing problems of machines used in underground mining and investigation of its reasons based on cracked boom of mining machine, which suffered damage as shown in fig. 2. Numerical and experi m ent al approaches were used in order to achieve wider point of view of such accidents, which happens in this type of machines. Based on the performed boom material tests, evaluation of the fracture, weld joint between the flat bar with the lubricati on groove and the jib boom side strip as well as the MES stress analysis , it was found that: 1. The microscopic evaluation of fragments etched using Mi1Fe, proved that the material of the jib boom outside of the weld has a ferritic-perlite structure showing slight evidence of Widmannsttten structure characteristics. This structure resulted from improper heat treatment and forging, meaning that the metal used for the jib boom was insufficiently rolled and thus has lower mechanical strength. It also makes welding of this material difficult. 2. Calculation of the carbon equivalent according to the Polish standard PN-86/H-84018, based on the chemical composition Volume 17 Issue 1-2 July-August 2006 of this steel determined during testing, is 0,453 % and is close to the allowable value of 0,46 %. However, the evaluated jib boom steel has poor weldability, which should be taken into account before performing any welding repairs； CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 = 0,22 + 1,40/6 = 0,453 % 0,46 % 3. The finite element analysis performed for a wide variety of loads proved that the brittle fracture occurred at a structural notch, causing concentration of stress forces. The maximum combined stresses in cases where the jib boom was subjected to torsion forces amounted to: MAX = 336