某汽車操縱系統(tǒng)的轉(zhuǎn)向橫拉桿故障研究課程畢業(yè)設(shè)計外文文獻翻譯@中英文翻譯@外文翻譯

Engineering Failure Analysis 14 (2007) 895 902 Failure investigation of a tie rod end of an automobile steering system A.H. Falah *, M.A. Alfares, A.H. Elkholy Mechanical Engineering Department, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait Received 30 May 2006; accepted 19 November 2006 Abstract A failure analysis of a tie rod end of a sports utility vehicle (SUV) steering mechanism has been carried out in this study. The tie rod end is composed of two parts fitted together: a threaded part and an embracing part. Failure took place in the threaded part which is made of AISI 8620 steel. The vehicle had been in service for approximately two years and accumulated less than 30,000 km. An evaluation of the failed part was undertaken to determine the cause of failure and assess its integrity. Visual examination, photo documentation, chemical analysis, hardness measurement, and metallographic examination were all conducted. The failure surface was examined with the help of a scanning electron microscope (SEM) equipped with EDAX facility that determines chemical composition at desired locations within the part. Results indicated that the tie rod end had failed by fatigue with a crack initiation at the throat (minimum) area of the threaded part due to material deficiency and improper heat treatment. Keywords: Fatigue; Crack initiation/propagation; Material property 1. Introduction Tie rods connect the center link to the steering knuckle on automobiles with conventional suspension systems and recirculating ball steering gears, Fig. 1. On automobiles with MacPherson strut suspension and rack- and -pinion steering gears, tie rods connect the end of the rack to the steering knuckle, Fig. 2. A tie rod consists of an inner and an outer end as shown in both previous figures. Tie rods transmit force from the steering center link or the rack gear to the steering knuckle, causing the wheels to turn. The outer tie rod end connects with an adjusting sleeve, which allows the length of the tie rod to be adjust -able. This adjustment is used to set a vehicles toes, a critical alignment angle, sometimes referred to as the caster and camber angles. Fig. 1. Conventional suspension. Fig. 2. McPherson suspension with rack and pinion. A vehicles steering and suspension systems should be checked regularly, at least once a year along with a complete wheel alignment. A worn tie rod end, due to rubbing and wearing, can cause wandering, erratic steering and excessive tire wear. If tie rod replacement is necessary, a wheel alignment is also required because tie rod replacement disturbs the toe setting. Tie rods may fail in many different ways, and except for a slight increase in noise level and vibration, there is often no indication of difficulty until total failure occurs. In general, each type of failure leaves characteristic clues, and detailed examination often yields enough information to establish the cause of failure. The general types of tie rod failure modes include fatigue, impact fracture, wear and stress rupture 1. Several causes of tie rod end failure have been identified. These include poor design, incorrect assembly, overloads, inadvertent stress raisers or subsurface defects in critical areas, use of incorrect materials and/or manufacture process, and improper heat treatment 2. Tie rods in automobile suspension are generally robust and reliable components .However, problems do occur particularly due to manufacture error or driver misuse 3. The case under investigation involves failure of the outer part of an automobile tie rod. It was brought for analysis by the investigation bureau of the Ministry of Interior over a legal dispute between the driver of an SUV and a local car dealer who sold him the vehicle. The vehicle was driven for nearly two years and had registered less than 30,000 km. The driver claimed that while he was driving the vehicle, a sudden bang was heard and he lost control of the vehicle and hit the median rail guard of the highway. The vehicle was damaged and the driver, though still conscious, was slightly injured. He believed that there was something went suddenly wrong with a mechanical component of the vehicle and that caused the accident. The local car dealer, on the other hand, disagreed with the drivers scenario on grounds that the manufacturer produced thousands of such vehicles every year and they were, and still are, running fine all over the world without any reported serious failure. The dealer attributed the accident to careless driving behavior that resulted in a loss of control over the vehicle, which in turn hit the guard rail and led to vehicle damage. To settle the dispute, it was decided to undertake a thorough failure analysis investigation of all components of the steering mechanism to determine the cause of failure. All steering components were found intact though badly bent, except for the outer tie rod end which was fractured at the throated area of its threaded part. The embracing part of the outer tie rod end, however, was intact, except for two scars at its rim that could have happened when the threaded part broke into two pieces. The general appearance of the parts of the failed tie rod end is shown in Fig. 3a, where the two fragmented pieces of the threaded part were brought together to show how the tie rod end appeared before failure. Fig. 3b shows the two fractured parts separated. Fig. 4 gives the visual appearance of the embracing part and one fragment of the threaded part, both facing up. It is clear that fracture took place at the throat area of the threaded part where stress is expected to be high due to reduced cross sectional area and stress concentration. Further examination of the threaded part was conducted to determine the exact cause of failure. Fig. 3. Parts of fractured tie rod end (a) assembled and (b) separated. Fig. 4. Threaded part fracture surface and rim scars on embracing part. 2. Experimental procedure The failed threaded part of the tie rod end was inspected visually and macroscopically taking care to avoid damage of fractured surface. The failed threaded part of the tie rod end was ultrasonically cleaned prior to microscopic examination, photo documentation, chemical analysis and hardness measurement at the fracture surface and away from it. Scanning electron microscope (SEM) equipped with EDAX facility and an optical microscope were both used in the investigation. 3. Results and discussion Chemical analysis using atomic absorption spectrophotometry was carried out at several locations of the failed threaded part of the tie rod end and the average values of the test results are given in Table 1 along with the specified chemical composition. Spectrum analysis revealed that the threaded part material was AISI 8620 steel which is usually used for main automobile steering components. The low percentage of manganese and of chromium in the tested sample suggests that the final hardness of the part would be substantially reduced. On the other hand, the high percentage of nickel in the tested sample would result in lower toughness thereby compromising the mechanical property that is required to withstand impact loads resulting from bumpy roads. The surface hardness of the fractured tie rod end was measured to be 45.6 HRC. This suggests that the tie rod was not hardened properly, since hardness of tie rods, in general, is expected in the range of fifties for such applications. Table 1 Chemical composition of failed threaded part of tie rod end and AISI 8620 steel Fig. 5. SEM micrograph showing crack propagation region. It is evident from Fig. 4 that there exist two distinct areas on the fracture surface; one is smooth while the other is relatively rough. This is a typical fatigue fracture where crack originates at the edge of the smooth area and propagates towards the rough area, which represents final failure. On the other hand, the smooth area of the fracture surface is dominant as seen in Fig. 4. This indicates that the tie rod end took some time to break from the instant of crack initiation till complete fracture; i.e. a high cycle fatigue failure case. This proves that the cause of the SUV accident was a lack of strength and low resistance to impact loads in the material of the threaded part of the tie rod end that initiated a crack and then took some time to reach complete separation. A specimen from the fractured surface was metallographically prepared and observed in a scanning electron microscope (SEM). Significant fatigue cracks were observed at the smooth area of the fracture surface. The origin of cracks was at the edge of the smooth area of the threaded part throat, suggesting that the stresses were highest at this region. Fig. 5 shows crack propagation on the fracture surface. Beach marks can be observed clearly which is a typical feature of fatigue failure 4. The origin of the crack was surrounded by beach marks. Also, the fracture surface at the fatigue region had a smooth appearance with a rippled beach mark pattern which indicates that fatigue had initiated at one point of the circumference and then grown across the fracture area. A small area has a rough, jagged look where the last portion of the throat broke away. No corrosion media were found on fracture surfaces. Fig. 6. SEM micrograph showing typical brittle fracture observed in final stage of crack propagation zone. Brittle fracture was observed in the final stage at crack propaga tion as seen in Fig. 6. Fig. 7 shows the micrography of the rough area of the fracture surface where variation of grain size combined with shallow dimples is evident. The light lines surrounding the grains in the figure indicate intergranular cracking that is usually observed with brittle fracture. A close-up of such grains revealed both intergranular and transgranular cracking at some locations as shown in Fig. 8. Fig. 7. SEM micrograph of the last portion of fracture surface to break away. Fig. 8. Close-up showing both intergranular and transgranular cracking. Quantitative chemical analysis was carried out by EDAX attached to SEM on the fracture surface to verify the presence of any other associated components. No presence of any detrimental foreign elements was observed. Metallographic view of a sample cut from the threaded part after polishing and etching with 2% Nital solution is shown in Fig. 9. As shown, the microstructure consists of pearlite (finger print appearance) and ferrite (which appears dark). This is typical of unhardened low carbon steel. No abnormality was observed in the microstructure. From the above observation, it can be ascertained that failure was caused by high stress concentration at the throat area mainly due to inadequate chemical composition which contributed to reduction in material strength and lack of toughness. Under the cyclic loading produced from driving the SUV on regular and bumpy roads, fatigue cracks had initiated at these stress concentration points, namely the throat, leading to fracture of the part at the instant when the local stress exceeded the material strength. It should be mentioned, as well, that every so often, the increased noise and vibration due to crack propagation go unnoticed till failure unexpectedly occurs. In order to further improve the durability of the tie rod end to stand the applied loads, it is suggested to increase the cross-sectional area of the threaded part throat and to enlarge the fillet radius. Fig. 9. Micrograph of thread part showing Pearlite (Finger prints Matrix) and a Ferrite (sample was etched with 2% Nital). 4. Conclusion This study was conducted on a failed tie rod end of a SUV. Spectrum analysis and hardness measurement revealed that the failed part was AISI 8620 steel. The composition and hardness did not conform to the specified standard. Fractographic features indicated that fatigue was the main cause of failure of the tie rod end. On the fracture surface of the threaded part of the rod, the crack initiation region and beach marks could be clearly identified. It was observed that the fatigue crack originated from destructive areas in the vicinity of the throat and propagated from there. Failure analysis results indicate that the primary cause of failure of the tie rod was likely material deficiency. Formation of the crack initiation and propagation together with a final rupture within the fractured area supported this hypothesis and are, thus, in agreement with the claim of the SUV driver that the accident took place as a result of incompatible mechanical part, in this instance, the tie-rod end. References 1 Sheldon GL. Unusual Failure of an automobile steering component, In: Failure prevention and reliability conference, Dearborn, Mich., USA, 1983; p. 2731. 2 Kim HR, Seo MG, Bae WB. A study of the manufacturing of tie-rod ends with casting/forging process. J Mater Processing Technol 2002;125126:4716. 3 Sidders PA. Linked mulhead machines for operations on tie-rod ends. Mach Prod Eng 1970;30(December). p. 105462. 4 Fatigue and fracture. ASM handbook, Metals Park (OH): American Society for Metals, 1996, vol. 19. Engineering Failure Analysis 14 (2007) 895 902 某汽車操縱系統(tǒng)的轉(zhuǎn)向橫拉桿故障研究 A.H. Falah *, M.A. Alfares, A.H. Elkholy Mechanical Engineering Department, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait 收稿日期 ： 2006.5.30 刊登日期： 2006.11.19 摘要 ： 本論文對 運動功能型車 (SUV)轉(zhuǎn)向橫拉桿末端操控機制進行了故障分析，其中轉(zhuǎn)向橫拉桿末端由兩部分組成：即螺紋連接組件和抱合桿組件。我們研究的這個 SUV是行使了大約兩年，總里程 3萬公里，其螺紋組件的成分是美國鋼鐵學(xué)會規(guī)定的 8620號鋼，故障就是在這個地方發(fā)生的。本論文采取了多種方法對故障部分進行評測，從而確定其發(fā)生原因并評定其故障后的完整性，其中有可視化檢測、圖像文件系統(tǒng)、化學(xué)分析、硬度測試和金相檢驗。通過帶 EDAX(能量彌散 X 線分析儀 )裝置的電子掃描顯微鏡 (SEM)可以檢測故障表面的任意部位的化學(xué)成分。結(jié)論指出如果螺 紋組件的材料性能不好且處于不合適的熱環(huán)境下，其結(jié)合處的一個初始裂縫會因疲勞效應(yīng)導(dǎo)致轉(zhuǎn)向橫拉桿末端的故障。 關(guān)鍵詞：疲勞效應(yīng) 初始裂縫 /裂縫蔓延 材料特性 1. 引言 配備 普通懸架系統(tǒng)和循環(huán)滾珠式轉(zhuǎn)向裝置的汽車，其轉(zhuǎn)向橫拉桿將中連桿 和轉(zhuǎn)向關(guān)節(jié)連接起來，如圖 1。配備 MacPherson型支柱懸架系統(tǒng)和帶齒條齒輪轉(zhuǎn)向裝置的汽車，其轉(zhuǎn)向橫拉桿將齒條和轉(zhuǎn)向關(guān)節(jié)連接起來，如圖 2。而轉(zhuǎn)向橫拉桿如圖 1、2所示分為內(nèi)端和外端。 轉(zhuǎn)向橫拉桿將來自轉(zhuǎn)向中連桿或齒條的力傳向轉(zhuǎn)向關(guān)節(jié)，從而使車輪轉(zhuǎn)動。 轉(zhuǎn)向橫拉桿的外端連接一個調(diào)節(jié)套 以伸縮拉桿的長度，這樣就可以調(diào)整汽車的車輪前端的角度，有時用來調(diào)整其轉(zhuǎn)向節(jié)銷的后傾角或外傾角。 汽車的轉(zhuǎn)向和懸掛系統(tǒng)應(yīng)該定期地檢查，一年至少進行一次全面的前輪校正。一個使用超期轉(zhuǎn)向橫拉桿，由于摩擦和磨損作用，會導(dǎo)致行車方向控制的不穩(wěn)定和輪胎額外的磨損。如果需要更換轉(zhuǎn)向橫拉桿，那么相應(yīng)地也要進行前輪校正，因為前者的更換會影響后者的定位。 圖 1 普通懸架系統(tǒng) 圖 2 帶齒條齒輪的 MacPherson懸架系統(tǒng) 轉(zhuǎn)向橫拉桿可能會有 很多故障，但它除了在噪聲等級和振動的少許增加上有所體現(xiàn)外，在其他方面常常沒有故障的明顯現(xiàn)象，除非橫拉桿的所有的故障都發(fā)生了。通常情況下，每種故障都會有自己的特征跡象，如果經(jīng)過詳細的檢查一般都會得到足夠的信息來判斷故障原因。轉(zhuǎn)向橫拉桿的故障類型常見的有：疲勞老化、沖擊破壞、磨損和斷裂 1。其故障原因有一些已經(jīng)得到了確認，包括設(shè)計缺陷、不合理的裝配、超載、不固定的應(yīng)力源、嚴酷環(huán)境下的隱患、材料使用錯誤、缺乏嚴格的制造過程和非正常受熱 2。一般說來，汽車懸架的轉(zhuǎn)向橫拉桿要能提供足夠的扭矩和保持良好的可靠性。但 是，故障還是時常發(fā)生，尤其是因為制造上的問題和不正確的駕駛方式 3。 本論文所研究的情況針對某輛汽車轉(zhuǎn)向橫拉桿的外側(cè)連接部分的故障。這個故障引發(fā)了一名 SUV司機和賣給他汽車的供應(yīng)商的一場官司，內(nèi)政部的調(diào)查局也對轉(zhuǎn)向橫拉桿的故障進行了調(diào)查分析。汽車已經(jīng)行駛了將近兩年，行程還不到 3萬公里。這名司機原告陳述說當(dāng)他正在駕駛汽車的時候，突然聽到一聲巨響，然后汽車就失去了控制并撞到了公路中間的隔離護欄。汽車被撞壞，司機受到一些輕傷。他認為導(dǎo)致事故的原因是這輛汽車的機械部件突然出現(xiàn)了問題，?？墒牵囦N售商認為完全不是 這樣的，他的理由是汽車制造商每年都生產(chǎn)大量的此型號的汽車，可是從來沒有接到如此嚴重故障的報告。汽車銷售商認為是司機的不正確駕駛導(dǎo)致了汽車的失控，這樣才造成了汽車撞上護欄并損壞。為了解決這個官司，就必須要對汽車操縱裝置進行全面徹底的調(diào)查分析，確定故障原因。所有現(xiàn)場散落的操縱部件都被收集起來，它們雖然扭曲的比較嚴重，但是都很完整。只有轉(zhuǎn)向橫拉桿末端螺紋連接頭部分斷裂，但是橫拉桿連接端的抱合桿部分除了有一些劃痕之外仍然完好。轉(zhuǎn)向橫拉桿的故障部分的照片如圖 3a所示，這里將螺紋連接頭斷裂的兩部分拼接起來，復(fù)現(xiàn)轉(zhuǎn)向橫拉 桿故障之前的形狀。圖 3b是斷裂的兩部分各自的狀態(tài)。 圖 3 轉(zhuǎn)向橫拉桿斷裂部件 a) 拼接后的組合 b)各自獨立 圖 4顯示了抱合桿部分以及螺紋桿的斷裂面朝上放置的照片。從這里可以很清楚的看到斷裂是在螺紋桿的連接頭部分發(fā)生的。因為這里橫截面積太小并且是應(yīng)力集中的地方，顯然裂紋要在這里發(fā)生。需要對螺紋桿進一步地檢查來確定故障原因。 圖 4 螺紋桿的斷裂面和抱合桿邊緣的劃痕 2.實驗流程 通過肉眼檢查轉(zhuǎn)向橫拉桿發(fā)生故障的螺紋桿 部分，并要避免對斷面的破壞。然后對其進行超聲波清洗，再采取金相試驗，圖像分析處理、化學(xué)分析，最后在斷面的近端和遠端進行硬度測量。在調(diào)查中使用了裝備 EDAX的電子掃描顯微鏡和光學(xué)顯微鏡。 3.分析結(jié)果和討論 取故障螺紋桿的幾個部位，使用原子吸收分光光度測定法進行化學(xué)分析，在表 1處給出了測試結(jié)果的平均值，同時給出了明確的化學(xué)成分。經(jīng)過光譜分析顯示出螺紋桿的材料是 AISI 8620號鋼，這種鋼普遍應(yīng)用于汽車操縱組件的主體部分。測試樣本顯示錳和鉻的含量較低，這樣這部分組件的硬度就大大地減小了。同時，高含量的 鎳將會使組件的硬度大大提高，從而使其機械性能中和，這樣就能承受住崎嶇道路顛簸的沖擊力。測量顯示轉(zhuǎn)向橫拉桿斷裂表面的硬度是 45.6 HRC。這說明轉(zhuǎn)向橫拉桿的硬度還不夠，因為一般在這種應(yīng)用環(huán)境下，其硬度應(yīng)該是 50 HRC。 由圖 4可以證明，斷裂表面存在兩種截然不同的區(qū)域：一種是光滑的，而另一種相對粗糙。這是典型的疲勞斷裂，即從光滑區(qū)域邊緣首先產(chǎn)生裂紋，然后向粗糙部位蔓延，這導(dǎo)致了最終的故障。另一方面，由圖 4可以看出，斷裂面大部分都是光滑區(qū)域。這說明轉(zhuǎn)向橫拉桿疲勞裂紋的延伸持續(xù)了很長時間，直到最后的故障發(fā)生。這 是一個高周疲勞的典型案例。因此， SUV發(fā)生故障是轉(zhuǎn)向橫拉桿的螺紋桿的材料缺乏剛性，抵抗沖擊的能力較弱造成的，從而螺紋桿產(chǎn)生了一個初始的裂紋，經(jīng)過一段時間之后就導(dǎo)致了斷裂。