Fatigue Failure of a Rear Axle Shaft of an Automobile

Engineering Failure Analysis 13 (2006) 1293–1302 www.elsevier.com/locate/eng www.elsevier.com/locate/engfailanal failanal

Fatigue failure of a rear axle shaft of an automobile Osman Asi

*

Department of Mechanical Engineering, Usak Engineering Faculty, Afyon Kocatepe University, 64300 Usak, Turkey

Received 26 August 2005; accepted 31 October 2005 Available online 2 February 2006

Abstract

This paper describes the failure analysis of a rear axle shaft used in an automobile which had been involved in an accident. The axle shaft was found to break into two pieces. The investigation was carried out in order to establish whether the failure was the cause or a consequence of the accident. An evaluation of the failed axle shaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that the axle shaft fractured in reversed bending fatigue as a result of improper welding. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Axle shaft; Reversed bending fatigue; Welding; Failure analysis

1. Introduction

All vehicles have some type of axle shaft-differential assembly incorporated into the driveline. Rear wheel drive is a common form of engine-transmission layout used in automobiles. Understand that rear wheel drive means the power from the engine and the transmission goes to the rear wheels. In automobiles, axle shafts are used to connect wheel and differential at their ends for the purpose of transm transmitti itting ng power power and rotatio rotational nal motion. motion. In operati operation, on, axle shafts shafts are genera generally lly subjec subjected ted to torsio torsional nal stress stress and bendin bending g stress stress due to self-we self-weigh ightt or weights weights of compon component entss or possib possible le mis misalig alignmen nmentt between between journal bearings. Thus, these rotating components are susceptible to fatigue by the nature of their operation and the fatigue failures are generally of the torsional, rotating-bending, and reversed (two-way) bending type [1] [1].. Fatigue failures start at the most vulnerable point in a dynamically stressed area particularly where there is a stress raiser. The stress raiser may be mechanical or metallurgical in nature, or sometimes a combination of the two. Mechanical stress raisers are non-uniformities in the shape of the shafts such as step step change changess in diamete diameter, r, sharp sharp corner corners, s, keyways keyways,, groove grooves, s, thread threads, s, spline splines, s, press-fi press-fitted tted or shrink shrink-fit -fitted ted *

Tel.: +90 276 2634195; fax: +90 276 2634196. E-mail address: [email protected] [email protected]..

1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.10.006

O. Asi / Engineering Failure Analysis 13 (2006) 1293–1302

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members and surface discontinuities like seams, nicks, notches and machining marks. Metallurgical stress raisers may be quench cracks, corrosion pits, gross metallic inclusions, brittle second-phase particles, weld defects, or arc strikes [1]. Also, the microstructure of the shaft material plays a vital role not only in the initiation of fatigue failures but also during the progressive growth of the fatigue crack to cause failure of the component. In the present study, a rear wheel drive axle shaft used in an automobile has been examined after failure, which resulted in the vehicle suddenly pulling right and crashing into a tree near the road. Significant impact damage was caused to the vehicle. When the vehicle was examined it was found that the rear wheel drive axle shaft had broken into two pieces close to the wheel hub. The vehicle was reported to be approximately nine years old. The general appearances of the failed axle shaft in the as-received condition are shown in Fig. 1. A schematic diagram of joint configuration and location of fracture is shown in Fig. 2. 2. Experimental procedure

The failed axle shaft was inspected visually and macroscopically; care was taken to avoid damage of fractured surfaces. The failed axle shaft was subjected to optical microscopy, photo documentation, chemical analysis and micro-hardness measurement both at the failure zone and away from the failure zone. The fractured surfaces were ultrasonically cleaned and examined with the help of a scanning electron microscope (SEM) equipped with EDX facility. Conventional tensile tests carried out on specimens machined from the failed axle shaft.

Fig. 1. (a) General view of the failed axle shaft in the as-received condition. (b) Close-up of fracture location.


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Fig. 2. Schematic diagram of joint configuration and location of fracture.

3. Results and discussion

Visual examination showed that there were two fillet welded region between the axle shaft and the bearing locking ring, as shown in Fig. 3. The bearing locking ring was probably fixed to the axle shaft at two region by gas welding process during repairing or maintenance of the vehicle (not contained in the original design) to secure the roller bearing moving axially along the axle shaft as more clearly illustrated in Fig. 2. Whereas, roller bearings and bearing locking rings must be press-fitted or shrink-fitted onto the axle shaft seat. Also, visual examination of the axle shaft revealed that the fracture had been initiated close to the welded areas. This fracture location is to be expected because the highest stress concentration and high residual stress would be anticipated to occur in this region [1,2]. Typical macro-fracture appearance of the failed axle shaft is shown in Fig. 4. The fracture surface of the failed axle shaft exhibits heat-affected regions (HAZ) where crack initiation sites (see mark A and B), progressive flat fatigue fracture regions (FF), and the final fracture region produced by overload (OL), as shown in Fig. 4. The fracture appeared to be typical of reversed bending fatigue under conditions of a low stress with a high stress concentration [3]. The zone of final fracture was located between two areas of fatigue propagation suggesting the presence of bending forces. The surface area of final fracture was approximately 15% of the total fracture surface suggesting that the axle shaft was not overload. Also, this indicates that the failure is of high cycle-low stress type. Examination of the fracture surface disclosed that cracks had originated at two locations approximately 180° apart on the outer surface of the drive axle shaft and propagated toward the centre. The crack initiation regions could be seen clearly with the naked eye. The origin of the cracks was at the surface close to welded regions. Beach marks could be observed clearly on the fracture surface, which is a typical feature of fatigue failure [3].

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Fig. 3. Optical photograph showing the welded regions in the failed axle shaft.

Fig. 4. Typical macro-fracture appearance of the failed axle shaft. Fracture surface showing HAZ where crack initiation sites (A, B), fatigue failure region (FF), and final overload fracture region (OL).

Chemical analysis using a spectrometer test machine was carried out and the results are given in Table 1. The ranges for the composition of AISI 4140 steel are also included in Table 1. The chemical composition of the failed axle shaft is the same as that of AISI 4140 (42CrMo4) steel. The micro-hardness distributions across the axle shaft in the HAZ, in the surface layer not influenced by the weld, and in the core were measured using a Vickers hardness tester with 500 g load on a polished and unetched surface of the sample prepared close to the fracture region, and the results are given in Fig. 5. The hardness value in the core was measured to be 285 HV. Spectrum analysis and micro-hardness measurements revealed that the failed axle shaft had hardened and tempered surface. The axle shafts are often induction


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Table 1 Chemical composition of the failed axle shaft and AISI 4140 steel Element

Failed axle shaft

AISI 4140 steel (individual values are maximums)

%C %Si %Mn %Cr %Mo %S %P

0.412 0.219 0.856 0.988 0.157 0.0091 0.016

0.38–0.43 0.15–0.35 0.75–1.00 0.80–1.10 0.15–0.25 0.040 0.035

region not influenced by the weld

600 550 V H , s s e n d r a H

HAZ

500 450 400 350 300 0

1 2 Distance from surface, mm

3

Fig. 5. Hardness distribution in the HAZ and in the region not influenced by the weld.

hardened to give proper hardness and case depth. In regions not influenced by the weld, hardness values are within the range of the AISI 4140 steel. Micro-hardness measurements showed that the hardness of the HAZ was lower than that of the unaffected area from weld. It is well known that welding of steels in the hardened and tempered condition, is not recommended and should be avoided if at all possible, as the mechanical properties will be altered within the weld heat-affected zone. For instance, the heat-affected zone usually contains areas with lower hardness than that of the base metal, thereby causing stresses in the shaft due to a so-called metallurgical notch. To minimize these stresses, the material should be slowly cooled after welding, and then tempered immediately using a temperature slightly lower than the original tempering temperature [1]. Tensile test was carried out using round specimens of 5 mm gauge diameter and 25 mm gauge length fabricated from the centre of the failed axle shaft along the axial (longitudinal) direction. The results are listed in Table 2, and show that the mechanical properties met the technical demands [4]. Light optical metallographic analysis was carried out on the failed axle shaft. Samples for optical microscopy were prepared by grinding and polishing, and were etched. The microstructure of the failed axle shaft in the HAZ, unaffected region from weld and core region are shown in Fig. 6. While the unaffected region from weld exhibits tempered martensitic structure, the HAZ have a tempered bainitic + martensitic structure and inclusions. The causes of the inclusion in the welded metal were related to the welding process and the welding procedure [2]. Core microstructure is pearlitic + ferritic structure.

Table 2 Mechanical properties of the failed axle shaft and AISI 4140 steel Material

Yield strength (MPa)

Tensile strength (MPa)

Elongation (%)

Failed axle shaft AISI 4140 steel

770 >765

1060 980–1180

12 >11

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Fig. 6. Optical micrographs of the failed axle shaft showing: (a) tempered martensitic structure in regions not influenced by the weld; (b) tempered bainite + martensite structure and inclusions in the HAZ and (c) pearlitic + ferritic structure in the core. Arrows in (b) indicate inclusions (200·).

The fractured surfaces were examined with the help of a scanning electron microscope (SEM) in order to characterize the fracture micromechanism(s). On the fracture surface two main crack initiation sites were present. Magnified views of the HAZ and crack initiation region marked by A in Fig. 4 are shown in Fig. 7 at different magnification. As shown in Fig. 7, heat-affected zone (HAZ) has a bearing on the cause of failure. The cracks almost certainly started at the welded region. This is because there is both a high residual stress and a very bad stress concentration at this location [1,3,5]. It can be seen that there are ratchet marks on the fracture surface. The presence of ratchet marks indicates multiple origins and relatively high total stresses.


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Fig. 7. Magnified views of the HAZ and the crack initiation region marked by A in Fig. 4 at different magnification. Note the presence of ratchet marks indicates multiple origins and relatively high total stresses.

Ratchet marks can result from either high stress on the part or from high stress concentrations. The combination of many ratchet marks and a small overload zone indicates that the load was light, but there were high stress concentrations [3]. Semiquantitative chemical analysis was carried out by EDX attached to SEM on the fracture surface to qualitatively determine the axle shaft chemistry and to verify the presence of any other associated components. As shown in Fig. 8(a), SEM examination revealed some inclusions in the surface close to the crack initiation region. Fig. 8(b) shows the EDX analysis diagram of these inclusion indicated white box area in Fig. 8(a). As shown, the main elements of these inclusions are O, Ca, Si, Al, and Mg, which are not contained in the original material (except Si). So the main chemical compositions of the inclusions are probably SiO 2 –CaO–Al2O3 – MgO. Such components are often seen in improper welded location of steels. These also act as crack initiation

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Fig. 8. (a) SEM micrograph showing inclusions in the fracture surface. (b) The EDX result of rectangular area.

points and in the present case can be an additive factor. Most of the inclusions are oxides that form during weld cooling in the range of 2500–1800 K. Inclusion characteristics such as average composition, volume fraction, size, and sequence of oxide formation depends on weld metal composition and welding process [2,6]. Fig. 9 shows the progressive flat fatigue fracture region marked by FF in Fig. 4. These fatigue appearances are characteristic of steels under combined bending and torsional stresses. Fig. 10 shows typical fracture features of the final overload fracture region marked by OL in Fig. 4. It can be seen that the fracture is composed totally of brittle cleavage facets, reflective of the low toughness of the material [3].


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Fig. 9. SEM micrograph showing crack propagation region marked by FF in Fig. 4.

Fig. 10. SEM micrograph showing the final overload fracture region marked by OL in Fig. 4. Note classical cleavage facets with river pattern and cleavage steps.

From the above observations, it is clear that fatigue cracks have initiated at stress concentration points leading to fracturing of the axle shaft. In general, axle shafts fracture in the spline portion. This is because the maximum induced stress is at the root of the spline [7]. But if there are badly welded region, not only can this lead to a high stress concentration, but can also cause fatigue crack initiation during a short service time. When the local stress exceeds the material yield strength, it is possible to form a fatigue crack. Since the resistance of steel to fatigue initiation in proportional to its yield strength, the low properties of the steel in this case left it open to fatigue initiation. The area of final overload fracture is small, approximately 15% of total area, indicating that the material was adequate for the applied stresses. Examination of the failed axle shaft also revealed that preheating and postheating were not used. Lower hardness values in the HAZ and formation of the inclusions close to the crack initiation regions supported this hypothesis. Whereas, preheat treatment prior to welding and postheat treatment after welding of medium-carbon steels are necessary to control the hardness level in the HAZ and minimize residual stress. Besides, cooling rates must be more carefully due to inclusion formation. 4. Conclusion

This study was conducted on a failed rear wheel drive axle shaft used in an automobile. Spectrum analysis and micro-hardness measurement revealed that the failed axle shaft material was AISI 4140 steel as hardened

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and tempered condition. The composition, microstructure, hardness values of the base metal were found to be satisfactory and within the specification. Fractographic features indicated that fatigue was the main cause of failure of the axle shaft. It was observed that the fatigue cracks originated from welded areas. Results indicate that the axle shaft fractured in reversed bending fatigue as a result of improper welding. The present study clearly indicates that improper welding of hardened materials involves low ductility in the HAZ, stress concentration points, and inclusions in the structure that served as nuclei for the fatigue cracks. We therefore conclude that the failure was the cause of the accident. References [1] Wulpi DJ. Failures of shafts. In: Failure analysis and prevention. ASM metals handbook, vol. 11. Metals Park (OH): American Society for Metals; 1986. p. 459–82. [2] Winsor FJ. Welding of low-alloy steels. In: Welding, brazing and soldering. ASM metals handbook, vol. 6. Metals Park (OH): American Society for Metals; 1993. p. 662–76. [3] ASM metals handbook. Fatigue and fracture, vol. 19. Metals Park (OH): American Society for Metals; 1996. [4] ASM metals handbook. Properties and selection: irons, steels, and high-performance alloys, vol. 1. Metals Park (OH): American Society for Metals; 1990. [5] Jones DRH, Macdonald KA. Fatigue failure of a rotating chemical vessel. Eng Fail Anal 1996;3:77–93. [6] Hsieh KC, Babu SS, Vitek JM, David SA. Calculation of inclusion formation in low-alloy steel welds. Mater Sci Eng A 1996;215:84–91. [7] Nanaware GK, Pable MJ. Failures of rear axle shafts of 575 DI tractors. Eng Fail Anal 2003;10:719–24.

Fatigue Failure of a Rear Axle Shaft of an Automobile

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