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1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 141 FAILURE OF FRONT SHOCK ABSORBER OF A MOTORCYCLE Yogesh Mahajan, A.A. Likhite, D.R. Peshwe Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology, Nagpur, Maharashtra, India ABSTRACT In vehicle shock absorber is a critical component and reduces the effect of traveling over rough ground, leading to improved ride quality and vehicle handing. Front Fork pipe is a vital component in the suspension assembly and failure can cause fatal incidences. This paper presents the classical failure analysis of a front fork pipe. The fractured surfaces as well as the surface of the fork pipe were examined in a scanning electron microscope at suitable magnifications. Optical microscopy was performed to evaluate the basic microstructure of the as received material. Detailed electron microscopic studies have indicated that the failure was due to the single overload. The presence of inclusions is responsible for lowering the strength of the steel. Keywords: Failure, Front Fork Pipe, Inclusion, SEM, Shock Absorber. I. INTRODUCTION Motorcycle fork connects a motorcycle’s front wheel and axle to its frame. It typically incorporates the front suspension and front breaks . A front suspension is a mechanical device designed to damp shock impulse, and convert kinetic energy to another form of energy usually thermal energy which can be easily dissipated inside the viscous fluid. Various shock absorbers are designed so as to damp the shock impulse in effective manner. Pneumatic, hydraulic and electromagnetic shock absorbers are available in the market. The motorcycle is a spatial mechanism composed of four rigid bodied: the rear assembly, front assembly, the front wheel, and the rear wheel . In front assembly of a vehicle shock absorbers reduce the effect of traveling over rough ground, leading to improved ride quality and vehicle handing. An automobile shock absorber contains spring-loaded check valves and orifices to control the flow of oil through an internal piston. This provides a cushioning action so road shocks INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN ENGINEERING AND TECHNOLOGY (IJARET) ISSN 0976 - 6480 (Print) ISSN 0976 - 6499 (Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME: www.iaeme.com/ijaret.asp Journal Impact Factor (2014): 7.8273 (Calculated by GISI) www.jifactor.com IJARET © I A E M E
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 142 have a minimal effect on the occupants and load in the vehicle. Mono suspension was introduced in the year 1970’s by YAMAHA motors . Shock absorbers help cushion vehicles on uneven roads. Front fork pipe has an application in shock absorber of motorcycle or scooter. In motorcycle’s fork tubes are employed in pairs. Usually Cam drum testing machines are designed with the global quality standards to test the front shock absorber of a motorcycle or scooter. Advanced machine can test four set of shock absorber at one time. The operation carried out on cam drum testing machine is to perform the endurance test on the shock absorbers for specific period of time at a given revolution per minute (RPM). The objective of this study was to examine the fracture surface and to carry out fractographic analysis to find out the reason for failure. II. FAILURE HISTORY The customer returned the fractured fork pipe samples after premature failure during testing. Fig. 1a shows the mechanism of testing the front shock absorber of a motorcycle. The shock absorber experiences repeated bumps or shocks while testing by a typical mechanism fitted at the bottom. There is an upper clamp bracket which holds the fork pipe. The design of the system as indicated in fig.1 shows the nature of loading and stress experienced by the system. The load experienced by the front fork is purely shock load. In one revolution it experiences the four bumps/shocks. During inspection the speed of revolution used was 120 RPM. To pass the test it has to run for 100 hr minimum cycle without failure or leakage. Assembly has to withstand 2880000 numbers of bumps minimum so as to qualify the test. Fig.1: a. Mechanism for testing Front fork pipe system of motorcycle or scooter by Cam drum testing machines, b. Magnified view of the front fork pipe of location A showing area of the crack and forces acting it Failures were reported during testing on Cam Drum testing machine. The details of failed fork pipes are indicated in the table 1. During testing it is found that one Left hand (LH) fork pipe was cracked after 20.50 hrs and other after 47.20 hrs. One failure was reported in right hand (RH) fork pipe after 23.54 hrs. Under clamp Mechanism for giving bumps / shocks Front fork Tensile component Resultant Compressive component Crack location (a) (a) Location A (b)
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 143 Table 1: Failure history of fork pipe Sr. No. Description Hrs: Mins 1 LH fork pipe (F 01/LH) 20:50 2 LH fork pipe (F01/LH) 47:20 3 RH fork pipe (F01/RH) 23:54 III. EXPERIMENTAL PROCEDURE The material under investigation was received after failure during testing. The detailed metallurgical investigation was carried out on the fractured fork pipe which includes visual examination of fractured surface and near area of failed components. Chemical analysis and mechanical tests such as hardness and tensile test were performed on the failed components. Microstructure analysis by optical microscopy and scanning electron microscopy were carried out. 1. Visual examination From the visual examination it reveals that the tube fails from 5mm below the bracket clamping edge. It is also observed that the failure started from the back side of front fork as indicated in the fig.1b. Dent marks were observed on the fork pipe just below the bracket clamp which can be seen from the removal of powder coating. The sample analyzed shows that failure occurred by the crack originating from the welded region. Slight burr is observed on the forging of under bracket. A white plated layer was observed on the cross sectional area of fork pipe as indicated in fig. 2. Visual examination of the sample revealed a generally smooth surface with small amount of deformation. However, the fractured portion of the sample revealed a brighter surface typical of brittle fracture. Fig.2: Photograph of cross section of the pipe indicating Ni hard Cr-plating on the surface at 23X magnification 1mm White plated layer
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 144 2. Chemical Analysis Chemical analysis was done and presented in the table 2 given below. It contains carbon, silicon and manganese. Sulfur and phosphorus are within the limits. From the chemical analysis, it is found that fork pipe material conforms to the specification Gr.SAE 1541. Table 2: Chemical analysis of fork pipe % C % Si % Mn % S % P Specified 0.36-0.44 0.25 Max 1.35-1.65 0.05 Max 0.04 Max Observed 0.41 0.21 1.39 0.0036 0.019 3. Mechanical Test Hardness test and Tensile strength were taken on the failed components. Hardness observed is in the range of 345-395 Hv on the surface of fork pipe. Samples from the failed components are taken and inspected for tensile test. Tensile strength of the tubes ranges from 87 to 89 Kgf/mm2 conforms to the specification. 4. Microscopic observation Microstructure shows the cold drawn structure of pearlite and ferrite matrix grains as shown in fig.3. The microscopic examination indicates the presence of Globular oxide type - D inclusions, thin series/ thick series - 1.5 as indicated in fig.4 Fig.3: Microstructure of fork pipe Fig.4: Inclusion content in fork pipe 5. Fracture observation The scanning electron microscopy was performed using a JEOL SEM model JSM 6380A. Different fractured surfaces are shown in SEM figs. 5, 6 and 7. The SEM image as indicated in fig.5 shows microvoid instability fracture which is fibrous in nature. Area marked by circles in fig.5, distributed at large on the fracture surface can be seen.
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 145 Fig.5: Fracture indicating Microvoid instability fracture Fig.6 is the areas encircled at higher magnification of fracture surface shown in figure no 5. The evidence of entrapment of a nonmetallic particles/ inclusions is seen in SEM image fig.6 while a void can be seen in the fig. 7 and 8. Fig.6: Fractrograph at high magnification indicating presence of oxide inclusions All the three SEM images 5, 6 and 7 though indicative of fibrous fracture are without any significant evidence of necking or plastic deformation. This confirms that though the material is ductile, fracture has occurred due to single overload with multiple origins at the locations of microvoids or entrapments. The arrows indicate the probable stress component at the time of single overload.
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 146 Fig.7: Fractrograph at high magnification Microvoids are observed under Scanning Electron Microscope (SEM) in the sample which shows the presence of oxides as indicated in fig. 9. Fig.8: Micrograph showing presence of microvoids
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 147 Fig.9: SEM Micrograph at high magnification IV. RESULTS AND DISCUSSION From the nature of loading and above observations, it is clear that the portion of Front fork pipe near Under bracket is under highly compressive condition while the other portion has developed equivalent tensile stresses. The conditions when this tensile component of the system exceeds the fracture strength of material, it leads to failure. The above crack nucleating condition can be created at voids / entrapments, where the stress concentration at the interface can lead to crack initiation, just below the under bracket due to single overload. The presence of Microvoids along with the entrapment of oxide particles in the steel, cleanliness of steel, weld seam offer least resistance for the propagation of crack. This is the causative factor for the initiation of crack during single overloading. From the observations it is clear that the failure is due to the uneven pressure on the assembly. The uneven pressure may be due to the uneven clamping pressure, or burr at Under bracket location. Dent mark on the under bracket indicate the excessive/uneven clamping pressure (fastening torque – should be 250-350 kgf cm) on the assembly which may be the causative factor for the initiation of crack on Front Fork pipe below the under bracket. Irregular vibration on machine due to improper air pressure in the tyres, misalignment of tyres with the centre of the drum, fitment problem with fixtures/fixture, etc could be the main factors which cause such type of failures. In addition to the above observations, it is found that the route of manufacturing the raw material for the fork tube is not specified on any part drawing. V. CONCLUSION From the above observations, we can conclude that the component failed in brittle manner. This is a single overload failure. The strength of the material is lowered due to the presence of the non metallic inclusions. Failure originated as the applied load exceeds the fracture strength of the material. Uneven pressure during clamping or burr at the under bracket location originate the crack on front fork pipe below the under bracket.
International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 – 6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 5, May (2014), pp. 141-148 © IAEME 148 REFERENCES  Arthita Dey, Sharmistha Dhara, Tanmay Bhttacharyya, Andip Bhattacharyya, Cracking of Telescopic Front Fork Tube during field operation, Journal of Failure Analysis and Prevention, 2013 13(3), 292-297.  Satish B. Purohit, S.R.Lapalikar, Niranjan Sharma, Methodology for the product specifications for motorcycle shock absorbers, International Journal of Emerging trends in Engineering and Development, 2(1), 2012, 147-156.  A.Shyam, R.Pachaiyappan, Sa.Paveethrun, M.Srinath, Design of Govern Arm Suspension System, Journal of Mechanical and Civil Engineering, 2320-334, 2014, 47-51.  Pravin Kumar .S, Venkatakrishnan.R and Vignesh Babu.S, “Process Failure Mode and Effect Analysis on End Milling Process- A Critical Study”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 5, 2013, pp. 191 - 199, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.  A. D. Lagad and Dr. K. H. Inamdar, “Root Cause Analysis of Field Failure Concern for Improvement in Durability of Vehicle System”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 3, 2013, pp. 232 - 243, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.  A.Mariajayaprakash, Dr.T. Senthilvelan and K.P.Vivekananthan, “Optimisation of Shock Absorber Parameters using Failure Mode and Effect Analysis and Taguchi Method”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 328 - 345, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.
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