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 vehicle’s 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 vehicle’s 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 driver’s 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.
