Fracture of an automobile anti-roll bar
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21-02-2011, 09:45 AM
Fracture of an automobile anti-roll bar
Anti-roll bar is a suspension element used at the front, rear, or at both ends of a car that reduces body roll by resistingany unequal vertical motion between the pair of wheels to which it is connected. In this study, fracture analysis of ananti-roll bar of ab automobile is carried out. The analysed type of the anti-roll bar is especially important as many casesare reported about the fracture after a 100,000 km of travel. Mechanical characteristics of the material are obtainedfirst. Then, the microstructure and chemical compositions are determined. Some fractographic studies are carriedout to asses the fatigue and fracture conditions. A stress analysis is also carried out by the finite element techniquefor the determination of highly stressed regions on the bar.IntroductionGround vehicle design typically represents a trade-off between performance and safety. Design parametersaffecting lateral dynamics can influence maneuvering ability, but also have some influence on the dynamicstability including spinout and rollover. In steering maneuvers, vertical loads on tires at the outertrack increase and those on the inner track decrease, which is called lateral load transfer. When momentequilibrium is broken at some conditions, a vehicle loses roll stability. Geometric dimensions, suspensioncharacteristics as well as maneuvering conditions influence the dynamic roll behavior of a car. To improvethe roll characteristics of a car, the customary approach is to increase the roll stiffness by using a stabilizerbar which affects the ride comfort with respect to high frequency isolation induced by road excitation.In order to enhance the vehicle performance, several expediences are studied. At present one of the mostefficient methods is the roll control with a semi-active suspension system in order to isolate the driver from roadway noise, road holding on irregular road surfaces and safe turning through steering. As is the casewith any vehicle system, an actual car is expected to operate in a highly variable environment. For instance,parameter variations resulting from loading pattern and driving condition will influence vehicle dynamics.The antiroll bar is a circular sectioned torsion bar mounted transversely on spaced out rubber bush swivelbearings to the body and having cranked arms. That is, bent extended ends which are attached to eachswing-arm via short vertical link-rods by means of rubber-bush joints. Thus the function of the antiroll baris to relieve the main suspension springs of some of their load every time the body rolls. An antiroll barstiffens the suspension springing when the body rolls or one wheel goes over a bump or dip in the road.The antiroll bar therefore permits softer suspension springing to be used when the vehicle is moving straightahead, as this responds far better than hard springing to small ripples or irregularities in the road, hence itimproves the ride comfort of the passengers. In the extreme, some antiroll bars are tuned to take as much as30–40% of the total vertical load imposed on the suspension when subjected to severe body roll . shows the anti-roll bar in case of the installation. The anti-roll bar picture analysed in this studyRecently many cases of fractured anti-roll bars after a 100,000 km of travel for a special passenger carare reported. All of the bars are fractured at nearly the same location (Fig. 3). In this study, fracture analysisof this anti-roll bar is carried out.Mechanical characteristics of the material are obtained first. Then themicrostructure and chemical compositions are determined. Some fractographic studies are carried out toasses the fatigue and fracture conditions. A stress analysis is also carried out by the finite element techniquefor the determination of highly stressed regions on the bar
Specimens extracted from anti-roll bar were subjected to various tests including tensile and hardnesstests and metallographic and scanning electron microscopy as well as the determination of chemical composition.All tests were carried out at room temperature.Results of the tensile test and hardness measurements are given in Table 1. The results are the mean ofthree specimens. The yield strength is the proof value at the 0.2% strain. The hardness measurements arecarried out by a MetTest-HT type computer integrated hardness tester. The load was 1471 N.
2.1. Chemical and metallurgical analysis
Chemical analysis of the fractured anti-roll bar material was carried out using a spectrometer. The chemicalcomposition of the material Chemical composition shows that the material is aspring steel of the AISI 9620 type.Preliminary microstructural examination of the failed anti-roll bar material It can beseen that the material has a mixed structure in which some ferrite exist probably as a result of slow coolingand high Si content. High Si content in this type of steel improves the heat treatment susceptibility as well asan improvement of yield strength and maximum stress without any reduction of ductility . If the microstructurecannot be inverted to martensite by quenching, a reduction of fatigue limit is observed. A temperedmartensitic structure is desirable for especially AISI 9260 type steel.
3. Inspection of the fracture
For the analysis of fractured region, a finite element analysis is carried out for the determination of thehighly stressed region. Then, the macro and microstructure of the region is analysed. For the microstructuralanalysis, the scanning electron microscopy is utilised.
3.1. Finite element stress analysis
The anti-roll bar is modelled via ANSYS 7.1/Mechanical module. After the definition of the geometry,the material properties developed by the mentioned experimental studies are entered into the program anda static stress analysis is carried out. For the simplification of the problem, only half of the geometry ismodelled with the symmetric boundary condition (Fig. 5). One kilo newton is selected for the entire solutionas the real load conditions on the road are unknown and the primary subject of the analysis is to determinethe highly stressed region of the cross-section. This determination allows us to assign the beginningpoint of the fracture.
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