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Dynamic bending fatigue life prediction of aluminum alloy wheels

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Aluminum alloy wheels are important safety components of automobiles, which have an important influence on the driving safety, ride comfort and occupant comfort of automobiles. Aluminum alloy wheels generally work under random dynamic loads, and the main form of failure of aluminum alloy wheels is fatigue damage. Aluminum alloy wheels must pass a number of performance tests before use, among which dynamic bending fatigue test is an important test link. The fatigue damage of aluminum alloy wheels is mainly caused by bending moment, and bending fatigue becomes the main reason for the damage of aluminum alloy wheels. If the fatigue life and failure locations of aluminum alloy wheels can be predicted in the design stage, it will be beneficial to structural improvement and optimal design. In recent years, aluminum alloy wheels have been widely used. Aluminum alloy wheels can not only provide higher carrying capacity, save energy consumption, but also meet the requirements of individual appearance design. The traditional design mode based on experience can no longer meet the requirements of modern development, and it has gradually transitioned to the development stage of using finite element technology. In this paper, a certain 22X8.5JJ aluminum alloy wheel is taken as an example to simulate the dynamic bending fatigue test process and predict the fatigue life and damage position of the aluminum alloy wheel.
1. The prediction and analysis process of the dynamic fatigue life of aluminum alloy wheels 
First, perform finite element static calculation on the aluminum alloy wheel, and obtain the result response of a load cycle under the test conditions, and then extract the stress and strain of each load step as the fatigue damage load. The principal strain criterion is used in the fatigue life analysis, and the influence of the average stress is considered. Finally, the Miner's law is applied to accumulate the damage caused by a single load cycle and calculate the fatigue life.

2. Dynamic bending fatigue test of aluminum alloy wheels Commonly used standards for dynamic bending fatigue test of aluminum alloy wheels include JWL standard, DOT standard and ISO standard, etc. These standards all simulate the load condition of aluminum alloy wheels under bending short action. The aluminum alloy wheel is fixed on the test bench, and the rotating bending moment is applied to the aluminum alloy wheel through the loading rod. The automobile industry standard QC/T 221-1997 stipulates that the calculation formula for the bending shortness of the light alloy wheel dynamic bending fatigue test is M = (uR + h) FS (1) where U —— the friction factor between the tire and the road R—— static Load radius (the static load radius specified by the automobile manufacturer to match the tire with the wheel) h-the inner or outer offset of the aluminum alloy wheel F-the rated load of the military wheel specified by the automobile manufacturer S-the strengthening test coefficient

3. The static finite element calculation of aluminum alloy wheels and the establishment of an accurate finite element model are the basis for analyzing the problem, which directly affects the accuracy of the calculation results. The hub flange and the loading shaft are connected by bolts. When modeling, the contact elements are used to simulate the contact surfaces of the bolt and the flange, the nut and the loading rod respectively, and the friction factor is 0.2; the connecting bolts use the special pretension provided by ANSYS software Unit (PRETS179) and solution method for simulation. In this model, the wheel hub, bolt structure and loading rod are all modeled by solid elements. The loading rod only serves to apply the bending moment. A larger element size is used to reduce the calculation amount; the flange part adopts a denser Unit division to obtain accurate calculation results

In the finite element calculation, the boundary conditions are set according to the way that the aluminum alloy wheel is fixed and the rotating load is applied at the end of the load. According to the fixed condition of the aluminum alloy wheel on the test bench, all the degrees of freedom of the nodes on the lower edge of the wheel copper are restrained.
During the bending fatigue test, the aluminum alloy wheel bolt hole is prone to stress concentration. The difference in the pre-tightening force of the connecting bolt between the wheel hub and the loading rod has a great influence on the result. Generally, the pre-tightening force is controlled by adjusting the torque applied to the bolt. size. The aluminum alloy wheel uses MIS x28.SX60 bolts, the applied torque is 110N·m, and the bolt pre-tightening force is. The bending moment applied to the aluminum alloy wheel is equivalent to applying a constant circumferentially varying load at the end of the load. , The load completes a load cycle every one rotation of the load. With the change of the load rotation angle, the stress and strain of the aluminum alloy wheel change continuously. At the end of the loading blast, the same loads are applied in the circumferential direction at 15° intervals in sequence, and 24 result sequences are obtained, which are used to approximate the force changes of each node of the aluminum alloy wheel within a load cycle. The calculation results show that the overall stress level of the aluminum alloy wheel under static load is not large, and the larger stress part appears at the connection of the spoke root, and the stress is less than the allowable stress of the material, indicating that the aluminum alloy wheel is safe under static load.

4. Calculation of bending fatigue life of aluminum alloy wheels
4.1. Life estimation model. When the fatigue life is calculated, the prediction accuracy depends not only on the accurate finite element model and the correct simulation of stress and strain response, but also on the reasonable use of the damage model. The dangerous sections of parts with complex geometric shapes are often subjected to multiaxial fatigue loads, and even under uniaxial loads, their parts may still be in a state of multi-transmission stress. The multiaxial fatigue damage model widely used at present is the critical plane method. This method is based on the fracture model and the crack initiation mechanism. It is believed that the crack occurs on a certain plane, and the accumulation of fatigue damage and life prediction are carried out on this plane. The physical meaning of. There are many methods to determine the critical plane, and different judgment criteria can be obtained according to different damage parameters. The damage models commonly used in engineering include principal strain criterion, maximum shear strain criterion and Brown-Miller criterion. For brittle metals, the principal strain criterion is generally adopted, that is, cracks appear first on the plane with the largest principal strain amplitude. In specific use, simplified methods are often used to convert complex stress and strain into equivalent stress and equivalent strain on the plane of the maximum principal strain amplitude, and then use the mature uniaxial fatigue analysis method to calculate the effect of the component on the multi-sleeve load. Fatigue life under. The cast aluminum alloy wheel material is A356, which is a brittle material, and it is more suitable to adopt the principle strain criterion in the fatigue life analysis.

The strain-life calculation criterion for uniaxial fatigue has been very mature, usually expressed by the Manson-Coffin equation

When applying the damage model, the stress and strain are first decomposed into a possible critical surface. The stress and strain on each surface are calculated by the rain flow counting method to calculate the fatigue damage of each cycle, and then the Miner cumulative damage criterion is used. Calculate the fatigue life. Take the material parameter 0.014, c= -0.67, and carry out similar repeated calculations on all possible critical planes, and take the shortest fatigue life as the fatigue life of the aluminum alloy wheel.

4.2, Fatigue life calculation
Fatigue life analysis generally requires the input of fatigue performance parameters and stress and strain history of the material. Material parameters can be obtained directly from the test according to relevant standards, or can be checked from the material manual or material database software, and some typical fatigue properties can be estimated from the elastic modulus and ultimate strength according to the empirical formula, but the accuracy is generally worse. . The elastic stress and strain under the unit load calculated by the finite element calculation or the test load can be extracted to obtain the stress and strain history. The calculation result obtained by the former is actually the proportional relationship between the stress generated by the external load, the strain output and the external load input. The measured load spectrum actually provides a time-varying scaling factor. The stress-strain spectrum of fatigue damage can be obtained by scaling the unit load response according to the numerical value of each point on the load spectrum, which is suitable for complex load spectrum loading; It is more suitable for simple constant amplitude loading. Under normal circumstances, components work in an elastic stress state. When yielding occurs locally, approximate correction methods are generally used to obtain elastic-plastic stress and strain responses. Commonly used correction methods include uniaxial or multiaxial Neuber criterion and Glink criterion. During the fatigue life analysis of the aluminum alloy wheel, the 24 load results obtained from the static analysis were extracted as a typical fatigue damage load spectrum, and the stress spectrum was corrected by Morrow average stress during calculation. By calculating the fatigue life distribution of aluminum alloy wheels, it can be seen that the low-life zone is basically concentrated in the connection part of the spoke root, which is also the area with higher stress under the static load state of the aluminum alloy wheel.

5. Test verification analysis
In order to verify the calculation results, the aluminum alloy wheels were tested on the B-600A dynamic bending fatigue testing machine, and the number of tests was set to 100,000 according to the design requirements. After cycling the corresponding period, the color penetration method is used to check whether the aluminum alloy wheel has visible cracks. The test found that cracks appeared in the contact part of the spokes near the flange and the predicted fatigue damage part basically coincided with the fatigue test crack. The results of the bending fatigue test of the wheel showed that the actual life was less than the design life, and the damage part appeared from the bolt hole to the wheel. The part in the middle of the load window, and there is no crack in the spokes. This failure mode is relatively common, mainly because the strength of the structure is weak, and the bending moment load has not been transmitted to the spoke direction before the damage occurs; or the structural strength of the spoke is too large, and the bending moment cannot be transmitted to the spoke direction. Concentrating on the middle disk surface causes the aluminum alloy wheel to break. When improving the structure, we should consider increasing the volume of the spoke weight reduction groove to reduce the spoke stiffness and facilitate the uniform distribution of the load. Measures such as increasing the rounded corners and improving the surface quality can also be adopted for the middle disk surface with greater load.

 6 Conclusion
(1) Through static finite element analysis, the stress distribution of aluminum alloy wheels can be understood, which is helpful to improve the structure, improve the bearing capacity of aluminum alloy wheels, and realize lightweight design. Comparing the conclusions of fatigue calculation and the test results, it shows that the simulation calculation of fatigue life can predict the fatigue failure position more accurately. It can be used for fatigue damage analysis in the product development stage, improve the one pass rate of the product, reduce R&D cost and shorten the R&D cycle.
(2) Using similar analysis methods, combined with the finite element calculation results under unit load and the vehicle road spectrum, it can also calculate the fatigue damage of the parts under the real working environment, which is beneficial to improve the structural design and increase the fatigue life of the parts.

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