Research On The Key Technologies Of Analysis, Test And Optimization For Vehicle Fatigue Durability

Most key components of the vehicle are damaged because of fatigue. As one of the main performances of vehicle product, fatigue durability is directly related to safety, reliability and economy. and has an important impact on the quality and reputation of vehicle product. Therefore, fatigue durability is paid more and more attention by enterprises as well as the users. How to validate and improve fatigue durability for the products designed becomes a research hotspot. The key technologies of CAE analysis for fatigue durability, road simulation test validation, structure optimization based on anti-fatigue, are studied deeply, and the integrated solutions for the development of vehicle fatigue durability are created. The effective guidance and technical support for the product development of vehicle enterprises are provided. The main work of the dissertation includes the following aspects:Road load spectrum of vehicle is pretreated and edited. Spike filter, drift correction, fourier filter are processed by pretreatment technology. The load spectrum is edited according to the frequency distribution and damage distribution of load cycles and the principles of load spectrum editing are put forward. To make the test time short and the edited load history replicated on the road test simulator smoothly, an accelerated load spectrum editing method is advanced considering damage reservation and PSD. Therefore, the distortion in frequency domain is reduced and iterations in road simulation are processed easily. Countermeasure of load spectral editing is put forward when the iteration ability of a simulator is poor.To replicate a variety of conditions in proving ground, the indoor road simulation test is carried out. The identification of the FRF (frequency response function) of the simulation system is performed with the white noise drive signals and the corresponding response signals,and the identification quality is evaluated immediately. The iterative process based on the FRF continues until the response errors of acceleration signals at the center of tires converge to an acceptable tolerance level, and the tire excitation signals from the test rig are obtained. The key technologies about gain factor adjustment and other critical points are summed up. It is proved that durability test by road simulation is valid and available by the indoor detected fault results which are in accordance with the road test results. The method of damage and crack identification based on variable strain signal and its statistics is established, and it is meaningful in road simulation test.The whole vehicle rigid-flexible coupled model is built to obtain the stress response on upper control arm based on MSR(modal stress recovery) method and road excitation signals from physical iteration results in the road simulation test. The flexible body of upper control arm and modal stress results are gained by CMS (component modal synthesis) analysis. The whole vehicle rigid-flexible coupled model equipped with the upper control arm as a flexible component is built and analyzed on virtual four-poster in ADAMS/Car Ride. The method with which the tire displacement excitations are acquired from the road simulation test is put forward. The rigid-flexible coupled simulation is processed using the tire displacement excitations as the boundary conditions and will improve the accuracy and validity of the input load in CAE. At the same time, this method achieves dual verification between CAE simulation and road test under the same work conditions. The stress responses are received using the modal stress results and the modal displacement histories from rigid-flexible coupled simulation. The CAE results are in accordance with those of road test, so the MSR method is accurate and valid to gain the stress response.As many components of vehicle are subjected to the multiaxial loads and their material properties are different from those under the uniaxial load, the multiaxial fatigue theory is researched. To simulate the actual road condition, a drive file which is prepared for dynamics analysis is acquired after the iteration process. The whole vehicle rigid-flexible coupled model is built and is validated with comparison between the acceleration signals at the center of tires in the virtual prototype and the target signals from the proving ground. After analysis, the load histories at the junctions of lower control arm are obtained. The method of inertia relief is used to finite element analysis for the lower control arm. Biaxiality analysis is processed considering the standard deviation of biaxial ratio, the time history of biaxiality ratio, the time history of angle Φp, biaxiality ratio distribution and angle Φp distribution, and determines that the lower control arm is in a non-proportional multiaxial loading state. The multiaxial fatigue life distribution of the lower control arm is achieved with the Wang-Brown method, and the results of multiaxial fatigue analysis are similar to those of proving ground test.As the junction between frame bracket and longitudinal beam cracked frequently, the structure optimization method is established considering fatigue life. The frame bracket fatigue life which is validated by the results in road test is analyzed by CAE and is prepared for the subsequent optimization. A reasonable distribution of material is received by topology optimization considering the constraint of fatigue life. According to the distribution result, the strengthened structure is designed accordingly. Size optimization for the frame bracket and the strengthened structure is processed considering fatigue life. The method which combines topology optimization and size optimization, can greatly enhance the fatigue life of the frame bracket, and substantially control the total mass of the frame.An anti-fatigue optimal design method based on the approximate model is established as fatigue analysis is a nonlinear, complex question and is difficult to carry out especially when the structure is complicated. With this method, the life of optimized structure is greatly improved and the effect of lightweight is achieved after DOE (design of experiments) by Latin Hypercubes method, foundation of approximation model of fatigue life and volume with moving least squares response surface and optimization solution by GA (genetic algorithm).