Fracture Simulation Of Engine Suspension Pads

It is well known that engine is a thermal power machine that widely be used on shipping, automobile, tractor, engineering mechanism and locomotives. As the engine develops in direction of high speed, little weight and high power, problems of its vibration being more and more concerned by people. Engine's vibration affects its life, working efficiency and the surrounding environment. Being the main vibration absorption source of engine, the engine suspending pads are usually ignored in daily overhaul for reasons as their working position, et al. And the failure can not be found until it occurs;this sometimes causes great economic expenses. So, it is highly indispensable to develop the research of fracture analysis of engine suspending pads. Obviously, this is a very significant subject. This thesis is established mainly on the existing hyperelastic constitutive model of rubber material and theories of linear fracture mechanism, the initial position where crack will occur is determined through strength analysis, subsequently, the crack propagation behavior is smulated numerically by using the finite element method and meshless method, and the crack trajectories of engine suspending pads are obtained.The engine suspension pads are located between engine bracket and vehicle frame, it is hard to know the initial position where crack occurs. So, to simulate the fracture behavior of engine suspending pads, the first task is to find the initial position where crack will occur. Rubber, as the main body of engine suspendsion pads, is a special engineering material. It is hyperelastic. The existing constitutive models of rubber are all based on strain energy density function. Since the recurrence of rubber-like materials is insufficient, plenty of parameters are not allowed to be evaluated accurately, so the practicability of high order strain energy density functions is very little. Here, the two-parameter Mooney-Rivlin model is selected to simulate rubber's constitutive behavior. When constructing the finite element model of engine suspension pads, contact pairs were created between rubber and steel plate, the type of contact pair is always bond together. This is to meet the manufacture's requirement, that is, when the rubber is sliced from the steel plate, there must be rubber adhere to the steel plate. According to the stress distribution in the component, the position where the initial crack will occur is determined. It also pointed out that, as the stress distributes in plane similarly, when simulating the fracture behavior, the three-dimensional problem can be simplified into two-dimensional problem on condition that the vehicle running smoothly, and the longitudinal shock can be ignored. Thus the difficulty of disposing problem can be reduced and thecalculation scale can also reduced greatly on the premise that the reliability of simulation is guaranteed.Finite element method is the most widely used numerical method in simulating crack propagation, although its precision is confined by factors as remeshing, et al. One of the difficulties for two-dimensional problem is that the load for finite element calculation can not be calculated theoretically, it needs special disposal. This thesis used the following method to obtain the load: Use the three-dimensional finite element model, and apply displacements constraints on nodes used to construct contact pairs, the allowable displacements in all degrees of freedom of these nodes are set to be zero. Get the nodal forces in a plane from finite element analysis results, and polyfit the nodal forces in segments, apply the fitted forces in form of segments polynomial functions on two-dimensional finite element model. According to the working environment of engine suspension pads, and combines with the mechanical characteristics of rubber material, which is although rubber is hyperelastic, and can experience large elastic strain, when the tension deformation or compress deformation is less than 25%, or the shear deformation is no more than 75%, stresses of each deformation can be approximated very well from traditional elastic analysis, fracture mechanics theory of linear elastic isotropic material is used in this thesis to determine the crack propagation angle of complex crack in plane. The continuous dynamical fracture process is also be discretized into a series of static fracture processes, and the crack is assumed to propagate along direct line in each static process. The new crack tip is determined according to the crack angle and the crack increment, and the shape of crack is updated after the crack propagated. Based on these theories, the two-dimensional fracture behavior of engine suspension pads are simulated using finite element method, and the crack trajectories of the engine suspension pads are obtained.When using finite element method to simulate fracture, the elements distort greatly soon so that the simulation can not go on. This is because the finite element method is based on element connection information. However, the meshless method is based on a set of discrete nodes, and needs not nodal connection information;it avoids the elements distortion phenomenon, and has profound usages in simulating crack propagation. The meshless local Petrov-Galerkin (MLPG) method, which is a truly meshless method, is used to simulate the crack propagation of engine suspension pads. The MLPG method is based on the local weak form of differential equation, all integrations can be implemented on local sub-domain without any background meshes. The visibility criterion is used to dispose the discontinuity caused by crack, and the essential boundaries are implemented by penalty parameter method. The moving least square approximation is used to construct the displacement trial function, in which the simplest linear basis function and radial enriched basis function are chosen to bethe basis function, and Gauss weight function with fine continuity and compact support domains is taken as the weight function. Using this method, the last part of this thesis simulated the fracture behavior of engine suspension pads, and got the crack trajectories that are similar to that obtained in finite element method, but the trajectories obtained using MLPG method are more smoothing than that obtained using FEM, which demonstrated the advantage of MLPG method in simulating structure's fracture behavior.