Static And Dynamic Characteristics Analysis And Fatigue Life Prediction On Rubber Mount Of An Automotive Powertrain

As a key attachment point between the powertrain and the car body,rubber mounts play the important role of support,limit and vibration isolation.They are asked not only to ensure that the distance between the powertrain and other components is always kept within a reasonable range,but also to dissipate vibration energy in the transmission path from road surface to the powertrain and from the powertrain to the car body.Whether the static and dynamic characteristics and the fatigue life of rubber mounts can meet the design requirements determines the NVH performance and comfort of vehicle.On the other hand,environmental factors can easily result in rubber’s performance degradation,especially the aging phenomenon caused by hot oxygen.So it is of great importance to study fatigue life and the static and dynamic characteristics of rubber mounts considering the aging factors,which can provide theoretical basis and data support for their performance design,structure optimization and formulation improvement.This paper takes the verification of the static and dynamic characteristics and fatigue life of a rubber mount as the research objective.By studying the basic mechanical properties,aging phenomenon and fatigue theory of rubber materials,and conducting uniaxial tensile test,simple shear test,hot oxygen aging test and crack propagation tests,then with the aid of technologies of FEA and fatigue life simulation,this paper focus on the constitutive models and elastic modulus of rubber,static and dynamic stiffness and fatigue life of mount.Firstly,summarize the basic mechanical properties and behaviors of rubber,and conduct key research on the theoretical basis,derivation process and test methods of hyperelastic and viscoelastic models.A dumbbell-shaped 45HA rubber specimen has been made and a uniaxial tensile test has been performed to obtain a hyperelastic constitutive model.The results shows that the Yeoh model was the best fit.In order to obtain the viscoelastic constitutive model,a simple shear test has been performed to obtain the modulus data within 1000Hz which has been eventually converted into the normalized shear relaxation data.Secondly,carry out hot oxygen aging test,establish the Peck-Yeoh model to describe the change law of Yeoh model parameters and initial elastic modulus of aging rubber,and analyze the influence of hardness property,test temperature and aging time on tensile properties.A hot oxygen aging test box has been used to perform aging tests on 60 groups of rubber materials and then uniaxial tensile test has been performed.The initial elastic modulus is derived to be about 6c₁₀.The Peck-Yeoh model of the Yeoh model parameters and the initial elastic modulus of the aging rubber material are established by nonlinear fitting,and the hardness curve,aging curve and time curve are drawn.The research shows that the Peck-Yeoh model describes the change law of the initial elastic modulus well.The increase in hardness property,test temperature,and aging time will all cause the initial elastic modulus to rise.Furthermore,the impact of hardness is the most significant.The speed of performance decline caused by time is first fast and then slow,which is still slower than the temperature’s.Thirdly,combine the FEA technology and stiffness test to study the static and dynamic characteristics of rubber mount.The 1/4 and 1/2 FEA models are obtained by simplification which can effectively shorten the calculation time.Then the stiffness tests have been perfromed through the dynamic servo testing machine.Research shows that the dynamic stiffness of the rubber mount gradually rises with the increase of frequency,and the hysteresis angle first rises and then falls,which conforms to the frequency dependent behavior.Moreover,the calculation error of FEA static stiffness in three directions is within 10%,and 20%for dynamic stiffness,both of which confirm the mount’s static and dynamic characteristics and the correctness of the FEA method.Then,take vertical static stiffness as the evaluation index to quantitatively analyze how mount’s vibration isolation performance will degenerate because of aging.With the aid of the Yeoh model of aging rubber,the vertical static stiffness under various aging conditions is obtained through FEA,and the ratio of aging static stiffness and original one is calculated.Then three-dimensional surface graph of the static stiffness and the static stiffness ratio at each hardness is drawn.Studies have shown that a proper increase in static stiffness will enhance the support and limit performance.But the static stiffness of 45HA rubber will increase by more than 60.05%after aging,which will lead to stiffness function failure and serious degradation of vibration isolation performance.Finally,based on the fatigue crack growth method of cracking energy density,the multi-axial fatigue life of the rubber mount is predicted.For rubber materials with strain crystallization,the crack growth rate is described by Thomas model and Mars-Fatemi model,whose parameters have been obtained through tear strength test and crack growth test.Then establish a fatigue simulation model with the help of Fe-safe/Rubber,and compare the results with the fatigue bench test of a mount with the same 3D model.The research shows that the minimum fatigue life occurs at the maximum stress unit,and the fatigue failure location and crack propagation direction are consistent with test result,which effectively predicts the fatigue failure information and verifies the correctness of the fatigue simulation at a certain extent.The prediction method in this paper can provide a reference in the early stage of rubber mount’s development and immensely save the time due to the small demand for test data.