| Connecting rod is the main movement and mechanical parts of engine,under reciprocating alternation of tension and compression load at work.And when the car is traveling in different road conditions, the working load of its engine connecting rod will also change,So the connecting rod in the case of meet the requirements of static strength , may occur fracture damage caused by fatigue.The load and working conditions material defects?surface roughness?machining errors of connecting rods are the direct causes of the initiation cracks of the connecting rod surface.The length of crack propagation is exponentially with the number of cycles of the connecting rod. When the crack size reaches a certain value, the connecting rod will break rapidly, resulting in the whole engine being unable to work or even scrapped. So it is very important to study the of engine connecting rod fatigue. Study failure position of the connecting rod,the relationship between fatigue life of connecting rod and the various causes, How to improve the fatigue life of connecting rod, This has always been a key of connecting rod fatigue problems.This paper use automobile engine connecting rod , Establish the classical rigid body dynamics equation of the connecting rod,import the gas pressure curve data at different speeds,obtain the theoretical velocity and acceleration curve of piston. In order to study the influence of motion flexible Connecting rod on the motion system, Combined with ANSYS and RecurDyn software to established a flexible body neutral file of connecting rod and imported into the engine of the coupled system, got the dynamic stress curve and ultimate load of flexible connecting rod under different speed conditions.Based on above, analyzed the static strength of connecting rod,Used the secondary development function of APDL, the bearing force loading method and inertial force of connecting rod are realized, and obtained the exact static solutions. Estimated the fatigue limit under different Stress ratio of connecting rod.The fatigue limit of connecting rod material under different stress ratio is solved by using the method of Ding,The fatigue limit of connecting rod is corrected twice, base on the influence factors such as roughness, hardness and stress concentration of connecting rod.The S-N curve of connecting rod is drawn by combining exponential fatigue curve theory, then combined static and dynamic calculation results to get the fatigue load spectrum curve of the car in uniform speed?variable speed and random variable speed.Through the rain flow count principle to statistics and analyze the stress spectrum under various operating conditions, finally got the connecting rod fatigue life.Finally come to a conclusion : The quality of engine connecting rod and the piston directly affect the engine power output, the greater the quality can cause the bigger the inertial force, the smaller pressure of gas is passed on to the crank ; In the maximum compression condition, the stress concentration mainly appear in the small end of connecting rod rounded corner. In the maximum tensile condition, stress concentration mainly appear in the big end of the fillet ; When the speed reaches 3000r/min the connecting rod stress and the output torque is the largest ; The fatigue strength of the connecting rod meets the design requirements, but the fatigue strength fillet Surface of the smaller tip of the connecting rod don t have surplus allowance of fatigue strength and is the most vulnerable to fatigue failure. Under the traffic load of the connecting rod fatigue life are meet the requirements of metal infinite life design (> 107), In this case than have some larger random load changes, nonlinear fatigue superposition theory should be used to reduce the deviation caused by load order. |
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