The Research Of The Prediction Method Of High-cycle Fatigue Life Based On The Connecting Rod Of Diesel Engine

With the development of science and economy, more and more people put forward thehigher requests to the life of automobile engine. The connecting rod as the core componentsof the engine, the fatigue failure of it which accounts for its fault ratio is about60%to90%.So in the early stages of design, through the finite element simulation combined with thefatigue life prediction model could predict the working life of the connecting rod, andensure the high performance of fatigue resistance. Fatigue of different link failure positionwill produce different stress state, which concludes uniaxial stress state or multi-axial stressstate. Therefore, before establishing the cumulative fatigue damage life prediction model, itshould be consider the different of the stress states. Aiming at this problem, throughtransient dynamic analysis of the connecting rod, this paper gets the point stress time courseof fatigue risk position, and judges the state of stress. By using the method of combinationof theory with experiment, seven kinds of damage parameters under uniaxial andmulti-axial stress of the specimen were used. Making the forecast cycle times compare withthe experimental data, the right life prediction model under the different stress state wasproposed. According to the conclusion of life prediction model which was obtained from thestandard specimen, the different dangerous parts of life for the connecting rod wascalculated combining stress life curve with cumulative damage theory.The main contribution can be summarized as follows:(1) The finite element model which contains the main parts of connecting rod wasestablished, including piston pin, crank radius, bushings, bearing and bolt model. By usingthe method of the transient dynamics analysis, the stress time history of the dangerous position node was extracted; through the multi-axial stress criterion, this paper analyzed thestress state of the connecting rod which were inspection parts, and provided a theoreticalfoundation which was used by select of the follow-up fatigue life prediction model later.(2) By means of standard specimen test results, the stress state under the condition ofdifferent loads by using of the finite element was simulated. Based on Basquin formula,selected seven kinds of damage parameters, including the absolute maximum principalstress, the maximum principal stress, the signed von Mises stress, the von Mises, the signedshear stress, shear stress and critical-plane parameters, made a life prediction respectively.The obtained results were compared with experimental value, and the reasonable lifeprediction models of fatigue damage with the uniaxial stress, and multi-axial high cyclewere proposed. The absolute value of the maximum principal stress damage parameter wasused to predict the position which was in the state of uniaxial stress; the critical-planeparameter was used to predict the position which was in the state of multi-axial stress. It isdemonstrated that each of the damage parameters gives satisfactory results.(3) The analysis of conclusion from the standard test was applied to the connecting rodof fatigue life calculation. Considering the size effect, surface processing state, papermodified material S-N curve to S-N parts curve. In this paper, the connecting rod body partsof life were calculated used by the absolute value of the maximum principal stress. Forother parts of the connecting rod was calculated by the model of critical-plane damageparameter. The worst fatigue position appears in groove position between the small end andthe body transition.