Study On Forecasting Method Of Fatigue Limit Load For Engine Crankshaft

The phenomenon of engine crankshaft stress concentration is serious, especially in the crankpin round and spindle neck round. The fatigue damage of crankshaft often occurs in these areas. Along with the rising of engine aggrandizement degree, the demand for crankshaft fatigue strength is also growing higher. The traditional method calculating crankshaft fatigue strength by the crankshaft shape coefficients and experience formula has failed to meet the design requirements of high reliability, low weight and low cost. Therefore, the researches including the fatigue strength influence analysis of crankshaft structure and process parameters, the accurate predicting study of component fatigue limit load, and the discussion for how to analyze the crankshaft fatigue test data effectively, have the practical significance for studying the crankshaft fatigue strength reliability.Therefore, around the research topic of crankshaft fatigue strength reliability, this paper mainly carries out the following contents:The impact analysis of key structure parameters on crankshaft fatigue strength. Based on the multi-disciplinary collaborative design optimization platform, combining the finite element analysis, variable structure parameters of design and design of experiment, the impact of crankshaft key structure parameters on fatigue strength is analyzed.The impact analysis of processing technic on crankshaft fatigue strength. By use of the parent standard deviation test-"f’test, parent average test-"t"test and rank sum test, the impact of nitridation time and rolling process on crankshaft fatigue strength is analyzed.Prediction of crankshaft fatigue limit load by integrated crack-modeling technique with test data. The standard crack growth threshold value-△Kth which is equivalent to the stress concentration of real component, is derived through the fatigue test data of a type of crankshaft. And by use of the value-△Kth and through the equivalent relationship between crankshaft with standard crack body, the fatigue limit load of the different crankshaft, which is made of the same material and has the same processing technic, can be predicted.Component fatigue limit prediction based on the integrated method of notched specimen equivalence and asymptotic interpolation. By the stress gradient matching method, the crankshaft stress concentration can be equivalent with the standard notched specimen, and through the asymptotic interpolation and introduction of virtual short crack correction, the fatigue limit load of crankshaft is predicted.Statistical analysis of crankshaft fatigue strength test data. Based on the experimental data, the statistical regression analysis is carried out. The degree of fitting effect between the experimental data with the statistical distributions is evaluated. And the crankshaft fatigue property curve is determined by the method of maximum likelihood estimation.The main conclusions were as followsIn order to improve the crankshaft fatigue strength, the effect should be more significant by increasing fillet radius or crank thickness than changing overlapping degree or crank radius. And it can be concluded that the length of nitriding time has little impact on the crankshaft fatigue strength and the rolling process can improve the performance of crankshaft fatigue strength.The crankshaft stress concentration can be equivalent to the standard crack model. The prediction accuracy of crankshaft fatigue limit load is closely related to the standard crack growth threshold value-△Kth And the prediction of the crankshaft fatigue limit load is accurate by the standard crack growth threshold value-△Kth, which is derived through the fatigue strength test data.Crack body and actual component differ in nature, the fatigue strength of actual component belonging to the stress concentration problem, is the same as the standard notched specimen. The equivalent fitting effect between actual component with standard notched specimen is better than the fitting effect between actual component with standard crack body. The advantage of standard notched specimen equivalence lies in that the actual component with processing technic can be equivalent to the standard notched specimen with the same processing technic.The six kinds of statistical distributions, including weibull(two-and three-parameter), normal, lognormal, extreme minimum value, extreme maximum value, have good fitting effect. And the crankshaft fatigue property curve can be determined by the method of maximum likelihood estimation with certain error.