Fatigue Strength Prediction Of Defective Railway Axle Based On Critical Distance Method

With the development of continuous overloading and high speed of railway transportation,railway vehicles have encountered more and more problems in the service and maintenance process,and the fatigue failure of axle is one of them.Surface defects such as scratches,corrosions,and impacts will inevitably occur during the service process.These defects cause stress concentration and reduce the fatigue strength of axles.Therefore,it is very necessary to study the effect of defects on the fatigue strength of axles,and to provide reference for the safety inspection of railway axles.Taking medium-carbon steel(JZ50)axle as the research object,three kinds of axle specimens,including smooth,ring-shaped V-notch and hole-notch,were prepared.The axial tension and compression fatigue strength was tested by a Swiss high frequency fatigue testing machine.At the same time,the crack growth threshold value of the material when the stress ratio R=-1 and fatigue limit,which are necessary for the application of the critical distance method,were tested.The test results show that with the appearance of defects,the fatigue strength of the axle specimens has decreased.The sharper the notch,the more the fatigue strength decreases.Based on the fatigue strength test data of the defective axle specimens and related fatigue performance parameters,and in order to find a fatigue strength prediction method suitable for application in engineering practice,the critical distance method,the redefined Modified W(?)hler Curve Method(MWCM),and the prediction method based on notch sensitivity are used to predict the fatigue strength of axle specimens with defects.It is found that the point method in the critical distance method has the highest prediction accuracy.The overall error is within 10%.This method,better than the line method,is easy to implement,and is suitable for application in engineering practice.Although the notched axle specimens bear the axial tension and compression loading simply,the stress state at the notch appears to be a multiaxial state due to the existence of defects.The critical distance method,combined with MWCM considering multiaxial fatigue,was used to predict the fatigue limit of axle specimens.The redefined MWCM method that could consider multiaxial fatigue can also predict fatigue strength accurately.But the accuracy is slightly worse than that of the point method,and the analysis process is more complicated.The analysis method,based on notch sensitivity,generally has large prediction errors(30%).Among the three methods,the point method is most suitable for engineering application.Based on foregoing research,the point method is used to predict the fatigue strength of railway axle with defects,and the critical size of the defect is calculated under the premise of considering the safety factor.Through related calculations,it can be found that for the scratch defects on the surface of railway axle,the fatigue limit of railway axle decreases as the depth of defect increases.When the sharpness of scratch is certain(0.04 mm),the safety factor of axle will no longer be able to ensure the safety of axle with the depth of defect reaching 0.11 mm.If the scratch depth is 0.1 mm immutably,the axle is also no longer safe when the notch radius is less than 0.038 mm.For local pit defects with different width and depth,according to Murakami’s relevant research conclusions,hole is drawn on the surface of the axle model to simplify defects of different shapes.Through related calculations,when the ratio of defect width to depth is constant at 2:1,the defective railway axle will no longer be within the allowable safety range with the diameter of the hole reaching 0.6 mm.The research achievement in this thesis provides a reasonable and effective method for the prediction of the fatigue strength of defective railway axles.It also provides the basis for safety evaluation of railway axle with defects.