Study On Short Fatigue Crack Behavior Of LZ50 Steel Under Tension-torsion Loading

With the railway transportation gradually entering the era of high-speed,heavy-duty and diversified forms of transportation,the reliability and safety of the structural strength of locomotives and vehicles has become one of the important research directions in this field.Tension-torsion loading is a compound load that often occurs in locomotive and vehicle structures.As the main bearing component of the vehicle equipment,the fatigue performance of axle is one of the key factors affecting the safety and ride quality of the vehicle.Under the alternating and compounding environment with different modes of multi-axial loads,the material may exhibit different fatigue resistance than conventional uniaxial loads.In this paper,only the material of LZ50 steel,a commonly used axle steel material,is taken as the research subject.The relevant fatigue performance of LZ50 steel is studied under tension-torsion loading based on the experimental reasults of the initiation and expansion behavior of short fatigue cracks of LZ50 steel under tension-torsion loading.Firstly,the basic concepts of short-crack and multi-axial fatigue is introduced,and the crack types based on crack size are classified.The mechanism of initiation and propagation of short cracks in existing studies and the theoretical system of multi-axial fatigue failure studies are summarized.The experimental study of the short fatigue cracks behavior of LZ50 steel under tension-torsion loading was carried out.From the observation of the replica membrane,it can be seen that short fatigue cracks originate in the ferrite grains or on the grain boundaries under tension-torsion loading.By analyzing the results of the replica test,the crack size increases slowly,and the rate of expansion slows down according to the number of grain boundary obstacles encountered at the MSC stage.the crack breaks through the ribbon spacing of the pearlite,leading to effective short crack formation,crack size and expansion rate are rapidly increasing at the PSC stage.Ttwo typical characteristic parameters of Dominate Effective Short Fatigue Crack scale and life span are statistically analyzed based on the fatigue failure mechanism and the safety of the tail prediction.The results show that the Minimum Value Distribution is a hypothetical distribution with good DESFC scale and life span.Furthermore,the statistical evolution of short crack under multi-axial stress is analyzed.The FP and crack angles are obtained from four fatigue parameter calculation models on the basis of the critical interface criteria.The comparison between experimental results and prediction results of the modified models corresponding to different FP parameters was carried out from two aspects of crack growth rate and crack failure life.The results show that the prediction spread rate calculated based on the FP parameters obtained by FS model and CCB model has small dispersion,and the predicted life is close to the test result.