Influence Of Tension-torsion Stress Amplitude Ratio On Short Fatigue Crack Behavior Of Axle Steel

More than half of the failure for metal materials results from fatigue.The initiation and propagation of short cracks in smooth structures or components subjected to alternating loads account for more than 70% of the total fatigue life.LZ50 steel is a common material for vehicles in China.The axle using this material has been serving in various harsh environments for a long time,so the axle is subject to complex and changeable load forms.Fatigue life prediction or safety assessment of practical engineering structures based on fatigue performance parameters under a single load mode or a single loading path may result in dangerous or over-conservative assessment conclusions.Therefore,the fatigue short crack replica tests of LZ50 steel using smooth hourglass shaped specimens under seven combined loading paths of tension and torsion were carried out,so as to study the effect of tension-torsion stress amplitude ratios on the short crack behavior.1.Short crack replica tests under seven loading paths were performed.The results show that the growth rate of short cracks under seven loading paths decreases twice,and the size of crack corresponds to the size of the microstructure barrier.At the same crack scale,the smaller the tension-torsion amplitude ratio is,the lower crack growth rate and the shorter the crack initiation and propagation period are.2.Compared with the seven different loading paths,with the decrease of the tension-torsion stress amplitude ratio,the angle between the fatigue fracture surface and the axial direction of the specimen decreases,and the life of the specimen increases gradually.According to the SEM observation of the fracture,it can be seen that the single crack source was transformed into multi-crack sources with the decrease of tension-torsion stress amplitude ratio;the morphology of the crack source region transits from the coexistence of river scattering and fatigue fringes to the coexistence of scratches and fatigue fringes;the morphology of the propagation zone changes from fatigue fringes to scratch marks;the morphology of the instantaneous fracture zone changes from a large number of circular or elliptic equiaxed dimples to a shallow parabolic dimple.3.The date of dominant effective short fatigue cracks(DESFC)size and life fraction of effective specimens under AT3 and AT7 typical paths were statistically analyzed under several common hypothetical distributions.The extreme minimum value distribution(EMVD1)is determined to be a good hypothetical distribution for describing DESFC size and life fraction for these two paths.4.Based on the growth rate model of multiple microstructure barriers,the data of short cracks growth rate under different paths are fitted.The curve can basically reflect the characteristics of the short crack growth rate affected by the microscopic barriers,and the fitting effect is good.Furthermore,a stress amplitude ratio effect function is introduced to revise the model,and a short crack growth rate model considering the effect of tension-torsion stress amplitude ratio is obtained.