Experimental And Theoretical Study On Stress Interaction Of Structural Steel Under Variable Amplitude Loading In Ultra-high Cycle Range

With the rapid development of the construction of high speed rail in China,it has been paid more and more attention in the research on the basic materials used for high speed rail by the engineering and researchers.In many applications,the lifetime of the mechanical components has to endure up to 10~9 cycles of loading without failure.In the study,based on a kind of acceleration fatigue test technique,called the ultrasonic fatigue testing,was used in this paper.It is proved to be a convenient and effective experimental method.In this study,fatigue behavior of the LZ50 and 50CrVa steels was studied with the ultrasonic fatigue testing technique,with a loading frequency of 20kHz.The fracture surface of specimens was examined by scanning electron microscopy(SEM).Experimental results showed that the S-N curves of normalized LZ50 steel and quenched and tempered and annealed 50CrVA high strength spring steel in the range of gigacycle fatigue were obtained under the service condition.Experimental results showed that the S-N curves of normalized LZ50 displays the characteristic of"continually decreasing type"in the range from 10~4 to 10~9 cycles,and exhibit no traditional horizontal plateau up to 10~9.The observation of the facture surface showed that fatigue cracks initiate from surface of all specimen.The results showed that different heating treatment process induced different microstructure of specimens so as to regulate ultrahigh cycle fatigue damage mechanisms.In QT state,fatigue cracks generally originated from inner oxide particles.It was probably due to that elastic interaction between micro-crack tip stresses and interstitial atoms attracted carbon atoms to be enriched around the particles,causing the degradation of performance of rich carbon region and initiation of fatigue cracks.In AD state,the enrichment of interstitial atoms was difficult due to that the special microstructure inhibited the diffusion of atoms approaching to impurity particles and inhomogeneity of deformation between the grains.So that,the cracks was prone to emanate from the surface of specimens.Variable amplitude fatigue properties of LZ50 steel were investigated with ultrasonic fatigue testing equipment.Fatigue tests were performed with a linear step accumulative loading.The results show that the fatigue life under variable amplitude loading is longer than that under constant stress amplitude.It is indicated that interaction is existed in fatigue damage under variable amplitude loading,and the article named this phenomenon as“stress aging”.In order to quantitatively study the effect of“stress aging”on the ultra-high cycle fatigue properties of the material,50CrVA spring steel with quenching and tempering and annealing heat treatment was selected.The experimental design was carried out around the preloading amplitude and preloading cycles,that two main factors of variable amplitude loading.The influence of preloading on the very high cycle fatigue properties of was obtained.In this paper,it is considered that the strain hardening of ferrite is the fundamental of the“stress aging”effect of the variable amplitude,and preloading affects the fatigue life of the subsequent loading stress by changing the microstructure of the material.With the analysis of the microstructure of the material,the mechanism of"stress aging"to material strengthening is that the interstitial carbon atoms in the ferrite have an elastic interaction with dislocation,which attract the interstitial atoms to migrate around the dislocation.The preloading process increases the dislocation energy field,increases the concentration of the gap atoms around the dislocation line,and enhances the pinning effect of dislocation.The plastic deformation resistance of the material is high,and the fatigue property is strengthened.Base on the results of two step loading ultra-high cycle fatigue test of 50CrVA spring steel at QT a fatigue life prediction method which takes into account the interaction effect of stress is proposed based on Miner's rule.This method introduces the concept of stress interaction effect factor,and divides the influence of preloading on the residual fatigue life into strengthening and damage,then gives the expression and parameters of the two influencing factors respectively.The results show that the modified Miner rule can give a better life prediction for 50CrVA steel and LZ50 steel.