Evaluation On Fatigue And Ratcheting Damage Of Austenitic Stainless Steel Based On Critically Refracted Longitudinal Wave

Austenite stainless steel?ASS?is widely used as engineering component in nuclear power,chemical industry and other fields because of its high strength,toughness and corrosion resistance.Because the working environment is usually serious,the components are prone to be damaged by fatigue,ratcheting and others,which seriously reduced the bearing capacity.Therefore,it is of great significance to use nondestructive testing technology to characterize and evaluate the key components in order to ensure the safe operation.Critically refracted longitudinal(L_CR)wave is a kind of longitudinal wave propagating along the material surface and subsurface.Compared with the traditional pulse echo method,it does not depend on the thickness information of the measured objects and is sensitive to defects,damage and stress.It has been paid more attentions by researchers.In this paper,aiming at fatigue and ratcheting damage of ASS,the effects of ultrasonic characteristics and surface roughness on L_CRR wave were studied by numerical simulation and experiments.On this basis,the mechanical damage evaluation was carried out and compared with the traditional pulse echo method.The main contents were as follows:?1?The ultrasonic propagation models of L_CRR wave in elastic isotropy and elastic anisotropy materials were established.It was found that the amplitude of L_CRR wave decreased and the ultrasonic velocity increased slightly in elastic anisotropy material.The results of ultrasonic propagation calculation indicated that the direction angle of the main ultrasonic beam of L_CRR wave was 68.2°.On this basis,the influence of surface roughness was analyzed.It was found that with the increase of root mean square surface roughness Rq,the amplitude of L_CRR wave decreased monotonously,and the numerical results were in good agreement with the experimental results.At the same time,it was found that compared with the traditional pulse echo method,the L_CRR wave had good repeatability and better stability in multiple measurements,which was beneficial to the practical engineering testing and application.?2?The results of ASS fatigue and ratcheting experiments indicated that the fatigue process could be divided into three stages:hardening in initial stage,cyclic softening in middle stage and secondary hardening in the last stage,which was reflected in the competitive relationship between dislocation multiplications and annihilations.The ratcheting process was different,hardening rapidly at the beginning of loading and stable plastic deformation until fracture.Both Rq increased monotonously with cyclic loading,of which the maximum of fatigue was 0.5?m,and that of ratcheting reached 7.0?m.Due to the accumulation of plastic deformation,the damage degree of ratcheting was greater than that of fatigue.?3?The fatigue and ratcheting damage of ASS were characterized by L_CRR wave technique.It was found that the normalization amplitude(A_nor)of L_CRR wave showed a linear downward trend with the loading cycle lg?n?for roughed surface specimens after damage.The trend was down-up-down for smoothed surface specimens,and downtend for the ratcheting,which indicated that the propagation of L_CRR wave was affected by the evolution of microstructure such as surface roughness,dislocation and grain boundaries.The attenuation coefficient and ultrasonic velocity of pulse echo method were nonlinear with the loading cycles,which were mainly affected by the evolution of microstructure such as dislocation.It was indicated that A_nor of L_CRR wave was more beneficial to the reverse evaluation of the mechanical damage level of engineering components.It provides the basis for mechanical damage evaluation of engineering components.