| Currently,2205 duplex stainless steel(DSS 2205)has been put into use in bridge structures due to its corrosion resistance,high temperature resistance,high strength,good ductility and low life-cycle cost.The reliability and safety of the bridge structure,which has been subjected to cyclic stress,such as vehicle load for a long time,are the dominant factors.Acoustic emission(AE),as an accurate and efficient real-time monitoring method,is widely used in the study of mechanical properties of metal materials which under fatigue loads.AE detection technology can be used for fatigue damage monitoring of the bridge main structure,which is convenient for timely understanding of the internal damage degree of the structure.Therefore,this paper combines theoretical analysis with experimental research,based on AE technology,analyzed the internal damage under different stress ratio conditions.Here are the primary research contents and results:Firstly,fatigue tests were conducted on the 2205 duplex stainless steel specimens under different stress ratios,while collecting AE signals during the test,and analyzing the fatigue tests results.It is found that when the stress ratio R is over zero,the fatigue crack growth rate da/d N of the specimen in the loading process gradually decreases,the internal damage process of the specimen gradually slows down,and the fatigue life gradually increases with the increase of the stress ratio R.Performing a linear fit on the double logarithmic coordinate axis to crack growth rate da/d N and stress intensity factor amplitude ?K,the Paris formula is used to verify the correctness of this fatigue test,and then the validity of the AE signals collected during the test is determined.Secondly,through the analysis of the history diagram and correlation of AE characteristic parameters collected during the test,it is found that under the effect of cyclic stress,the AE characteristic parameters generated by the test specimens describe a cyclic pattern of "continuous high-continuous low-continuous high".As the stress ratio increases,the internal damage levels in the specimens gradually decrease.With the phase characteristics of the generated AE signals decline,the correlation between the AE characteristic parameters weaken.Through the analysis of the AE amplitude using statistical indicators,it is found that the margin index and kurtosis index showed a "high-low-high" cyclic change law during loading.With the increase of stress ratio,the internal damage process of the specimens slows down,the AE signals gradually stabilize,and the variation range of the margin index and kurtosis index gradually decreases.Finally,AE ringing count is used as the characterization parameter of specimen damage based on critical slowing down theory,and analyzing the precursor signals of specimen failure.It is concluded that different window lengths and lag steps have effects on the variance and autocorrelation coefficients of the AE ring count sequence that represents the destructions of the specimen.The critical slowing down arises along with the variance and the autocorrelation coefficient increase rapidly before the specimens break.Meanwhile,based on the damage model about the AE cumulative ring count-fatigue crack length,it is observed that the mean error between the actual crack length and the calculated crack length is less than 0.1 and the variance is less than 0.02 under different stress ratios,and the error range gradually decreases with the increase of stress ratio.The fatigue crack growth length is characterized by the cumulative ring count,which can predict the damage degree of the specimens more intuitively.This paper provides data support for the application of AE technology to fatigue monitoring and life warning of duplex stainless steel. |
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