Crack Location In Orthotropic Steel Bridge Decks Based On Improved Delta-T Mapping Of Acoustic Emission Waves

Orthotropic steel bridge decks are widely used in large-span bridges all over the world because of its advantages such as good integrity,large bearing capacity,light weight,fast construction,and beautiful appearance.However,the fatigue problem of orthotropic steel bridge decks is increasingly prominent under the long-term repeated integral structural loads and local wheel loads.There is no effective method to diagnose and monitor the fatigue cracks of orthotropic steel bridge decks in their operational status.Considering the mechanical characteristics,geometric properties and operational noise of steel bridge decks and taking the advantages of acoustic emission(AE)technology in crack detection,this thesis proposed an improved delta-t mapping(DTM)method to identify and locate the fatigue cracks in orthotropic steel bridge decks.The main contents of this thesis are listed as follows:(1)The off-the-shelf AE source location methods were reviewed,including time of arrival(TOA),single sensor modal analysis location(SSMAL),beamforming analysis and delta-t mapping(DTM).Among these methods,DTM was proved to be more suitable for the crack location in large-scale and complex structures,such as orthotropic steel bridge decks.After measuring the arrival times of AE sources simulated by pencil lead breaks at all grid nodes,DTM establishes time-difference distribution maps for the structure of interest,and then use these maps to locate crack-induced AE sources through the weighted algorithm.DTM is able to achieve higher localization accuracy,since it takes into account velocity change,reflection and dispersion phenomenon of AE waves during their propagation.(2)Finite element simulation of the generation and propagation of AE waves in the steel plate was carried out.The propagation characteristics of AE waves in the steel plate were investigated by analyzing the waveforms at different propagation distances.The dispersion and multi-mode phenomena of Lamb wave were observed and their effects on the AE source location were also discussed.(3)Source location experiment on a full-scale orthotropic steel bridge deck was done in the laboratory.The area to be monitored on the plane side of the bridge deck was of 1.2m×0.8m,and that on the ribbed side was of 1.2m×1.2m.The location results of TOA and DTM were compared.The average localization error of DTM was 91 mm on both sides of the bridge deck.It was far less than that of TOA,which was 293 mm.(4)In order to further depress the influence of dispersion phenomena and operational noise and to improve the interpolation efficiency of time-difference distribution maps,an improved DTM based on continuous wavelet transform(CWT)and radial basis function(RBF)interpolation was proposed.The arrival times of AE waves were determined by CWT.The time-difference matrix was interpolated by RBF to obtain more accurate time-difference distribution maps.Source location experiment was carried out on the orthotropic steel deck of a suspension bridge in the field.In an area of 3m×3m,the location errors of TOA,DTM and improved DTM were 547 mm,219mm,and 146 mm respectively.The results showed that the improved DTM proposed in this study has higher AE source location accuracy and could be used for crack monitoring of orthotropic steel bridge decks in the field.