Ultrasonic Guided Wave Detection Methods For Fatigue Cracks In Orthotropic Steel Bridge Decks

Orthotropic steel decks have been widely used in long-span bridges owing to their light weights,high load-carrying capacities and rapid erections.However,fatigue cracking in orthotropic steel bridge decks during their long-term service has posed a serious threat to the safety and normal operation of structures.Therefore,efficient and accurate detection of fatigue cracks in orthotropic steel bridge decks is of great significance to ensure the safety of service and operation of long-span bridges and extend their service life.Generally,the locations of cracks in orthotropic steel bridge decks are random.Some fatigue cracks are also concealed and cannot be found by conventional visual inspection.Moreover,fatigue cracks usually do not affect the global static or dynamic characteristics of the structure,which makes it difficult for general damage identification methods to work,and further increases the difficulty of detection.Ultrasonic guided wave-based damage detection is an accurate and effective method in plate-like structures,because guided waves can travel a long distance in waveguide structures(such as plate-like structures),with lower energy loss and more sensitive to small damage.In this paper,according to the characteristics of fatigue cracks at different positions in the orthotropic steel bridge deck,the corresponding guided wave detection methods for fatigue cracks are studied respectively.The detailed research contents are as follows:(1)Regarding to the troublesome problem of low signal-to-noise ratio of the received signal when guided waves are used for long-distance detection in actual orthotropic steel bridge decks,an eigen value decomposition-based guided wave array signal processing approach for weak signals was proposed.Utilizing the orthogonal characteristic of the signal subspace and the noise subspace,the damage information was extracted from the array receiving signals with low signal-to-noise ratio.The effectiveness of the method was verified by numerical and experimental studies.(2)Since the resolution of the traditional delay-and-sum imaging approach cannot satisfy the requirement of fatigue crack detection in trapezoidal rib of orthotropic steel bridge decks,an eigen value decomposition-based algorithm combined with the phase coherence imaging approach was proposed.The eigen value decomposition was used to extract the weak damage scattering signal and suppress the interference of noise.On this basis,the phase coherence coefficient was used to suppress the unfocused components with large phase dispersion to further improve the imaging resolution.The test results in an actual bridge confirmed that the performance of the proposed approach was far superior to the delay-and-sum imaging method.(3)Based on the dual-array multiple signal classification(MUSIC)algorithm,this study develops a baseline-free approach to detect penetrating root-deck fatigue cracks in the orthotropic steel deck.Two linear arrays were used to receive the diffracted wave signals from both ends of the crack in the deck plate,and the MUSIC algorithm was used to identify the arrival direction of the two diffraction sources.According to the characteristics of the crack occurrence location and the relative position information of the two arrays,the positions of the two diffraction sources(that is,the two ends of the penetrating crack)were finally determined.The proposed method can detect such hidden deck cracks without removing the pavement,greatly save manpower and material resources,and ensure the safe service of the orthotropic steel bridge deck.(4)The phase difference between the detected signal and the baseline signal is proposed as an indicator to identify the initial root-deck crack in the orthotropic steel deck.The effectiveness and applicability of the proposed guided wave-based approach are verified by simulation firstly,laboratory experiments and in-situ detection tests on an actual bridge.An automatic crack detection system based on the proposed guided wave-based approach is invented.This crack detection system can detect root-deck cracks in the initial stage of cracking,so that the hidden hazard can be controlled and repaired as soon as possible,and more serious damage to the orthotropic steel bridge deck can be avoided.