Fatigue Crack Detection And Life Evaluation Of U-Rib To Deck Weled Structures Based On Nonlinear Lamb Wave

The orthotropic steel bridge deck(OSD)has the advantages of light weight,high bearing capacity and strong spanning capacity,and is the first choice for long-span bridges.However,in the process of manufacturing in the factory,various plates are connected by many welds,which makes the bridge panel prone to weld fatigue cracking.There are two reasons for the cracking: one is the internal cause,there is an initial welding defect during welding;the other is the external cause,the bridge deck directly bears the vehicle load,and under the repeated action of the vehicle load,fatigue cracks occur at the weld.In the orthotropic steel bridge deck,among the fatigue failure modes of the deck-to-rib joint,the fatigue cracks of the weld root accounts for a large proportion.In the crack of weld root,there is a pavement layer on the bridge deck and its cracking should not be observed,and it is concealed.Once cracking occurs,a crack is formed through the roof,which seriously affects the safety of the bridge deck.Therefore,the early detection of the failure mode of rib-to-deck welding root is particularly important.According to the principle of "early detection,early treatment",it is of great significance to detect the early warning of cracking,timely reinforcement and maintenance,and grasp the best reinforcement time.Aiming at the fatigue failure mode of its welding root,the fatigue crack detection of its deck-welding failure mode based on ultrasonic guided wave technology is carried out in this paper.The main research contents and conclusions are as follows:(1)According to the force mechanism and performance of the orthotropic steel bridge deck,based on the theory of linear elastic fracture mechanics,the deck-weld root failure mode was analyzed,the entire crack propagation process was prefabricated,and the size of each step of crack propagation was obtained,which provides a basis for ultrasonic guided wave detection of cracks.(2)To obtain the nonlinear ultrasonic effect,a dispersion curve with a thickness of 18 mm was drawn.According to the phase velocity matching conditions,the central excitation frequency is determined to be 80 k Hz,and a suitable excitation waveform is excited to obtain the interaction with the crack surface.Nonlinear ultrasonic effect after action-high order harmonic phenomenon.(3)Fracture mechanics prefabricated crack size,ultrasonic guided wave detection,and sweep detection along the direction of the weld.The guided wave signal in the cracked area and the non-cracked area have obvious changes,and the nonlinear coefficient β is used to determine the position of the cracked area.There is a closed-breathing crack in the cracked area,its nonlinear coefficient amplitude β is higher than other places,there is a sudden change,the undetected crack provides a basis,and the nonlinear coefficient β is used to evaluate the entire crack propagation,As the crack grows,its nonlinear coefficient increases continuously.(4)In order to further calibrate the crack damage area,reconstruction algorithm for probabilistic inspection of damage(RAPID)is used,and the cross excitation-receiving sensor network diagram of multiple detection paths is arranged.Carry out flaw detection with different damage degrees for each detection path,compare the waveform difference in the time domain,the damage exists in the path with a large difference coefficient,and use the difference coefficient to evaluate the crack growth.The larger the size,the further the damage location is located according to the tomography,and the damage location is located on the intersecting path formed by multiple detection paths.