Study On Damage Imaging Of Steel Bridge Deck Based On Laser Ultrasonic

Laser ultrasonic uses the pulse laser excite ultrasonic waves,which can propagate on the structural surface or within the structure.The ultrasonic waves generated with this non-contact method are broadband and multi-mode,and can detect the microcracks on the surface of or within the structures.It is considered as a convenient,flexible and promising nondestructive testing technique.In this paper,the laser ultrasonic method is applied to the nondestructive testing of civil engineering structures,especially the steel plate structures.The main contents are as follows:First of all,the basic mechanism of laser induced ultrasonic wave is summarized.The excitation mechanisms were analyzed,mainly including thermal elasticity and ablation.A physical model of laser induced ultrasonic propagation in homogeneous isotropic solids is obtained and the working principle of four commonly used laser ultrasonic testing methods is introduced.Secondly,the numerical simulation of laser action on metal sheet is carried out based on the finite element thermoelastic coupled analysis method.The influence of the mesh size,the thickness,the half width of the source line and the pulse width on the displacement signal of the laser ultrasonic signal is obtained.More reasonable parameter settings can be used to obtain more efficient and signal-to-noise ratio ultrasonic displacement signals.Subsequently,a laser ultrasonic system with automatic scanning function was built by means of integrated pulse laser,galvanometer,signal acquisition system and LabVIEW software platform,which can be controled by programming voltage,control the laser with the signal generator,and synchronize the signal data acquisition and storage,and realize the function of laser ultrasonic excitation and reception.Finally,Burst test was done by laser external control mode and the time history analysis,frequency spectrum analysis and wavelet time frequency analysis are carried out,and the basic parameters such as propagation speed and frequency components at each time are obtained.Subsequently,a set of laser ultrasonic scanning system is used to carry out the regional irradiation of the damaged aluminum plate surface,and the whole wave field information is obtained according to the reciprocity of the acoustic wave.The whole process of wave propagation is reproduced by wave field imaging method,and the damage location imaging of circular hole defect and slender crack defect is carried out.Furthermore,the linear scanning test of the T bridge deck was carried out by laser ultrasonic method,and the cracks in the weld were identified and located effectively.