The Design Of An Array Flexible Sensor For Weld Defect Detection Based On Ultrasonic Guided Wave

Weld joint is widely used in large plate structure and plays an indispensable role,especially in large structures such as pressure tanks,ships,offshore platforms,et al.Because of the special structure of the weld joint,stress accumulation and stress concentration often occur,which leads to the occurrence of defects,aging and even fracture more easily at the weld than at other locations in the plate,and would result in very serious consequences.Therefore,the defect detection of weld quality is particularly important.The ultrasonic guided wave technology has unique advantages which can detect the whole weld joint in a single sensor position at once fast and efficiently.Based on the theoretical study of ultrasonic guided waves,this paper explored the propagation characteristics of typical modes of ultrasonic guided waves,designed the type of weld feature guided wave sensors,excitation modes and arrangement modes of single sensor element,et al,and aimed to design a weld guided wave sensor array composed by piezoelectric wafers.The main research results obtained as followed:?1?Based on the finite element analysis model,the propagation characteristics of Lamb wave and SH wave in the plate and the"energy trapping effect"of weld-feature guided wave in weld structure were studied.According to dispersion characteristics and"energy trapping effect",it was found that the dispersion characteristics of Lamb wave and SH wave is quite different.SH wave has a single mode in a lower frequency range,while Lamb wave has more than two different modes propagating simultaneously in any frequency range.By establishing finite element simulation analysis,the"energy trapping effect"and propagation characteristics of SH wave and Lamb in weld joint were further compared and analyzed.The feature guided wave SH_w0 mode which analogous to the vibration mode of SH₀ mode in plate is better than the feature guided wave S_w0 mode whose vibration mode like Lamb wave in a plate.?2?Based on piezoelectric theory,it was completed that piezoelectric excitation elements which are the core component of array flexible sensor,the arrangement of array and the excitation mode were designed.According to the vibration and the main propagation direction of SH_w0 mode,the vibration mode of piezoelectric ceramics,the electric field direction,polarization direction and size of piezoelectric ceramic were further determined.According to the principle of space filling,the array sensors were designed as 2D matrix type.The effect of direction signal fusion was analyzed and improved:the signal superposition between the elements in axial which was better used simultaneous excitation directly,while the signal between the elements in circumferential direction used by asynchronous excitation.On the basis of theoretical and simulation analysis,the arrangement and excitation mode of array flexible sensors were designed,and the problem of signal fusion was solved,so that the sensor can produce a signal without signal aliasing and with high energy.?3?It was completed that the whole package of array flexible sensor was designed,and the series-parallel connection mode and spacing between excitation elements were determined.In addition,the performance of array flexible sensor was evaluated.The arrangement spacing of sensor arrays in the axial and circumferential directions,the series or parallel connection modes between sensor elements,and the packaging materials of sensor arrays were studied and determined.The spacing of the elements in a same row is 3 mm and that in a same column is 3.26 mm,polyimide film is used to encapsulate the upper and lower surfaces of the whole flexible sensor,which ensure the good electrical,thermal properties,and the flexibility of the sensor.By building an experimental platform for ultrasonic guided wave weld detection,the time-sharing excitation of the designed sensor is carried out by using signal generator and FPGA Spartan6 development board,and the detection results of different defects are compared with existing sensors.The results showed that the designed array flexible sensor had higher signal energy,higher end echo rate and defect echo rate,and the weld guided wave excited had more concentrate propagation in the weld,so it is more suitable for long-distance weld detection.The research results in this paper can provide important reference for the application of ultrasonic guided wave technology in the rapid health diagnosis of large weld joints.