Design Of Focused Electromagnetic Acoustic Phased Array Transducer For Plate Flaw Detection

The metal plate is widely used in the industrial field,but defects is unavoidable to it,and the ultrasonic nondestructive testing(NDT)technology can detect the inner defects effectively,thus the quality of the metal plate can be ensured.The traditional single-element ultrasonic transducer,with the poor beam control ability,owns the low detection efficiency,but the multi-array phased array ultrasonic transducer,which is easy to control the deflection and focus of the beam,has the higher detection efficiency.However,the universal piezoelectric ultrasonic phased array technology depends on the couplant,so its application area is limited.Electromagnetic ultrasonic phased array technology,combining the couplant independent advantage of the electromagnetic ultrasonic and the easy beam control advantage of the phased array,has a wider application prospect.But the sound filed theory of the electromagnetic ultrasonic phased array is complex and the design difficulty of multi-channel electromagnetic ultrasonic phased array device is huge,so the development of the electromagnetic ultrasonic phased array technology is quite slow.Aiming to solve above problems,the focused sound field of the phased array EMAT is studied and the 8-channel electromagnetic ultrasonic phased array detection system is designed in this paper.To solve the problem that it is difficult to describe the acoustic field of the EMAT in analytic methods and low-efficiency in numerical methods,a method is proposed to approximate the far-focusing field of phased array EMAT based on the function-fitting result to the sound field of the single-element EMAT.The approximate analytic solution of the single-element EMAT is obtained by fitting the finite element simulation results.The beam directivity function of the far-focusing field of the phased array EMAT is deduced by combining the analytic solution of sound field of the single-element EMAT and the far-focusing delay law of the phased array.The design method of the far-focusing phased array EMAT is obtained by analyzing the relation between element number,array spacing,deflection angle and the beam directivity function in analytic methods.To solve the problem that it is impossible to describe the near-focusing acoustic field of the phased array EMAT in analytic methods,a method that setting numerical analysis methods based on the finite element simulation model is proposed.Two indexes,which are the axial resolution and the radial resolution,are extracted,through analyzing the two-dimensional simulation model of the near-focusing phased array.The design method of the near-focusing phased array EMAT is obtained by analyzing the relation between key parameters,including element number,array spacing,array aperture,element width,working frequency,deflection angle and focusing position,and the beam directivity function in simulated methods.To solve the problem that it is difficult to design the electromagnetic ultrasonic phased array device,an 8-channel electromagnetic ultrasonic phased array detection system is designed.Aiming at the problem that the energy-tansitting efficiency is low,and the echo signal is weak,the phased array EMAT with high signal intensity,the phased array transmitter circuit with high power and the controllable receiving circuit with high gain and low noise are designed.Aiming at the problem that the range of phased array parameters is wide of and its control is complex,the control program with easy-control feature is designed.The experimental platform of electromagnetic ultrasonic phased array is set up to test the focus-field distribution of the phased array EMAT and verify the theory and simulation of focused sound field of the phased array EMAT.The experimental platform is used to image for the slot defect of 2x2 mm at the bottom surface of the aluminum plate and the hole defect with 6mm diameter in the inner of the aluminum plate by fan sweep.The metho that combining the sub-array to synthesis phased array is used to make up the disadvantage that the channel number of the detection system is small and to improve the focusing ability of phased array EMAT.