Study On The Detection Of Sub-millimeter Pinhole Corrosion By High-frequency Electromagnetic Ultrasonic Excited High-order Lamb Wave

Metal materials are widely used in petroleum,chemical,aerospace,shipbuilding,nuclear industry,high-speed trains,machinery industry,pressure vessels and other industrial fields.However,long-term use will be accompanied by cracks,corrosion,wear and other defects.The defects are initially manifested as sub-millimeter pinhole corrosion defects.Sub-millimeter pinhole corrosion is common in pressure pipelines,heavy machinery and other special equipment and industrial metal materials.It has the characteristics of small size and is not easy to be found,and its diameter is about 0.5 mm ? 1 mm.It is a difficult problem in engineering testing.Electromagnetic ultrasonic detection method has the advantages of high detection accuracy and simple sensor structure,which is widely used in various fields.In this paper,the method of electromagnetic ultrasonic excitation and receiving high-order Lamb wave mode is proposed to realize the detection of sub-millimeter pinhole defects.The points with small dispersion degree at high frequency are analyzed by phase velocity dispersion curve,and then the design parameters of the special electromagnetic ultrasonic transducer with folded coil are determined according to the design formula of the folded coil.The line spacing is 1 mm and 0.75 mm,respectively.The frequency characteristics of the two transducers are studied.The optimal working frequency of the two transducers on different aluminum plates is determined by sweep experiment.The high-frequency Lamb wave mode is effectively excited and received between 2.8 MHz and4 MHz.By selecting different operating points on the dispersion curve,the influence of different operating frequencies on the received signal is studied.The influence of different excitation periods on the received signal and signal spectrum is studied by changing the number of excitation signal cycles.Due to the dispersion and multi-mode characteristics of the guided wave,the dispersion makes the guided wave packet wider,the amplitude decreases,the detection sensitivity decreases,and the multi-mode phenomenon will appear multiple modes,which makes it difficult to distinguish the characteristics of each wave packet.The dispersion and multi-mode effects of different working frequencies on high-frequency modal signals are studied by setting different distances between the excitation and receiving sensors,so as to obtain the effective detection distance of two kinds of sensors to excite high-order Lamb waves.By establishing a two-dimensional simulation model to simulate the excitation,propagation and reflection process of high-order Lamb waves in aluminum plates,the propagation characteristics of high-frequency Lamb waves are studied,and the propagation law of high-order Lamb waves is analyzed,which is used to compare with experiments.Different types of defects,including through holes,cracks and sub-millimeter-level pinhole defects with different depths,are processed to study the detection sensitivity of high-frequency Lamb waves.The experimental results show that the high-order mode of Lamb waves can effectively detect pinhole defects,and the detection sensitivity can achieve sub-millimeter-level pinhole defects of ?0.5×0.5 mm.