Research On Non-contact Laser Ultrasonic Testing Method Using Large-dimension Flexible Photoacoustic Transducer

Metal materials are widely used in many sectors of life,thanks to the rapid growth of modern industrial technology.Metal component defect detection is a critical step in preventing minor metal flaws from escalating into major flaws that cause property damage and even death.Metal flaws can be detected using the laser ultrasonic technique.High-power lasers can readily burn the surface of the component to be tested in current laser ultrasonic applications,and the signalto-noise ratio of the ultrasonic signal generated by the laser under non-destructive conditions is so low that it cannot be employed.Detection accuracy and depth are guaranteed.As a result,a simple and effective laser ultrasonic detection technology that can reliably find faults under non-destructive conditions is urgently needed.The National Natural Science Foundation of China's key project "Research on the Key Technologies of Intelligent Detection of Large-scale Aeronautical Composite Materials Complex Profile Adaptive Phased Array 3D Focusing Imaging" and the Shenzhen Key Laboratory of Intelligent Sensing and System Detection have both contributed to this paper.Using large-scale photoacoustic transducers(PAT)and laser ultrasonic methods to achieve large-area rapid detection of the metal surface and deep defects as the research object,a new method is proposed that uses large-scale photoacoustic transducers and laser ultrasonic methods to achieve large-area rapid detection of the metal surface and deep defects.To begin,a largescale flexible photoacoustic transducer with uniform performance was designed and constructed on the aluminum plate's surface,and the photoacoustic transducer's acoustic excitation performance was tested in situ.The transducer construction is optimized,and ultimately,utilizing the optimized photoacoustic transducer and the synthetic aperture focusing approach,focused imaging of the surface and deep defects of the aluminum plate are achieved.The findings of the experiments reveal that using a photoacoustic transducer and laser ultrasonic testing together can provide accurate fault detection in non-destructive conditions.This work is unique in that it proposes a large-scale flexible photoacoustic transducer preparation and optimization approach with consistent performance,as well as the non-destructive identification of surface and deep defects in metal components using a laser ultrasonic method.The following are the paper's main points and innovations:(1)The laser ultrasonic testing technology is introduced,and the research status of the PAT is summarized from two aspects of theoretical research and experimental application.The physical process of photoacoustic conversion of PAT is theoretically analyzed,and a twodimensional simplified model of PAT is designed.Based on classical heat transfer and acoustic wave propagation theory,the temperature distribution and sound pressure output characteristics in the photoacoustic transducer are theoretically explained with the adoption of the method of replacing the light source with an equivalent heat source.Finally,the sound pressure distribution expression on the surface of the PAT is given based on the theory and model.(2)A large-dimension flexible PAT is designed and fabricated.The robotic arm is used to precisely control the movement of the substrate along the serpentine route so that the candle soot deposition paths are superimposed in an orderly manner to obtain a uniform thickness candle soot nanoparticles(CSNPs)layer.Then,based on the liquid spin-coating process,the thermoelastic material polydimethylsiloxane(PDMS)is combined with a light-absorbing layer of uniform thickness,and a large dimensional PAT is fabricated on a 3 mm thick aluminum plate.A laser ultrasonic testing system is built to characterize and analyze the samples(the PAT and the aluminum plate).The test results show that the waveform and frequency characteristics of the sound waves generated at different positions on the sample are the same,which successfully verifies the working effectiveness and control reliability of the preparation method.(3)The performance of the PAT to excite acoustic waves is optimized.The influence of the three-layer structure of the PAT on the excitation acoustic wave performance is studied by experiments and simulations: 1)he photoacoustic conversion layer: the acoustic excitation performance of the PAT with different thicknesses of the photoacoustic conversion layer is experimentally fabricated and tested,and analyze the test results from the three processes of the PAT generating sound waves respectively,and calculate the optimal thickness;2)the protective layer and the acoustic matching layer: the photoacoustic conversion layer is established by COMSOL software.The two-dimensional axisymmetric model of the energy device is used to analyze the relationship between thickness and acoustic waves,and the calculation formulas for the optimal thickness of the two are given respectively.Finally,a large dimensional PAT is fabricated under the optimal thickness parameters of each layer,and the results show that the structure-optimized PAT is suitable for defect detection of metal components.(4)Detect and locate the surface and deep defects in the aluminum block using optimized PAT.For surface defects,a certain area of the surface of the aluminum plate is covered by the method of arranging the detection points in a ring.By calculating the flight time of the acoustic wave from the acoustic excitation source to the acoustic detection point,the signal amplitudes are corresponding to all points in the area,and process the data based on the synthetic aperture focusing technology(SAFT),and obtain the two-dimensional imaging location image of the defect;for the deep defect of the aluminum plate,the transmission and reflection experiments are carried out on the test aluminum block by using the high-amplitude longitudinal wave excited by the PAT,process the test data respectively,and draw a two-dimensional image to determine the landscape position and portrait depth of the defect.The non-contact laser ultrasonic testing method using large-dimension flexible photoacoustic transducers presented in this paper realizes the position detection of surface and deep defects of aluminum plates,which has certain application potential in defect detection of metal components.