Detection And Calibration Of Phased Array Ultrasonic Total Focusing Method Imaging For Curved Structures

There are a lot of curved structures in engineering applications.When using Phased Array Ultrasonic Testing（PAUT）technology to detect curved surface structures,there are some problems,such as coupling difficulty,low detection resolution and artifact interference,which make defect signal difficult to identify.Ultrasonic post-processing imaging technology such as the Total Focusing Method（TFM）developed by PAUT can usually be used to improve the detection sensitivity and resolution,reduce the detection blind area,and improve the detection effect of defects.However,due to the influence of the shape and structure of the surface,there are still some problems such as artifact residue and uneven energy distribution in the imaging area.In this paper,TFM is used to study the imaging detection of the surface structure contour and its internal defects,and TFM calibration is used to reduce the artifact interference in the surface contour imaging.A two-layer medium TFM calibration model suitable for the surface structure is proposed and applied to improve the imaging quality of the cross-through hole.Specific research contents are as follows:In this paper,the samples with different heights Amp_mand lengthsΨ_nwere designed and processed.The five height parameters of the samples ranged from 2 mm to 14.44 mm,expressed as Amp_1-5respectively,and the five length parameters ranged from 27 mm to108.12 mm,denoted asΨ_1-5respectively.The height and length parameters were arranged and combined to obtain a total of 25 segments of sinusoidal surface structure.The maximum curvatureσ_maxrange of the sample is 6.6°≤σ_max≤73.45°.In addition,on the Amp₃sample with a height of 8.22mm,aΦ2 mm side drilled hole was processed 10 mm below the maximum curvature position of each surface,and there were 5 in total.The acoustic parameters such as longitudinal wave velocity of the sample material were measured,and the PAUT simulation model of the sample with curved surface structure was established to optimize the ultrasonic testing parameters.The influences of the probe center frequency（f）,array element width（a）and array element number（N）on the sound field are analyzed.Combined with the features of curved surface structure,a linear array probe with center frequency of 5 MHz,array element width of 0.52 mm and 64 array elements was selected for detection.In addition,by analyzing the influence of the distance h from the probe to the sample on the contour imaging of the surface,the value of this parameter suitable for different samples was determinedThe samples with different curvatures of sinusoidal surfaces were simulated and tested by TFM,and the missing surface contour information was fitted by linear interpolation.The simulation results show that the error of contour reconstruction gradually increases with the decrease of Amp_mand the increase ofΨ_n,that is,with the increase of the maximum curvatureσ_maxof the surface,the error of contour reconstruction presents an exponential trend.The variation trend of experimental contour reconstruction error with the increase of curvature is basically consistent with the simulation results.According to the experimental results,the calibration model based on array element directivity was used to suppress the artifact interference in the TFM image.After the calibration,the average error of surface contour reconstruction was reduced by 0.55 mm at most,and the maximum error was reduced by 4.8mm at most.Based on the extracted surface contour information,TFM was used to detect the image of the side-drilled holes inside the Amp₃sample with a surface height of 8.22 mm.The simulation results show that the imaging amplitude decreases with the increase of curvature,the maximum quantitative error is 0.3 mm,and the Array Performance Index（API）fluctuates from 1.67 to 7.82.A TFM calibration model is proposed and applied to two-layer media with curved structure.It can be concluded from the experimental results that the API index fluctuation range is reduced to 0.81～1.31 after calibration.The experimental results show that,except the side drilled hole with the largest curvature,which is affected by acoustic attenuation factors,can not obtain good imaging results,other side drilled holes get clear images.The image amplitude of the side-drilled holes decreases with the increase of the curvature of the surface,and the quantitative error range is 0.1mm～0.7mm.After calibration,the quantitative error is reduced to 0.1～0.5mm,and the API index range is reduced from6.5～12.14 to 0.63～0.87,which is in good agreement with simulation results.