Study On The Application Of Synthetic Aperture Focusing Imaging In The Detection Of The Defects Inside Lead Block By Laser Ultrasonic Shear Wave

Lead is used in cable sealing and construction industry frequently with good corrosion resistance and sealing property.There might be small defects inside lead products during manufacturing process,so it is of great value to test them to ensure its reliability.Due to the variety of lead products,the traditional ultrasonic detection is limited.Therefore,the laser ultrasonic detection is taken into consideration.However,when the size of the defects inside lead products is submillimeter or less,the signal-to-noise ratio of echo is low,so it is difficult to locate small defects by the traditional laser ultrasonic technology.In this paper,the synthetic aperture focusing technology(SAFT)is applied to the laser ultrasonic detection.The result shows that this method can improve the signal-to-noise ratio and the detection accuracy effectively,providing a more reliable basis for the qualitative analysis of defects.Firstly,the traditional SAFT is optimized according to the characteristics of multi-mode,broadband and directivity of laser ultrasound,and the sensitivity and resolution ratio of the laser ultrasound detection combined with SAFT are discussed.According to the propagation characteristics of laser ultrasound,the scanning method used in large area imaging is discussed.The results show that this method can improve the recognition ability of micro defects effectively compared with traditional laser ultrasound.Secondly,the imaging difference of laser ultrasonic SAFT in inversion of different characteristic defects in lead is studied.The sensitivity of laser ultrasonic SAFT to two kinds of defects with different geometric characteristics(volumetric type and area type)is discussed.The influence of the area type defects' dip angles on the ultrasound propagation characteristics and the imaging quality are discussed.In addition,the imaging of the area type defects is realized by this method.The results show that the method can reflect the geometric characteristics of area type defects.Then,the influence of multi-mode and wide-band characteristics of laser ultrasound on the ability of laser ultrasonic SAFT to detect defects in different depth is analyzed.The result shows that the multi-mode and wide-band characteristics of laser ultrasound must be considered in the imaging of near-surface defects;while the influence of these characteristics on the imaging of deep defects is very weak so it can be ignored.Finally,the imaging of the internal defects inside freeform-surface sample is realized.Laser ultrasonic SAFT can be used to detect defects under the condition of low SNR of echo waves,which has a certain application value in the field of laser ultrasonic detection.This work can provide a theoretical basis for the application of laser ultrasonic SAFT.