Non-contact And Nondestructive Testing Of Cartridge Case Internal Manufacturing Defects By Using Laser Ultrasound Technology

During the manufacture of 7050 aluminum alloy cartridge case,it is possible to produce transverse tensile defects due to the process.The nondestructive testing of cartridge case is a very important step before the case leaving the factory,and has important military and economic significance.At present,it is difficult to realize non-contact testing of the internal micro defects in cartridge case by the existing methods.Laser ultrasound technology has the advantages of non-contact excitation and detection,high spatial resolution,high signal-to-noise ratio and so on.In this paper,the propagation characteristics of laser ultrasound and the interaction between ultrasound and internal defects in cartridge case are studied theoretically and experimentally.Finally the non-contact detection and localization of the internal defects in cartridge case are realized.Based on the classical heat conduction equation and thermo-elastic theory,the physical model of the excitation and propagation of ultrasonic wave in cartridge case is established and solved by the finite element method.The propagation characteristics of ultrasonic wave of axial and circumferential are analyzed,and the influence of cartridge case structure on the ultrasonic wave characteristics is discussed.Besides,the propagation characteristics of ultrasonic waves in uniform thick plate and wedge plate are discussed.Finally,the influence of the thickness on the characteristics of acoustic wave is verified.On this basis,the physical model of the interaction between ultrasonic wave and defect in cartridge case is established.The Pulse-echo method and Time of Flight Diffraction method are used to detect the internal defects,respectively.The interactions between defect and ultrasonic wave are discussed,and the recognition basis of internal defect in cartridge case is given.An experimental system is established to study the propagation characteristics of the ultrasonic wave in different regions of cartridge case by the method of scanning laser line source.The artificial defect in cartridge case is detected experimentally,and the results are in agreement with the theoretical results.Furthermore,the defects which have the same width and varying depth in 7050 aluminum alloy plate with 2mm thickness are also tested.The detectable depth of the internal defects under the current laser parameters is given.The research results can provide theoretical and experimental reference for the study of cartridge case structure and defects detection by laser ultrasound technology.