Study On Excitation Characteristics Of Laser Ultrasonic Array Source And Its Application In Metal Internal Defect Detection

With the rapid development of the "post technology" era,various industries and technologies are developing rapidly with great momentum,and human life is changing with each passing day.In order to ensure that the mechanical workpiece operates in good condition,it is necessary to detect the subtle fracture and defect on its surface or inside in time.The traditional inspection technology often needs to smear the coupling agent on the workpiece to achieve the purpose of inspection.This not only affects the detection efficiency,but also pollutes the specimen to some extent.In extreme cases,it is impossible to complete the detection task.Laser ultrasonic nondestructive testing technology is a new testing technology developed rapidly in recent years.It has the characteristics of no coupling agent,rich wave modes,no harm to human body,strong environmental adaptability and so on.It has objective development prospects in many fields,such as machinery,aerospace,shipbuilding and so on.In order to accelerate the large-scale and industrialization of laser ultrasonic nondestructive testing technology in the industrial field,it is necessary to constantly enrich and improve the laser ultrasonic theoretical system,fully study and study the excitation and propagation laws of ultrasonic signals under different light source modes,summarize the optimal detection strategies for different types of defects,and summarize effective methods to improve the excitation efficiency of laser ultrasonic nondestructive testing technology,so that laser ultrasonic nondestructive testing technology can meet the needs of different fields NDT requirements for different environments and structures.At present,researchers have done a lot of research on point light source,line light source and ring light source,while the research on array light source is still in a relatively shallow stage.In order to explore the excitation characteristics of laser ultrasonic array source and its effective method for detecting metal internal defects,the work of this paper is as follows:Based on the principle of thermoelastic coupling and the basic principle of finite element method,the simulation model of ultrasonic excitation on aluminum surface by array linear light source with different parameters is created by using finite element software.The concept and calculation method of excitation efficiency are proposed.The effects of the number of laser beams,total excitation energy and laser beam spacing on the excitation efficiency at different depths inside the metal are summarized.The effects of different excitation parameters on the excitation efficiency of ultrasonic S-wave and ultrasonic P-wave are studied,and the effective schemes to improve the excitation efficiency of ultrasonic S-wave and ultrasonic P-wave are summarized.The structural characteristics of array ring light source are analyzed.The effects of superposition wave and time modulation method on excitation efficiency are determined as the research objects,and the finite element models are established respectively.Through the analysis of the simulation results,the relationship between the superposition wave characteristics and the geometric parameters of the light source is summarized.By extracting and comparing the signal amplitudes of superimposed wave and above and below superimposed wave,it is concluded that the occurrence of superimposed wave will improve the excitation efficiency.By comparing the changes of the signal amplitude of the target point before and after using the time modulation method,it is concluded that the time modulation method can make the superimposed waves generated by the ring light sources with different radii converge at the same point,which solves the problem that it is difficult for the superimposed waves with different depths generated by the rings with different radii to intersect.Based on the theoretical study of ultrasonic superimposed wave generated by array light source,combined with the characteristics of array line light source and array ring light source,a combined scanning method of array light source is proposed to detect internal defects.The theoretical model of this method is established,the relationship between the change trend of echo signal and the information of defect location and size is analyzed,and the specific calculation method is given.It provides a feasible scheme for the application of array light source in defect detection.This paper creatively puts forward the calculation method of excitation efficiency,and studies the relationship between the excitation parameters and excitation efficiency of the array line light source and the array ring light source acting on the metal surface,which provides a reference for the laser acting on the metal surface to efficiently generate ultrasonic signals for the detection of metal internal defects.