Research On Laser Ultrasonic Characteristics Of Surface Defects Of Coated Cylindrical Parts

Cylindrical metal parts are important load-bearing parts in the industry.In engineering practice,micro-cracks are often generated on the surface of the parts,which has a serious impact on the performance of the parts.The traditional non-destructive testing method belongs to the contact detecting method,and it has the disadvantage that it cannot adapt to the harsh environment such as high temperature and corrosiveness.Laser ultrasonic is a non-destructive technology developed in recent years.It has the advantages of non-contact,no damage,high time and spatial resolution,and has become one of the hotspots in the field of non-destructive testing.In this paper,the laser ultrasonic characteristics of the surface defects of coated cylindrical parts are studied based on laser ultrasonic technology.The main research contents are as follows:In this paper,the basic principle of laser ultrasonic excitation is analyzed based on the basic thermoelastic theory.The basic methods of laser ultrasonic nondestructive testing and the basic principles of laser ultrasonic finite element analysis are expounded.In order to study the laser ultrasonic properties of coated cylindrical surface defects,a finite element model of coated cylindrical parts was established.The relationship between surface coating,surface defects and ultrasonic surface wave signals was developed by numerical simulation methods.In order to study the influence of metal surface coating,a cylindrical finite element model with different thicknesses was established,and the temperature field and displacement field results were analyzed.When the coating exists,there is a big difference in temperature distribution due to the differences in material parameters.The results of the displacement field show that the metal nickel coating causes dispersion in the ultrasonic surface wave signal.At the same time,the surface acoustic wave velocity is different at different coating thicknesses.In order to detect the surface defects,a finite element model of different defect depths and widths was established,and the reflected and transmitted surface wave signals were analyzed.The results show that there are characteristic peaks RR and RS in the reflected signal without coating and with coating thickness of 200μm and 400μm.There is a phased linear relationship between the time difference ?t and the defect depth.In the transmission signal after cracking,the time-domain signal with or without coating has poor signal-to-noise and is difficult to be used for defect identification.When a coating is present,the amplitude of the frequency domain signal of the transmitted signal decreases as the depth of the defect increases.The research results in this paper will provide a meaningful reference for the detection of surface defects in coated cylindrical parts by laser ultrasonic testing.