Simulation Analysis Of Defect Characteristics Of High-strength Screw Billet Based On Laser Ultrasound

High-strength bolts are widely used in aerospace,weapons and equipment,production equipment and other fields because of their high strength,high stress bearing and long service time.As an important part of the production process of high-strength bolts,high-strength screw blanks are prone to micro-defects such as micro-cracks,micro-pores and folds that are hard to distinguish with naked eyes during the cold heading process.Timely and effective detection of these defects is of positive significance for subsequent parts and improving the stability and reliability of mechanical equipment.At present,the defect detection of high-strength screw billet is mostly based on the traditional ultrasonic detection technology,the detection accuracy and efficiency are relatively low,and coupling agent is needed.As a new type of ultrasonic testing technology,laser ultrasound has a broad application prospect in the field of nondestructive testing with the advantages of non-contact,high precision and long-distance receiving.Based on the mechanism of laser ultrasonic thermoelastic excitation,the propagation law of ultrasonic wave in the circumferential and axial direction of high-strength screw blank and the method of defect location and characterization are studied by means of numerical simulation,so as to realize the nondestructive evaluation of the specimen.The specific research contents are as follows:1.Based on the basic theory of laser ultrasound,the model of circular surface defect and the model of axial defect were established by finite element method,and the transient temperature and stress fields were simulated.The influences of different laser excitation radius,laser rising time and the output energy of the light source on the ultrasonic signal are discussed.2.According to the transmission principle of laser ultrasound on the circumferential surface of metal specimens,the scanning detection source method and scanning excitation source method were respectively used to analyze the location of defects on the circumferential surface of screw billet.By comparison,it was found that the amplitude of ultrasonic wave peak near the defects changed greatly when scanning excitation source method was used,which was more conducive to the identification of defects.By analyzing the relationship between the time difference of Rr and R'r characteristic echoes generated by the circumferential surface and the defect depth,the quantitative calculation method of the defect depth was studied.By numerical fitting and comparison,the method can achieve the defect depth characterization within the error tolerance range of 0-1.5mm.3.According to the propagation principle of laser ultrasound on the axial surface and inside of metal specimens,the influence of two-dimensional and three-dimensional screw defects on ultrasonic wave amplitude and transit time was analyzed,and the echo of three-dimensional screw defects was verified by experiments.And the location and size of the defect rough judgment.Rr amplitude is positively correlated with the defect depth,while transmitted wave is negatively correlated with the defect depth.The arrival time of Rr wave and the amplitude of transmitted wave are negatively correlated with the defect height.