Numerical Study Of Shaft Surface Crack By Laser Ultrasonic Detection

Shaft parts under stress concentration and alternating loads for a long time are prone to produce surface cracks.Cracks not only reduce the mechanical properties of shaft parts,but also cause major safety accidents.Because the traditional non-destructive testing technology can not overcome the difficulties of measurement in non-contact and harsh environment,and some shaft parts need shoulders to locate gears,bearings and other parts,the structure presents a more complex staircase shape,which also brings new challenges to the testing.Laser ultrasound has many advantages,such as non-contact,no coupling agent,long-distance excitation and reception.It has a wide application prospect in detecting surface cracks of shaft parts.In this paper,the finite element method is used to establish shaft models with different shaft radius,different transition modes and different crack parameters.The temperature field of different shaft models is calculated numerically.The displacement field of different shaft models is calculated by indirect coupling method.The generation,propagation and interaction between laser-induced surface acoustic wave and crack on shaft surface are studied.In order to study the influence of shaft radius and stepped shaft transition mode on surface acoustic wave propagation,the time-frequency domain characteristics of surface acoustic wave were analyzed by comparing several groups of data.The numerical results show that the dispersion of Rayleigh wave and the group velocity of Rayleigh wave become smaller when the radius of the shaft is larger;the dispersion of Rayleigh wave becomes smaller when the radius of the corner transition is larger,and the group velocity of Rayleigh wave becomes larger;and the attenuation of surface acoustic wave becomes more and more severe when the height difference of the right angle transition is larger.In order to study the influence of crack width and depth on the propagation of surface acoustic wave,the time-frequency characteristics of surface acoustic wave were analyzed by comparing several groups of data.The numerical results show that the time difference of arrival of reflected Rayleigh wave and shear Rayleigh wave is linearly related to the crack depth for both the optical shaft and the stepped shaft of corner transition.The numerical results provide valuable references for laser ultrasonic testing of surface cracks on shafts.