Research On Nonlinear Ultrasound For Mechanical Structure Microdefects

During the process in the manufacture and use of metal plates of the mechanical structures,the defects such as cracks,pores,wrinkles and segmentation are produced.In the long run,it leads to the formation of the micro-defects inside the metal structure and the macro-cracks eventually cause the structural fracture.It is significant for the safety service to perform the routine damage detection on the running machinery and equipment,timely discover and eliminate the hidden failures of the equipment and to keep in the healthy condition.Compared with the traditional linear ultrasonic detection technology to detect the macro defects,the non-linear ultrasonic detection technology has higher sensitivity to the micro-structure characteristics of the material.It has the particular advantages for the assessment and detection of the incipient degradation of the material and early damage inside structures.Lamb wave is very suitable for the defect detection of the sheet metal because of its advantages such as small attenuation,long detection distance and fast propagation speed.The detection method used in this thesis is a nonlinear ultrasonic detection based on the Lamb waves?The thesis summarizes the detection mechanism of Lamb wave in detail,introduces the multi-mode and dispersion propagation characteristics of Lamb wave,and draws its dispersion curve and excitation angle curve.The nonlinear dynamics theoretical background in solids is introduced.The equations of ultrasonic nonlinear coefficients are derived.The experimental system for the ultrasonic nonlinear detection based on nonlinear Lamb wave is established.It is determined about the parameters of the excitation probe the used emission probe and the used and the choice of excitation signal.The system is used to detect two different types of aluminum alloy plates with defects.The relationship between the amplitudes of the second harmonics and the non-linear coefficients,and the ultrasonic nonlinear coefficients and the depths of the defects and the sizes of the fatigue cracks are analyzed.The pattern of the Lamb wave signal with defects in aluminum alloy sheet is demonstrated.The experimental results show that the nonlinear Lamb wave detects the sizes of defects in aluminum alloy sheet.The depths of artificial cracks and the degree of fatigue crack damage have a good linear relationship with the nonlinear coefficients.The depth reaches the certain distance or the occurrence of macro-cracks limits the detection results.It shows that the detection system based on Lamb wave has requirements for the thickness of the plate.The nonlinear effects in fatigue cracked plates are mainly generated by the fatigue cracks.The macro-cracks have little effect on the nonlinear effects.