Abstract
Compared with conventional linear ultrasonic inspection methods, which are sensitive only to severe defects, nonlinear ultrasonic inspection methods are better for revealing micro-cracks in thin plates. However, most nonlinear ultrasonic inspection methods have only been experimentally investigated using bulk or Rayleigh waves. Numerical studies, especially numerical simulations of Lamb ultrasonic waves, have seldom been reported. In this paper, the interaction between nonlinear S0 mode Lamb waves and micro-cracks of various lengths and widths buried in a thin metallic plate was simulated using the finite element method (FEM). The numerical results indicate that after interacting with a micro-crack, a new wave-packet was generated in addition to the S0 mode wave-packet. The second harmonics of the S0 mode Lamb waves and the new wave-packet were caused by nonlinear acoustic effects at the micro-crack. An amplitude ratio indicator is thus proposed for the early detection of buried micro-cracks.
Highlights
Cracks can be a major source of concern in safety-critical structures, such as the vital components of aircraft, nuclear power plants, chemical plants and refineries, because they can lead to serious damage or fractures
It can be clearly seen that a new wave-packet has appeared in the time-domain signal obtained from the micro-cracked plate
The point force method was applied to approximate piezoelectric ceramic transducer (PZT) wafer transmitters; the micro-crack was modeled as an oval shape with hard contact and frictionless surfaces; an optimal exciting centered frequency was selected via a tuning curve, and proper element size and time step were selected to ensure the model’s accuracy and efficiency
Summary
Cracks can be a major source of concern in safety-critical structures, such as the vital components of aircraft, nuclear power plants, chemical plants and refineries, because they can lead to serious damage or fractures. Both the conventional ultrasonic bulk wave inspection method and the Lamb wave technique are based on linear theory, and both depend on measuring particular parameter, such as sound velocity, attenuation, or the transmission and reflection coefficients of the propagating waves These parameters are sensitive only to gross defects, opened cracks, or macro-cracks within structures. In that particular investigation the micro-crack introduced into the inspected specimen was visible and on the surface, it has been experimentally shown that the nonlinear Lamb wave technique has potential for detecting structural micro-cracks. Shen and Giurgiutiu [19,20] adopted FEM to simulate the interaction between nonlinear Lamb waves and a surface-breathing crack in a plate We used finite element analysis to simulate the interaction between nonlinear Lamb waves and a buried micro-crack perpendicular to the upper and lower surfaces of a thin metallic plate.
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