Xiang Yanxun

Cumulative second-harmonic generation of Lamb waves propagating in a two-layered solid plate ⁄

The physical process of cumulative second-harmonic generation of Lamb waves propagating in a two-layered solid plate is presented by using the second-order perturbation and the technique of nonlinear reflection of acoustic waves at an interface. In general, the cumulative second-harmonic generation of a dispersive guided wave propagation does not occur. However, the present paper shows that the second-harmonic of Lamb wave propagation arising from the nonlinear interaction of the partial bulk acoustic waves and the restriction of the three boundaries of the solid plates does have a cumulative growth effect if some conditions are satisfied. Through boundary condition and initial condition of excitation, the analytical expression of cumulative second-harmonic of Lamb waves propagation is determined. Numerical results show the cumulative effect of Lamb waves on second-harmonic field patterns.

Time-Domain Second-Harmonic Generation of Primary Lamb-Wave Propagation in an Elastic Plate

We present an experimental observation of the generation of the time-domain second harmonic by propagation of the primary Lamb-wave tone-burst. For a case where the phase velocity matching between the primary and the double frequency Lamb waves is satisfied but the group velocity matching between them is not, our observation clearly shows that the duration of the time-domain second-harmonic tone-burst, as well as its integrated amplitude, increases with the increasing propagation distance. This experimental result is consistent with the theoretical prediction and demonstrates that group velocity matching is not absolutely necessary for the generation of the cumulative time-domain second harmonic by primary Lamb-wave propagation.

Evaluation of material damage using nonlinearultrasonic wave

Generally speaking, damages and defects are inevitable with the operating life consuming of engineer structures. Engineer structures would fail or have to retire ultimately when their load bearing capacity is inadequate to accommodate the working load. It has been revealed that, to ensure the safety and reliability of components, it is essential to accurately evaluate the damages and defects during the service process. Frankly speaking, the traditional linear ultrasonic testing technology is not sensitive to micro-defects being large than 0.2 mm, which usually occur perior to 70%–80% of the material life. By contrast, the nonlinear ultrasonic testing based on the technology of ultrasonic harmonic method provides a possible way for the early evaluation of damages and defects. With the outstanding detective effectiveness and efficiency and high sentivitiy to micro-nano defects, such as dislocations, micro cracks, the nonlinear ultrasonic wave technology has attracted much attention in recent years. In this work, the evaluation of material damages using the nonlinear ultrasonic wave technology was reviewed systematically. Firstly, the basic principle of nonlinear ultrasonic wave technology in the detection of material damages was introduced. One-dimensional longitudinal wave was used as an example to describe how nonlinear ultrasonic wave was generated in damaged solids. Secondly, the interation between the propagating nonlinear ultrasonic wave and microstructures was marked. Three kinds of nonlinear utrasonic wave models were introduced, namely dislocation string models, dislocation dipole models and plastic-dependent models. The advantage and disadvantages of the existing models were discussed comparatively. After that, experimental studies on this topic are presented according to different types of material damage, such as tensile (or compressive) plastic damage, fatigue damage, thermal damage, creep damage, adhesive damage and other damages. Experimental results show that the nonlinear ultrasonic wave technique can characterize the degrees of above damages reasonably accurately. Furthermore, the responses of acoustic nonlinearity depended on the type of damages, materials and loading conditions. Finally, further studies on the evaluation of material damages using nonlinear ultrasonic wave technology were proposed, including the interaciton of nonlinear ultrasonic wave with the discontinuous medium, creation of more robust detective methods and signal processing methods, etc.