Harmonic Lamb Waves Research Articles

Evaluation of fatigue damage using nonlinear guided waves

This research develops an experimental procedure for characterizing fatigue damage inmetallic plates using nonlinear guided waves. The work first considers the propagation ofnonlinear waves in a dispersive medium and determines the theoretical and practicalconsiderations for the generation of higher order harmonics in guided waves. By usingresults from the nonlinear optics literature, it is possible to demonstrate that both phaseand group velocity matching are essential for the practical generation of nonlinear guidedelastic waves. Next, the normalized acoustic nonlinearity of low cycle fatigue damagedaluminum specimens is measured with Lamb waves. A pair of wedge transducers is used togenerate and detect the fundamental and second harmonic Lamb waves. The results showthat the normalized acoustic nonlinearity measured with Lamb waves is directly related tofatigue damage in a fashion that is similar to the behavior of longitudinal and Rayleighwaves. This normalized acoustic nonlinearity is then compared with the measuredcumulative plastic strain to confirm that these two parameters are related, and toreinforce the notion that Lamb waves can be used to quantitatively assess plasticitydriven fatigue damage using established higher harmonic generation techniques.

A nonlinear-guided wave technique for evaluating plasticity-driven material damage in a metal plate

This research develops an experimental method for evaluating material damage due to plastic deformation in a metal plate using nonlinear-guided waves. An improved version of a previously proposed measurement technique is used. The material nonlinearities of aluminum specimens loaded to produce different levels of plastic strain are measured with Lamb waves. A pair of wedge transducers is used to generate and detect the fundamental (s1) and second harmonic (s2) Lamb waves. The weak amplitude of the second harmonic wave is extracted from the spectrogram of a received signal. The measured acoustic nonlinearity increases monotonically with the level of plasticity (plastic strain), which is similar to the behavior of longitudinal and Rayleigh waves. This result indicates that Lamb waves can be used to assess plasticity-driven material damage in the established framework of the second harmonic generation technique. The effects and the importance of signal processing in determining the nonlinearity parameter are also discussed.

Numerical description of the edge mode at the beveled extremity of a plate

This paper deals with the reflection of a two-dimensional harmonic Lamb wave at the beveled end of a plate. The existence of a resonant edge mode is described by a numerical model. It is proved that the edge mode is the resonance of different complex modes. The behavior of this mode as a function of the bevel angle is studied. Its amplitude decreases and its resonance frequency shifts as the bevel angle decreases from 90 to 85 deg. An unexpected strong variation of the repartition of the reflected energy is linked to this phenomenon.

Scattering of lamb waves by a normal rectangular strip weldment

A combined finite element and Lamb wave modal expansion method is presented here for analysing scattering of time harmonic Lamb waves by material and geometric irregularities in an isotropic linearly elastic infinite plate. All the irregularities are assumed to be contained in a bounded region. The method is to replace this region with a finite element mesh. A nodal force-displacement relation is developed to satisfy the continuity conditions along the artificial boundaries separating the inner finite element region from the exterior regular region. The method is illustrated by solving the problem of scattering of Lamb waves by a normal transversely anisotropic weldment. The reflection and transmission coefficients are computed for the first antisymmetric and symmetric incident modes. The validity and accuracy of the results are checked by satisfaction of the energy conservation principle.