The fundamental ultrasonic edge wave mode: Propagation characteristics and potential for distant damage detection.

Engineering structures are often composed of thin elements containing features such as free edges, welds, ribs, and holes, which makes distant safety inspections based on guided waves difficult due to wave scattering. However, these features can themselves generate so-called 'feature-guided' waves, which can potentially be utilised for damage detection. One such example are flexural wedge waves, which have been investigated extensively both theoretically and experimentally in the past. Another example is edge waves. These waves, which are a natural analogue of Rayleigh waves propagating in a finite thickness plate, have received relatively little attention, specifically with respect to their possible use in distant damage inspections and Structural Health Monitoring systems. The current paper is aimed to address this gap, and it is focused on the investigation of the fundamental mode of edge waves (ES0), which is the most promising for practical applications. The features of the transient ES0 mode are investigated experimentally and numerically, and compared with previous theoretical studies. It was demonstrated that the ES0 mode can be effectively excited with the wedge excitation method, and distant damage detection with this wave mode at low frequency-thickness values (FTV < 5) is readily achievable. In particular, in a laboratory environment the ES0 mode propagated several meters with almost no decay. However, at higher frequency-thickness values, a wave amplitude modulation, significant energy decay and strong coupling between the ES0 and S0 wave modes were observed. These phenomena may restrict the defect resolution as well as the range of damage inspections based on the fundamental edge wave mode.

Large acoustoelastic effect for Lamb waves propagating in an incompressible elastic plate.

In this paper, the effect of a large pre-stress on the propagation of small amplitude Lamb waves in an incompressible elastic plate is investigated. Using the theory of incremental elasticity, the dispersion equations, which give the phase velocity of the symmetric and anti-symmetric wave modes as a function of the wavenumber, plate thickness, and pre-stress state, are derived for a general strain energy function. By considering the fourth-order strain energy function of incompressible isotropic elasticity, the correction to the phase velocity due to the pre-stress is obtained implicitly to the second order in the pre-strain/stress, and depends on the second, third, and fourth-order elastic constants. Numerical results are presented to show the dependence of the phase velocity of the Lamb wave modes upon the applied stress. These are compared to the first-order correction, and agree well with the limiting and asymptotic values obtained previously. It is envisaged that the present results may well find important practical applications in various guided wave based ultrasonic techniques utilising gels and rubber-like materials.

On the determination of the third-order elastic constants of homogeneous isotropic materials utilising Rayleigh waves.

This paper presents a new method for determining the third-order elastic constants (TOECs) of a homogeneous isotropic material utilising the acoustoelastic effect associated with Rayleigh waves. It is demonstrated that the accuracy of the evaluation of TOECs can be substantially improved by supplementing the classical equations of acoustoelasticity, which describe the effect of applied stress on bulk wave speeds, with the nonlinear characteristic equation for the propagation of Rayleigh waves in pre-stressed media. The developed method can be readily implemented for Structural Health Monitoring applications; for example, the measurement of applied stresses based on the acoustoelastic effect, or the monitoring of near-surface microstructural damage based on the change in magnitude of the TOECs.