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The in situ monitoring of stresses provides a crucial input for residual life prognosis and is an integral part of structural health monitoring systems. Stress monitoring is generally achieved by utilising the acoustoelastic effect, which relates the speed of elastic waves in a solid, typically longitudinal and shear waves, to the stress state. A major shortcoming of methods based on the acoustoelastic effect is their poor sensitivity. Another shortcoming of acoustoelastic methods is associated with the rapid attenuation of bulk waves in the propagation medium, requiring the use of dense sensor networks. The purpose of this article is twofold: to demonstrate the application of Rayleigh (guided) waves rather than bulk waves towards stress monitoring based on acoustoelasticity, and to propose a new method for stress monitoring based on the rate of accumulation of the second harmonic of large-amplitude Rayleigh waves. An experimental study is conducted using the cross-correlation signal processing technique to increase the accuracy of determining Rayleigh wave speeds when compared with traditional methods. This demonstrates the feasibility of Rayleigh wave-based acoustoelastic structural health monitoring systems, which could easily be integrated with existing sensor networks. Second harmonic generation is then investigated to demonstrate the sensitivity of higher order harmonics to stress-induced nonlinearities. The outcomes of this study demonstrate that the sensitivity of the new second harmonic generation method is several orders of magnitude greater than the acoustoelastic method, making the proposed method more suitable for development for online stress monitoring of in-service structures.

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The paper presents outcomes of a finite element (FE) study of acoustoelastic effect associated with Lamb wave propagation in plates subjected to homogeneous bi-axial and bending stresses.In particular, the change of the phase velocity of the fundamental Lamb wave modes is obtained for different stress levels, bi-axial stress ratios and wave propagation angles. A comparison of the obtained numerical results with an analytical solution demonstrates a very good agreement.Moreover, the influence of bending stress on the wave velocities and wave front profile is further investigated numerically. There are currently no analytical results for this case. The developed and validated FE modelling approach can help to address several issues in the current non-destructive inspections including: in the investigation of changing stress conditions on the defect detection as well as in an adaptation of the existing Lamb wave-based defect evaluation systems to monitoring of stress too. The latter may have many benefits from sharing the same hardware for the purpose of maintaining structural integrity of thin-walled structural components. . Acoustoelastic effect of Lamb wave propagationAcoustoelastic effect is defined as the effect of the applied stress on the wave propagation in a media. It has been studied since the development of the finite deformation theory by Murnaghan [40], who formulated the material nonlinearity using third order elastic constants. Some pioneering studies in this area include the research of Hughes and Kelly [41], who derived equations relating the wave velocity to the applied stress and experimentally measured the acoustoelastic effect. Another experimental study of Egle and Bray [42] demonstrated how to obtain higher-order elastic constants from experiments with bulk waves. In the literature, many developments and studies in the acoustoelasticity mainly focused on and utilised bulk waves (Pau and Scalea [43]). The acoustoelastic effect is usually quantified = (20) So, equation (19), with the energy function presented in equation (16) can be translated to, ¬ = J -. © © = J -. © © © = J -. %)( ) % ©where T is the second Piola-Kirchhoff (PK2) stress. The stress in VUMAT must be updated with the equation at the end ( + ∆ ) of an integration step and stored in stressNew(i), based on the values of F and U given in the subroutine at the end of previous step ( ). Numerical Case Studies 3D Finite Element ModelA 3D FE model of a 6061-T6 aluminium plate was developed with ABAQUS software and the wave propagation problem was solved by the explicit integration approach [49]. The materialproperties of the 6061-T6 aluminium are listed in Table 1. The thickness of the plate is 3.2mm and the in-plane dimension is 240mm×240mm. The element type used in the model is the 8-

Detection of mechanical damage using Lamb or Rayleigh waves is limited to relatively simple geometries, yet real structures often incorporate features such as free or clamped edges, welds, rivets, ribs and holes. All these features are potential sources of wave reflections and scattering, which make the application of these types of guided waves for damage detection difficult. However, these features can themselves generate so-called ‘feature-guided’ waves. This article details the first application of the fundamental mode of transient edge waves for detection of mechanical damage. The fundamental edge wave mode (ES0) – a natural analogue to Rayleigh waves – is weakly dispersive and may decay with propagation distance. The phase and group velocities of the ES0 wave mode are close to the fundamental shear horizontal (SH0) and symmetric Lamb (S0) wave modes, at low and high frequencies, respectively. It is therefore quite challenging to excite a single ES0 mode and avoid wave coupling. However, it was found experimentally that at medium range frequencies the ES0 mode can be decoupled from SH0 and S0 modes, and its decay is small, allowing for distant detection of defects and damage along free edges of slender structural components. This article provides a brief theory of edge waves, excitation methodology and successful examples of distant detection of crack-like and corrosion damage in I-beam sections, which are widely applied in engineering and construction.

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

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