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.

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.

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This study proposes an amplitude-modulation vibro-acoustic (AMVA) technique to track the evolution of thermal damage in pristine graphene mortar. In the proposed amplitude-modulation technique, the pump wave is amplitude-modulated using three different methods: (1) the pure amplitude-modulated (PAM) method, (2) the suppressive amplitude-modulated (SAM) method and (3) the transmitted amplitude-modulated (TAM) method. The nonlinear parameters [Formula: see text], [Formula: see text] and [Formula: see text] corresponding to the PAM, SAM and TAM methods, respectively, are theoretically established and experimentally demonstrated that the nonlinear parameters associated with the material nonlinearity, and they can be used to quantitatively characterize different damage stages of thermal damage. Meanwhile, the resonant frequency (RF) and ultrasonic pulse velocity (UPV) tests are conducted. The linear measurements, dynamic elastic modulus [Formula: see text] obtained from RF test and ultrasonic pulse velocity [Formula: see text] collected by UPV test, are used to compare with the nonlinear parameters. The results show that the proposed AMVA technique is more sensitive and feasible to serve as the tool for thermal damage detection in cement-based material compared with the conventional RF and UPV techniques.

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