a b s t r a c tThe continuum modeling of the mechanical behavior of nanowires has recently attracted much attention due to its simplicity and efficiency. However, there are still some critical issues to be solved. In this paper, we demonstrate the importance of accounting for the effects of initial stresses in the nanowires that are caused by deformation due to surface stresses; we note that such initial stresses have previously been neglected in most existing continuum models. By considering the local geometrical nonlinearity of strains during the incremental flexural motion, a new formulation of the Euler-Bernoulli beam model for nanowires is developed through the incremental deformation theory, in which effects of the surface stress, the surface-induced initial stress and surface elasticity are naturally incorporated. It is found through comparisons to existing experimental and computational results for both fcc metal and ceramic nanowires that the surface-induced initial stresses, which are neglected in the Young-Laplace model, can significantly influence the overall mechanical properties of nanowires. We additionally demonstrate and quantify the errors induced by using the Young-Laplace model due to its approximation of surface stresses acting on only the top and bottom surfaces of nanowires.

Due to the complex nature of such composite structures, an understanding of the guided wave propagation mechanism in honeycomb composite panels with different frequencies inherently imposes many challenges. In this paper, a numerical simulation is first conducted to investigate the wave propagation mechanism in honeycomb sandwich structures using piezoelectric actuators/sensors. In contrast to most of the previous work, elastic wave responses based on the real geometry of the honeycomb core are obtained by using the finite element method (FEM). Based on the simulation, the global guided waves in the composite can be observed when the loading frequency is low and the leaky guided waves in the skin panel are found when the loading frequency is sufficiently high. The applicability of the homogenization technique for a celled core is discussed. The effects of cell geometry on the wave propagation are also demonstrated. Experimental testing is finally conducted to validate the results of numerical simulation and very good agreement is observed. Specifically, some guided wave propagation characteristics such as group velocity dispersion and mode tuning capabilities with the presence of a honeycomb core are discussed.

Concrete structures have been used widely in civil infrastructural systems. Due to the complex nature of the microstructure, nondestructive testing (NDT) of concrete inherently imposes many challenges, which can cause severe limitations on the resolution and the sensitivity of the signals observed. In this study, a numerical simulation based on the finite element (FE) model is first performed to investigate surface wave generation and reception using piezoelectric actuators/sensors, especially in relatively higher frequency cases. The results provide a basic understanding of some features of the microstructure effects on the surface wave propagation. Experimental testing is then conducted to validate the numerical simulation. The group velocity dispersion curves of the surface waves, which are very useful for future damage detection in concrete materials, are obtained from both numerical and experimental results. The good agreement shows the great potential and feasibility of using piezoelectric actuators/sensors to generate and receive surface waves for quantitative damage detection in concrete structures.

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The time-reversal technique has been successfully used in structural health monitoring (SHM) for quantitative imaging of damage. However, the technique is very time-consuming when it is implemented in the time domain. In this paper, we study the technique in the frequency-wavenumber (f-k) domain for fast real-time imaging of multiple damage sites in plates using scattered flexural plate waves. Based on Mindlin plate theory, the time reversibility of dispersive flexural waves in an isotropic plate is theoretically investigated in the f-k domain. A fast damage imaging technique is developed by using the cross-correlation between the back-propagated scattered wavefield and the incident wavefield in the frequency domain. Numerical simulations demonstrate that the proposed technique cannot only localize multiple damage sites but also potentially identify their sizes. Moreover, the time-reversal technique in the f-k domain is about two orders of magnitude faster than the method in the time domain. Finally, experimental testing of an on-line SHM system with a sparse piezoelectric sensor array is conducted for fast multiple damage identification using the proposed technique.