Impact detection in anisotropic materials using a time reversal approach

This article presents an in situ imaging method able to detect in real-time the impact source location in reverberant complex composite structures using only one passive sensor. This technique is based on the time reversal acoustic method applied to a number of waveforms stored in a database containing the impulse response (Green's function) of the structure. The proposed method allows achieving the optimal focalization of the acoustic emission source in the time and spatial domain as it overcomes the drawbacks of other ultrasonic techniques. This is mainly due to the dispersive nature of guided Lamb waves as well as the presence of multiple scattering and mode conversion that can degrade the quality of the focusing, causing poor localization. Conversely, using the benefits of a diffuse wave field, the imaging of the source location can be obtained through a virtual time reversal procedure, which does not require any iterative algorithms and a priori knowledge of the mechanical properties and the anisotropic group speed. The efficiency of this method is experimentally demonstrated on a stiffened composite panel. The results showed that the impact source location can be retrieved with a high level of accuracy in any position of the structure (maximum error was less than 3%)

Modelling of multiscale nonlinear interaction of elastic waves with three-dimensional cracks

This paper presents a nonlinear elastic material model able to simulate the nonlinear effects generated by the interaction of acoustic/ultrasonic waves with damage precursors and micro-cracks in a variety of materials. Such a constitutive model is implemented in an in-house finite element code and exhibits a multiscale nature where the macroscopic behavior of damaged structures can be represented through a contribution of a number of mesoscopic elements, which are composed by a statistical collection of microscopic units. By means of the semi-analytical Landau formulation and Preisach-Mayergoyz space representation, this multiscale model allows the description of the structural response under continuous harmonic excitation of micro-damaged materials showing both anharmonic and dissipative hysteretic effects. In this manner, nonlinear effects observed experimentally, such as the generation of both even and odd harmonics, can be reproduced. In addition, by using Kelvin eigentensors and eigenelastic constants, the wave propagation problem in both isotropic and orthotropic solids was extended to the three-dimensional Cartesian space. The developed model has been verified for a number of different geometrical and material configurations. Particularly, the influence of a small region with classical and non-classical elasticity and the variations of the input amplitudes on the harmonics generation were analyzed

Design and development of a heatsink for thermo-electric power harvesting in aerospace applications

In recent years, the growing interest of aerospace companies in wireless structural health monitoring (SHM) systems has led to the research of new energy efficient sources and power harvesting solutions. Among available environmental power sources, temperature gradients originated at different locations of the aircraft can be used by thermo-electric generators (TEGs) to create electrical voltage. TEGs are lightweight, provide high-energy conversion and do not contain movable parts. Thermal diffusion systems, commonly known as heatsinks, can be combined with TEGs to enhance their performance by increasing heat dissipation from a high temperature surface to the ambient air. This paper focused on the enhancement of TEG performance by developing an air-cooled heatsink for low-power wireless SHM applications. The design, manufacturing and testing of the proposed thermal diffusion system was investigated by evaluating the increase of the temperature gradient between the opposite surfaces of a commercial TEG element. The thermal performance of the heatsink was assessed with numerical finite element thermal simulations and validated with experimental tests. Experimental results revealed that the proposed thermal diffusion system provided higher temperature differences and, therefore, higher output power in comparison with traditional cylindrical pin-fin heatsinks. A hybrid heat diffusion system composed by copper heatsinks and highly oriented pyrolytic graphite layers was also here proposed in order to allow TEG reaching wireless SHM operative power requirements of tens of mW and, at the same time, adapt the assembly to the complexity of aerospace SHM arrangements. Experimental results revealed that the proposed heatsink-TEG arrangement was able to generate an output power over 25 mW.</p

Phase symmetry analysis for nonlinear ultrasonic modulated signals

SummaryNonlinear ultrasonic experiments typically require digital pass‐band filters and advanced signal processing tools to highlight low‐amplitude nonlinear elastic effects such as harmonics, subharmonics, and sidebands, which are used as signatures for the presence of damage. However, current signal processing techniques cannot be used with dual periodic excitation without reducing signal frequency resolution and severely altering measured waveforms. This paper reports the theoretical development of phase symmetry analysis for nonlinear ultrasound with dual periodic transmission. The proposed signal postprocessing technique consists of determining the phase angles of transmitted waveforms that allow filtering modulated nonlinear ultrasonic waves from the measured signal spectrum. Experimental results validated theoretical predictions and revealed that phase symmetry analysis method provides an easy‐to‐implement and reliable procedure to extract sidebands from the measured signal noise. Phase symmetry analysis with dual excitation has, therefore, the potential to enable sensitive and efficient nonlinear ultrasound testing for various materials, damage scenarios, and applications