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%)

Linear and nonlinear ultrasound time reversal using a condensing raster operation

One of the weaknesses of nonlinear methods is the excitation power required to generate nonlinear effects such as higher order harmonics (single frequency) and sidebands (dual excitation). In conjunction with the high-power prerequisite, is the required sensitivity of sensors to capture nonlinear effects which can be multiple orders of magnitude lower than the fundamental/linear response of the system. In this work a semi-air coupled nonlinear ultrasound modulation method combining phase symmetry analysis and time reversal is developed, in the aim of alleviating some of these issues. Phase symmetry analysis is used to characterise the second order and modulated nonlinear responses of a dual frequency excitation signal containing a single frequency (f1) and a sweep burst (f2). Time reversal allows for optimal focusing, in this case at locations on composites panels with barely visible impact damage. Time reversed signals typically focus at single locations at a time, in this work, a raster time reversal methodology is proposed which focuses on multiple locations simultaneously. The raster methodology reduces distortions which may affect air-coupled techniques, reduces inspection time, and shows clear enhancement of damage imaging when compared against the raw fundamental response and standard time reversal.</p

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

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