Continuous waves probing in dynamic acoustoelastic testing

Consolidated granular media display a peculiar nonlinear elastic behavior, which is normally analysed with dynamic ultrasonic testing exploiting the dependence on amplitude of different measurable quantities, such as the resonance frequency shift, the amount of harmonics generation, or the break of the superposition principle. However, dynamic testing allows measuring effects which are averaged over one (or more) cycles of the exciting perturbation. Dynamic acoustoelastic testing has been proposed to overcome this limitation and allow the determination of the real amplitude dependence of the modulus of the material. Here, we propose an implementation of the approach, in which the pulse probing waves are substituted by continuous waves. As a result, instead of measuring a time-of-flight as a function of the pump strain, we study the dependence of the resonance frequency on the strain amplitude, allowing to derive the same conclusions but with an easier to implement procedure

Propagation of guided waves in a hollow circular cylinder application to non destructive testing

In this paper, we study the propagation of guided waves in a tube and their interaction with surface defects. Axisymmetric (longitudinal and torsional) modes and nonaxisymmetric (flexural) modes are found and represented in phase and group velocity dispersion diagrams. The mode chosen to be generated is the second longitudinal mode L (0, 2), for the frequency-thickness product equal to 1 MHz.mm, at this product, the radial component of displacement is very small with regard to the axial component so, the attenuation is less important. According to our experimental results the L (0, 2) mode is sensitive to the depth variation and to the defect circumference. A relation of proportionality between the amplitude of the signal reflected by the defect and its depth was established. The results of the modal decomposition method, concerning the amplitude of the modes diffracted by circumferential defects according to the circumference of the defect, were presented and compared with the experimental measures results

Wavelet Based Clustering of Acoustic Emission Hits to Characterize Damage Mechanisms in Composites

International audienceClustering data is an important topic in acoustic emission (AE) health monitoring. Identification of damage mechanisms in composites via AE technique requires an automatic process of classification. In this work, we propose a novel and simple supervised method to discriminate AE signals produced by fracture mechanisms in polymer composites. The novelty of this work is to propose new pertinent descriptors offered by using the continuous wavelet transform, where signals of learning are decomposed and the corresponding wavelet coefficients are calculated. In addition, the entropy criterion is applied to select the most correlated wavelets associated to each failure mechanism. This process allows to establish a filter in the form of vectors for each class of signals and descriptors denote the reconstruction errors calculated by involving the filter associated to each damage mechanism. The k-means algorithm is executed to calculate the center of each class. The technique is applied to AE signals recorded from specific mechanical tests to demonstrate the performance of the proposed descriptors

Continuous waves probing in dynamic acoustoelastic testing

Consolidated granular media display a peculiar nonlinear elastic behavior, which is normally analysed with dynamic ultrasonic testing exploiting the dependence on amplitude of different measurable quantities, such as the resonance frequency shift, the amount of harmonics generation, or the break of the superposition principle. However, dynamic testing allows measuring effects which are averaged over one (or more) cycles of the exciting perturbation. Dynamic acoustoelastic testing has been proposed to overcome this limitation and allow the determination of the real amplitude dependence of the modulus of the material. Here, we propose an implementation of the approach, in which the pulse probing waves are substituted by continuous waves. As a result, instead of measuring a time-of-flight as a function of the pump strain, we study the dependence of the resonance frequency on the strain amplitude, allowing to derive the same conclusions but with an easier to implement procedure

Nonlinear coda wave analysis of hysteretic elastic behavior in strongly scattering media

Strongly scattering elastic media, such as consolidated granular materials, respond to ultrasonic pulse excitations with a long response signal with peculiar properties. The portion of the signal at late times, termed coda, is due to multiple scattering. It contains information about the elastic properties of the material, a d it has been proven to be very sensitive to small variations in the modulus. Here we propose a technique based on a nonlinear analysis of the coda of a signal, which might be applied to quantify the nonlinear elastic response in consolidated granular media exhibiting a hysteretic elastic behavior. The method proposed allows for an intrinsic definition of the reference signal which is normally needed for applying coda-based methods

Experimental Study of Lamb Waves Propagation inside an Impact Damage in the Size of the Used Wavelength

The present work is an experimental study of Lamb waves propagation in an aluminum plate with an impact damage in the size of the S0 mode wavelength. The aim of this study is to visualize the wavefield near the defect in the case of extreme diffusion, as well as the interference of the modes inside of it and their transformation. The results were obtained by applying the continuous wavelet transform (CWT) on the wavefield data recorded by two ultrasonic scanning techniques: the air coupled ultrasonic (ACU) and the Laser doppler vibrometry (LDV), to obtain a C-scan and a B-scan of the plate respectively. Space-wavenumber representations showed the behavior of Lamb waves in the plate as well as the reduction in thickness of the impacted area. The width of the latter could be estimated and the modes present at each position of the plate could be identified

Assessment of trabecular bone tissue elasticity with resonant ultrasound spectroscopy

International audienceThe material properties of the trabeculae (tissue-level properties), together with the trabecular architecture and the bone volume fraction determine the apparent millimetre-scale bone mechanical properties. We present a novel method to measure trabecular tissue elastic modulus E t using resonant ultrasound spec-troscopy (RUS). The first mechanical resonance frequency f e of a freestanding cuboid specimen is measured and used to back-calculate E t. The steps of the back-calculation are (1) the apparent stiffness tensors C(˜ E t) is computed using micro-finite elements for a set of trial values of tissue Young's modulus˜Emodulus˜ modulus˜E t based on the computed tomography image of the specimen; (2) the modeled free-vibration resonance frequencies f m (˜ E t) of the specimen is calculated with the Rayleigh-Ritz method using C(˜ E t); (3) finally, E t is obtained by interpolation using f e and f m (˜ E t). Four bovine bone specimens were tested (nominal size 5×6×6 mm 3). Average (standard deviation) of E t was 13.12 (1.06) GPa. The measurement of a single resonance frequency enabled an estimation of tissue elasticity in line with available data. RUS is a non destructive technique relatively easy to implement compared to traditional mechanical testing. The novel method could contribute to a better documentation of bone tissue elasticity which is an important parameter of micro-finite element analyses for the * Corresponding author clinical assessment of bone strength

Investigation of the validity of Dynamic AcoustoElastic Testing for measuring nonlinear elasticity

International audienceMeasurement of the dependence of the elastic moduli on the strain, i.e., the characterization of nonlinear elastic properties of solid media, poses intrinsic experimental difficulties. The Dynamic AcoustoElastic Technique has been recently developed as an efficient tool for the determination of the modulus in both compression and tension. The goal of the present paper is to discuss the limitations of the experimental implementation and the interpretation of the measured quantities in terms of nonlinear parameters. For this purpose, simulation results will be presented for both classical and nonclassical nonlinear elastic media