Characterization of defects in plates using shear and Lamb waves

This work investigates the interaction of shear and Lamb waves with different kinds of defects in plates, in view of applications to defect characterization purposes. Using a finite element model, the reflection and transmission coefficients of shear and Lamb waves are determined, as a function of size parameters of the defect related to its extension and depth. Notches with elliptical and rectangular profile are examined, together with internal voids, covering both symmetric and asymmetric cases. Low and high frequency times height (2hf) regimes are considered in order to clarify how mode conversion can provide information on the shape of the defect. In this regard, also the role of symmetric and asymmetric waves is elucidated

Damage characterization in waveguides with ultrasonic shear waves

Damage can be viewed as a continuum discontinuity. In order to identify and quantify damage, the present study investigates the dependence of reflection and transmission coefficients from a discontinuity in a waveguide using a finite element model. A parametric study of a fiber reinforced polymer laminate beam is presented with five different ratios of reduced cross-section with two different kinds of geometry: symmetric and asymmetric, in three discontinuity lengths. What is more, finite element results are compared with the corresponding results of a semi- analytical model based on the principle of reciprocity in elastodynamics. The analysis shows that: a) depending on the kind of discontinuity, the diagnostic potential of the used guided wave depends on the mode existence and on the magnitude of its wavelength in relation to the discontinuity extension, b) reflection and transmission coefficients can be used to identify and characterize damage both in symmetrical and asymmetrical damaged cross sections

Effective filtering of modal curvatures for damage identification in beams

In this work, we investigate the effectiveness of a damage identification technique recently proposed in [1] and assess how it is affected by the number and position of the sensors used. Mode shapes and curvatures have been claimed to contain local information on damage and to be less sensitive to environmental variables than natural frequencies. It is known that notch-type damage produces a localized and sharp change in the curvature that unfortunately could be difficult to detect experimentally without the use of an adequate number of sensors. However, we have recently shown that even a coarse description of the modal curvature can still be employed to identify the damage, provided that it is used in combination with other modal quantities. Here, by exploiting the perturbative solution of the Euler-Bernoulli equation, we consider the inverse problem of damage localisation based on modal curvatures only and we ascertain the feasibility of their sole use for recostructing the damage shape. To do so, we set up a filtering procedure acting on modal curvatures which are expressed in a discrete form enabling further investigation on the effect of using a reduced number of measurement points. The sensitivity of the procedure to damage extension is further assessed

Stress monitoring of plates by means of nonlinear guided waves

We investigate the propagation of nonlinear guided waves in plates in view of their application to the identification of the state of stress. The study is performed modelling via Finite Elements the propagation of Lamb waves in a prestressed plate, whose equations of motion are a second-order approximation accounting for both geometric and material nonlinearities. Different intial prestress conditions are investigated, parallel and orthogonal to the direction of wave propagation. A couple of physical phenomena presenting remarkable sensitivity to initial stress emerged using the resonant couple of S1-S2 modes. First, the increase of the secondary wave amplitude with propagation distance is clearly observed together with the sensitivity of the second-harmonic amplitude to the initial state of prestress. Second, a subharmonic and superharmonics are observed, due to the interaction between primary and secondary waves

A match coefficient approach for damage imaging in structural components by ultrasonic synthetic aperture focus

Ultrasonic Synthetic Aperture Focus (SAF) techniques are commonly used to image structural defects. In this paper, a variation of SAF based on ideas borrowed from Matched Field Processing (MFP) is evaluated to reduce artifacts and sidelobes of the resulting images. In particular, instead of considering the full RF ultrasonic waveforms for the SAF time backpropagation, only selected features from the waveforms are utilized to form a “data vector” and a “replica” (expected) vector of MFP. These vectors are adaptive for the pair of transmitter-receiver and the focus point. The image is created as a matched filter between these two vectors. Experimental results are shown for an isotropic and homogenous metallic plate with simulated defects, probed by six piezoelectric patches used as receivers or transmitters

DESIGN OF MECHANICAL METAMATERIALS BASED ON BIPHASIC PERIODIC MICROSTRUCTURES

The passive control of elasto-acoustic wave propagation is a very active field of research, currently fuelled by the theoretical advancements in multiscale design, accompanied by the technological development of additive manufacturing techniques. In this work, we present a mechanical metamaterial, characterized by a biphasic (fluid and solid) periodic cell, that exhibits acoustic as well as elastic bandgaps in the dispersion spectrum, which – in principle – could provide insulation from both sound and vibration in prescribed frequency ranges. Bandgaps arise when voids and channels open in the repetitive cell. We aim at studying the geometric parameters that influence the metamaterial performance. Through a tuning of the mechanical properties of the metamaterial, waves of given nature and frequency can be remarkably attenuated simultaneously in the two different domains, the fluid domain where acoustic waves propagate and the solid domain where elastic waves propagate. A finite element model is used to determine the dispersion curves and investigate the frequency band structure, which is found to be governable through the selection of the geometric parameters of the repetitive cell

Monitoring prestress in plates by sideband peak count-index (SPC-I) and nonlinear higher harmonics techniques

Propagating guided waves in a homogeneous, isotropic, prestressed, hyperelastic plate show nonlinear characteristics that depend on the state of initial prestress. These nonlinear phenomena include higher harmonic generation, occurring when Lamb wave modes of different frequencies (ωa and ωb) are allowed to mix within the material generating secondary waves at frequencies 2ωa, 2ωb and ωa ± ωb. Further, if prescribed internal-resonance conditions are satisfied, the amplitude of secondary waves increases in space, providing a response quantity which is dependent on prestress and easy to be observed. Using the finite element method, in this paper we investigatthe time and space evolution of higher harmonics arising in one-way wave mixing. The influence of prestress on the response is elucidated, observing the nonlinear parameter β. It is further shown that the nonlinear ultrasonic technique called Sideband Peak Count-Index (SPC-I) can provide an effective monitoring tool for prestress

Genetic association study of QT interval highlights role for calcium signaling pathways in myocardial repolarization.

The QT interval, an electrocardiographic measure reflecting myocardial repolarization, is a heritable trait. QT prolongation is a risk factor for ventricular arrhythmias and sudden cardiac death (SCD) and could indicate the presence of the potentially lethal mendelian long-QT syndrome (LQTS). Using a genome-wide association and replication study in up to 100,000 individuals, we identified 35 common variant loci associated with QT interval that collectively explain ∼8-10% of QT-interval variation and highlight the importance of calcium regulation in myocardial repolarization. Rare variant analysis of 6 new QT interval-associated loci in 298 unrelated probands with LQTS identified coding variants not found in controls but of uncertain causality and therefore requiring validation. Several newly identified loci encode proteins that physically interact with other recognized repolarization proteins. Our integration of common variant association, expression and orthogonal protein-protein interaction screens provides new insights into cardiac electrophysiology and identifies new candidate genes for ventricular arrhythmias, LQTS and SCD

Detection of a concentrated damage in a parabolic arch by measured static displacements

The present paper deals with the identification of a concentrated damage in an elastic parabolic arch through the minimization of an objective function which measures the differences between numerical and experimental values of static displacements. The damage consists in a notch that reduces the height of the cross section at a given abscissa and therefore causes a variation in the flexural stiffness of the structure. The analytical values of static displacements due to applied loads are calculated by means of the principle of virtual work for both the undamaged and damaged arch. First, pseudo-experimental data are used to study the inverse problem and investigate whether a unique solution can occur or not. Various damage intensities are considered to assess the reliability of the identification procedure. Then, the identification procedure is applied to an experimental case, where displacements are measured on a prototype arch. The identified values of damage parameters, i.e., location and intensity, are compared to those obtained by means of a dynamic identification technique performed on the same structure