Damage spectral element for condition assessment of one-dimensional waveguide

The stress waves generated with piezoelectric actuators can propagate along a path defined by the material boundaries of a structure. If there are damages or discontinuities in the path, the received wave will be greatly affected. The changes of the waveform can provide enormous characteristics which will indicate the conditions of damage in the structure. Based on this theory, the guided wave (GW)-based methods are developed to detect local damages in a structure. In order to investigate wave propagation in damaged waveguides, many numerical modelling methods have been developed. Among them, spectral element method (SEM) solves the governing partial differential equation of wave propagation problem in the frequency domain using fast Fourier transformation (FFT). It is not only efficient for computation but also accurate for analysis in comparison with the conventional finite element formulation. In this paper, a damage spectral element is developed to model local discontinuity by using reflection and transmission coefficients. Numerical simulations show that the method is efficient to simulate local damage in one-dimensional waveguides

Some effects of different constitutive laws on simulating mitral valve dynamics with FSI

In this paper, three different constitutive laws for mitral leaflets and two laws for chordae tendineae are selected to study their effects on mitral valve dynamics with fluid-structure interaction. We first fit these three mitral leaflet constitutive laws and two chordae tendineae laws with experimental data. The fluid-structure interaction is implemented in an immersed boundary framework with finite element extension for solid, that is the hybrid immersed boundary/finite element(IB/FE) method. We specifically compare the fluid-structure results of different constitutive laws since fluid-structure interaction is the physiological loading environment. This allows us to look at the peak jet velocity, the closure regurgitation volume, and the orifice area. Our numerical results show that different constitutive laws can affect mitral valve dynamics, such as the transvalvular flow rate, closure regurgitation and the orifice area, while the differences in fiber strain and stress are insignificant because all leaflet constitutive laws are fitted to the same set of experimental data. In addition, when an exponential constitutive law of chordae tendineae is used, a lower closure regurgitation flow is observed compared to that of a linear material model. In conclusion, combining numerical dynamic simulations and static experimental tests, we are able to identify suitable constitutive laws for dynamic behaviour of mitral leaflets and chordae under physiological conditions

Identification of de-bonding between steel bars and concrete using wavelet techniques : comparative study

The interface between steel bar and concrete plays an important role in retaining the strength of reinforced concrete structures. When the interface is damaged by cracking, de-bonding between the two materials and/or other kinds of damage, signifi cant degradation of the structural performances will occur owing to loss of composite actions. In this study, wave propagation on several steel bars embedded in two concrete plates with different de-bonding scenarios is tested. Piezoelectric actuators and sensors are attached to steel bars for recording input and response signals. This paper uses the wavelet transform combined with the wavelet packet decomposition, component energy and Shannon entropy to analyse the experimental results. The results demonstrate that both damage index and relative entropy are sensitive to the existence of de-bonding damage and change linearly with de-bonding length when there is only a single de-bonding damage, while energy and entropy distribution are largely infl uenced by the property of the excitation signal. When multiple de-bonding exists, however, both the damage index and relative entropy have no such clear tendency

Condition assessment of shear connectors in slab-girder bridges via vibration measurements

The paper presents a field study on condition assessment of the shear connectors in a full slab-girder bridge via vibration measurements. First, a model updating technique is employed to assess the condition of the whole structure, including boundary conditions, bearings, girders, slab, and shear connectors, from the accelerations on the slab measured in vibration testing. Then, a new damage index based on the difference of frequency response functions on the slab and the corresponding points on the girder is developed to evaluate the condition of shear connectors. The advantage of the new method lies in the fact that it does not need any reference data (undamaged data) for the structure. Compared with the results obtained using the model updating technique, the method is more reliable and accurate in assessing the condition of the shear connectors between the slab and girders. The effects of measurement noise on the damage identification results and the damage quantification are also studied through numerical simulation.Department of Civil and Environmental Engineerin

Structural, elastic, electronic and dynamical properties of Ba2MgWO6 double perovskite under pressure from first principles

Ab initio calculations within the framework of density-functional theory employing the local density approximation have been performed to study the structural, elastic, electronic and dynamical properties for cubic double perovskite Ba2MgWO6 under hydrostatic pressure. The calculated ground-state properties and compression curve are in good agreement with the available experimental results. Pressure-induced enhancements of elastic constants, aggregate elastic moduli, elastic wave velocities and Debye temperature are observed, without any softening behaviors. Upon compression, the fundamental indirect energy gap EgΓ−X first increases slightly and then monotonically decreases. A linear-response approach is adopted to derive the full phonon-dispersion curves and phonon density of states. Evolution with pressure of the zone-center phonon frequencies for Raman- and infrared-active modes is analyzed. A pressure-induced soft optically silent T1g phonon mode is identified near the Γ point, signifying a structural dynamical instability. Our calculated results reveal that, when the pressure is high enough, besides bond shortening, the W-O-Mg bond becomes nonlinear, resulting in octahedral tilting distortion and thus a slight departure from the ideal cubic symmetry