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In this study, the wavefield radiated from a building to its surroundings is identified and extracted from M4.6 earthquake recordings collected by sensors installed in a building and on the nearby athletic field in Matera (Italy) using a new approach for soil-structure interaction assessment. The proposed approach for earthquake data analysis combines in an innovative way two methods already used in seismology and engineering seismology: deconvolution and polarization analysis. The approach enables the identification, reconstruction, and characterization of the wavefield radiated from a vibrating building into its surroundings, and the estimation of the amount of energy associated with it. The approach consists of four steps: estimation of the resonant frequencies of the building, deconvolution of the earthquake recordings from a building and its surroundings, identification of the seismic phases, reconstruction of the signal transmitted from the building to its surroundings, and estimation of its energy, and polarization analysis. Analysis of recordings of the M4.6 event highlighted that the motion related to the wavefield radiated from the building to the ground was mostly linearly polarized in the radial and transverse planes, while a clear ellipticity was observed only in the horizontal plane. The wavefield radiated from the building might be dominated by unconventionally polarized surface waves, i.e., quasi-Rayleigh waves or a combination of quasi-Rayleigh and quasi-Love waves. The results indicated that the energy transmitted from the analyzed vibrating building to its surroundings was significant and decreased the ground motion shaking due to the out-of-phase motion.
It can be difficult to efficiently model ultrasonic waves in 3D structures, especially when the computational model needs to account for complex geometries. This contribution presents a solution based on the Scaled Boundary Finite Element Method (SBFEM). It is a numerical tool suitable for elastodynamic problems. A space-tree discretisation, namely quad-trees, is used. This technique allows the decomposition of an image into quadrilaterals or quads, which are extruded to generate the 3D plate geometry. In particular, small quads resolve regions with discontinuities, allowing them to represent fine details in the structure. Moreover, this meshing technique allows for exploiting cell similarities, making the calculation procedure more efficient. The space-tree discretisations are generated from a high-resolution image containing all the information about damaged regions or boundary conditions. The resulting SBFEM polyhedral domains employ transition elements to ensure correct coupling between cells of different sizes. The analytical solution of a cylindrical scatterer serves as a reference to validate the proposed approach. Other examples also demonstrate the validity of the methodology and its flexibility.
Damage Assessment of Civil Structures Using Wave Propagation Analysis and Transmissibility Functions
(2022)
A common damage detection method in civil engineering is to monitor dynamic characteristics such as natural frequencies and modal shapes, which are directly related to the structural stiffness. However, the discrepancy in measured natural frequencies may result from environmental changes, the soil-structure interaction or effects caused by earthquakes. This makes it difficult to decide whether the local change in structural properties is caused by damage or other factors. To address this challenge and thus improve the current damage detection method, the wave propagation analysis method and the transmissibility relationship were considered. In our study, the wave propagation field in structures was reconstructed by applying the Normalized-Input-Output-Minimization (NIOM) method to vibration recordings. Wave velocities and transmissibility functions were considered as reference values for the damage indicators. This paper demonstrates the evaluation of local property changes in two examples of large-scale structures: a 14-story RC building and a 64 m long pedestrian bridge. The proposed damage indicators show a clear correspondence to structural changes.
Damage assessment of civil structures using wave propagation analysis and transmissibility functions
(2021)
This presentation shows the applicaiton of seismic interferometry to vibration recordings. The Normalized-Input-Output-Minimization (NIOM) method to reconstruct the virtual impulse wave propagation is introduced. We obtain wave velocities and transmissibility functions and consider them as reference values for the damage detection method. At the end, the evaluation results are presented by two examples: a 14-story RC building and a 64 m long pedestrian bridge.
Guided waves hold great potential for applications in the field of ultrasonic nondestructive testing. Examples of possible applications are the ultrasonic testing and structural health monitoring of wheelset-axles as used in trains. Depending on the particular type, these axles can be described as either thick cylindrical rods or thick walled hollow cylinders with varying thickness. Wheelset-axles are safety relevant components that have to be inspected on a regular basis. The use of guided waves would allow a full inspection while accessing only the front faces of the axle, thus potentially speeding up the inspection procedure. In order to develop such an inspection technique, however, detailed knowledge of wave propagation through the axle is required. Established mesh-based procedures, like the finite element method, could be used to simulate guided wave propagation in such structures. However, due to the size of the axle itself and the comparatively fine mesh that is dictated by the wavelengths usually applied in ultrasonic testing, these mesh-based procedures would be very expensive in terms of computation times. The multimodal approach seems to be a very promising alternative that can be expected to provide results significantly faster. The multimodal method uses the guided wave modes of a corresponding waveguide with a constant cross-section as basis in which the local sound field at any given position in a waveguide with varying thickness can be expressed. Thereby the numerical effort is reduced to solving the one dimensional differential equations that govern the evolution of the coefficients in the mode spectrum along the waveguide. Once the sound field has been calculated, a time dependence can easily be included, which allows the simulation of pulse propagation through the waveguide. In this thesis, the multimodal approach, as described for the calculation of Lamb-waves in plates with non-constant thickness, is extended to other types of elastic waveguides such as cylindrical rods and thick walled hollow cylinders. For the sake of simplicity, investigations are restricted to axially symmetric wave modes. The results obtained with the multimodal approach are validated against FEM-simulations. It is shown that the multimodal method potentially holds a great advantage in terms of computation time over commercially available software based on the finite element method. Finally, the multimodal method is evaluated with respect to possible future applications on wheelset-axles.
Elastic wave propagation of ultrasound in bituminous road surfaces – simulations and measurements
(2015)
Maintenance costs of road infrastructure are increasing steadily. Adverse environmental impacts on infrastructure get more and more important as well. Therefore, it is important to determine how limited financial resources can be directed with an optimum pay-out. The present study takes first steps towards the usage of low-frequency ultrasound as a tool to evaluate the road condition.
Applications of fibre reinforced plastic (FRP) composites in modern industries are increasing due to their considerable advantages such as light weight and excellent mechanical properties. Accordingly, importance of operational safety of modern structures made of advanced composites by ensuring the material quality has led to increasing demands for development of non-destructive evaluation (NDE) systems. In the context of a European project entitled “Validated Inspection Techniques for Composites in Energy Applications” (VITCEA), the aim is to develop and validate traceable procedures for novel NDE techniques with contrasting damage detection capabilities in energy related applications such as wind and marine turbine blades, nacelles, oil and gas flexible risers. Accordingly, VITCEA focuses on optimization of ultrasonic tests (UTs) for quantitative defect detection and quality characterization of FRP structures. In this context, the present study describes the ultrasound field in heterogeneous composite materials. The theoretical predictions are compared with simulation results obtained from CIVA a software package dedicated to NDT simulations based on the asymptotic ray theory.
Lamb waves travel in plates and hollow cylinders over large distances and propagate with multiple mode shapes. Therefore the waves can be used for integrity tests of large scale structures. Each propagating wave mode has a unique dispersive character and a frequency dependent vibration pattern. Therefore, by interacting with flaws each mode is assumed to generate an individual reflection pattern depending on the flaw type, flaw orientation and size. To extract this information a frequency as well as wave number dependent mode excitation is required ensuring a single mode excitation. The challenge, however, is to control the selective mode excitation electronically to allow a sequence of consecutive tests with different wave modes to collect the data. A method is proposed using phased array wedge transducers with fluid coupling for normal force excitation of Lamb wave modes. The excitation principle is investigated analytically on plates by modelling the wave generation in the wedge and the coupling between wedge and structure. The analytical results of the excitation principle are validated by corresponding experiments where the field patterns on the wedge surface and on the plate were scanned using a laser vibrometer. The directivity of the radiated field is investigated together with the direction dependent analysis of the mode content.
When dealing in ultrasonic testing with inhomogeneous and anisotropic material structure such as
diverse types of components made from austenitic or nickel based cast, which are currently used
for modern power plant concepts, data interpretation is quite difficult. For better understanding of
the complex interaction between the sound field and the component under test, the mathematical
modeling of sound propagation in solids is a substantial task to increase the probability of
detection of relevant defects.
First we present a mathematical approach for modeling the three dimensional transient
particle displacement as a function of time in each point in a half space excited by an impulsive
point load at the surface. The transient ultrasonic field of a rectangular array element is calculated
with this approach by a point source synthesis. Based on this solution we model the wave
propagation of a phased array transducer by time delayed superposition of the wave field of the
transducer elements.
Next we use an electrodynamic technique to visualize the grazing sound field at the surface
of a test block radiated by a phased array probe. By detecting the grazing beam at the samples
surface with a small electrodynamic probe, we measured the particle displacement as a function of
time. It allows for measuring the displacement in all three spatial directions. This comprises the
detection of the horizontal and vertical particle displacement with respect to the surface and thus
also the detection of longitudinal and shear waves is possible.
The calculated and measured wave fields will be compared for different delay laws in
isotropic and transversely isotropic media. The results support the theoretical activities to model
the wave propagation and to find optimal testing parameters for different components and
configurations.