Optical coherence elastography (OCE) has been used successfully for characterizing changes tissue mechanical properties particularly in breast tissue and the eye. Many dynamic ultrasound shear wave elastography (SWE) methods have been developed over the past three decades that use propagating waves with different dynamic excitations. We have successfully translated excitation and analysis methods from SWE for applications using OCE. We report here recent developments that utilize focused ultrasound to produce acoustic radiation force or mechanical vibration. We have explored characterizing the rheological properties such as surface tension and viscosity of various fluids. Additionally, we have applied these OCE methods to soft tissues such as blood clots, aorta samples, and porcine kidneys. These techniques have opened new areas for tissue characterization that take advantage of the sensitivity and resolution of optical coherence tomography and the strengths of wave-based approaches for quantifying material properties.
Tissue mechanics have been widely studied for the past two decades because they are significantly associated with disease states. The variations of mechanical properties in soft tissues are considerable indicators used for clinical diagnosis and disease monitoring. Optical coherence elastography (OCE) has been extensively developed to characterize the mechanical properties of various tissues. However, these methods are generally based on time-domain data and measure the time-offlight of the localized shear wave propagations. Here, we propose a method to evaluate phase velocity in four-dimensional (4D) space (x, y, z, f), called 4D-OCE phase velocity, and applied it to a heterogeneous phantom with a 4 mm diameter inclusion. A 7.5 MHz highly focused transducer transmitting a toneburst of 500 μs was used to provide an acoustic radiation force (ARF) for transient elastography. A customized 4D acquisition pattern was developed under Microsoft Visual C++ 2019 development environment. The 3D-OCE group velocity was reconstructed to compare with 4D-OCE phase velocity as cross-verification. The experimental result from the heterogeneous phantom showed that 4D-OCE phase velocity is capable of accurately reconstructing a 4D-OCE phase velocity map and provides good contrast between two gelatin materials with different concentrations, 7% versus 14%. The proposed 4D-OCE phase velocity can provide further information to better understand the delicate change of mechanical properties of various biological tissues in each voxel over a range of temporal frequencies.
Nonlinear ultrasonic techniques provide improved damage sensitivity compared to linear approaches. The combination of attractive properties of guided waves, such as Lamb waves, with unique features of higher harmonic generation provides great potential for characterization of incipient damage, particularly in plate-like structures. Nonlinear ultrasonic structural health monitoring techniques use interrogation signals at frequencies other than the excitation frequency to detect changes in structural integrity. Signal processing techniques used in non-destructive evaluation are frequently supported by modeling and numerical simulations in order to facilitate problem solution. This paper discusses known and newly-developed local computational strategies for simulating elastic waves, and attempts characterization of their numerical properties in the context of linear and nonlinear media. A hybrid numerical approach combining advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE) is proposed for unique treatment of arbitrary strain-stress relations. The iteration equations of the method are derived directly from physical principles employing stress and displacement continuity, leading to an accurate description of the propagation in arbitrarily complex media. Numerical analysis of guided wave propagation, based on the newly developed hybrid approach, is presented and discussed in the paper for linear and nonlinear media. Comparisons to Finite Elements (FE) are also discussed.
Nonlinear features of elastic wave propagation have attracted significant attention recently. The particular interest herein relates to complex wave-structure interactions, which provide potential new opportunities for feature discovery and identification in a variety of applications. Due to significant complexity associated with wave propagation in nonlinear media, numerical modeling and simulations are employed to facilitate design and development of new measurement, monitoring and characterization systems. However, since very high spatio- temporal accuracy of numerical models is required, it is critical to evaluate their spectral properties and tune discretization parameters for compromise between accuracy and calculation time. Moreover, nonlinearities in structures give rise to various effects that are not present in linear systems, e.g. wave-wave interactions, higher harmonics generation, synchronism and | recently reported | shifts to dispersion characteristics. This paper discusses local computational model based on a new HYBRID approach for wave propagation in nonlinear media. The proposed approach combines advantages of the Local Interaction Simulation Approach (LISA) and Cellular Automata for Elastodynamics (CAFE). The methods are investigated in the context of their accuracy for predicting nonlinear wavefields, in particular shifts to dispersion characteristics for finite amplitude waves and secondary wavefields. The results are validated against Finite Element (FE) calculations for guided waves in copper plate. Critical modes i.e., modes determining accuracy of a model at given excitation frequency - are identified and guidelines for numerical model parameters are proposed.
Numerical tools, which are used to simulate complex phenomena for models of complicated shapes suffer from either long computational time or accuracy. Hence, new modeling and simulation tools, which could offer reliable results within reasonable time periods, are highly demanded. Among other approaches, the nonlocal methods have appeared to fulfill these requirements quite efficiently and opened new perspectives for accurate simulations based on crude meshes of the model's degrees of freedom. In the paper, the preliminary results are shown for simulations of the phenomenon of temperature-dependent crack-wave interaction for elastic wave propagation in a model of an aluminum plate. Semi-nonlocal finite differences are considered to solve the problem of thermoelasticity - based on the discretization schemes, which were already proposed by the authors and taken from the previously published work. Numerical modeling is used to examine wave propagation primarily in the vicinity of a notch. Both displacement and temperature fields are sought in the investigated case study.
The paper illustrates a general equation in a new form, which allows calculating the characteristic frequencies of any kind of epicyclic gear sets with a ring, a sun, and planets. Moreover, presented equation takes into account corrected teeth (i.e. where the equality 2P+S=R is not fulfilled). This happens when gearboxes contain gears where corrected teeth procedure was adopted during designing stage. Presented solution can refine the configuration modules of the Condition Monitoring Systems (CMS) in such a way that allows to configure systems into larger groups than now available, i.e. multistage gear sets systems with epicyclic gears. Such CMS are capable of early mechanical faults detection, which prevents from costly critical repairs. For instance, fault detection of wind turbines is typically based on vibration and process signals analysis. Illustrated possible enhancement of configuration module is the basis for determining the energy bands in the spectra and envelope spectra in the process of identifying characteristic frequencies caused by gear defects.
A large number of commercial systems for condition monitoring of most common planetary gearboxes used in wind turbines and mining machinery have been developed for years. However nowadays, multistage constructions are encountered in industries. These are not necessarily planetary, but generally epicyclic. Current state of the art, according to the authors knowledge, does not give general equations for a case where multistage systems are considered, where some of the gears consist all moving parts. Hence, currently available CMS systems are not suitable for condition monitoring of these kinds of systems.
The paper presents a new general equation, which allows calculating the characteristic frequencies of any kind of multistage gear sets, as a result of theoretical investigation. Illustrated solution does not assume a fixed speed of any element. Moreover, presented equation takes into account corrected teeth, making developed equations most general from all available in tribology science. Presented scientific development is currently implemented in a modern European CMS.
Attractive properties of guided waves provides very unique potential for characterization of incipient damage, particularly in plate-like structures. Among other properties, guided waves can propagate over long distances and can be used to monitor hidden structural features and components. On the other hand, guided propagation brings substantial challenges for data analysis. Signal processing techniques are frequently supported by numerical simulations in order to facilitate problem solution. When employing numerical models additional sources of errors are introduced. These can play significant role for design and development of a wave-based monitoring strategy. Hence, the paper presents an investigation of numerical models for guided waves generation, propagation and sensing. Numerical dispersion analysis, for guided waves in plates, based on the LISA approach is presented and discussed in the paper. Both dispersion and modal amplitudes characteristics are analysed. It is shown that wave propagation in a numerical model resembles propagation in a periodic medium. Consequently, Lamb wave propagation close to numerical Brillouin zone is investigated and characterized.
The last few decades have seen a significant increase in research interest related to nonlinearities in micro-cracked and cracked solids. As a result, a number of different nonlinear acoustic methods have been developed for damage detection. The paper investigates nonlinear crack-wave interactions used for damage detection in plate-like structures. Semi-analytical modelling is used to investigate wave propagation in the vicinity of the crack. The focus is on non-classical crack model leading to wave modulations. Various physical phenomena (including fluctuation of temperature gradient) associated with these modulations are investigated. The work presented can be used for better understanding of nonlinear crack-wave interactions that are used for damage detection in structural health monitoring applications.
The paper presents a novel damage detection method that combines Lamb wave propagation with nonlinear acoustics.
Low-frequency excitation is used to modulate Lamb waves in the presence of fatigue cracks. The work presented shows
that the synchronization of the interrogating high-frequency Lamb wave with the low-frequency vibration is a key
element of the proposed method. The main advantages of the proposed method are the lack of necessity for baseline
measurements representing undamaged condition and lack of sensitivity to temperature variations. Numerical
simulations and experimental measurements are performed to demonstrate the application of the proposed method to
detect fatigue crack in aluminum beam.
An approach based on Macro-Fiber Composite (MFC) transducer rosettes and ultrasonic guided waves is proposed for
damage location in plate-like structures. By using the directivity behaviour of the three MFC sensors in each rosette, the
direction of an incoming wave generated by scattering from damage can be estimated without knowledge of the wave
velocity in monitored structures. Two rosettes suffice to identify the location of a scatterer in a planar structure. The
technique does not have any drawbacks of time-of-flight triangulation that requires information on wave velocity and
thus complicates damage location when testing anisotropic materials, tapered sections, or any structure under
temperature fluctuations. The effectiveness of the piezoelectric rosette method is tested experimentally using an
aluminium plate with a simulated damage subjected to temperature variation.
The University of California at San Diego (UCSD), under a Federal Railroad Administration (FRA) Office of Research and Development (R&D) grant, is developing a system for high-speed and non-contact rail integrity evaluation. A prototype using an ultrasonic air-coupled guided wave signal generation and air-coupled signal detection in pair with a real-time statistical analysis algorithm has been realized. This solution presents an improvement over the previously considered laser/air-coupled hybrid system because it replaces the costly and hard-to-maintain laser with a much cheaper, faster, and easier-to-maintain air-coupled transmitter. This system requires a specialized filtering approach due to the inherently poor signal-to-noise ratio of the air-coupled ultrasonic measurements in rail steel. Various aspects of the prototype have been designed with the aid of numerical analyses. In particular, simulations of ultrasonic guided wave propagation in rails have been performed using a LISA algorithm. Many of the system operating parameters were selected based on Receiver Operating Characteristic (ROC) curves, which provide a quantitative manner to evaluate different detection performances based on the trade-off between detection rate and false positive rate. Experimental tests have been carried out at the UCSD Rail Defect Farm. The laboratory results indicate that the prototype is able to detect internal rail defects with a high reliability. A field test will be planned later in the year to further validate these results. Extensions of the system are planned to add rail surface characterization to the internal rail defect detection.
Although damage detection using Lamb waves has been investigated for many years, real engineering applications are
limited due to practical aspects related to implementation. Temperature effect is one of the major problems. It is well
known that temperature variations influence Lamb wave propagation response parameters. In practice it is important to
compensate for this effect. Experimental tests are often required to understand how temperature influences wave
propagation. Numerical simulation can ease this task preventing many time-consuming experiments. Simulated Lamb
wave responses can be used to develop new methods for temperature compensation.
The effect of temperature variations on piezoceramic transducer responses is investigated using finite element modelling.
The model takes into account temperature-dependent physical properties of low-profile PZT transducers and transducer
bonding layers. The model is used to predict the S0 and A0 Lamb response in aluminium plate for the temperature range
from -60 to +40°C. The study shows relevant changes in Lamb wave amplitude response caused by temperature
fluctuations. This approach can provide the basis for temperature compensation in ultrasonic guided wave damage
detection systems used for structural health monitoring applications.
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