Structural Mechanics, Dynamics and Control, Smart Infrastructure, Intelligent Material Applications, Structure Acoustic Interactions, Power/Energy Harvesting, Visual Suppression
My research focuses on the general areas of structural dynamics, vibration, control, testing, adaptive structures and smart materials.
Structural Dynamics
Our work focuses on several aspects of structural dynamics such as modeling, vibration suppression, vibro-acoustics, nonlinear systems, control, testing and validation. Our work strongly focuses on bridging the theoretical and experimental field in order to provide well-validated and trustworthy systems. Our testing labs feature several distinct capabilities such as 3D laser non-contact measurements, non-contact excitation, high-bay labs, overhead cranes, temperature-controlled vacuums, and ground isolation platforms.
Key words: Model Updating, Model Reduction, Experimental Modal Testing, Modal Analysis, Model Validation
Adaptive Structures
By adaptive structures we refer to structures that have the ability to adapt, evolve or change their properties or behavior in response to the environment around them. Much of this work is accomplished with domain-coupled material such as piezoceramics and shape memory alloys and use much of the techniques we use in structural dynamics to achieve, for example, high precision control and structure integration in a native way. This takes in the Vibrations, Adaptive Structures and Testing (VAST) Laboratory directed by me.
Key words: Structural Health Monitoring (SHM and NDE), Control, bio-inspired concepts, smart material integration, PZT, Shape memory alloys
Smart Infrastructure
As founder and co-Director of the Virginia Tech Smart Infrastructure Laboratory this area of work focuses on research in topics that utilize sensor information to improve the design, monitoring and daily operation of civil and mechanical infrastructure as well as to investigate how humans
Publications (14)
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Track defects are a major safety concern for the railroad industry. Among different track components, insulated rail joints, which are widely used for signaling purposes, are considered a weak link in the railroad track. Several joint-related defects have been identified by the railroad community, including rail wear, torque loss, and joint bar breakage. Current track inspection techniques rely on manual and visual inspection or on specially equipped testing carts, which are costly, timeconsuming, traffic disturbing, and prone to human error. To overcome the aforementioned limitations, the feasibility of utilizing impedance-based structural health monitoring for insulated rail joints is investigated in this work. For this purpose, an insulated joint, provided by Koppers Inc., is instrumented with piezoelectric transducers and assembled with 136 AREA rail plugs. The instrumented joint is then installed and tested at the Facility for Accelerated Service Testing, Transportation Technology Center Inc. The effects of environmental and operating conditions on the measured impedance signatures are investigated through a set of experiments conducted at different temperatures and loading conditions. The capabilities of impedance-based SHM to detect several joint-related damage types are also studied by introducing reversible mechanical defects to different joint components.
Research over the past few decades has shown that the ear exhibits an important, nonlinear amplification called the “cochlear amplification.” It is responsible for boosting faint sounds and improving frequency sensitivity, which allows the ear to process a larger range of sound intensities (from about 20 micro-Pa to 20 Pa). In contrast, typical microphones, accelerometers, and other dynamic sensors are designed to operate linearly in the sensor’s quasi-static response region. Instead, the cochlea operates in the resonance region, where weak inputs are significantly amplified. The goal of our research is to develop unique, piezoelectric-based MEMS sensors that mimic the function of hair cells in the mammalian cochlea. Inspired by the geometry of the hair cells, a set of artificial hair cells (AHCs) are designed based on piezoelectric cantilever beams.
Developing piezoelectric, MEMS AHC arrays capable of mimicking the cochlea’s behavior is the main scope of this work. The design consists of a substrate material and two layers of Lead Zirconate Titanate (PZT) deposition that represents artificial hair cells. Abaqus finite element software is used to model the AHC arrays. Fundamental frequencies of the AHCs and also frequency response function due to a base excitation of the array are obtained by linear perturbation frequency analysis and steady-state linear dynamic analysis, respectively. As a proof of concept, a series of dynamic tests are conducted on larger scale single AHCs and an array of four AHCs to measure the response of the sensors to an external stimulus.
The present work generates steady-state traveling waves in fin-like continuous structures with the help of Macro-Fiber Composite (MFC) piezoelectric actuators. To produce traveling waves, two MFCs simultaneously excite a clamped-free beam at a common frequency with a preset phase difference. Previous research has shown that optimal traveling waves are developed in structures when the common frequency lies halfway between two adjacent resonant frequencies and the phase difference between the two inputs is 90°. These traveling waves closely replicate the undulatory patterns that propel aquatic animals but at a low amplitude linear regime. The present work studies the generation of underwater traveling waves and investigates the range of propulsive forces generated through such mechanism. The ability of continuous structures to produce thrust using undulations is the first step towards mimicking the bio-kinematics and biomechanics of aquatic animals.
The field of event classification and localization in building environments using accelerometers has grown significantly due to its implications for energy, security, and emergency protocols. Virginia Tech’s Goodwin Hall (VT-GH) provides a robust testbed for such work, but a reduced scale testbed could provide significant benefits by allowing algorithm development to occur in a simplified environment. Environments such as VT-GH have high human traffic that contributes external noise disrupting test signals. This paper presents a design solution through the development of an isolated platform for data collection, portable demonstrations, and the development of localization and classification algorithms. The platform’s success was quantified by the resulting transmissibility of external excitation sources, demonstrating the capabilities of the platform to isolate external disturbances while preserving gait information. This platform demonstrates the collection of high-quality gait information in otherwise noisy environments for data collection or demonstration purposes.
The problem of estimating the location of an impact force in a dispersive medium is complicated given the dispersion-related distortion of the generated traveling wave. The problem cannot be solved, with reasonable accuracy, using conventional time difference of arrival (TDOA) techniques. A building floor is an example of a dispersive medium that is being loaded by occupant footsteps. If more accurate localization algorithms are obtained, then they can be used to localize and track occupants in a building using floor vibration sensors measuring the footstep-induced traveling waves. This paper presents the evaluation of a new localization approach, in a simulated aluminum plate (dispersive waveguide), using a network of sensors measuring the plate's vibration. Average signal power is calculated for all the sensors over a fixed time period, and then used to generate a location estimate. Two different location estimation solutions are presented and compared; a constrained least squares solution (CLS), and a non-linear root finding solution generated using the Levenberg-Marquardt (LM) algorithm. A finite element (FE) thin plate model is used as a testbed to evaluate the performance of the developed localization algorithm by estimating the location of virtual hammer impacts acting on the plate. The results encourage further future development.
The Goodwin Hall Smart Infrastructure facility at Virginia Tech is a five-story “smart building" with an integrated network of 213 wired accelerometers. We utilize a subset of 68 sensors to perform high-resolution Operational Modal Analysis (OMA) of the structure under windy conditions. The low-rise, L-shaped construction and high mass, high stiffness properties of Goodwin Hall provide a unique case study in comparison to typical cases of building OMA in literature, which generally feature high-rise buildings with rectangular architectures. Our work focuses on data acquisition and feature extraction, which are two critical steps within a complete structural health monitoring approach. Our detailed methodology establishes guidelines for sensor selection and data processing applicable to this and more general cases. Modal parameters extraction using Stochastic Subspace Identification shows the first four natural frequencies, damping values, participation factors and mode shapes of the building. We hypothesize that high damping values and large differences in the participation of fundamental modes are related to the nature of the wind excitation.
KEYWORDS: Source localization, Buildings, Smart structures, Machine learning, Vibrometry, Sensing systems, Sensors, Dispersion, Data acquisition, Research management, Signal attenuation, Environmental sensing, Signal to noise ratio
Recent years have shown prolific advancements in smart infrastructures, allowing buildings of the modern world to interact with their occupants. One of the sought-after attributes of smart buildings is the ability to provide unobtrusive, indoor localization of occupants. The ability to locate occupants indoors can provide a broad range of benefits in areas such as security, emergency response, and resource management. Recent research has shown promising results in occupant building localization, although there is still significant room for improvement. This study presents a passive, small-scale localization system using accelerometers placed around the edges of a small area in an active building environment. The area is discretized into a grid of small squares, and vibration measurements are processed using a pattern matching approach that estimates the location of the source. Vibration measurements are produced with ball-drops, hammer-strikes, and footsteps as the sources of the floor excitation. The developed approach uses matched filters based on a reference data set, and the location is classified using a nearest-neighbor search. This approach detects the appropriate location of impact-like sources i.e. the ball-drops and hammer-strikes with a 100% accuracy. However, this accuracy reduces to 56% for footsteps, with the average localization results being within 0.6 m (α = 0.05) from the true source location. While requiring a reference data set can make this method difficult to implement on a large scale, it may be used to provide accurate localization abilities in areas where training data is readily obtainable. This exploratory work seeks to examine the feasibility of the matched filter and nearest neighbor search approach for footstep and event localization in a small, instrumented area within a multi-story building.
Embedded and surface bonded piezoelectric wafers have been widely used for control, energy harvesting, and structural
health monitoring applications. The basis for all these applications is the energy transfer between the piezoelectric wafer
and the host structure, which takes place through the adhesive bonding layer. The characteristics of the bonding layer are
found to have an important impact on the sensing and actuation capabilities of piezoelectric-based applications.
In this paper, the high-frequency dynamic response of an elastic beam coupled with a piezoelectric wafer is investigated,
including the bonding layer in between. A previously developed three-layer spectral element model, with high-frequency
capabilities, is utilized for this purpose. Timoshenko beam and elementary rod theories are adopted to describe axial and
lateral deformations in each of the three layers. A parametric study is conducted to evaluate the effects of bonding layer
characteristics on the steady-state dynamic response of the coupled system, including frequency response functions and
electromechanical impedance. The frequency-dependent nature of bonding layer effects is highlighted and discussed.
KEYWORDS: Wave plates, Wave propagation, Actuators, Superposition, Wavefronts, Microsoft Foundation Class Library, Beam propagation method, Skin, Aluminum, Video
Structural traveling waves have potential applications in numerous areas such as propulsion and skin friction drag reduction. Recent research has shown that via the two-mode excitation method, traveling waves can be generated in both one- and two-dimensional structures via the use of low-profile piezoelectric actuators. Traveling waves on a one-dimensional beam propagate in a single direction, while those on a two-dimensional structure, such as a plate, do not necessarily propagate uniformly across the surface. The propagation patterns can include unidirectional traveling waves with spatial phase shifts, wave fronts moving in opposing directions, or even rotationally moving waves. These propagation patterns depend on the participating modes and vary based on the excitation frequency, thus if multiple frequency traveling waves are generated on a plate, multiple propagation patterns are superimposed. In this study, traveling waves were generated in a plate at two different frequencies. Those frequencies were then simultaneously excited on the plate to generate a propagation pattern containing traveling waves at both frequencies. The superimposed propagation pattern was then analyzed by comparing it with a numerical combination of the individual frequency patterns. The experimentally superimposed traveling waves were found to be a linear combination of the individual frequency waves. In addition, by combining multiple frequency waves, the percentage of the plate containing traveling waves increased.
KEYWORDS: Actuators, Wave propagation, Wave plates, Wind measurement, Microsoft Foundation Class Library, Composites, Aluminum, Velocity measurements, Fluctuations and noise
A major technological driver in current aircraft and other vehicles is the improvement of fuel efficiency. One way to increase the efficiency is to reduce the skin friction drag on these vehicles. This experimental study presents an active drag reduction technique which decreases the skin friction using spanwise traveling waves. A novel method is introduced for generating traveling waves which is low-profile, non-intrusive, and operates under various flow conditions. This wave generation method is discussed and the resulting traveling waves are presented. These waves are then tested in a low-speed wind tunnel to determine their drag reduction potential. To calculate the drag reduction, the momentum integral method is applied to turbulent boundary layer data collected using a pitot tube and traversing system. The skin friction coefficients are then calculated and the drag reduction determined. Preliminary results yielded a drag reduction of ≈ 5% for 244Hz traveling waves. Thus, this novel wave generation method possesses the potential to yield an easily implementable, non-invasive drag reduction technology.
Active Fluid Flow Control (AFFC) has received great research attention due to its significant potential in engineering
applications. It is known that drag reduction, turbulence management, flow separation delay and noise suppression through
active control can result in significantly increased efficiency of future commercial transport vehicles and gas turbine
engines. In microfluidics systems, AFFC has mainly been used to manipulate fluid passing through the microfluidic device.
We put forward a conceptual approach for fluid flow manipulation by coupling multiple vibrating structures through flow
interactions in an otherwise quiescent fluid. Previous investigations of piezoelectric flaps interacting with a fluid have
focused on a single flap. In this work, arrays of closely-spaced, free-standing piezoelectric flaps are attached perpendicular
to the bottom surface of a tank. The coupling of vibrating flaps due to their interacting with the surrounding fluid is
investigated in air (for calibration) and under water. Actuated flaps are driven with a harmonic input voltage, which results
in bending vibration of the flaps that can work with or against the flow-induced bending. The size and spatial distribution
of the attached flaps, and the phase and frequency of the input actuation voltage are the key parameters to be investigated
in this work. Our analysis will characterize the electrohydroelastic dynamics of active, interacting flaps and the fluid
motion induced by the system.
The complex nature of tires requires very precise test data to model the structure accurately. The highly damped
characteristics, geometric features and operational conditions of tires cause various testing difficulties that affect the
reliability of the modal testing. One of the biggest challenges of tire testing is exciting the whole tire at once.
Conventionally, impact hammers, shakers, and cleats are used as an excitation input. The shortcomings of these
excitation methods are the directional and force inconsistency of hammer impacts, coupled dynamics of shakers and
speed limitations of cleat excitation. Other challenges of modal testing of tires are the effect of added mass due to sensor
placements and difficulty of vibration measurement of a rotating tire with accelerometers. In order to remedy these
problems, we conduct experimental modal analysis (EMA) using a non-contact measurement technique and piezoelectric
excitation. For non-contact measurement, a 3-D scanning laser doppler vibrometer (SLDV) is used. For the piezoelectric
excitation, Micro Fiber Composite (MFC) patches are used due to their flexible nature and power capacity. This
excitation method can also be crucial to the excitation of rotating tires since the cleat excitation is not adequate for low-speed
measurements. Furthermore, the piezoelectric actuation could be used as sensors as well as noise controllers in
operating conditions. For this work, we run experiments for a loaded tire in non-rotating condition. Experiments are
carried out for the frequency bandwidth up to 500Hz to capture the structural behavior under high-frequency excitations
and its potential coupled behavior to airborne noise.
KEYWORDS: Composites, Microsoft Foundation Class Library, Vibrometry, Sensors, Satellites, Laser applications, Ferroelectric materials, Signal attenuation, Vibration control, Positive feedback
Composite booms are an emerging low weight structural alternative for on-orbit satellites. With any satellite, vibrational control of the structure is a concern. Using traditional measurement techniques, possible inaccuracies may result due to the lightweight natures of the composite boom. Laser vibrometry techniques are investigated in this paper as an alternative to standard accelerometer measurements. The advantages of non contact measurement are presented in two different experimental setups. Using the dynamic measurements obtained from these experiments, a positive feedback controller is developed to attenuate the vibration of the strut.
KEYWORDS: Microsoft Foundation Class Library, Composites, Sensors, Structural health monitoring, Transducers, Satellites, Actuators, Damage detection, Packaging, Ferroelectric materials
Inflatable-rigidizeable composite space structures are an emerging technology that could revolutionize the design of large on-orbit satellites. These structural systems have the advantages of low mass, high packaging efficiency, low life cycle cost, low part counts, and high deployment reliability. As they are rigidized on-orbit, they do not depend on internal pressure to maintain their shape once deployed. However, as thin-walled structures, micrometeoroids and orbital debris (MMOD) are still a potential threat to their structural integrity. Such impacts will create punctures on the structure of varying sizes related to the size and kinetic energy of the debris/meteorite. For closed-cell geometries, such as booms or struts, MMOD objects can penetrate the outer wall twice, once on initial impact and once upon exiting the
structure. As impact damage and structural degradation will be cumulative over time, being able to monitor the structural integrity of these satellites would be of great interest. Impedance-based structural health monitoring schemes using distributed piezoelectric transducers are one possible approach. In this study, several Macro-Fiber Composite (MFC) piezoelectric devices were installed on a representative space-inflatable rigidizeable composite boom and used in ground tests as collocated sensor-actuators for detecting
and assessing simulated micrometeoroid/orbital debris strike damage. Electrical impedance signatures were compared before and after application of the simulated damage to determine the extent of the damage sustained. Both small and large footprint MFC piezocomposite sensor/actuators were shown to be effective in characterizing simulated MMOD punctures along the entire length of the boom.
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