Publications

A Review: Non-Contact and Full-Field Strain Mapping Methods for Experimental Mechanics and Structural Health Monitoring

Non-contact and full-field strain mapping captures strain across an entire surface, providing a complete two-dimensional (2D) strain distribution without attachment to sensors. It is an essential technique with wide-ranging applications across various industries, significantly contributing to experimental mechanics and structural health monitoring. Although there have been reviews that focus on specific methods, such as interferometric techniques or carbon nanotube-based strain sensors, a comprehensive comparison that evaluates these diverse methods together is lacking. This paper addresses this gap by focusing on strain mapping techniques specifically used in experimental mechanics and structural health monitoring. The fundamental principles of each method are illustrated with specific applications. Their performance characteristics are compared and analyzed to highlight strengths and limitations. The review concludes by discussing future challenges in strain mapping, providing insights into potential advancements and developments in this critical field.

Experimental investigation of vibration attenuation using nonlinear tuned mass damper and pendulum tuned mass damper in parallel

3D printed metamaterials for damping enhancement and vibration isolation: Schwarzites

Next-generation 2D optical strain mapping with strain-sensing smart skin compared to digital image correlation

This study reports next generation optical strain measurement with "strain-sensing smart skin" (S4) and a comparison of its performance against the established digital image correlation (DIC) method. S4 measures strain-induced shifts in the emission wavelengths of single-wall carbon nanotubes embedded in a thin film on the specimen. The new S4 film improves spectral uniformity of the nanotube sensors, avoids the need for annealing at elevated temperatures, and allows for parallel DIC measurements. Noncontact strain maps measured with the S4 films and point-wise scanning were directly compared to those from DIC on acrylic, concrete, and aluminum test specimens, including one with subsurface damage. Strain features were more clearly revealed with S4 than with DIC. Finite element method simulations also showed closer agreement with S4 than with DIC results. These findings highlight the potential of S4 strain measurement technology as a promising alternative or complement to existing technologies, especially when accumulated strains must be detected in structures that are not under constant observation.

Torsion in Base‐Isolated Structures with Elastomeric Isolation Systems

Torsion in base‐isolated structures with inelastic elastomeric isolation systems due to bidirectional lateral ground motion is studied. In a companion paper by the writers, torsional coupling in sliding base‐isolated structures was investigated. In this paper, which is the second part of the sequence, torsional coupling in elastomeric base‐isolated structures is investigated. Various multistoried structural systems with elastomeric isolation systems are investigated, with the objective of studying the influence of: (1) The flexibility of the superstructure; (2) the ratio of uncoupled torsional to lateral frequencies; (3) stiffness eccentricity in the superstructure; (4) eccentricity in the isolation system; (5) higher mode effects; and (6) number of bearings in the isolation system. Response to different ground motions is also studied. The results are used to explain: (1) The behavior of actual buildings; and (2) some inconsistencies in the conclusions of previous studies. It is shown that, although the total superstructure response is reduced significantly due to the effects of elastomeric base isolation, torsional amplification can be significant depending on the isolation and superstructure eccentricity and the lateral and torsional flexibility.

Latching control: Experimental study on pendulum latched mass damper

Seismic Response Prediction of Multiple Base-Isolated Structures for Monitoring

A novel crosswind mitigation strategy for tall buildings using negative stiffness damped outrigger systems

This study presents a new crosswind mitigation strategy by using negative stiffness damped outrigger (NSDO) system for tall buildings. Using the "assisting motion" feature of negative stiffness, NSDO amplifies the motion of viscous damper resulting in and significant improvement of energy dissipation for tall buildings. Systematical evaluation and comparison on crosswind performance are carried out for tall buildings using (a) conventional outrigger (CO), (b) conventional damped outrigger (CDO), and (c) NSDO. It is shown that the proposed NSDO is able to achieve a damping amplification factor larger than a unit, which would never be achieved by CDO in practical due to the actual deformation of perimeter columns. In this way, NSDO has the most satisfactory crosswind reduction effect, especially when perimeter column stiffness is insufficient. For example, replacing CDO with NSDO not only attenuates crosswind-induced harmful drift and structural acceleration by 22% and 36%, respectively, but also further saves about 75% of the size of viscous dampers. In other words, NSDO adopts less outrigger damping coefficient but reduces more crosswind-induced vibration.

Output only modal identification and structural damage detection using time frequency & wavelet techniques

Bi-directional semi-active tuned mass damper for torsional asymmetric structural seismic response control

Tracking of Stiffness Variation in Structural Members Using Input Error Function Observers

This study evaluates input error function observers for tracking of stiffness variation in real-time. The input error function is an Analytical Redundancy (AR)-based diagnosis method and necessitates a mathematical model of the system and system identification techniques. In practice, mathematical models used during numerical simulations differ from the actual status of the structure, and thus, accurate mathematical models are rarely available for reference. Noise is an unwanted signal in the input–output measurements but unavoidable in real-world applications (as in long span bridge trusses) and hard to imitate during numerical simulations. Simulation data from the truss system clearly indicates the effectiveness of the proposed structural damage detection method for estimating the severity of the damage. Optimization of the input error function can further automate the stiffness estimation in structural members and address critical aspects such as system uncertainties and the presence of noise in input–output measurements. Stiffness tracking in one of the planar truss members indicates the potential of optimization of the input error function for online structural health monitoring and implementing condition-based maintenance.

Control of structures with friction controllable sliding isolation bearings

MWCNT Based Thin Film Strain Sensor

In this study, the real time strain response of the Multiwalled Carbon Nanotube (MWCNT) film to different kinds of tensile loading and response of the MWCNT film to temperature changes at macroscale is studied experimentally. The strain is applied to the MWCNT film by attaching it to a brass specimen using vacuum bonding, and subjecting it to tensile loading. Voltage output from MWCNT film is obtained using four-point probe in conjunction with a sensitive voltage measurement device. Experimental results demonstrate a linear relationship between change in voltage across the film and change in strain in the brass specimen when subjected to tensile load. The observed electromechanical characteristics also exhibit excellent reversibility evident from the way the voltage of the MWCNT film recovers to its unstressed state upon unloading. The MWCNT film exhibited a stable and linear response to changes in temperature with the resistance observed to be decreasing as temperature is increased. The study of MWCNT response to changes in temperature was done over a limited range. The experimental results presented in this paper demonstrate the effectiveness of MWCNT thin film in strain sensing as well as highlight the effect of temperature on it.

On the effectiveness of principal component analysis for decoupling structural damage and environmental effects in bridge structures

Negative stiffness device for seismic protection of MDOF yielding fixed base structures: Experimental and analytical study

Shake table tests of single degree of freedom elastic/inelastic structures have confirmed that by adding a negative stiffness device (NSD), capable of exhibiting nonlinear elastic negative stiffness, in conjunction with a viscous damper, the acceleration, inter-story drifts, and base shear can be reduced significantly. However, little is known about how the presence of NSD/damper in the first story influences the response of higher stories of a multistory structure. In this paper, shake table test results are presented that demonstrate the advantages of NSD/damper in the first story of a multidegree of freedom, three-story, fixed-base structure (MDOF-3SFS). Results confirm that deployment of NSD/damper at the first story leads to significant reductions of acceleration as well as base shear and inter-story deformations in the MDOF-3SFS. Essentially, an NSD and a damper in the first floor prevents the transmission of the input energy from the ground motion to the second and third story of the multistory structure by deflecting and dissipating energy. If the first-story displacements become excessive, NSD stiffens and prevents collapse. The efficacy of the NSD/damper system is further demonstrated by comparing it with the performance of a structure with passive viscous dampers deployed in the first story. In addition to the reduction of base shear and maximum accelerations, the NSD/damper system also restricts large deformations to the first story only, leading to minimal damage to the whole structure. Finally, an NSD-based bracing system is introduced that can be deployed in new and existing structures.

Mixed-Sensitivity Controller Design for Repetitive Control in Hysteretic Systems

Smart actuators, like piezoceramic materials, have been increasingly used in many engineering fields. One such application of piezoceramic actuators is ultrahigh precision positioning and tracking. Applicability of these materials in high precision devices is hampered due to the presence of nonlinearities such as hysteresis. Tracking control of such hysteretic systems has received considerable attention in the past two decades. In this work, a systematic approach is developed to represent the hysteretic systems as time-invariant, parameter-dependent uncertain systems assuming variable stiffness and damping as uncertain parameters. And also, design of robust controller for tracking periodic signals by minimizing the mixed sensitivity H∞ norm of the closed loop system. The effectiveness of proposed method, in compensating hysteresis nonlinearities, is validated through experiments on Thunder actuator. The effectiveness of designed controller is demonstrated experimentally for tracking sinusoidal signals. Experimental results substantiated the improved tracking performance of closed loop system. Effective width of hysteresis loops is reduced to a great extent after incorporating the proposed controller. Robustness of the developed H∞ controller is demonstrated experimentally by verifying the performance of controller at different input amplitudes, input frequencies and change in physical properties of the system.

Structural Health Monitoring of Civil Infrastructure Using Applied Recursive Bayesian Estimation Methods

Apparent-weakening by adaptive passive stiffness shaping along the height of multistory building using negative stiffness devices and dampers for seismic protection