Sourav Gur

An improved tuned mass damper (SMA-TMD) assisted by a shape memory alloy spring

The tuned mass damper (TMD) is a well acclaimed passive control device for vibration control of structures. However, the requirement of a higher mass ratio restricts its applicability for seismic vibration control of civil engineering structures. Improving the performance of TMDs has been attempted by supplementing them with nonlinear restoring devices. In this regard, the ability of shape memory alloy (SMA) in dissipating energy through a hysteretic phase transformation of its microstructure triggered by cyclic loading is notable. An improved version of TMD assisted by a nonlinear shape memory alloy (SMA) spring, referred as SMA-TMD, is studied here for seismic vibration mitigation. Extensive numerical simulations are conducted based on nonlinear random vibration analysis via stochastic linearization of the nonlinear force–deformation hysteresis of the SMA. A design optimization based on minimizing the root mean square displacement of the main structure is also carried out to postulate the optimal design parameters for the proposed system. The viability of the optimal design is verified with respect to its performance under recorded earthquake motions. Significant improvements of the control efficiency and a reduction of the TMD displacement at a much reduced mass ratio are shown to be achieved in the proposed SMA-TMD over those in the linear TMD.

Compliant liquid column damper modified by shape memory alloy device for seismic vibration control

Liquid column dampers (LCDs) have long been used for the seismic vibration control of flexible structures. In contrast, tuning LCDs to short-period structures poses difficulty. Various modifications have been proposed on the original LCD configuration for improving its performance in relatively stiff structures. One such system, referred to as a compliant-LCD has been proposed recently by connecting the LCD to the structure with a spring. In this study, an improvement is attempted in compliant LCDs by replacing the linear spring with a spring made of shape memory alloy (SMA). Considering the dissipative, super-elastic, force-deformation hysteresis of SMA triggered by stress-induced micro-structural phase transition, the performance is expected to improve further. The optimum parameters for the SMA-compliant LCD are obtained through design optimization, which is based on a nonlinear random vibration response analysis via stochastic linearization of the force-deformation hysteresis of SMA and dissipation by liquid motion through an orifice. Substantially enhanced performance of the SMA–LCD over a conventional compliant LCD is demonstrated, the consistency of which is further verified under recorded ground motions. The robustness of the improved performance is also validated by parametric study concerning the anticipated variations in system parameters as well as variability in seismic loading.

Optimal performance of base isolated building considering limitation on excessive isolator displacement

The past studies on optimisation of isolation bearings emphasise on the allowable limit on the isolator displacement without incorporating it in the optimisation process. Large bearing displacement of the isolator during strong motion is often linked to isolator damage and pounding to adjacent units. The present study addresses the limitation on the excessive isolator displacement in the optimisation process of the isolator. It is intended not only to assure optimal performance in terms of vibration mitigation, but also to reveal the importance of constraining excessive isolator displacements. The stochastic response required for the optimisation study is evaluated through nonlinear random vibration analysis. The consistency of the optimal response behaviour obtained through stochastic optimisation is validated for recorded earthquake ground motions. A Lead Rubber Bearing (LRB), isolating a multi-story shear building is used to elucidate the effect of this constraint on the optimum parameters and system performance. The results clearly indicate the existence of significant disparities among the conventional unconstrained and the proposed constrained optimisation approach.

Thermo-mechanical strain rate–dependent behavior of shape memory alloys as vibration dampers and comparison to conventional dampers

A study on shape memory alloy materials as vibration dampers is reported. An important component is the strain rate–dependent and temperature-dependent constitutive behavior of shape memory alloy, which can significantly change its energy dissipation capacity under cyclic loading. The constitutive model used accounts for the thermo-mechanical strain rate–dependent behavior and phase transformation. With increasing structural flexibility, the hysteretic loop size of shape memory alloy dampers increases due to increasing strain rates, thus further decreasing the response of the structure to cyclic excitation. The structure examined is a beam, and its behavior with shape memory alloy dampers is compared to the same beam with conventional dampers. Parametric studies reveal the superior performance of the shape memory alloy over the conventional dampers even at the resonance frequency of the beam-damper system. An important behavior of the shape memory alloy dampers is discovered, in that they absorb energy from the fundamental and higher vibration modes. In contrast, the conventional dampers transfer energy to higher modes. For the same beam control, the stiffness requirement for the shape memory alloy dampers is significantly less than that of the conventional dampers. Response quantities of interest show improved performance of the shape memory alloy over the conventional dampers under varying excitation intensity, frequency, temperature, and strain rate.

Response of Bridges Isolated by Shape Memory–Alloy Rubber Bearing

AbstractThe present paper deals with optimum performance of the shape memory–alloy-based rubber bearing (SMARB) compared with the conventionally adopted lead rubber bearing (LRB) for isolating the bridge deck against a random earthquake. Specifically, a systematic study on the optimum performance of SMARB is conducted and compared with the LRB to minimize the acceleration of the isolated deck subjected to a stochastic earthquake by addressing the limitation on the excessive isolator displacements. The optimal characteristic strength of the bearing and respective responses are obtained by solving a bi-objective optimization problem. The responses required for this are obtained by nonlinear random vibration analysis via stochastic linearization of the cyclic nonlinear force-deformation behavior of the shape memory?alloy restrainers. The robustness of the improved performances of the SMARB was studied through extensive parametric studies with respect to the variation of the system parameters and the scenario...

Linking simulations and experiments for the multiscale tracking of thermally induced martensitic phase transformation in NiTi SMA

University of Arizona; THALES/INTERMONU-Project from European Union (European Science Foundation-ESF) [68/1117]

Atomistic Study on Size Effects in Thermally Induced Martensitic Phase Transformation of NiTi

The atomistic study shows strong size effects in thermally induced martensitic phase transformation evolution kinetics of equiatomic NiTi shape memory alloys (SMAs). It is shown that size effects are closely related to the presence of free surfaces; thus, NiTi thin films and nanopillars are studied. Quasi-static molecular dynamics simulations for several cell sizes at various (constant) temperatures are performed by employing well-established interatomic potentials for NiTi. The study shows that size plays a crucial role in the evolution of martensite phase fraction and, importantly, can significantly change the phase transformation temperatures, which can be used for the design of NiTi based sensors, actuators, or devices at nano- to microscales. Interestingly, it is found that, at the nanometer scale, Richard’s equation describes very well the martensite phase fraction evolution in NiTi thin films and nanopillars as a function of temperature.

Probabilistic Assessment of Container Crane Under Wind Loading

Container cranes are highly susceptible to damage or even failure during severe windstorms. Damage of container crane causes significant amount of economic loss both in terms of repair/replacement and downtime. This chapter focuses on the effect of uncertainty of different parameters of wind field on the performance of container cranes. A representative crane of tower height 72.60 m and total boom length of 131.00 m is chosen and modeled in a commercial software considering both material and geometric nonlinearities. Stochastic fluctuating wind fields have been simulated by means of spectral representation method using the Kaimal and Simiu power spectra in conjunction with along and across wind coherence functions. Nonlinear time history analyses are carried out using simulated wind fields, and the performance of the crane is assessed in terms of fragility curves. Further, a sensitivity analysis is conducted by means of a tornado diagram and first-order second-moment analysis to rank different uncertain parameters of wind field. Based on these results, a few considerations for design have been provided.

Vulnerability assessment of container cranes under stochastic wind loading

Container cranes, an essential component in any seaport facility system, are highly susceptible to failure during natural disasters such as severe windstorms. Damage or collapse of these structures can lead to significant amount of economic loss in terms of both repair or replacement and downtime. To study how various parameters such as boom position, yaw angle, gradient height and gradient wind speed affect the failure vulnerability of a crane, in this work, a representative container crane has been modelled in a commercial software considering both material and geometric nonlinearities. Stochastic wind fields are simulated as a stationary process by means of the spectral representation method using power spectrum in conjunction with along- and across-wind coherence functions. Wind fields are also simulated as a non-stationary stochastic process utilising the modulation functions from the available literature to consider the situations of thunderstorm downbursts. Nonlinear time history analyses are then performed and a few responses are compared for stationary and non-stationary wind field cases. It is observed that for the given simulation parameters, the responses considering stationary wind fields are larger than those of the non-stationary wind fields. Failure vulnerability of the crane is then assessed in terms of fragility curves. The results of this study show that the failure vulnerability may not be maximum for the set of parameter values for which the aerodynamic forces are maximum. This study provides a better understanding of crane vulnerability that may be utilised to achieve a better crane design.

Computational simulations for the development of novel solid-state smart NiTi-Al thermal diodes:

In this study, NiTi shape memory alloys coupled in series with Al are considered as building blocks for thermal diodes. It is shown that the strong nonlinearity in the temperature-dependent thermal properties of NiTi in conjunction with the very different thermal properties of Al can result into a thermal diode of high thermal rectification ratio. As a first level of study, Ni50Ti50 is considered and the effects of various NiTi-Al geometrical configurations, initial temperature, and temperature difference at two ends on the thermal rectification ratio are studied numerically. Within the adopted temperature range (300–400 K, where phase transformation in NiTi occurs), it is shown that NiTi-Al thermal diodes are feasible with rectification ratio up to 4.8, which is quite higher than the ratios in currently known solid-state thermal diodes. This fundamental computational study could provide an important basis and motivation for the development of the next generation of high-temperature solid-state thermal diodes based on smart material such as NiTi shape memory alloys or others.