Publications

The peak deformation, acceleration, and the base shear experienced by the structures can be reduced by simulating yielding in an elastic system—also referred as apparent weakening. The negative stiffness device (NSD), used in this study, exhibits nonlinear-elastic negative stiffness behavior; by adding NSD to the elastic structure (primary structure), the resulting structure-device assembly behaves like a bilinear-elastic structure. In an elastic structure, the acceleration and base shear experienced by the structure can be reduced by adding the negative stiffness device, and the additional deformations caused from the reduced stiffness can be contained by adding viscous dampers. Previously, the authors have carried out experimental studies to demonstrate the effectiveness of apparent weakening in elastic structures, but little is known about the behavior of these systems when the primary structure itself yields. This paper focuses on the issues that may emanate with the addition of NSD to the systems prone to yielding while highlighting the response reduction achieved by proper design of the NSD. Shake-table studies of a three-story fixed-base structure (3SFS) braced in top two stories, that is acting as a single degree of freedom system (SDOF) are presented in this paper. Two NSDs and a viscous damper are installed in the first story of the SDOF-3SFS. The structure is subjected to strong ground motions, with and without the NSDs, so that the columns of the SDOF-3SFS in the first story yield. Analytical models for all the components, that can capture the observed experimental behavior, are also presented in this paper. To demonstrate the advantages of apparent-weakening in yielding SDOF structures, experimental and analytical responses of four different systems, (1) 3SFS, (2) 3SFS with damper, (3) 3SFS with NSD, and (4) 3SFS with NSD and damper, are compared for a suite of ground motions. Shake-table tests on SDOF-3SFS have confirmed that by adding the NSD and damper, the acceleration and base shear and deformation of the bilinear inelastic structure and NSD assembly will be reduced by more than 20% for moderate ground motions, and the collapse of structure can be prevented for severe ground motions.

Strain measurements are essential in structural health monitoring. Traditional strain gages require physical contact between the sensor and read-out device, perturb the surface being monitored, and allow measurement only at the specific location and orientation axis of the sensor. We demonstrate a novel non-contact, multi- point, multi-directional strain sensing approach that overcomes these limitations. In our method, the surface is coated with a thin film of "smart skin" containing individualized single-walled carbon nanotubes in a polymeric host. After curing, substrate strains are transmitted through the polymer film to embedded nanotubes. This induces axial strains in the nanotubes, systematically shifting the wavelengths of their characteristic near-infrared fluorescence peaks. To measure strain, a visible laser excites nanotubes at points of interest on the surface, and the near-infrared emission is collected and spectrally analyzed. Observed spectral shifts reveal quantitative strain values. Laboratory tests show sensitivity down to ~400µm, limited by mechanical properties of the polymeric host film. We also vary excitation beam polarization to find the axis of substrate strain. Our method provides spatial resolution down to its gage length of ~100µm. Because the entire substrate is coated with nanoscale strain sensors, measurements can be made at arbitrary locations to construct a full strain map. We will describe recent smart skin refinements involving selection of polymer host, nanotube surfactant, nanotube dispersion method, and preparation protocol. Finally, we characterize the orientational distribution of nanotubes using a probabilistic model.

Abstract Cement is one of the most widely used building materials due to its strength and durability. However, conventional cement has a very high setting time, which makes it less attractive for applications requiring quick-setting behavior, such as rapid construction, emergency repairs, underwater construction, and 3D printing. The present study proposes hexagonal boron nitride (hBN) as a potential accelerant to impart quick-setting behavior to conventional cement. hBN is a two-dimensional material renowned for its exceptional thermal conductivity, chemical stability, and mechanical strength. Our study investigates the incorporation of hBN nanoparticles into class G Portland cement to enhance its mechanical, thermal, and rheological properties. Our experimental investigation demonstrates that hBN acts as an excellent accelerant in cement by reducing the dormancy period by up to 2 hours and enhancing the overall setting kinetics. This makes hBN a promising candidate for quick-setting cement applications. Further thermal analysis reveals an improved heat dissipation capability, with lower surface temperatures and enhanced structural integrity due to reduced porosity and microcrack formation. Mechanical testing demonstrates substantial improvements in compressive strength (up to 29%), compressive modulus (up to 45%), and energy absorption capacity (up to 31%) for 1% hBN-reinforced cement compared to neat cement. Moreover, hBN-reinforced 3D-printed cement structures exhibit a 72% increase in compressive strength. The hBN-reinforced cement ink also demonstrates enhanced printability, characterized by superior flow stability, better structural recovery, and reliable shape retention, making it ideal for 3D printing applications.

Structural weakening and damping is an approach previously proposed for the reduction of seismic forces and drifts in structures. While this approach is very efficient, it requires a reduction in strength, which will result in inelastic excursions and permanent deformation of the structural system during the seismic event. This paper describes a true Negative Stiffness Device (NSD) that can emulate weakening of the structural system without inelastic excursions and permanent deformations. The Negative Stiffness Device (NSD) is a self contained device that produces a force which is in the same direction as the imposed displacement, thus the name “negative stiffness”. The device is inherently unstable but when installed in a structure transfers a force at its installation level which in turn reduces all forces, drifts and accelerations above that level. The NSD is therefore capable of limiting the forces developed in a structure without any yielding which could lead to permanent deformations. The NSD was developed at the University at Buffalo (UB), Rice University and Taylor Devices Inc. and tested on a shake table by authors in a three storied structural model isolated with elastomeric bearings. The NSD consists of a highly compressed spring in a double negative stiffness magnification mechanism. In order to ensure a considerable amount of positive stiffness of the global system around small displacements to minimize wind vibrations a so called gap spring assembly (GSA) mechanism is implemented which delays the engagement of the device by a prescribed displacement. Moreover, the device employs double containment chevron braces that contain the large vertical forces needed for the development of negative stiffness without transferring these forces to the structure. This paper describes the development of analytical and computational tools that describe the behavior of the device and presents results for the experimental verification of these tools. Apostolos A. Sarlis, Dharma Theja R Pasala, Michael C. Constantinou, Andrei M. Reinhorn, Satish Nagarajaiah and Douglas Taylor 2

One of the most distinctive properties of semiconducting single-walled carbon nanotubes (SWCNT) is their emission of structure-specific fluorescence (band gap photoluminescence) in the short-wave infrared (SWIR) spectral region. The fluorescence peak from each ( n , m ) structural species shifts in wavelength as the nanotube’s band gap is altered by axial strain. These shifts are known from theory and experiment to depend on the nanotube’s chiral angle and on its mod ( n-m ,3) value, and they are linear with strain for realistic axial deformations. We have previously shown that this effect may lead to a new strain measurement technology (called S 4 , for strain-sensing smart skin) in which individualized SWCNTs embedded in a polymeric film serve as strain sensors and readout is achieved by non-contact optical methods. 1 Recent research progress aimed at enabling such an implementation will be described. Focus areas include processing and dispersing SWCNTs for effective use in S 4 ; comparing suitability of different polymer formulations as S 4 hosts; applying and stabilizing the S 4 films; understanding factors that influence interfacial adhesion between SWCNTs and host molecules; miniaturizing the optical hardware to allow excitation and capture of SWCNT fluorescence spectra with hand-held devices; refinement of data analysis methods for deducing strains from spectral data; tests of sensitivity and reproducibility for S 4 measurements with plastic, aluminum, and steel substrates; and construction of two-dimensional strain maps from sets of single-point measurements. 1 P. A. Withey, V. S. M. Vemuru, S. M. Bachilo, S. Nagarajaiah, and R. B. Weisman, Nano Lett. 12 , 3497 (2012).