Kenneth J. Loh

A summary review of wireless sensors and sensor networks for structural health monitoring

In recent years, there has been an increasing interest in the adoption of emerging sensing technologies for instrumentation within a variety of structural systems. Wireless sensors and sensor networks are emerging as sensing paradigms that the structural engineering field has begun to consider as substitutes for traditional tethered monitoring systems. A benefit of wireless structural monitoring systems is that they are inexpensive to install because extensive wiring is no longer required between sensors and the data acquisition system. Researchers are discovering that wireless sensors are an exciting technology that should not be viewed as simply a substitute for traditional tethered monitoring systems. Rather, wireless sensors can play greater roles in the processing of structural response data; this feature can be utilized to screen data for signs of structural damage. Also, wireless sensors have limitations that require novel system architectures and modes of operation. This paper is intended to serve as a summary review of the collective experience the structural engineering community has gained from the use of wireless sensors and sensor networks for monitoring structural performance and health.

Multifunctional layer-by-layer carbon nanotube-polyelectrolyte thin films for strain and corrosion sensing

Since the discovery of carbon nanotubes, researchers have been fascinated by their mechanical and electrical properties, as well as their versatility for a wide array of applications. In this study, a carbon nanotube–polyelectrolyte composite multilayer thin film fabricated by a layer-by-layer (LbL) method is proposed to develop a multifunctional material for measuring strain and corrosion processes. LbL fabrication of carbon nanotube composites yields mechanically strong thin films in which multiple sensing transduction mechanisms can be encoded. For example, judicious selection of carbon nanotube concentrations and polyelectrolyte matrices can yield thin films that exhibit changes in their electrical properties to strain and pH. In this study, experimental results suggest a consistent trend between carbon nanotube concentrations and strain sensor sensitivity. Furthermore, by simply altering the type of polyelectrolyte used, pH sensors of high sensitivity can be developed to potentially monitor environmental factors suggesting corrosion of metallic structural materials (e.g. steel, aluminum). (Some figures in this article are in colour only in the electronic version)

Performance monitoring of the Geumdang Bridge using a dense network of high-resolution wireless sensors

As researchers continue to explore wireless sensors for use in structural monitoring systems, validation of field performance must be done using actual civil structures. In this study, a network of low-cost wireless sensors was installed in the Geumdang Bridge, Korea to monitor the bridge response to truck loading. Such installations allow researchers to quantify the accuracy and robustness of wireless monitoring systems within the complex environment encountered in the field. In total, 14 wireless sensors were installed in the concrete box girder span of the Geumdang Bridge to record acceleration responses to forced vibrations introduced by a calibrated truck. In order to enhance the resolution of the capacitive accelerometers interfaced to the wireless sensors, a signal conditioning circuit that amplifies and filters low-level accelerometer outputs is proposed. The performance of the complete wireless monitoring system is compared to a commercial tethered monitoring system that was installed in parallel. The performance of the wireless monitoring system is shown to be comparable to that of the tethered counterpart. Computational resources (e.g. microcontrollers) coupled with each wireless sensor allow the sensor to estimate modal parameters of the bridge such as modal frequencies and operational displacement shapes. This form of distributed processing of measurement data by a network of wireless sensors represents a new data management paradigm associated with wireless structural monitoring. (Some figures in this article are in colour only in the electronic version)

Tailoring Piezoresistive Sensitivity of Multilayer Carbon Nanotube Composite Strain Sensors

In recent years, carbon nanotubes have been utilized for a variety of applications, including nanoelectronics and various types of sensors. In particular, researchers have sought to take advantage of the superior electrical properties of carbon nanotubes for fabricating novel strain sensors. This article presents a single-walled carbon nanotube (SWNT)-polyelectrolyte (PE) composite thin film strain sensor fabricated with a layer-by-layer (LbL) process. Optimization of bulk SWNT-PE strain sensor properties is achieved by varying various LbL fabrication parameters, followed by characterization of strain-sensing electromechanical responses. A resistor and capacitor (RC)-circuit model is proposed and validated with electrical impedance spectroscopy to fit experimental results and to identify equivalent circuit element parameters sensitive to strain. Experimental results suggest consistent trends between SWNT and PE concentrations to strain sensor sensitivities. Simply by adjusting the weight fraction of SWNT solutions and film thickness, strain sensitivities between 0.1 and 1.8 have been achieved. While SWNT-PE strain sensitivity is lower than some metal-foil strain gauges (

Spatial conductivity mapping of carbon nanotube composite thin films by electrical impedance tomography for sensing applications

2), the LbL method allows for precise tailoring of the properties (i.e., strain sensitivity, resistivity, among others) of a high-capacity (±10,000 μm m-1) homogeneous multilayer strain sensor.

Detection of spatially distributed damage in fiber-reinforced polymer composites:

This paper describes the application of electrical impedance tomography (EIT) to demonstrate the multifunctionality of carbon nanocomposite thin films under various types of environmental stimuli. Carbon nanotube (CNT) thin films are fabricated by a layer-by-layer (LbL) technique and mounted with electrodes along their boundaries. The response of the thin films to various stimuli is investigated by relying on electric current excitation and corresponding boundary potential measurements. The spatial conductivity variations are reconstructed based on a mathematical model for the EIT technique. Here, the ability of the EIT method to provide two-dimensional mapping of the conductivity of CNT thin films is validated by (1) electrically imaging intentional structural defects in the thin films and (2) mapping the films response to various pH environments. The ability to spatially image the conductivity of CNT thin films holds many promises for developing multifunctional CNT-based sensing skins.

Nanoengineering Ultra-High-Performance Concrete with Multiwalled Carbon Nanotubes

This work describes a novel method of embedded damage detection within glass fiber–reinforced polymer composites. Damage detection is achieved by monitoring the spatially distributed electrical conductivity of a strain-sensitive multiwalled carbon nanotube thin film. First, thin films were spray-deposited directly upon glass fiber mats. Second, using electrical impedance tomography, the spatial conductivity distribution of the thin film was determined before and after damage-inducing events. The resolution of the sensor was determined by drilling progressively larger holes in the center of the composite specimens, and the corresponding electrical impedance tomography response was measured by recording the current–voltage data at the periphery of the monitored composite sample. In addition, the sensitivity to damage occurring at different locations in the composite was also investigated by comparing electrical impedance tomography spatial conductivity maps obtained for specimens with sets of holes drilled at different locations in the sensing area. Finally, the location and severity of damage from low-velocity impact events were detected using the electrical impedance tomography method. The work presented in this study indicates a paradigm shift in the available possibilities for structural health monitoring of fiber-reinforced polymer composites.

Piezoelectric Characterization of PVDF-TrFE Thin Films Enhanced With ZnO Nanoparticles

Ultra-high-performance concretes (UHPCs) are characterized by extremely high packing densities. These densities can be achieved with optimization of grain size distribution by incorporating a homogeneous gradient of fine and coarse particles during mixing. Addition of steel fibers can increase the ductility under tensile loading tremendously. The packing density is a key parameter for the high bond strength between steel fibers and the UHPC matrix. The objective of this study is twofold: to enhance the behavior of the bond between the steel fibers and the matrix by increasing packing density with the inclusion of nanometer-sized particles while preserving the workability of the concrete mix. Multiwalled carbon nanotubes (MWNTs) were chosen for their unique physical and impressive mechanical properties (i.e., ultimate strength, stiffness, and ductility). However, because of the nanotubes’ inherent tendency to agglomerate, an extensive study was conducted to optimize their dispersion in solutions suitable for concrete mixing. An experimental validation study was then conducted to explore the effects of incorporating MWNTs in mixes with only 0.022% by cement weight and how they affect the bond behavior of two different high-strength steel fibers. Mechanical characterization studies revealed that low concentrations of MWNTs can significantly improve the bonding behavior of single pulled-out high-strength steel fibers. No conclusive evidence can be drawn with regard to MWNTs’ influence on the UHPCs’ compressive and bending strengths [i.e., 200 and 13 MPa (30 and 1.9 ksi), respectively].

Spatial Sensing Using Electrical Impedance Tomography

The objective of this letter is the fabrication and piezoelectric characterization of poly(vinylidene fluoride)-trifluoroethylene (PVDF-TrFE) thin films enhanced with zinc oxide (ZnO) nanoparticles. The incorporation of piezoelectric ZnO nanoparticles will enhance bulk film piezoelectricity while preserving the mechanical flexibility of PVDF-TrFE. The nanocomposites are fabricated by spin coating. Piezoelectric properties are measured by applying electric fields while measuring their response using a Sawyer-Tower circuit.

Passive wireless strain and pH sensing using carbon nanotube-gold nanocomposite thin films

The need for structural health monitoring has become critical due to aging infrastructures, legacy airplanes, and continuous development of new structural technologies. Based on an updated structural design, there is a need for new structural health monitoring paradigms that can sense the presence, location, and severity with a single measurement. This paper focuses on the first step of this paradigm, consisting of applying a sprayed conductive carbon nanotube-polymer film upon glass fiber-reinforced polymer composite substrates. Electrical impedance tomography is performed to measure changes in conductivity within the conductive films because of damage. Simulated damage is a method for validation of this approach. Finally, electrical impedance tomography measurements are taken while the conductive films are subjected to tensile and compressive strain states. This demonstrates the ability of electrical impedance tomography for not only damage detection, but active structural monitoring as well. This paper acts as a first step toward moving the structural health monitoring paradigm toward large-scale deployable spatial sensing.