Structural monitoring and new nanoscale phenomena

Citation

(2012), "Structural monitoring and new nanoscale phenomena", Sensor Review, Vol. 32 No. 3. https://doi.org/10.1108/sr.2012.08732caa.001

Structural monitoring and new nanoscale phenomena

Article Type: Nanosensor update From: Sensor Review, Volume 32, Issue 3

The first part of the update explores the role of nanosensors in structural monitoring, including wireless sensing applications. The second part considers some recently reported and intriguing nanoscale phenomena which have potential in future generations of nanosensors.

Structural monitoring

Critical structures such as bridges, buildings and oil rigs are routinely monitored to ensure their integrity and safety and all manner of sensing techniques are employed. Nanosensors, most notably those based of carbon nanotubes (CNTs), are poised to play a role in future structural monitoring practices and the possibility of wireless operation is also being studied. These sensors frequently exploit the piezoresistive properties of CNTs which makes they well suited to detecting structural strains, cracks and deformations. Expressing sensitivity to strain ε as a gauge factor G, where G=(ΔR/R)/ε, it has been shown that CNTs have values of up to 2,900; by contrast, the figures for other materials used in strain gauges are far lower, e.g. −125 to +200 for silicon and 2-5 for metal foils. Research is also investigating the role of nanosensors to monitor corrosion, another major cause of structural failure.

Research in the Department of Construction Engineering and Management at the North Dakota State University has studied the role of CNT-based nanosensors to monitor wirelessly strains and crack formation in concrete structures. The sensors were based on a Portland cement matrix containing 1 per cent of randomly dispersed single-walled CNTs (SWCNTs) and thin copper wire electrodes. To evaluate their response, the sensors were subjected to monotonic and cyclic tensile loads at rate of 33 N/min. The load was continuously applied up to the failure point of the sensors. During loading, the electrical resistance was determined using a two-point measurement scheme and the tensile strain along the longitudinal axis of the sensor was measured with conventional strain gauges. The resistance was found to increase linearly and monotonically up to 125 με and then exhibited a non-linear behaviour up to specimen failure. Under applied tensile load, the thickness of the cement matrix between adjacent nanotubes tends to increase, resulting in an increase in the effective resistance of the sensor. As the applied stress continues to develop in the sensor, matrix damage occurs at the nanotube contact points due to the high strain concentrations. This nanoscopic damage increases the gap between the nanotubes where electrical tunnelling take place and thus increases the effective resistance of the sensor. When embedded into a concrete structure, this sudden change in the sensor’s resistance could be used to detect the onset of damage such as cracks and delaminations. The sensors were integrated into a wireless and embeddable sensing system consisting of a rectifier, a 16-bit ADC with a sampling rate of 80 Hz, a 110 dB amplifier, a 16-bit memory and coil antennas. The interrogator consisted of an oscillator, demodulator/level shifter, data logger and antennas and the system uses an inductive link for sensor powering and data collection. The interrogator emits an electromagnetic pulse at 916.5 MHz through the transmitter antenna which is picked up by the sensor’s receiver antenna and is used to power the data acquisition system. These wireless sensor systems were successfully embedded into concrete beams and their response described well the behaviour of the beams in terms of crack propagation, with the crack greatly affected the resistance. These findings have demonstrated the concept of using wireless and embedded nanosensors for monitoring the integrity of concrete structures.

A group at the University of Strathclyde’s Department of Civil Engineering has developed a potentially lower cost alternative, a so-called “smart paint”, which could form the basis of a novel structural monitoring technology. The paint is based on a combination of fly ash and aligned CNTs which, as with the above, change their resistance when subjected to stress. It has a cement-like consistancy which, the group argues, makes it particularly useful for applications in harsh environments. The idea is to spray-coat the structure with the paint and attach electrodes which would detect changes in the current flowing (Figures 1 and 2). The electrodes would in turn be attached to small, battery-powered, wireless transmitter nodes although the batteries may eventually be replaced by an energy harvesting technique such as vibration scaveging or solar power. A master transmitter will be used to receive and communicate the whole structure’s responses. In this way, data covering large parts of a structure could be acquired in real-time, thus providing warning of the developent of microcracks or other defects. The team is also studying the possibility of incorporating electrical impedance tomography (EIT, see below) into the system. This technique creates a spatial conductivity map, so if a crack appears it would be possible to locate it on a finite element model of the structure.

Figure 1 Testing the smart paint technology on a concrete sample

Figure 2 A sample of the smart paint with electrodes attached

A very different approach but one which also involves CNTs is being studied by a group from LETI, the University of Paris and other French research institutes. Here, the CNTs are used as the basis of nanoscale resonators to generate ultrasound. The concept is to probe individual water- or air-filled pores (10 nm to 10 μm) within cementitious materials with the familiar pulse-echo technique by using ultrasonic waves generated by embedded sensors. The sensors are dubbed “capacitive micromachined ultrasonic transducers” (cMUTs) and are based on the vibrations of a conducting membrane suspended above an actuation electrode. To reduce the size of the sensors, the membrane is fabricated from a dense assembly of well-aligned SWCNTs. The cMUTs are constructed in a transistor configuration whereby the CNTs are caused to vibrate by applying a varying gate voltage. The vibrations were measured using scanning laser Doppler vibrometry and trials in air showed vibrations of the membrane over a large range of frequencies, from 100 kHz to 5 MHz. According to a numerical model of the cMUTs, their low frequency vibration amplitudes are expected to be four orders of magnitude higher in air than in a liquid and their first resonance frequency in water should be in inverse proportion to the pore size. Hence, if the proposed sensors were embedded in large numbers in a multiphase, porous material such as concrete, they could differentiate air-filled micropores from other liquid-filled pores and also determine the size of the water-filled pores (Figure 2). Such in situ data could be exploited to monitor hydration and water-related structural degradation with unprecedented spatial resolution.

Monitoring corrosion

A group from the Faculty of Built Environment and Engineering at Australia’s Queensland University of Technology and the Korean Pukyong National University is studying the role of CNTs for monitoring corrosion in structures. Presently, a range of non-destructive methods are used to detect corrosion, such as ultrasonics, radiography and various electromagnetic techniques but these tend to be costly and difficult to implement. In this work, the CNTs are combined with Nafion, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer with ionic properties. The sensors were produced using a traditional solution-casting technique from a dispersion of multi-walled CNTs in a Nafion solution in the ratio 5:95 per cent by weight. They were tested by attaching them to steel plates and immersing them in various buffer and saline solutions to simulate corrosion and measuring the change in resistivity and conductivity. It was found that significant changes occurred in the saline solution and after about 35 h of exposure, the change in resistance of sensors attached to a steel specimen was about three times higher than that of a sensor attached to a non-metallic, control specimen. These results confirm the hypothesis that, as corrosion occurs on the metal surface, the corrosive ions penetrate the surface of the CNTs and change their electrical resistivity. The authors maintain that resistance measurements based on simple CNT-based nanosensors can be used to assess the onset of structural corrosion.

Figure 3 The wireless impedance analyser

Research conducted by workers from the University of Michigan and the University of California, Davis involves a CNT-based sensing skin for the wireless monitoring of corrosion. The skin is a thin conformable film, assembled at the nanoscale by a layer-by-layer method, using SWCNTs and polyelectrolyte (PE) species. The SWCNT-PE film can be designed to sense mechanical (e.g. strain) and chemical (e.g. corrosion) stimuli in a distributed manner. EIT is adopted to provide spatial mapping of the sensing skin’s conductivity based on electrical measurements taken at the skin boundary. This involves making measurements of the test body at multiple locations along its boundary, with corresponding electrical potential measured for each unique current injection. However, to do this manually would be extremely time-consuming, thereby ruling out the possibility of employing the skin for autonomous, real-time monitoring. Consequently, the group has developed a low-cost, wireless impedance analyser for automated measurements and acquisition of EIT data. The device is designed with a high-precision current source offering control over the generated current’s amplitude and frequency. A multiplexing interface is integrated with the analyser to offer 32 independently addressable channels into which the current can be injected, allowing the sensing skin’s boundary potential to be simultaneously measured. The heart of the analyser is a low-power, eight-bit microcontroller that operates the device and measures the boundary electric potential using its internal ten-bit ADC. In addition, a wireless transceiver is integrated with the data acquisition package to allow communication with other wireless sensors within a structure’s monitoring system. The wireless analyser (Figure 3) is fabricated using four separate PCBs, connected using interlocking headers and flat flexible cables. The system has been tested by using the layer-by-layer technique to apply a 50-layer thin film of the sensing material onto a carbon steel plate. Eight conductive electrodes are formed along each side of the film boundary to form a total of 32 electrodes for the entire film. Each electrode is a thin slice of copper tape bonded to the sensing skin by silver paste. A coaxial wire is attached to each electrode with each of the 32 wires terminated at the impedance analyser’s multiplexer board. Using sodium chloride solutions to promote corrosion, it has been shown that the EIT spatial conductivity maps exhibit localised decreases in conductivity corresponding to rust formation. It was found that the sensor’s conductivity decreases at an exponential rate of 0.031/min for 1.0 M NaCl and 0.024/min for 0.1 M NaCl. This work has shown that it is possible to use EIT to map the conductivity of nanosensing films and the wireless impedance analyser provides a cost-effective and rapid method for EIT data acquisition. In addition to corrosion, the sensing skin technology has been shown to identify structural damage due to strain, cracks, dents, bending, impacts (Figures 4 and 5) and changes in pH.

 

Figure 4 The sensing skin deposited on an aluminium plate and impacted at four different intensities

Figure 5 EIT spatial conductivity images of the damaged plate