Siu Chun Michael Ho

A Review of Rock Bolt Monitoring Using Smart Sensors

Rock bolts have been widely used as rock reinforcing members in underground coal mine roadways and tunnels. Failures of rock bolts occur as a result of overloading, corrosion, seismic burst and bad grouting, leading to catastrophic economic and personnel losses. Monitoring the health condition of the rock bolts plays an important role in ensuring the safe operation of underground mines. This work presents a brief introduction on the types of rock bolts followed by a comprehensive review of rock bolt monitoring using smart sensors. Smart sensors that are used to assess rock bolt integrity are reviewed to provide a firm perception of the application of smart sensors for enhanced performance and reliability of rock bolts. The most widely used smart sensors for rock bolt monitoring are the piezoelectric sensors and the fiber optic sensors. The methodologies and principles of these smart sensors are reviewed from the point of view of rock bolt integrity monitoring. The applications of smart sensors in monitoring the critical status of rock bolts, such as the axial force, corrosion occurrence, grout quality and resin delamination, are highlighted. In addition, several prototypes or commercially available smart rock bolt devices are also introduced.

A fiber Bragg grating sensor for detection of liquid water in concrete structures

Concrete structures rely greatly on the integrity of their supporting rebars to remain serviceable for their intended purposes. Unfortunately, rebars are vulnerable to corrosion due to the ingress of water from the environment, which often also carries a multitude of ionic particles that encourage corrosion. Liquid phase water may enter the structure through cracks that may not be obvious to human observation. Thus, a fiber Bragg grating sensor was designed that is able to detect (i.e. ‘on‐off’) the presence of liquid water in order to provide an early warning signal for the ingress of water. Two tests were conducted to verify the functionality of the sensor: the first experiment tested the repeatability of the sensor to cyclic input of various volumes of water, and the second tested the sensor’s response to flooding conditions. The sensor showed a good repeatability, with fast response times (<10 min to reach a level guaranteeing the presence of water) and a recovery time of 11‐15 h, depending on the input volume. The flooding test showed similar performance and viability of the sensor during flooding conditions. (Some figures may appear in colour only in the online journal)

Corrosion detection of steel reinforced concrete using combined carbon fiber and fiber Bragg grating active thermal probe

Steel reinforcement corrosion is one of the dominant causes for structural deterioration for reinforced concrete structures. This paper presents a novel corrosion detection technique using an active thermal probe. The technique takes advantage of the fact that corrosion products have poor thermal conductivity, which will impede heat propagation generated from the active thermal probe. At the same time, the active thermal probe records the temperature response. The presence of corrosion products can thus be detected by analyzing the temperature response after the injection of heat at the reinforcement-concrete interface. The feasibility of the proposed technique was firstly analyzed through analytical modeling and finite element simulation. The active thermal probe consisted of carbon fiber strands to generate heat and a fiber optic Bragg grating (FBG) temperature sensor. Carbon fiber strands are used due to their corrosion resistance. Wet-dry cycle accelerated corrosion experiments were performed to study the effect of corrosion products on the temperature response of the reinforced concrete sample. Results suggest a high correlation between corrosion severity and magnitude of the temperature response. The technique has the merits of high accuracy, high efficiency in measurement and excellent embeddability.

Real time bolt preload monitoring using piezoceramic transducers and time reversal technique—a numerical study with experimental verification

Bolted joints are ubiquitous structural elements, and form critical connections in mechanical and civil structures. As such, loosened bolted joints may lead to catastrophic failures of these structures, thus inspiring a growing interest in monitoring of bolted joints. A novel energy based wave method is proposed in this study to monitor the axial load of bolted joint connections. In this method, the time reversal technique was used to focus the energy of a piezoelectric (PZT)-generated ultrasound wave from one side of the interface to be measured as a signal peak by another PZT transducer on the other side of the interface. A tightness index (TI) was defined and used to correlate the peak amplitude to the bolt axial load. The TI bypasses the need for more complex signal processing required in other energy-based methods. A coupled, electro-mechanical analysis with elasto-plastic finite element method was used to simulate and analyze the PZT based ultrasonic wave propagation through the interface of two steel plates connected by a single nut and bolt connection. Numerical results, backed by experimental results from testing on a bolted connection between two steel plates, revealed that the peak amplitude of the focused signal increases as the bolt preload (torque level) increases due to the enlarging true contact area of the steel plates. The amplitude of the focused peak saturates and the TI reaches unity as the bolt axial load reaches a threshold value. These conditions are associated with the maximum possible true contact area between the surfaces of the bolted connection.

Feasibility study of using smart aggregates as embedded acoustic emission sensors for health monitoring of concrete structures

Acoustic emission (AE) is a nondestructive evaluation technique that is capable of monitoring the damage evolution of concrete structures in real time. Conventionally, AE sensors are surface mounted on the host structures, however, the AE signals attenuate quickly due to the high attenuation properties of concrete structures. This study conducts a feasibility study of using smart aggregates (SAs), which are a type of embedded piezoceramic transducers, as embedded AE sensors for the health monitoring of concrete structures. A plain concrete beam with two surface mounted AE sensors and two embedded SAs was fabricated in laboratory and loaded under a designed three-point-bending test. The performance of embedded SAs were compared with the traditional surface mounted AE sensors in their ability to detect and evaluate the damage to the concrete structure. The results verified the feasibility of using smart aggregates as embedded AE sensors for monitoring structural damage in concrete. Potentially, the low cost smart aggregates could function as embedded AE sensors, providing great sensitivity and high reliability in applications for the structural health monitoring of concrete structures.

Acoustic emission monitoring and finite element analysis of debonding in fiber-reinforced polymer rebar reinforced concrete:

The acoustic emission technique is widely used for mechanical diagnostics and damage characterization in reinforced concrete structures. This article experimentally investigated the feasibility of debonding characterization in fiber-reinforced polymer rebar reinforced concrete using acoustic emission technique. To this end, carbon-fiber-reinforced polymer rebar reinforced concrete specimens were prepared and they were subjected to pullout tests to study the interfacial debonding between concrete and reinforcement. Test results showed that the debonding failure between concrete and reinforcement was characterized by the total peeling off of the helical wrapping layer of the carbon-fiber-reinforced polymer reinforcement. The response of acoustic emission activity was analyzed by descriptive parameters, such as cumulative acoustic emission hits, amplitude, and peak frequency. The evolution of debonding failure is thus characterized by these acoustic emission parameters. The results demonstrated a clear correlation between the damage evolution of carbon-fiber-reinforced polymer rebar pullout and the acoustic emission parameters. In addition, finite element analysis was adopted to study the stress field during the pullout of the reinforcement. The simulation results agreed well with the experimental investigations.

Dynamic fiber Bragg grating sensing method

The measurement of high frequency vibrations is important in many scientific and engineering problems. This paper presents a novel, cost effective method using fiber optic fiber Bragg gratings (FBGs) for the measurement of high frequency vibrations. The method uses wavelength matched FBG sensors, with the first sensor acting as a transmission filter and the second sensor acting as the sensing portion. Energy fluctuations in the reflection spectrum of the second FBG due to wavelength mismatch between the sensors are captured by a photodiode. An in-depth analysis of the optical circuit is provided to predict the behavior of the method as well as identify ways to optimize the method. Simple demonstrations of the method were performed with the FBG sensing system installed on a piezoelectric transducer and on a wind turbine blade. Vibrations were measured with sampling frequencies up to 1 MHz for demonstrative purposes. The sensing method can be multiplexed for use with multiple sensors, and with care, can be retrofitted to work with FBG sensors already installed on a structure.

Wind turbine blade damage detection using an active sensing approach

The wind energy sector is one of the fastest growing parts of the clean energy industry. As the wind energy sector grows, so does an increasing concern for the damage detection of wind turbine blades. This paper proposes an active sensing approach by utilizing piezoceramic transducers as actuators and sensors. The influence of the crack quantity, location, length and depth on the wave propagation was experimentally studied. Sweep sine signals ranging from 1 khz to 50 khz were used as input signals for active sensing. The change in the energy that propagated through the cracks was verified as feasible in detecting crack-related damage. An innovative polar plot analysis method based on Fast Fourier transform was developed to compare the minuscule difference between the damage signals and the baseline signal. The polar plot was able to make apparent differences in both the magnitude and the phase of the signals, which could be correlated to crack depth and plane geometry, respectively, based on the observation of the damage.

Interfacial debonding detection in fiber-reinforced polymer rebar–reinforced concrete using electro-mechanical impedance technique

For reinforced concrete structures, the use of fiber-reinforced polymer rebars to replace the steel reinforcement is a topic that is receiving increasing attention, especially where corrosion is a serious issue. However, fiber-reinforced polymer rebar–reinforced concrete always carries the risk of structural failure initiated from the debonding damage that might occur at the reinforcement–concrete interface. This study employed an electro-mechanical impedance–based structural health monitoring technique by applying lead–zirconate–titanate ceramic patches to detect the debonding damage of a carbon fiber–reinforced polymer rebar reinforced concrete. In the experimental study, a carbon fiber–reinforced polymer rebar reinforced concrete specimen was fabricated and it was subjected to a pullout test to initiate the debonding damage at the reinforcement–concrete interface. The impedance and admittance signatures were measured from an impedance analyzer according to the different debonding conditions between the reinforcement and the concrete. Statistical damage metrics, root-mean-square deviation and mean absolute percentage deviation, were used to quantify the changes in impedance signatures measured at the lead–zirconate–titanate patches due to debonding conditions. The results illustrated the capability of the electro-mechanical impedance–based structural health monitoring technique for detecting the debonding damage of fiber-reinforced polymer rebar–reinforced concrete structures.

Structural health monitoring of wind turbine blade using piezoceremic based active sensing and impedance sensing

Wind energy has recently risen to the forefront of green, sustainable energy. The structural health monitoring of wind turbine blades are important for optimal operational safety and costs. In this paper, the piezoceramic based active sensing method and impedance method were proposed and applied for the monitoring of wind turbine blades. A tensile test on a customized blade specimen and a destructive test on commercial full blade were performed to simulate the damage process. Piezoceramic patches were mounted on the surface to monitor the damage process. Wavelet packet analysis was used to build the energy vector for the propagated wave during active sensing. Root Mean Square Deviation (RMSD) based damage index were built for both approaches. Results indicate that both methods were able to detect and locate damage on wind turbine blades.