With recent advancements in Sensor technology, Structural Health Monitoring (SHM) systems have been developed and implemented in various civil structures such as bridges, buildings, tunnels, power plants, and dams. Many advanced types of sensors, from wired to wireless sensors, have been developed to continuously monitor structural condition through real-time data collection. However, there are still a remarkable number of questions associated with the use of SHM sensors. For designing a SHM system, one of the critical missions is discovering how to determine an appropriate type of sensor that can efficiently meet the scopes of the designed sensing system. This article aims to present a brief review to different types of sensors for structural health monitoring and real-time condition assessment of structures.
Smart Sensors for Structural Health Monitoring
Structural health monitoring heavily relies on collecting accurate and high quality real-time measurements of structural element condition, communicating this information with control system, and signalling necessary warnings should an irregular pattern is ever observed. Sensors for structural health monitoring are designed to facilitate the monitoring process, and enabling maintenance engineers with decision-making tools, which will ensure the safety of the facility, and the public. A typical health monitoring system is composed of a network of sensors that measure the parameters relevant to the current state of the structure as well as its surrounding environment, such as stress, strain, vibration, inclination, humidity, and temperature.
The latest advances in research on sensor technology for structural health monitoring has been resulted in various types of SHM sensors. The following provides a brief review to the most widely used SHM sensors for structural monitoring.
1. Fiber Optic Sensors
Fiber optic sensors have been under great development in recent years. In civil engineering, these sensors can be used to measure different parameters such as strains, structural displacements, vibrations frequencies, acceleration, pressure, temperature, humidity and so on. The monitoring of the structure can be either local, concentrating on the material behavior or global, concentrating on the whole structural performance. Fiber optic sensors have been tested for different applications such as strain monitoring of concrete components in a bridge [G. T. Webb et al, 2017].
2. Accelerometer for Structural Health Monitoring
An accelerometer is an electromechanical device that measures acceleration forces. Such forces can be static, like the continuous force of gravity on structural components, or dynamic to sense motions or vibrations like when a truck crossing a bridge. The application of accelerometers extends to multiple disciplines, from smart phones to rotating machinery and civil infrastructure. In the context of structural monitoring, the accelerometers can be used for real-time monitoring the variations of structural dynamic characteristics due to damage or change in structural performance [Chih-Hsing Lin et al]. The accelerometers are manufactured in single or multi-axis models to detect magnitude and direction of the proper acceleration. Accelerometers have also a wide use in constructions where there is a need to control the dynamic behaviour of the structure, either short or long term [Ref].
Piezoelectric accelerometer: A piezoelectric accelerometer is an accelerometer that employs the piezoelectric effect of certain materials to measure dynamic changes in mechanical variables (e.g., acceleration, vibration, and mechanical shock). This type of vibration transducers offers a very wide frequency and dynamic range. Piezoelectric accelerometers are used in many different industries, environments and applications. Piezoelectric measuring devices are widely used today in the laboratory, on the production floor, and as original equipment for measuring and recording dynamic changes in mechanical variables including shock and vibration [Ref].
3. Vibrating Wire Traducers
Vibrating wire sensors are a class of sensors that are very popular in geotechnical and structural monitoring. The fundamental component of the vibrating wire sensor is a tensioned steel wire that vibrates at a resonant frequency that depends on the strain in the wire. This mechanism is used in a variety of sensor configurations to measure static strain, stress, pressure, tilt, and displacement. The use of frequency, rather than amplitude, to convey the signal means that vibrating wire sensors are relatively resistant signal degradation from electrical noise, long cable runs, and other changes in cable resistance. This has contributed to their reputation for long term stability and wide usage for monitoring structures such as dams, tunnels, mines, bridges, foundations, piles, unstable slopes, and excavations.
Vibrating Wire Strain Gauge: Vibrating wire strain gauge is an old technique with its roots in the early 20th century. A thin steel wire held in tension between two end blocks makes a vibrating wire. A transverse vibration is excited by a short pulse of an electromagnet with surrounding coil positioned near the midpoint of the wire. The frequency of the vibration varies with the tension of the wire. If the distance between the end blocks changes, the natural frequency will change as well. The coil measures both the natural frequency and its changes. This gauge measures strain in a variety of materials and it can be easily cast or embedded in concrete. The frequency signal can be transmitted over long led cables to a readout unit and monitored. These sensors are widely used in different applications like bridges, tunnels and other large structures.
Vibrating Wire Displacement Transducer: Vibrating wire displacement transducers are designed to measure displacements across joints and cracks in concrete, rock, soil and structural members. In essence, the transducer consists of a vibrating wire in series with a tension spring. Displacements are accommodated by a stretching of the tension spring, which produces a commensurate increase in wire tension. These sensors are mostly used for crack width measures for example in bridges and tunnels.
4. Linear Variable Differential Transformer (LVDT)
An LVDT (linear variable differential transformer) is an electromechanical sensor used to convert mechanical motion or vibrations, specifically rectilinear motion, into a variable electrical current, voltage or electric signals, and the reverse. One can use LVDT in the applications where displacements to be measured are ranging from a fraction of mm to few cm's. LVDT position sensors are frequently used in testing and structural monitoring applications. These sensors are ideal for recording displacements on structural members due to live loads and temperature variations.
5. Load Cells
A load cell is a type of transducer, specifically a force transducer. It converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. As the force applied to the load cell increases, the electrical signal changes proportionally. Load cells have found their applications in a variety of fields that demand accuracy and precision. These sensors are employed in many historic buildings, where various building materials such as stone and brick have been used [Ref].
6. Strain Gauges in Structural Health Monitoring
The most common type of load cell used in structural monitoring is strain gauge. A strain gauge is a device used to measure strain on an object. The most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change is related to the strain by the quantity known as the Gauge factor. These sensors are widely used to monitor strain in steel structures and reinforced concrete structures [Ref].
Strain Gauge Rosette: A strain gauge rosette is one type of strain gauge composed of two or more strain gauges that are positioned closely to measure strains along different directions of the component under evaluation. Single strain gauges can only measure strain effectively in one direction, so the use of multiple strain gauges enables more measurements to be taken, providing a more precise evaluation of strain on the surface being measured [Ref]
7. Inclinometer (Tiltmeter)
An inclinometer or clinometer is an instrument used for measuring angles of slope (or tilt), elevation, or depression of an object with respect to gravity’s direction. These sensors are suited for monitoring structures, the towers of vertical lift bridges, and monitoring twist in structural elements [Ref].
8. Acoustic Emission Sensor
Acoustic Emission (AE) Sensors measure high-frequency energy signals that are generated from local sources of stress waves. Discontinuities and defects in a material produce stress waves, which then propagate to the material’s surface and are picked up by the active AE sensor. By converting these waves into electrical signals, AE is an ideal technique to effectively assess the behavior of materials under stress. These sensors are mainly used to detect the onset or growth of existing cracks in structural components [Ref].
9. Temperature Sensors
Civil engineering structures are subjected to the environmental changes and therefore it is necessary to measure the temperature that affects to some extend every physical process [Ref]. Thermocouples are one of the most widely used temperature sensors to control the temperature in certain points of the structure. Most of the large concrete structures have a lot of thermocouples installed while casting and under construction in order to have a full control over temperature changes under curing.