Fiber bragg grating sensors for structural health monitoring in harsh conditions

Piezoelectric transducers have been traditionally used in structural health monitoring. These devices convert pressure to an electric voltage. The transducers are typically reliable in normal working conditions, however, they become less accurate in harsh environments, like high temperatures – over 300 degrees Celsius – or highly corrosive environments.

With the development of laser technologies the popularity of fiber Bragg grating sensors have been increasing due to their numerous advantages over traditional means of measuring strain, pressure, and temperature: 1) FBG sensors work reliably under very high temperatures because of the optical fiber material – fused silica; 2) fiber Bragg grating sensors are immune to electromagnetic interference; 3) the sensors are not susceptible to corrosion; 4) FBG sensors are very small and can be easily mounted on any surface. The development of fiber-optics technology will enhance structural health monitoring in harsh conditions, such as nuclear power plants and modern superheaters.

The following types of FBG sensors are used in structural health monitoring:

  1. FBG temperature sensors;

The main advantages of fiber Bragg grating temperature sensors are their reliability and accuracy under high temperatures, absolute temperature measurements, fast response, immunity to electric sparks. The fast response rate is particularly useful in cases where temperature shifts are extreme.

2. FBG strain sensors;

FBG sensors are the best strain sensors at the moment due to multiple advantages that are inherent to all FBG products. These sensors can determine structure parts that are stressed.  

3. FBG displacement sensors.

Fiber Bragg grating displacement sensors can detect shifts in structures and the appearance of cracks.

There are many applications of fiber Bragg grating sensors for structural health monitoring. For example, one of the possible uses of FBG sensors is pipeline monitoring. P19 pipelines, that are used in the power industry, are usually used to transmit corrosive high-temperature, high-pressure steam. The integrity of the pipes may deteriorate over time leading to eventual failure of the steam transmitting system.

If you would like to purchase FBG (Fiber Bragg Gratings) sensors, please contact us: info@optromix.com or +1 617 558 98 58

FBG strain sensors in structural health monitoring

Fiber Bragg grating sensors are used over conventional sensors in structural health monitoring based on a number of advantages they provide:

  • FBG sensors are more stable;
  • fiber Bragg grating sensors are more durable, namely, they don’t rust;
  • FBG sensors can be highly multiplexed;
  • the sensors can be used in highly explosive atmospheres, like natural gas or oil.

These features are desirable for the health monitoring of complex structures. FBG structural health monitoring systems are essential in the design of smart buildings. The FBG sensor systems help to ensure building safety and performance. The systems are of particular use for buildings situated in earthquake-prone zones.

The mechanism behind FBG sensors allows to monitor complex structures based on the measurements of the wavelength of the reflected light; its wavelength changes under the influence of pressure, building incline, etc. Due to the fact that a single FBG sensor reflects a narrow region of the light, it is possible to multiplex them, which provides higher data accuracy. However, the number of FBG elements should be carefully designed and spaced in accordance with the range and bandwidth of the input light source.

The FBG strain sensors are fragile on their own, therefore they need to be placed on a base of some kind to protect them from outside physical influence. Usually, the sensors have a metal protective cover that prevents damage to the sensors from environmental factors. The sensors are then welded or glued to a structural member. FBG strain sensors, or FBG deformation strain sensors, can be used in both steel and concrete structures; they offer high accuracy and resolution. The measurements received from the sensors are often used to detect the behavior of structural materials under different environmental influences. The detection of segments that are stressed is essential for proper maintenance of the building and its structural parts and prevents premature aging and failure of these structures.

FBG strain sensors may also be used in the monitoring of tunnel ribs, pipelines, ship hulls, etc. Their high accuracy, easy installation, and compact nature provide a wide area of applications.

Optromix, Inc. is a U.S. manufacturer of innovative fiber optic products for the global market, based in Cambridge, MA. Our team always strives to provide the most technologically advanced fiber optic solutions for our clients. If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com or +1 617 558 98 58

Fiber Optic Strain Sensors Technology and Applications

Nowadays, different types of fiber optic strain sensors have attracted attention from all over the world. Fiber Bragg Grating (FBG) has become the most widespread technique, directly applicable to bridges, concrete, and dams for strain measurement.

To create the actual strain sensor, the optical fiber is inscribed during production with a Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.

The material interferences are placed at certain intervals. When the fiber is stretched or compressed and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa. Not only does the reflected light take a little longer or shorter to travel back when the Fiber Bragg Grating is under strain, but the wavelength that is reflected also changes. In scientific terms, the Fiber Bragg Grating has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.

For measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum.  In order to ensure the safety of personal and public property, the precise and real-time monitoring of strain becomes more and more important in all kinds of engineering applications, such as chemical plants, gas stations, power stations, bridges, tunnels, oil pipelines, etc.. In general, these application environments full of poisonous gas, intense radiation, and elevated temperature are dangerous to human health, so safe and efficient remote monitoring of strain is of great significance. Compared with conventional electrical sensing methods, an optical fiber strain sensor is more suitable for present applications because of its compact size, high sensitivity, multiplexing capability, immunity to electromagnetic interference, high-temperature tolerance, and resistance to harsh environments.

Fiber optic strain sensors are welded directly to the surface of the metal structure (pipes, beams, etc.), and it has a protective silicone cover. Fiber optic strain sensors are durable and stable, widely used for civil engineering constructions, particularly they reinforce concrete structures exceptionally well.

If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com  or +1 617 558 98 58