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Fiber Optic Strain Sensors
There has been quite a bit of research into fiber optic strain sensors, leading to us finding quite a few papers on this topic. Fiber optic strain sensors have become much more popular lately, mainly due to their sensitivity and relatively low cost. They are also very useful in the aerospace field. If implemented correctly, fiber optic strain sensors can help us better understand fatigue characteristics and structural performance, two very important topics for an aircraft or spacecraft. Measuring these characteristics can help determine maintenance and replacement of specific systems on the aircraft. The theory behind fiber optic sensors is that the sensor contains fiber Bragg gratings (FBG) which are distributed along the length of the fiber. We will elaborate more on the theory behind how to create FBGs and how fiber optical sensors are used in industry during our full paper and presentation.
The idea with fiber optic sensors is that the sensor contains fiber Bragg gratings (FBG) which are distributed along the length of the fiber. The way an FBG is created is by using an ultraviolet interference pattern to create a periodic change in the core index of refraction, which is done by exposing an optical fiber to this ultraviolet interference. The periodic changes from the ultraviolet interference then cause a reflection when the light in the waveguide is a specific wavelength. This is because of constructive interference of grating plane reflections, while other wavelengths can be transmitted to the fiber itself. The reflected wavelength is dependent on the FBG’s period of core index of modulation (Λ) and the effective core index of refraction (
). The reflected wavelength is called the Bragg wavelength (
During experimentation, when an FBG is strained it either stretches or contracts. This causes Λ and
to either increase or decrease. This then creates a differential increase or decrease ( ) in
, which is directly related to Λ and
From the articles we read there seem to be many different techniques for using fiber optic strain sensors effectively. The chosen technique depends on what results one is looking to get, along with the preferred mounting method. We will be outlining a specific technique that we read from one of our sources, “Strain Measurement Validation of Embedded Fiber Bragg Gratings”, which is our fourth listed source. In this experiment the strain-optic coefficient was verified experimentally at the NASA Dryden Flight Research Center, and the strains themselves were calculated using a processing system from the same Research Center. This processing system uses a method based on a demodulation scheme which was patented by NASA Langley. The process involves using lasers and Fourier transforms (normal and inverse) to calculate strains. The actual technique used to implement the fiber optic strain sensors is as follows. The optical fibers were embedded into a laminated piece of carbon fiber/ epoxy unidirectional prepregs. This lay-up was quasi-isotropic. The optical fiber was embedded along the mid-plane and made to be parallel with the structural fibers. Since there is a stress concentration at the points the optical fiber enters and exits the composite, they decided to place the optical fiber in a Teflon tube. The embedded optical fibers were placed to have a centrally located fiber in each specimen, running parallel to the long side of the composite (the composite is rectangular). They used X-ray photographs to ensure the optical fibers were placed correctly prior to performing the experiment. Two strain gauges (specifically Vishay Micro-Measurements) were then mounted on the surface above and below the optical fibers which had been previously embedded. After that, an Instron load frame was used to perform the test on the composite. They applied an initial grip pressure, and then ramped up the uniaxial load at a predetermined rate in megapascals per minute. Load and strain data were collected at ten samples every second. They then did finite element analysis (FEA) using ANSYS to find theoretical values in order to compare with the collected data from the experiment using the optical fibers. The results from the experiment and FEA were consistent with each other, leading to the conclusion that fiber optic strain sensors are a good tool for this type of test.
One of the key factors of any strain gauge is their sensitivity, which shows the relationship between the variations of the sensor output and the measurand. Fiber optic sensors are sensitive to both strain and temperature. With an optical sensor, the sensitivity can be defined as the optical power at the receiver’s variation. Fiber optic sensors have a high sensitivity which means that when large variations in output occur, they correspond to the small measured variations in the magnitude. When fiber optic strain sensors have uniaxial strain, field applied they experience a linear relationship between the wavelength drift and the axial strain. Fiber optic sensors are sensitive to temperature by a linear thermal expansion of the grating and by changes in the refractive index. This effect needs to be corrected like common strain gauges to maintain accuracy. For temperatures greater than 100 °C the gratings need to be annealed in order to work reliably. Overall fiber optic strain gauges can withstand extremely large variations in temperature.
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Another key factor for strain gauges is resolution, which is defined as the capability of the sensor to detect small variations in the measurand. In terms of fiber optic sensors, resolution can change depending on the interrogation/read-out method being used and can vary as high as 0.005 ɛ and as low as 700 nɛ. This shows the variability and versatility of the fiber optic strain gauges that allows them to also be used as sensors for vibration, electric, acoustic and magnetic fields, acceleration, rotation, pressure, linear and angular position, humidity, viscosity, chemical measurements, and more.
fiber optic sensors have become increasingly popular as replacements for current sensor technology due to advantages such as high sensitivity, light weight, long term stability, being immune to electromagnetic interference, and the ability to be embedded without degradation. Because of these advantages and their ability to withstand tough environments, they are well suited for aerospace, transportation, energy, and civil applications.
From our seventh article we studied some of the best applications of fiber sensors towards aerospace industry products. Some of the areas being investigated for use include monitoring the deformation in the shape of the wing during flight, being used as a monitor to check the structural health of the leading-edge during flight, life-cycle monitoring for spars and corner parts, and detection of failure in composite patches and lap joints. In particular, the ability to detect the failure of stiffeners has become an increasingly popular application for fiber based distributed sensing systems. A study by Minakuchi et al. showed that by embedding just one optical fiber between the stiffener and Carbon Fiber Reinforcement Polymer (CFRP) panel, they were able to estimate the damage that occurred during impact and any residual strains created through the manufacturing process.
Another potential use within the aerospace/energy fields is the health monitoring of composite structures of a wind turbine blade manufactured using the Vacuum Assisted Resin Transfer Molding (VARTM) method. The initial purpose of the sensors was to detect matrix shrinkage during the cure process of the resin in order to avoid delamination or failure. However, since a sensor was needed to monitor the in-use strain of the blades under a load, it was proposed to use fiber optic sensors (specifically FBGs) due to their ability to be embedded into the composite. This versatility eliminates the need for extra sensors without a loss of accuracy. The fiber optic sensor also has a far greater life span than commonly used sensors while also being far more rugged.
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