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Optical Fiber Instrumentation

Fiber Optic Sensing Opens New Capabilities for Structural Analysis

Beyond finite element analysis, fiber optic sensors can be used to give graphic analysis for complex structures and soon may even be embedded in composite materials for real-time monitoring of stress and strain during actual operations such as in aircraft.

BY ALEX TONGUE, 4DSP

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Understanding how engineering structures respond to loads and their environment is of paramount importance for their successful design and reliable operation. Analyzing the strains, stresses, temperatures and deflections in a bridge during rush hour, a composite aircraft’s wing spar during a storm, or a high-end bicycle during the Tour de France, is what allows engineers to predict a structure’s lifetime, increase its safety and optimize its performance. Armed with a complete picture of these quantities from the early design stages to the end of a product’s lifecycle, designers would undoubtedly create safer, stronger and more efficient engineering structures. 4DSP has teamed up with NASA Dryden to create a fiber optic sensing instrument capable of providing this information in real time. Their distributed sensing technology provides strains, temperatures, stresses, out-of-plane deflections and three-dimensional shape; all in a small and lightweight package. 

Fiber optic sensing has emerged in recent years as an effective solution for characterizing structures in harsh environments. Optical fiber is extremely lightweight, immune to electromagnetic interference, and is resistant to extreme temperatures and corrosive chemicals.  The sensing elements within these fibers are called Bragg gratings. These elements are fabricated using high-intensity UV laser light to locally destroy the silica-oxygen bonds within the core of single mode optical fiber. The disruption caused by these broken bonds slightly decreases the speed at which light travels through the core, or it increases its index of refraction. If this process is done periodically over a short distance along the fiber, it creates a Bragg grating. The interesting and useful characteristic these elements exhibit is that they act as selective optical mirrors; they reflect a specific wavelength and allow all others to pass. The grating’s periodicity is what determines the wavelength that it reflects. Thus, when these fibers are stretched, compressed, or undergo thermal expansion, the periodicity of the grating and its reflected wavelength change proportionally. By using a light source to interrogate them and a detector to sense the reflected signal, Bragg gratings turn plain optical fiber into passive and robust strain gages and thermocouples. 

Bragg grating sensing systems have actually been around for quite some time. Off-the-shelf instruments combining grated fiber and reflected optical signal demodulation techniques such as wavelength division multiplexing or time division multiplexing are offered by several established manufacturers. However, severe processing limitations associated with these technologies have hindered their widespread adoption in industry. For example, wavelength division multiplexing requires that each grating on a single fiber reflect its own unique wavelength. To accommodate the expected shifts due to strain and temperature changes, each grating must also be allocated its own wavelength band. This puts a limit on the number of sensors one can have on a single fiber since light sources have limited bandwidth. Time division multiplexing allows the gratings to be written at the same wavelength. However, it runs into extreme processing speed limitations with an increasing number of sensors on a single length of fiber. Low sensor count and sample rates, while useful in some scenarios, aren’t an effective solution for large scale distributed sensing.  

These roadblocks have been recently overcome through collaboration between NASA Dryden and 4DSP. Together they have developed and implemented a new processing technique that expands on the capabilities of existing Bragg grating technologies. Their efforts led to 4DSP licensing the patented technology and integrating their platform into an off-the-shelf product. Their RTS150 is a multichannel optical sensing instrument capable of interrogating a significantly larger number of sensors at higher rates than past Bragg grating systems (Figure 1). Utilizing the newly developed demodulation technique, engineers can now simultaneously monitor up to 65,536 optical strain gages and thermocouples, each at 100 samples per second.This opens the door to a vast number of structural problems being solved by distributed optical sensing.

Figure 1
4DSP RTS150 8-channel system.

The fundamental physical parameters that define the state of a structure are stress and strain. These are the quantities that determine if a bridge will collapse, whether a pressurized tank will burst, or how long an aircraft can continue to be in service. Traditionally, the stresses and strains in large and complex structures such as aircraft and spacecraft are modeled using various finite element methods (FEM). These are used to simulate different loading scenarios and engineers use their results to determine structural soundness. FEM models are also used to locate critical points to monitor with conventional foil strain gages during testing and operation. While finite element methods have been a groundbreaking technology in structural engineering, there are still some problems associated with using them. Modern structures are becoming increasingly more complex as new materials and technologies continue to be discovered and introduced to industry. Despite this increasing complexity, FEM models are often simply assumed to be accurate, and, if they are validated, it’s done using a few discrete points with strain gages. 

Any practicing engineer will know a lot of unexpected things can occur between any two discrete sensing points. Distributed optical sensing allows engineers to determine precisely what happens over a continuous sensing length and acquire finite element-like experimental data (Figure 2). Fiber can be applied along the entire length of a wing spar, wound around the perimeter of stress-concentrators such as doors and windows, or fiber grids can be applied to large planar sections to obtain a far more detailed picture of a structure’s actual behavior. With the ability to sense stress, strain and temperature at spatial resolutions less than a millimeter over entire structural components, this new technology provides the means for FEM models to be fine-tuned, and can help establish the utmost confidence in a model’s accuracy.

Figure 2
RTS150 Sensing Engineering Strain.

Distributed fiber optic sensing also provides new solutions for carbon fiber composites. Their unique fabrication processes allow Bragg grating sensors to easily be embedded straight into the structure. This technology enables real-time embedded monitoring of highly flexible composite structures from the early design stages to the end of their operational lifecycle. Optical sensing’s small spatial resolution and high strain accuracy allows engineers to discover, locate, quantify and track the various complex failure modes carbon fiber composites exhibit. Anomalies like delamination between layers, matrix cracking, fiber breakage and fiber buckling all produce stress and strain concentrations at their origin.

Because 4DSP and NASA’s new technology turns the entire length of fiber into one continuous strain gage, these concentrations would appear along the fiber’s output strain profile. As delaminations or cracks propagate under increasing loads or fatigue, the concentration point at the crack front can be tracked along the fiber and quantified at each point along the way. Optical fiber is also resistant to temperatures ranging from cryogenic to hundreds of degrees Celsius. Embedded fibers can be monitored during the high temperature cure phase of composite fabrication to determine the through-thickness residual stresses and strains.

Continuous monitoring of composite structures could be a far reaching advancement in industry. Commercial airlines will always know the state of fatigue damage on critical components, allowing them to fine-tune maintenance schedules and minimize aircraft downtime. Composite rotor blades, skins, beams and pressure vessels across various industries can benefit from knowledge of the physical state of a structure or structural component during operation.

This new technology also enables out-of-plane measurements. The majority of sensing instruments in the past have only been able to provide in-plane quantities. Strain gages and traditional fiber Bragg grating systems sense strain only along the primary directions of a structural component—along the length of a beam or in the plane of a plate or shell. From this, mechanical stress and planar deformations are readily obtained. However, there are very few solutions for obtaining distributed out-of-plane measurements such as deflections or applied loads, especially during operation of a structure. As industry continues to move toward lighter and more flexible structures, out-of-plane information is becoming a critical factor in design and operation. Transfer functions have been developed for use with this new technology that utilize the high spatial density planar strain measurements along a fiber bonded to a structural component. These algorithms yield both out-of-plane deflections and applied loads, all in real time. When applied to a wing spar, engineers can simultaneously obtain the strain along its length and the deflection of the aircraft’s wing. This can provide information on flutter, natural frequencies and mode shapes, lifting loads and moments.

Three dimensional shape sensing of a continuous fiber is another new application enabled by distributed fiber optic sensing. These shape sensors can accurately determine deviations from a straight line, or they can be manipulated into complex shapes with tight twists and bends with radii less than half of an inch (Figure 3). This unique application can be beneficial to such fields as petroleum engineering, where oil pipelines may contain blockages that need to be located and inspected, or in satellite technology where an antenna is undergoing complex bending in multiple directions. While the 2-D shape sensing solution requires the fiber optic cables to be bonded to an underlying structure, the 3-D shape sensor is an independent entity, and can be used by itself or integrated with an existing structure such as a catheter used for invasive surgery. With a diameter down to 450 micrometers, these sensors can be put through complex, tightly turning tracks like arteries, pipelines and mining boreholes to accurately determine the shape of their path. Human limb movement can be recorded far more accurately than current digital image correlation techniques, or the shape of the tether to an underwater vessel can be tracked and used to determine the vehicle’s location and whether or not the umbilical is wrapped around an unseen obstacle.  

Figure 3
3D shape rendering.

The future of fiber optic instruments lies in miniaturization and speed. As the size of the interrogation system is reduced, the number of applications that can use it increases tenfold. Arriving in 2014, the next generation product will be the size of a 5-inch cube, more than capable of fitting within a typical flight instrumentation box. With such a small weight penalty, the future may see every commercial airliner being constantly monitored by distributed fiber optic sensing. Or, a system may be installed in the housing of every wind turbine around the world, continuously monitoring blade shapes and structural integrity. In the long-term, 4DSP intends to reduce the size to the equivalent of a deck of playing cards. Another leap forward to be taken is an order of magnitude increase in sample rates. While the current rates have already left past fiber optic sensing technologies behind, 4DSP has its eyes on sensing high frequency vibrations such as those on rotor blades. Paired with the continuous sensing capabilities, high sampling rates would also allow engineers to study stress wave propagation in a whole new way. Distributed sensing offers a unique solution to a wide spectrum of engineering problems and appears to have a very bright future. 

4DSP

Austin, TX.

(775) 830-2059.

[www.4dsp.com].