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Fiber Optics Sensing vs. Strain Gauges – Episode 1

Posted on by Pierrick Vulliez

The medium used for strain gauges is typically a metal alloy connected to a few copper wires. The same type of technology enabled the telephone network to interconnect the world starting in the second half of the 19th century. At the advent of the internet, copper became a bottleneck to transmit data (as those of you who are familiar with dial up internet will remember). Fiber optics made it possible to start streaming information across significantly longer distances at bandwidth levels orders of magnitude higher than any copper transmission line. Because of the sheer amount of data that can be sent using fiber optics, an entire new set of applications and services became available to users, such as high speed video streaming.

Similarly, Sensuron’s fiber optics sensing technology enables a paradigm shift to take place in the area of structural testing, specifically for strain and temperature sensing. Spatially continuous measurements along the length of thin, flexible, and virtually weightless optical fiber make it possible to develop a new set of tools that provide deeper insights into how materials react to both strain and temperature. One great example of this is the structural testing of full-scale aerospace structures. Thousands of fiber optic strain gauges can be installed on an aircraft in a fraction of the time required to install traditional strain gauges, making it economically feasible to collect strain measurements at thousands of sensing points. Another one is the structural health monitoring of weight sensitive structures, such as a lightweight UAV, due to the remarkable reduction in cabling required for FOS sensors. The contrast in cabling requirements is illustrated in Figure 1.

Figure 1: Instrumentation comparison of strain gauge and fiber optic sensing technology

 

Trade studies performed at NASA Armstrong Flight Research Center (NASA AFRC) show that a typical FOS installation is 0.1% to 1% the weight of a traditional foil strain gauge installation [2]. While FOS sensors have created a paradigm shift in the structural testing arena, their operation is still analogous to that of an electric strain gauge. Instead of monitoring changes in electrical resistance, changes in reflected optical wavelength are monitored and correlated to mechanical strain via a gauge factor:

Strain gauge technology was developed nearly a century ago and has been considered the gold standard for performing experimental strain measurements for the past several decades. However, the use of FOS technology continues to become more prevalent as engineers increasingly take advantage of the advanced testing technologies available in the 21st century. The primary advantages of FOS technology will be discussed in subsequent blog posts.

Sensuron enables real-time structural health monitoring onboard X-56 NASA’s flexible wing aircraft

Posted on by Pierrick Vulliez

In contrast to the stiff, rigid wings found on most commercial aircraft, flexible wing technology is considered essential to next generation, fuel efficient aircraft. However, flexible wings are susceptible to “flutter,” or highly destructive aeroelastic instability. The Lockheed Martin X-56A multi-utility technology testbed (MUTT) unmanned aerial vehicle (UAV) – a modular high-altitude, long-endurance (HALE) technology demonstrator – was specifically designed with long, flexible, very high-aspect ratio wings in order to investigate and test active flutter suppression technology.

To better understand and mitigate flutter, engineers at NASA’s Armstrong Flight Research Center (AFRC) equipped the X-56A UAV, or “drone,” with fiber optic sensing (FOS) technology.

Learn more about flutter

For the last decade, AFRC has utilized FOS technology to perform distributed strain sensing and real time structural health monitoring during flight. Compared to traditional sensors, FOS technology provides an unprecedented level of insight into the behavior of a structure: a single hair-like optical fiber spanning up to 40 feet can act similarly to 2,000 or more strain gauges without the cumbersome and weight prohibitive instrumentation wire. In addition to strain, FOS can be used to measure temperature, deflection, stress, load, stiffness, and various other critical engineering parameters.

 

A repackaged version of the RTS125+ FOS interrogator flew onboard X-56 to acquire the FOS data. The overall weight of the interrogator (orange chassis) was reduced from 13 pounds to 6.7 pounds. (Image courtesy: Sensuron)

Engineers installed a two-line fiber system on both the top and bottom wing surfaces of the X-56A to simultaneously monitor the bending and cross-sectional rotations of each wing. The distributed strain data was acquired along the wingspan and used to derive the oscillation velocity of the wings which is indicative of flutter onset. As each fiber line is comprised of hundreds of sensing points, the FOS setup provided significantly more data and greater insight than traditional sensors.

Learn more about finite element analysis

Over the course of testing, the largest deflections occurred at the wing tips and the largest bending strains occur a few feet outboard the wing root. Historically, obtaining this level of insight into the overall strain distribution was only possible computationally through finite element models (FEM). Using FOS, finite element-like data is acquired experimentally on the actual structure. This allows critical areas to be confidently identified instead of just predicted, as well as serving as thorough FEM validation.

A histogram of bending strain and deflection data acquired through the leading-edge fibers over the duration of the FOS test flight. The x-axis corresponds to the sensor number corresponding to a discrete location along the leading-edge fiber on the. The color indicates frequency of that level in the data, where pink is the mean over time and blue is less frequent. (Image courtesy: Sensuron)

A repackaged version of Sensuron LLC’s RTS125+ real-time FOS interrogator flew onboard the X-56A to acquire the strain and deflection data. Considering the specific operating environment, the RTS125+ system’s size and weight was modified – while still offering a level of ruggedization – to ensure optimal performance throughout the mission.

 

Both the top and bottom of the X-56A wing surfaces received fiber applications. (Image courtesy: Sensuron)

Sensuron’s RTS125+ acquires continuous strain and temperature distributions on up to 320 feet of small, virtually weightless, and long-lasting fiber optic cable. Optimized for flight applications at AFRC, this technology provides test and instrumentation engineers a new level of insight into the strain distributions, fatigue life, and in-operation performance of aerospace structures.

In addition to real-time, in-flight structural health monitoring, there are a multitude of applications within the aerospace industry where FOS technology is being utilized, including the subcomponent level through full-scale structural testing, crack detection, composites embedment and cure monitoring, finite element validation, flight loads monitoring, and many more. For applications that necessitate the use of hundreds or even thousands of electric strain gauges, a significant reduction in installation time, weight, and complexity is realized by switching to distributed FOS technology.

For smaller scale applications, significantly more sensors can be installed in the same amount of time that it takes to install only a few traditional strain gauges. Regardless of the application, fiber optic sensing technology enables engineers to capture significantly more data than they can practically with traditional sensors, ensuring they literally cover all angles.

How Fiber Optics Sensing works – Let’s take a look under the hood

Posted on by Pierrick Vulliez

New technologies can often be intimidating to implement for a number of reasons, but one of the most common hurdles is a lack of understanding how the technology works or how it can be applied. Fiber optic sensing (FOS) is such a technology, but it’s not that hard to understand the basics. A look inside FOS can help open your imagination to the possibilities.

Although FOS technologies have existed for a couple of decades, advancements in the past five years have propelled the technology into new markets by expanding addressable applications. Historically, FOS technologies have been developed for specific solutions in niche markets. Today, there are a handful of FOS technologies that are being developed that can be deployed in a myriad of applications ranging from monitoring how composite materials cure to determining the deflection of an aircraft wing in real time.

Specifically, FOS systems that use fiber Bragg gratings (FBGs) have undergone tremendous advancements in the past few years. Two notable innovations are the transition from point sensing solutions to spatially continuous solutions as well as the addition of real-time multi-parameter sensing functionality. Here, we’ll take a look at the operating principles behind optical frequency domain reflectometry (OFDR), one of the types of FBG-based technologies that make distributed multi-sensing possible.

The core of most FOS technologies, whether they use FBGs or not, is interferometry. Simply put, interferometry is a family of techniques in which waves are superimposed to extract information about the waves. In FBG-based systems, light reflected back to the interrogator (light source) gets superimposed with other reflected signals, and the resulting interference signals get translated into strain or temperature data. To understand the operating principles behind OFDR, a better understanding of optical fiber and FBGs is necessary.

Fiber Optic Cables and FBG Manufacturing

Fiber optic cables are composed of an outermost protective coating, and then two layers of glass. The outer layer of glass is referred to as the cladding, and the very small inner portion is called the core. The core is where the light travels and where the FBGs are inscribed. Fiber is typically produced on a draw tower, which is a standard fiber manufacturing installation that heat up glass tubes (preforms) and draws them out, creating the thin, multi-layer optical fiber.

During the manufacturing process, the core is made to be reactive to UV radiation so that a UV laser can inscribe the gratings. There are several techniques to execute this process of writing gratings. One manufacturing process inscribes the FBGs on a completed fiber. As a result, inscribing the gratings requires the fiber coating be stripped, the gratings written, and the fiber recoated. This process leads to reduced mechanical strength and can limit how the fiber is used in the field.

A more cost-effective method inscribes the gratings during initial fiber creation. A UV laser installed on the draw tower writes the gratings as the fiber is being drawn. The fiber with gratings is then coated and spooled. This process allows for separate or continuous gratings to be written and produces fiber with much higher mechanical strength.

Fiber Bragg Gratings—The Sensing Element

The fiber Bragg grating (FBG) forms the sensing element. FBGs are essentially microscopic, wavelength-selective mirrors, meaning they reflect a single, specific wavelength and transmit the rest of the optical signal that the interrogator generated. One way to think about this is with white light. White light consists of the entire color spectrum, or in other words, many different wavelengths. If white light was sent down a fiber incorporating a FBG, one would see a single color reflected, while everything else is transmitted.

To use the FBG together with OFDR, gratings are written continuously throughout the fiber. This means that reflections are being sent back to the interrogator from every point along a fiber’s length. The reflected wavelength at each location is referred to as the Bragg wavelength. When a fiber (hence, the grating) is stretched, compressed, or undergoes thermal expansion and contraction, the Bragg, or reflected wavelength, changes. The interrogator then uses a demodulation technique to observe the change in wavelength and translate this into strain and temperature measurements. The relationship between mechanical strain and the Bragg wavelength is described in the figure below.

Fiber bragg grating working principle

Figure 1 – Diagram of the working principles behind fiber Bragg gratings and the relationship between the Bragg wavelength, strain and temperature.

The technique used to interpret the superimposed wave created by the optical signals the FBGs have reflected largely determines a FOS system’s capabilities. Two of the most prevalent interpretation techniques are wavelength division multiplexing (WDM) and, as stated before, OFDR. The biggest difference to note between the two techniques is that OFDR allows for fully distributed sensing while WDM offers tens of discrete sensors per fiber.

The demodulation technique being used has a significant impact on the performance that is possible with a FOS system. The demodulation technique largely determines system’s refresh rate, sensing length, number of sensors, spatial resolution, and the interplay these parameters. Certainly, there are other variables that impact the performance of a FOS system, but understanding the capabilities and limitations of demodulation techniques is essential when evaluating a FOS system for an application.

Wavelength Division Multiplexing (WDM)

For WDM systems, each grating must be written at a different Bragg wavelength, which means only tens of FBGs can be multiplexed on a single channel. Most WDM systems in the market have twenty to thirty FBGs on each channel. While WDM systems typically have more than one channel, they can generally only interrogate one channel at a time.

The number of sensors per channel is limited because the reflected wavelengths cannot be written close to each other. If the Bragg wavelengths are too similar, a grating under strain could shift to reflect the same wavelength as another grating, making the data useless. While WDM fiber sensors can have long lead lengths, only points of information can be obtained. If an event occurs between sensors, the user will miss critical data.

WDM systems can have high refresh rates, but each additional sensor added to a channel will significantly reduce its capabilities. Another limiting factor for WDM systems is that the user must specify exactly where the gratings need to be written on the sensing fiber. This means that each fiber must be customized for each project, a cumbersome process. The process of identifying where the gratings need to be inscribed, as well as lead times for custom fibers, can be lengthy.

Applications well suited for using WDM systems are those that only require a handful of sensing points but sense at very high speeds. For example, WDM systems can be used to measure strain on crash test dummies during the moments of impact or used to monitor detonation velocity in an explosion.

WDM working principle

Figure 2 – Diagram of a basic configuration for a WDM fiber optic sensing system.

 

Optical Frequency Domain Reflectometry (OFDR)

The other demodulation technique, OFDR or swept laser interferometry, can be used to determine both what and where events are occurring all along an optical fiber. There are two separate things that are being interrogated with OFDR: what wavelength of light is being reflected at any given point along a fiber’s length, and at what distance along the fiber that particular wavelength is being reflected from.

OFDR interrogation starts with a narrow linewidth, continuously swept light source. By narrow linewidth we mean that the light source emits roughly a single wavelength at any given instant in time. Continuously swept means that the light source sweeps its output across a given wavelength range. One full sweep across the wavelength range corresponds to one full acquisition of the FBG sensor array along the length of the fiber.

When the interrogation signal launches, the light travels down the fiber and at every point along its length, a single wavelength within the sweeping range gets reflected. The strain or temperature at each point along the fiber, because they have altered the spacing of the grating lines, determines the wavelength that gets reflected at that point. Therefore, where reflections are seen during the sweep of the laser the wavelength being reflected reveals details about the strain or temperature.

One full sweep of the laser yields reflected wavelengths coming from each of the gratings along the entire fiber – so interferometry comes into play to separate the multitude of signals. On its way back to the system, all of the reflected light is allowed to interfere with a reference signal and the result goes to an optical detector. The interference between the reference optical wave and each of the reflected waves results in frequency modulated signal information from the sensor. Reflections coming from early parts of the fiber (nearer the interrogator) modulate, or beat, at low frequencies. Reflections from far down the fiber modulate at high frequencies. Spectral analysis then reveals which reflections are returning from what point along the fiber.

Because the FBG sensing is occurring in a spatially continuous manner down the fiber’s length, the OFDR system enables the acquisition of fully distributed strain and temperature profiles along the fiber. This information can then be processed to reveal further detail. Other measurements that can be derived from these profiles include deflection, 3D shape, liquid level, pressure, and magnetic fields.

OFDR working principle diagram

Figure 3 – Diagram of the operating principle behind OFDR laid out from the laser pulse to the signal for analysis.

Because the gratings have been continuously written, OFDR technology offers significantly higher spatial resolution and exponentially more sensors than other sensing technologies in the market. Such spatially continuous information provides a number of advantages to engineers across multiple industries. For example, distributed data from OFDR sensing systems equips engineers with the data necessary to confidently validate their thermal, vibration, or strain models of a design and avoid costly failures after production has begun. Sensuron’s OFDR FOS systems, for instance, have been used in a number of applications including locating and tracking crack propagation in fatigued wind energy blades, monitoring folding and wrinkling of composite materials during manufacturing, and even determining the degree of unforeseen plastic deformation in critical load-bearing aircraft components.

Another benefit of OFDR FOS systems is that they can have multi-sensing capabilities, or the ability to monitor multiple parameters on different channels simultaneously. For example, the technology has been used to simultaneously monitor strain, deflection, temperature, and load of an aircraft wing in real time during flight testing.

The multi-sensing abilities and distributed data breakthrough that FOS sensing has made in the last several years will empower engineers to solve the design problems they face today and innovate beyond the problems of tomorrow. Now that you understand how it works, there is no longer a need to be intimidated by the technology.

Fiber optic sensing: The past, present, and exciting future

Posted on by Pierrick Vulliez

Over the past 60 years, fiber optic sensing (FOS) has been used to enhance and test the integrity, efficiency, safety, and durability of structures, vehicles, medical devices, and more across a multitude of industries. Advancements over the past five years have enabled FOS to expand its abilities to include unprecedented levels of data and sensing density across applications in aerospace, energy, and even the medical field. This is helping engineers solve problems they are faced with today, and innovate to advance their designs. Today there are a vast number of real-world implications for fiber optic technology, as well as a realm of possibilities for the future.

This article will discuss the recent advancements in intrinsic FOS technology, including 3D shape sensing and optical frequency domain reflectometry. It will also address how engineers can utilize the technology today, and provide a preview of what to expect in the future.

A brief history

The first fiber optic sensor was patented in the 1960s and relied on free space optics. Roughly 10 years later, researchers developed the first intrinsic fiber optic sensors. This enhancement offered significant engineering benefits over free space sensors for obtaining reliable mechanical measurements. The use of fiber allows signals to be transmitted inside a deployable medium whereas free space optics relies on line of sight, and cannot be deployed in operating structures or vehicles. Commercialized in the 1980s, the fiber optic gyroscope was one of the earliest applications of fiber optic sensors and has become a critical component in stabilization and navigation systems. In the early 1990s, the civil industry began implementing various types of fiber optic sensors in multiple applications to measure temperature, strain, pressure, and more.

Engineers also began experimenting with fiber bragg grating (FBG) based sensors. FBG sensors, with their multiplexing and quasi-distributed capabilities, had a distinct advantage over existing fiber optic sensing technologies. By 2000, some common applications in the civil industry included deflection monitoring of critical elements in historic buildings, monitoring strain on critical points on bridges, and observing the behavior of concrete as it cures. Most of these applications used a variety of interferometric sensors, most of which were not able to be multiplexed.

FBG sensors have largely replaced these technologies in civil, oil and gas, and aerospace applications. For instance, FBG sensors found a place in oil and gas by monitoring pressure and other parameters on critical down hole tools. Similarly, the aerospace industry continues to use FBG based sensors primarily for structural health monitoring, load testing, and fatigue testing.

In the early 2000s, distributed sensing, another fiber optic sensing technology, emerged and has shown the greatest potential in the oil and gas industry. These technologies are used primarily to take temperature measurements along the entire length of the fiber to help improve various down hole processes, including leak detection, monitoring the injection process, and creating flow profiles. While they provide distributed measurements, these technologies have slow refresh rates (a few seconds between acquisitions at best) and spatial resolution on the order of meters.

Recent FOS advances

Intrinsic and extrinsic sensors are two broad categories of fiber optic sensors. Extrinsic sensors use the fiber to guide the light to a sensing region where the optical signal leaves the waveguide and is modulated in another medium. In the case of intrinsic sensors, the light remains within the waveguide so that it measures the effects of the optical signal as it moves down the fiber.

Intrinsic FOS technology, whereby the fiber optic cable itself is the sensor, has undergone significant advancements in recent years. Within the division of intrinsic sensors, there are two different technologies: scattering or FBG. While scattering techniques offer fully distributed data points along a fiber, FBG techniques can have a handful of sensing points or can be fully distributed. By placing FBGs throughout the fiber, engineers can analyze the changes in the way the light reflects and interpret this information to provide accurate measurements. Scattering techniques do not use FBGs at all, but depend on naturally occurring random imperfections in the fiber optic cable to attain readings. Since FBGs are fabricated to be well-defined sensors, they have a much higher signal to noise ratio than scattering techniques.

While strain gages, thermocouples, and liquid level sensors only look at critical points, distributed FOS can provide a profile between critical points, which allows engineers to obtain precise measurements of full strain fields, temperature distributions, and other parameters. Both scattering and FBGs use different demodulation techniques. Scattering techniques obtain data by observing changes in naturally occurring Raman, Brillouin, or Rayleigh backscattering patterns. Wavelength division multiplexing (WDM) is the most common demodulation technique for FBG based technology. However, optical frequency domain reflectometry (OFDR) offers significant advantages over WDM in certain circumstances.

WDM can cover large distances and obtain data quickly, and the technology supports multiple gratings on a fiber; however, every grating that is added significantly reduces the refresh rate. Typical parameters measured by WDM include strain and temperature, although it is also possible to attach a single accelerometer or pressure sensor in certain circumstances. WDM also only allows users to monitor critical points rather than the entire field of information. For this reason, an application requiring very high-speed acquisition and only a handful of data points, such as monitoring components in automobile crash testing, would be a good fit for WDM.

Raman, Brillouin, or Rayleigh scattering techniques can cover kilometers of distance and provide a distributed profile of information. Unlike WDM, the scattering techniques are fully distributed which means that they obtain data along the entire length of the fiber instead of at critical points. Although Rayleigh scatter can obtain strain data, many systems on the market only measure temperature or acoustics and are referred to as distributed temperature sensing (DTS) or distributed acoustic sensing (DAS). Applications that must cover multiple kilometers, but do not require high precision and refresh rates work well with scattering technology. For example, monitoring a pipeline for tampering only requires spatial resolution on the order of meters and does not require high-speed acquisition.

OFDR is a different demodulation technique primarily used with FBG based sensors where gratings are placed end-to-end resulting in a fully distributed sensing fiber. OFDR has significantly higher spatial resolution than the scattering techniques and has exponentially more gratings than WDM. One advantage that is unique to OFDR is that it is able to maintain high refresh rates even as the number of sensors increases. The combination of high spatial resolution, quick refresh rate, massive number of sensors, and full distribution set OFDR apart as one of the most sophisticated sensing technologies on the market today. Unlike scattering and WDM, some applications of OFDR can consolidate multiple technologies into a single, robust platform. In addition to sensing strain and temperature, OFDR technology can determine 2D deflection, 3D shape, liquid level, pressure, operational load, and magnetic fields. Due to the versatility of the platform, engineers can solve multiple problems with a single system, which can make the industry more efficient and effective.

Real-world use cases for OFDR

Aerospace

Stress and strain are the parameters that determine the longevity and operational safety of any flight-bound vehicle. Airlines and aerospace agencies are constantly searching for safer equipment and processes. However, existing technologies make it difficult and costly to monitor and maintain the structural safety of planes and spacecraft. In addition, existing methodologies don’t clearly indicate when a plane or shuttle has reached its end-of-life cycle.

With thousands of sensors contained in a hair-thin fiber, FOS solutions can provide a detailed picture of the health of an aircraft. For example, by using FOS in aerospace, engineers can:

 

By using FOS technology, engineers can test, monitor, and analyze the integrity of structures and capture aircraft component positioning feedback through the continuous monitoring of strain, temperature, stress, loads, out-of-plane deflections, and 3D shapes. Armed with this data, engineers can improve safety, prolong the life cycle, reduce maintenance, and enhance in-flight efficiency of aircrafts – all resulting in reduced costs.

Medical

The small diameter and chemical inertness of optical shape sensors make the technology an excellent fit for medical applications. These features allow FOS to be integrated with existing minimally invasive technologies. By utilizing FOS technology, surgeons are provided with information about the location of the entire length of the instrument without the use of x-rays or ultrasound. The 3D data can be plotted in real-time and displayed visually on a monitor to show the position of the instrument. The image can also be compared to known coordinates of locations within the body, enabling physicians to pair the visual reference from the tip of an endoscope with knowledge of how and where the rest of the instrument is positioned. This improved positional awareness can help with real-time guidance of the instrument, minimize the injection of foreign material into a patient’s body, and do away with radiation.

Benefits of using FOS in the medical industry include:

Energy

FOS is ideally suited for subsea riser monitoring because it enables the advanced collection of real-time tension, torsion, and shape information. Subsea risers are designed to withstand some of the most complex loads and harsh environments that engineers have ever faced. The dynamic nature of the riser, its components, and its environment subjects it to structural stresses, fatigue stresses, material wear, mechanical degradation, impacts, and environmentally induced loads. Due to these and other factors, the ability of sensors and instrumentation to measure the riser’s structural response to loads is critical.

By using FOS across a variety of energy applications the industry can:

FOS in the future

The price point of FOS and its size are two of the barriers to adoption that the technology currently faces. Once these issues are resolved, we can expect more use cases across new industries.

Take, for example, the fashion industry. In the future, one could insert sensors into meshes in a piece of clothing, providing data and insight into everything about an individual’s shape, height, weight distribution, etc. This data could then be used to create clothing specific to the wearer. This would completely disrupt the fashion industry, changing the very method in which clothing is made. Imagine shopping online and having your clothes tailored to fit you perfectly before arriving on your doorstep.

Let’s also consider the automobile industry. By inserting FOS throughout the structure of a car, we could receive real-time feedback on how the car reacts to changes in its surroundings, or monitor when a car part needs to be replaced. This would be done in real-time, alerting the passenger to a possible emergency situation before it happens.

In construction, fibers could be placed into buildings or roads to monitor and determine how the structures are affected by the environment over time to detect structural issues before they occur.

Conclusion

The advancements of intrinsic FOS technology, its spatial resolution, refresh rate, and sensing length, have helped to progress the problem-solving capabilities of a multitude of industries. The data and insight FOS can collect is helping engineers progress beyond the problems of today and innovate into the future. As the technology continues to evolve, so will the designs and sophistication of applications across sectors like aerospace, energy, and medical. As engineers continue to push the boundaries of technology with their innovations, they will need sensing systems that can adapt to solve problems that don’t exist yet. FOS is flexible enough to be implemented as a platform that can be incorporated into designs as a component of critical systems where real-time monitoring is necessary or stand alone as an advanced testing suite.

What Lies Within…How Distributed Strain and Temperature Sensing Plays a Role in Complete Lifecycle Testing for Composites

Posted on by Vanessa McMillan

Composite materials are taking over the aerospace, automotive, marine, aviation, civil engineering and sports/leisure industries. And with good reason. Composites include materials which are usually stronger, lighter, or less expensive than traditional manufactured materials. They usually consist of a reinforcement (often a high-performance fiber such as carbon or glass) and a matrix (such as epoxy polymer). Typical engineered composites find their place in reinforced concrete and masonry, wood, plastics, ceramics, and metals. They are generally used for buildings, bridges, and structures such as boat hulls, swimming pool panels, racing car bodies, storage tanks, and even bathroom showers and countertops. The most advanced examples perform routinely on spacecraft and aircraft in demanding environments. Researchers have even begun to create “smart composites” known as robotic materials that mimic the human central nervous system. What’s more, design engineers like the material for its ability to mold into a wide range of shapes and a surface texture that can be altered to mimic any finish, from smooth to textured.

Despite their strength, composite material structures are frequently subjected to harsh environmental conditions and are still vulnerable to fatigue damage and failures (buckling, splitting, cracking, fracturing, and bending). It is for that reason they must be monitored, preferably in real time, for the duration of their lifecycle. Historically, this diagnostic and condition monitoring was performed with resistive strain gauges or several different types of point sensors. However, these relatively inexpensive solutions have shown their drawbacks. Single sensors are point solutions that do not measure along the entire length of the structure. Therefore, a lack of cracks or deformations where the sensor is placed does not mean that the entire composite structure is stable. The same goes for individual strain gauge installations which also have other limitations, namely:
• Strain gauges are generally more susceptible to installation faults such as glue voids, inclusions, and glue line thickness. On top of this, strain gauge accuracy can suffer with incorrect selection of excitation voltage, gauge metal selection, self-heating, gauge size selection, as well as incorrect amplification and filtering methods.
• Strain gauges are susceptible to fatigue and plastic deformation which results in hysteresis and inaccurate measurements.
• Strain gauges are susceptible to electromagnetic interference and are difficult or impossible to use in underwater, corrosive, or explosive environments.
• They can exhibit loss of repeatability and reduction in accuracy with prolonged use.

Distributed fiber optic sensing is an excellent solution for structural health monitoring of composite materials because it is very compact and can be embedded within the composite, actively taking diagnostics and condition monitoring during operation of the structure. Distributed sensing technology is also immune to electromagnetic interference, does not corrode and, because it does not require much labor to install, is extremely cost effective. Finally, true distributed sensing systems can take multiple measurements (such as strain and temperature) simultaneously. They can even sense 2D and 3D shape variations. A single fiber can be used to conduct continuously spatial infrastructure monitoring. One example is the fiber embedded within the composite panels of an automobile that sends an alert if the fiber demonstrates sudden strain, movement, or the temperature of the fiber puts the infrastructure at risk of damage or failure. According to the Fiber Optic Sensing Association (FOSA) “Fiber optic sensing is not constrained by line of sight or remote power access and, depending on system configuration, can be deployed in continuous lengths…with detection at every point along its path. Cost per sensing point over great distances cannot be matched by competing technologies…”.

As Aristotle said, “the whole is greater than the sum of its parts” and that sentiment certainly applies to the use of composites as a building tool. As costs come down and design flexibility improves, fiber-reinforced composites like carbon fiber and fiberglass are opening new design opportunities and gaining popularity with manufacturers who rely on fiber optic sensing to keep these new structures functioning and safe.