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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.

We’re All Getting Older: Accelerated Lifecycle and Fatigue Testing

Posted on by Vanessa McMillan

Shrinking military budgets often necessitate the use of vehicles and aircraft well beyond their service lives. For example, according to STARS and STRIPES, among the Navy’s concerns is a rapidly aging Ready Reserve Force of 46 transport ships, which each average 44 years of age. Similar problems face the Air Force. In 1980, the average age of the Air Force’s bomber force has increased from under 20 years to 39 years and its tanker fleet from about 20 years to 38 years. It’s hard to imagine that Air Force pilots are flying planes designed in the 1970s. What’s more, when these vehicles are decommissioned, many are passed down to the US Forestry Service (as observational or fire-fighting aircraft) or to rural municipalities who need ambulance, police and fire vehicles. People’s lives (and sometimes the outcome of wars) hang on the accuracy and dependability of these vehicles. Being able to predict their fatigue life with accuracy can save time, maintenance costs, and ultimately lives.

Fatigue testing or accelerated-life testing is the process of testing a product by subjecting it to conditions (stress, strain, temperatures, voltage, vibration rate, pressure etc.) in excess of its normal service parameters in an effort to uncover faults and potential modes of failure in a short amount of time. In other words, the simulation of wear and tear over time. By analyzing the product’s response to such tests, engineers can make predictions about their potential service life and recommend maintenance intervals. Two popular methods of fatigue or life-cycle testing include accelerated hardware-in-the-loop simulation and accelerated field testing. Field testing is labor intensive, time consuming and expensive. Hardware in the loop is none of the above, however, not many pilots would feel entirely comfortable relying on the integrity of a plane that had only been tested using a software model. For more advanced testing, many engineers are turning to distributed sensing as the mechanism to help alleviate the guess work in determining an accurate end-of-life scenario. The reason they are embracing this advanced technology is that distributed sensing solutions provide a more detailed picture of the true health of a vehicle. With thousands of sensors contained in a single hair-thin fiber, distributed sensing solutions obtain real-time, spatially continuous information about multiple parameters (strain, temperature, deflection, etc.). All these parameters can be measured simultaneously using a single system – thus saving considerable time and money. In addition, they can also be measured in the actual simulated conditions such a vehicle encounters in the real world, without subjecting them to real-world field testing. This includes, rugged environments with extreme temperature fluctuations, radiation, and high EMI/RFI readings.

Some applications that utilize distributed sensing for end-of-life assessment include crack detection, deformation monitoring, deflection monitoring, composite health, liquid-level monitoring, corrosion sensing, temperature elevation, and more. Each of these assessments allow an engineer to predict – often down to the number of flight hours or miles of service – how long a vehicle will remain safe and functional, enabling decisions to retire or decommission a vehicle or aircraft before a serious accident occurs.

For more information:
View our Applications page about Structural Health Monitoring
View our Case Studies page about Structural Analysis, Structural Health Monitoring, and Nondestructive Evaluation
View our Whitepapers page for an Introduction to Fiber Optic Sensing

$2 Trillion in US Infrastructure Repairs Will Spur Nondestructive Evaluation Initiatives

Posted on by Vanessa McMillan

Earlier this year, the White House agreed to spend up to $2 trillion to help fix our nation’s aging infrastructure. While the largest chunk will likely go to roads, bridges, and transit, there is also talk of improvements for wastewater, harbors, and airports, as well as a more efficient energy grid to transmit energy over longer distances. This much-needed, enormous undertaking will not only involve demolishing and rebuilding existing infrastructures, but also assessing structural integrity and repairing structures that may not require drastic measures such as demolition or replacement.

With that in mind, engineering firms, construction companies and manufacturers will likely see a surge in nondestructive testing (NDT) or nondestructive damage evaluation (NDE) over the next few years. NDE consists of a variety of non-invasive inspection techniques used to evaluate material properties, components, or entire structures and assemblies. The techniques can also be utilized to detect, characterize, or measure the presence of damage (e.g. corrosion or cracks). The purpose of NDE is to inspect a component or structure in a safe, reliable, and cost-effective manner – without causing further damage. This contrasts destructive testing, where the part being tested is damaged or destroyed during the inspection process. NDE can be performed during or after manufacture/construction, or even on edifices or structures that are currently in use. NDT inspections can be used to assess the current state of a structure, monitor its ongoing health, and make informed evaluations on its remaining lifespan.

While there are many methods to conduct NDT, such as eddy current, radiography, magnetic particles, etc., a practical alternative to these methods which is generating excitement within the civil, nuclear, and oil and gas industries is distributed strain and temperature sensing using optical fiber-based sensors. There are several factors driving interest in distributed sensing. First, old methods fail to provide the spatial coverage required to locate every conceivable potential damage area, as well as the global response to the presence of localized damage. In other words, the exact location of the fault and the effect it has on the rest of the structure. Distributed sensing provides ultra-fine spatial resolution across tens of meters of sensor length, providing spatially continuous information over large areas. Another reason is the technology’s ability to assess a wide variety of structures and subsystems. Distributed sensing is just as useful detecting cracks on a large building as it is quantifying damage to the interior of a pipe or determining loads on and within concrete and concrete-embedded rebar. Distributed sensors are also able to be installed on, around, and in complex geometries and difficult-to-reach spaces. Finally, with traditional coarse sensor layouts, large-scale structures typically experience a significant amount of damage prior to revealing the existence and location of that damage. With distributed sensing, near continuous information detects failure modes far in advance of any damage.

How is this accomplished? Sensuron’s sensing systems monitor strain and temperature distributions by transforming an optical fiber into a spatially continuous sensor that performs like thousands of strain gauges or thermocouples installed adjacent to one another. The sensors are easier and require far less time to install, and last orders of magnitude longer than traditional technologies. This allows engineers to detect early stages of material degradation related to fatigue, overloading, aging, thermal cycling, and corrosion rapidly, reliably, and without jeopardizing the health or safety of the structure they’re charged with evaluating. This is good news for a nation about to undergo an infrastructure overhaul.

For More Information:
View a NDE demonstration video

Read a case study about NDE

Visit our Resources page

FIU Bridge Collapse: New OSHA Report Says Key Signs Were Ignored

Posted on by Pierrick Vulliez

It’s been a year and a half since a pedestrian bridge collapsed at Florida International University in March of 2018, killing six people. Recently, OSHA came out with a 115-page report concluding that the tragedy could very well have been prevented. Just hours before the bridge collapsed, its design engineers inspected the deep structural cracks and asserted that they posed no safety concerns. The bridge’s contractor, who was also made aware of safety concerns by its employees on the morning of the collapse, also failed to alert authorities and close the street below the weakening bridge. Now, not only is the engineering firm being implicated, but also the construction management company that built the bridge. Ultimately, where does the fault lie and how could this have been prevented?

Those who study structural health know one thing, a structure’s integrity begins way before construction starts. It begins in the design phase. Before starting a design, the structural engineer must determine the criteria for acceptable performance, including the loads or forces to be resisted. Types of analysis include equilibrium, stress, strain, and elasticity. Once the correct types of tests for that structure have been identified , final design can proceed. Deflections and allowable stresses or ultimate strength must be checked against criteria provided either by the owner or by the governing building codes. That’s when safety at working loads is calculated. Several methods are available, depending on the types of materials used. One method is the Finite Element Model (FEM) or Finite Element Analysis (FEA). FEA is the process of using mathematical models to simulate displacement, stresses, strains, and other parameters to predict the performance and limits of a design. FEA models rely on statistical assumptions to determine how a design will behave in a real-world environment, but reality often does not match the scenario assumed by models. Reasons could include tiny cracks in the material, retrofitting situations, and/or unexpected environmental impacts. This design analysis phase is where the engineering firm that designed the FIU Pedestrian Bridge fell short. In fact, the OSHA report concluded that the cracks were “due to deficient structural design.” The engineers probably performed the necessary tests on their model using traditional resistive strain gauges. They could and should have used distributed sensing technology to validate the efficacy of their design. Unlike traditional strain gauges which report information at critical points in the structure, distributed sensing obtains data along the entire length of the structure. In addition to collecting data at the critical points, it measures between these points. This provides an unprecedented level of insight into the behavior of a structure and allows engineers to perform comprehensive model validation at thousands of discrete points, instead of just a few, identifying just about any source of weakness long before the structure is constructed.

In addition, once construction of the bridge was complete, the managing contractor should have immediately checked the bridge’s structural integrity, as well as instituted a continuous plan to monitor structural health. Once again, the engineers probably used individual resistive strain gauges placed at various critical points along the span of the bridge to test its structural integrity. However, with this discrete point solution for checking, how could they really be sure that the entire length of the bridge was structurally sound? Again, distributed strain sensing for structural health monitoring and safety is key. Forward and safety thinking engineers perform continuous real-time monitoring of critical structures on an ongoing basis. Mechanical strain, stress, loads, temperature, and deflection are just a few of the various engineering parameters that they continue to test and monitor. With distributed sensing, a hair-like optical fiber with spatial resolution down to 1.6mm, leaves almost no point undetected . Multiple fibers can be deployed across a structure to create an effectively weightless network of sensors, capable of continuously monitoring structural health at several thousand locations – thus protecting critical assets and lives.

According to the Miami Herald, the OSHA report confirms what independent experts who examined the bridge design plans and engineering calculations hypothesized: “that the collapse was triggered by sending work crews to re-tension internal support cables at the critical structural connection where the cracks appeared.” What if they were aware of the exact portion of the bridge that held the critical weakness – not just where the cracks appeared? They may have made changes in the structural design of the bridge, they certainly would have built the bridge differently and, with structural health monitoring, they would have had advance notice of critical weaknesses forming and been able to take necessary safety steps to avoid the six deaths that resulted that day.

For more information, visit our Resources page.