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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.

Distributed Strain and Temperature Measurement – A Thousand Points of Light

Posted on by Pierrick Vulliez

Organizations that put a premium on safety and reliability have been using test and measurement solutions for years to validate structural integrity (strain), moderate temperature, and gain insight into places not visible with the human eye. Traditionally, strain and temperature were measured using electrical sensing technologies, such as foil (strain) gauges and electrical vibrating wire. In the past few years, new advances in technology have challenged those traditional measurement methods by requiring more stringent accuracy, reliability, and durability, as well as the need to measure on a continuum, rather than being limited to a single sensing point.

Following a UAV prototype crash a few years ago, NASA realized that it had innovated beyond its capability for monitoring and keeping airborne assets safe. A paradigm shift in sensing technology was sorely needed to ensure that such an event would not occur again. Since then, NASA and Sensuron have jointly developed and brought to market a new technology that afforded them the ability to take more accurate measurements in order to validate an aircraft’s structural integrity before it ever leaves the ground. That technology is now commonly known as fiber optic sensing – the gold standard of strain and temperature measurements. But how does this new distributed strain and temperature measurement platform work and what are its uses?

Photo courtesy of NASA

With fiber optic sensing, a single fiber spanning 10 meters can perform the measurements of thousands of individual strain gauges or thermocouples simultaneously. The technology utilizes Optical Frequency Domain Reflectometry (OFDR) to achieve this process. With OFDR, a wavelength source (continuously tunable laser) is used to spectrally interrogate a multitude of sensors (also known as FBG) along a fiber. The reflected light from these sensing elements is then detected, demodulated, and analyzed. Because the optical path difference between the reference reflector and each individual sensor is different, the reflected signal from each sensor is modulated by a unique frequency that is directly dependent upon the grating’s location in the fiber. This technique enables detection from hundreds to thousands of discretized points along a single fiber, allowing it to collect data from a dense array of spatially distributed sensors that is impossible to replicate with other techniques currently available.

Distributed fiber optic sensing can measure continuous strain, temperature, deflection, 3D shape, liquid level and magnetic fields. It is ideally suited for applications that are exposed to harsh environments, require long-range, long-term deployments, and measurements that are in close proximity to noise sources (such as power transformers, electric motors, antennas, etc.). Moreover, it enables new sensing capabilities. One of these is 3D Shape Sensing, which benefits medical applications by guiding devices safely within the human body during invasive procedures.

In the coming months, we’ll explore some exciting applications currently utilizing fiber optic sensing for distributed strain and temperature measurement.
For more information, visit our Resources Page.