Sensuron’s fully distributed fiber optic sensing technology enables a paradigm shift to take place in the areas of structural testing and monitoring. One frequently overlooked aspect of this shift is the potential cost savings when compared with the use of traditional sensors. For example, thousands of fiber optic strain gauges can be installed on an aircraft in a fraction of the time required to install traditional strain gauges. Due to the laborious nature of the installation process, strain gauges are often deployed in limited numbers at probable critical points throughout a structure. Fiber optic sensors are installed using similar methods, but at a significantly faster pace. For the same amount of labor required to install two quarter bridge strain gauges, approximately 85 fiber optic strain gauges can be installed.
Thom Rollins, a principal engineer at Northop Grumman said it best: “ A single fiber allows us to replace thousands of strain gauges, saving significant man-hours of effort on a single project and providing us with new insight we would not have gotten using legacy sensing technology”. For applications that require or can benefit from the use of multiple strain sensors, the cost effectiveness and reduced installation time of fiber optic sensing technology is clearly attractive. This is demonstrated in much greater detail in a white paper available on our website titled Fiber Optic Sensing vs. Strain Gauges.
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.
Distributed fiber optic sensing has traditionally been a steep entry price technology to get behind. It has been a common issue for many years that has led many engineers to look for other alternatives when they see the price tag. This has adversely affected the technology’s penetration in the marketplace.
Fortunately, Sensuron is pleased to announce the release of Strain Sense, our low-cost distributed strain sensing platform capable of monitoring strain across thousands of sensing points simultaneously by using a single fiber optic cable. The system replaces single-point solutions such as strain gauges and enables users to measure spatially continuous and finite element-like strain distributions. Provided with a clear picture of deformation, load paths, gradients, and stress concentrations, engineers gain far more insight into the performance of their structures, components, and materials. Additionally, customers who replace single-point sensing solutions with a true distributed sensing platform enjoy significantly reduced installation time as well as labor cost savings. Strain Sense excels in Structural Health Monitoring applications where the ability to continuously observe, assess, and address how a structure or an object reacts under load means the difference between catastrophic failure and safe operation. The technology is especially of interest for the monitoring of critical assets in Civil Engineering, such as bridges, and is becoming a key tool in Nondestructive Testing. With a price point of $15,000 Strain Sense has the potential to revolutionize how Engineers test, monitor, and ensure the safety and reliability of their structures.
Strain Sense offers reduced installation effort compared to traditional strain gauges, increased sensor density, excellent fatigue life, insensitivity to EMI, minimal measurement drift, corrosion resistance, minimal lead cabling, and is available at a much lower price than any other product delivering the same performance! More information about Strain Sense can be found on the product page.
One widespread application where FOS technology exhibits superior performance over foil strain gauges is in the fatigue testing of components, sub-assemblies, and full-scale structures. Demand for full-scale fatigue testing continues to increase, specifically in the aerospace industry where there is significant interest in extending the service life of aging aircraft well past the intended design life.
In order to extract as much life out of these aging aircraft as possible, new fatigue tests are required to appropriately plan future maintenance schedules. Strain gauges are not ideal for this purpose as they often fail prematurely when subjected to fatigue or cyclic loading. As strain gauges are sufficiently fatigued, a drift of the zero signal occurs due to permanent damage in the gauge, known as a “zero-shift”. Depending on the stress amplitude and number of cycles that the strain gauge is subjected to, the “zero-shift” can range from ten to several hundred microstrain before the sensor stops working entirely. Regardless of the application, the fatigue performance of FBG optical fiber is vastly superior. For example, the nominal fatigue life (where zero-drift remains below 100 μϵ) of a typical commercial strain gauge is 1500 – 2500 μϵ at 10^6-10^7 cycles . In comparison, optical fiber is essentially insensitive to fatigue. FBG fiber commonly used with Sensuron equipment has proven capable of withstanding over 20,000 μϵ at several million cycles . Thus, FOS strain sensors are ideal for fatigue testing as the fatigue limit for fiber is well above the strain amplitudes witnessed during the testing of common structural materials. Summary Reduced installation effort, increased sensor density, and excellent fatigue life are only a few of the unique advantages of FOS technology. Additional benefits include insensitivity to EMI, minimal measurement drift, corrosion resistance, and minimal lead cabling. Although FOS technology provides a variety of distinct advantages, it is not always optimal for all structural testing applications. For example, hard to reach areas or locations with restrictive space constraints are often best suited for traditional strain gauges. Additionally, traditional strain gauge rosettes are recommended to measure principal strains in areas with limited space. FOS technology has demonstrated its relevance as a critical structural testing tool. When used in conjunction with strain gauges, it provides the ability to thoroughly characterize the behavior of a structure or a component.
Due to the time and resource intensive installation process, traditional electrical strain gauges are often deployed in limited numbers. Because of budgetary or schedule constraints, engineers are often forced to determine critical points throughout the structure where individual strain gauges will be installed. This lack of comprehensive coverage creates unforeseen risks that can translate into catastrophic failure. Ignorance is not bliss when it comes to testing. FOS technology enables engineers to capture significantly more data than they can practically with strain gauges and ensure that potential problematic areas on a structure are not missed.
One application amongst many where FOS has demonstrated its usefulness is in the structural testing of a large-scale sandwich composite cylinder (CTA8.1) at NASA Marshall Space Flight Center, shown below in Figure 1.
Figure 1: (a) CTA8.1 Test Assembly (b) Finite element model of CTA8.1 Test Assembly.
The testing was completed in 2016 under NASA’s Shell Buckling Knockdown Factor (SBKF) Project. The goal of the project was to improve thin-walled shell buckling design guidelines widely used throughout the aerospace community that have not been updated since the 1960s. For modern launch vehicles, buckling is often the critical failure mode and the current guidelines have proven to be overly conservative in most cases. Under improved design guidelines, structural margins can be confidently reduced while maintaining safety of the structure, yielding significant weight savings. For modern launch vehicles, significant costs savings per launch are realized due to reduced material costs and increased payload capacity.
The composite cylinder (CTA8.1) was instrumented with 16 optic cables (40-foot long), each containing over 1,000 individual fiber optic strain sensors (0.50” gauge length). The 16 fibers were directly connected to a modified version of the Sensuron RTS125+ FOS system. The installation layout, shown in Figure 2 (a), included eight axial (vertical) runs and five circumferential (horizontal) runs. Additionally, 144 individual fiber optic strain rosettes were implemented near the top and bottom interface rings (Figure 2 (b)). Note that the installation is mirrored on both the outer mold line (OML) and inner mold line (IML) surfaces (not obvious in the Figure).
Figure 2: (a) FOS Instrumentation layout on the outer mold line (b) FOS rosette layout.
The axial and circumferential fiber optic cables (Figure 8 (a)) are installed in order to capture the global strain distribution throughout the cylinder for the subcritical load cases as well as to identify the failure location for the critical load case. The FOS strain gauge rosettes are installed near the top and bottom attachment rings (Figure 8 (b)) to monitor the uniformity of the load introduction onto the composite cylinder. In addition to the 16,000 fiber optic strain sensors, traditional measurements were collected from 256 electric strain gauges, 28 displacement transducers, and 14 digital image correlation (DIC) photogrammetry systems. All 3 technologies used as part of this experiment were complementing one another.
The test article was initially subjected to a series of subcritical axial compression load cases (ranging from 20% to 50% ). At these subcritical load steps, the axial fiber runs confirmed that the axial strains were being distributed essentially uniformly. During the final load case, uniform axial compression was incrementally applied until buckling failure occurred. As the applied load approached , deformations throughout the structure began to produce a non-uniform internal load distribution due to stiffness changes.
As thin-walled shells are loaded critically in compression, the amount of bending present is a useful parameter to monitor as it is indicative of the amount of radial deformation occurring in each panel, leading to buckling. The FOS sensors were purposely installed on the OML and IML surfaces to characterize the bending strain in each panel. In the Figure below, the bending strain distribution throughout the cylinder is shown just prior to the failure.
Figure 3: (a) Distributed bending-strain measurements just prior to failure (b) Bending-strain intensity color scale.
The bending strains shown in Figure 9 are calculated as one-half the difference between the IML and OML strain measurements. Since the FOS system was operated at a 0.50” spatial discretization, the IML and OML strain measurements were aligned within 0.50”. As shown in the Figure, the maximum bending strain occurred at approximately 10 inches above the mid-height of the cylinder at the 45° circumferential position. Pre-test buckling predictions often deviate from actual buckling, making it difficult to identify critical points for traditional strain gauges to be installed. The large spatial coverage provided by the 16,000 FOS sensors greatly increased the probability of capturing bending strain from the critical panel and location.
The distributed data from the FOS measurements provided a high resolution strain map of the axial and circumferential strain occurring throughout the structure. Additionally, the mirrored FOS installation made it possible to monitor the bending strain occurring throughout each panel as the applied load approached Pcr.