In-flight sensor tests a step toward Structural Health Monitoring for safer flights

Senior scientist Dennis Roach (6620), center, works inside the cabin of a B737 test bed installing and acquiring data from Structural Health Monitoring sensors with mechanical engineers Stephen Neidigk, left, and Tom ice (both 6621). (Photo by Randy Montoya)

by Heather Clark

Nine commercial aircraft flying regular routes are on the frontier of aviation safety, carrying sensors that monitor their structural health along with their routine maintenance. These flight tests are part of a Federal Aviation Administration (FAA) certification process that will make the sensors widely available to US airlines.

"The flight test program is underway," says Dennis Roach, a senior scientist in Sandia's Transportation, Safeguards & Surety Program 6620 who has worked in aviation safety for 25 years. "We have moved past laboratory research and are looking for certification for actual on-board usage. Our activities are proving that the sensors work on particular applications and that it is safe and reliable to use these sensor systems for routine aircraft maintenance."

Flight tests complement lab tests

Delta Air Lines Inc. and a non-US aircraft manufacturer have partnered with Sandia researchers in two separate programs to install about 100 sensors on commercial aircraft. These teams worked together to provide the installation procedures for technicians and now oversee monitoring of the in-flight tests.

The flight tests complement laboratory performance testing at Sandia to provide the critical step in a decade-long journey to enhance airline safety through a more comprehensive program of Structural Health Monitoring. SHM uses nondestructive inspection principles — technologies that examine materials for damage without affecting their usefulness — and built-in sensors that automatically and remotely assess an aircraft's structural condition in real-time and signal the need for maintenance.

Dennis says the goal of monitoring the sensors installed on the aircraft is to accumulate successful flight history to show that the sensors can sustain the operating environment, while providing the proper signals for flaw detection.

SHM eventually could help airlines save money by basing maintenance on the actual condition of the aircraft, rather than fixed schedules and inspection routines that might not be necessary, thereby reducing airplanes' downtimes, Dennis says.

The team says that so far the sensors installed on the aircraft are working as expected.

By 2015, Sandia intends to present the flight and laboratory test results to the FAA for approval and certification. Should the FAA approve the sensors, they would be available for specific applications across the entire airline industry and the process for certifying future applications should be more efficient because of the research being conducted now.

In September, a team from the Airworthiness Assurance Nondestructive Inspection Validation Center (AANC) operated by Sandia for the FAA will receive the 2014 Airlines for America Nondestructive Testing Better Way Award for establishing the sensitivity, durability, and repeatability of applying SHM solutions on commercial aircraft. The award honors team members from the FAA; Delta; Boeing Co.; Structural Monitoring Systems Inc.; and Canadian-based Anodyne Electronics Manufacturing Corp. The award recognizes the year's most outstanding innovation for aircraft maintenance based on technology advancement and cost-effectiveness.

Two SHM systems reach maturity for use on regular flights

Sandia began its work in the aviation safety arena in 1991 when the FAA, in response to a number of aviation incidents, increased its research efforts to improve inspection, maintenance, and repair of commercial aircraft. Among the projects to improve aviation safety, the FAA created the AANC, operated by Sandia, to conduct research into nondestructive inspection (NDI), advanced materials, engines, structural integrity, and a wide range of other airworthiness assurance areas.

The center provides a way to develop, evaluate, and bring new aircraft technologies to the airline industry, Dennis says. "We work to make the technology viable and often focus on that last phase of technology validation and certification."

The current SHM program is testing two sensors: Comparative Vacuum Monitoring (CVM) sensors manufactured by Structural Monitoring Systems and piezoelectric sensor arrays produced by Sunnyvale, Calif.-based Acellent Technologies Inc.

  • CVM sensors improve crack detection by monitoring "galleries," or 0.025-inch channels etched by laser into the Teflon sensor. CVM sensors are then mounted in areas of the aircraft known to experience fatigue. The sensors are bonded to the surface of the structure with an adhesive surface preparation that seals out the atmosphere, creating a vacuum inside the gallery. When a tiny crack intersects the gallery, the pressure changes, much like the pressure in a vacuum cleaner changes when the hose has a leak. The sensor records the pressure change and alerts inspectors, well before the crack becomes a safety issue.
  • Piezoelectric sensors (PZT) are strategically distributed in polyimide films, called Acellent's SMART Layers, which adhere to an airplane's surface to monitor specific regions for damage. The array of PZT sensors communicate with one another by transmitting and receiving ultrasonic surface waves called Lamb waves. This creates a mini-communications network. Damage to the aircraft disrupts or changes the signal patterns from the baseline communication signals. Acellent's software measures and analyzes any changes, called the "damage index," and sends an alert to the inspector. Work is ongoing on the best spacing and placement for these sensors on aircraft, Sandia mechanical engineer Stephen Neidigk (6621) says.

Both of these on-board sensors must meet the same performance and reliability standards as those required for current maintenance inspections, Dennis says. "The SHM systems also help eliminate some of the concerns about human factors associated with manually deployed NDI," he says. "You have the sensor in place, you know it works and it's giving you a proper signal, whereas an inspector must manually orient the inspection probe properly each time and there are always concerns about human vigilance when inspections become time-consuming or tedious."

The sensors are custom built to fit an aircraft's parts, they are verified to be in working order before they are sealed inside the aircraft, and the readouts provide inspectors with a "pass" or "fail" decision so the results can't be misinterpreted, the researchers say.

Sandia also is researching wide-area monitoring using piezoelectric and fiber optic strain sensors for composite materials used in today's aircraft. Impacts don't always show dents in composite materials, so SHM techniques are needed to find structural damage within what appears to be a smooth, undamaged surface, Stephen says.

Field tests bridge gap from lab to routine use in aircraft

The field tests have helped fine-tune the sensors so that they can withstand the harsh environments aircraft fly in and the environment aircraft mechanics work in, neither of which is as pristine as the laboratories where the sensors were initially tested.

For example, field testing showed that mechanics working in the cramped bowels of an aircraft couldn't see well enough to connect the sensors' tubes together by hand, Stephen says. So the team designed snap-clip type connectors for the CVM sensors, like those used to plug a telephone landline into a wall outlet.

"With the snap-click connectors, they are able to feel them click together, which is easier than the previous method of connecting tiny tubes individually by hand," Stephen says.

Growing realistic cracks part of Sandia performance tests

Complementing the in-flight tests, Sandia is looking at the sensors' ability to detect cracks and at how well they perform in extreme environmental conditions.

In the laboratory, Sandia engineer Tom Rice (6621) breaks things for a living, but that's not as easy as it sounds. The cracks he "grows" have to represent cracks found on an airplane. So, for example, he places a pale green wing box fitting on a load frame that mimics the stress conditions that part would experience on an aircraft. After about four hours of accelerated fatigue cycles, a crack begins to show.

"We literally have to grow the crack enough to where it stays open (without the load on it), so our sensors can detect the crack when the aircraft is in an unloaded state in the maintenance hangar," Tom explains.

Once the sensors have detected an array of cracks, Sandia assembles various test scenarios and collects the data to calculate the statistical probability of detection for cracks of various lengths, typically fractions of inches.

In hundreds of laboratory tests, the sensors have never issued a false call, Tom says.

Future of SHM can reduce costs, enhance safety for airline industry

If flight tests verify that the sensors can be used to help monitor airliners' structural health, the Sandia researchers hope to see a more comprehensive SHM program follow.

In addition to safety enhancement, SHM would save the airline industry time and money, particularly if sensors are mounted in hard-to-reach areas and used widely throughout an aircraft, Dennis says.

With today's routine maintenance, inspectors often need to remove a cabin's interior seats or galleys to conduct inspections. But with the on-board sensors mounted in place, the mechanics can plug in from a convenient location to acquire the sensor data without the time and cost of removing items, Dennis says. Such part removal also introduces the possibility of damaging the structure during disassembly.

Researchers hope SHM eventually will permit the real-time condition of the aircraft to dictate maintenance. "The ultimate goal is to monitor it in-flight and have it tell you 'I need some attention, I've got a problem here.' So you do condition-based maintenance rather than time-based maintenance," Dennis says. "That's downstream a ways, but these are all building blocks working toward that."

Tom adds that with SHM abnormal problems that show up prior to scheduled maintenance would be detected with real-time sensors. "With condition-based maintenance, you could find damage earlier than normal," he says. "It's rare that it happens, but it could."

Such early damage detection and repairs provided by SHM also are cost-effective because they reduce the need for subsequent major repairs, Dennis says.

-- Heather Clark

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Sandia magnetized fusion technique produces significant results

“Here, a miracle occurs!” — No, it’s just a series of equations and graphs describing Z, being batted about between Sandia’s Patrick Knapp (1688), left, and Paul Schmit (1684).  (Photo by Randy Montoya)

by Neal Singer

Sandia researchers have produced a significant output of fusion neutrons, using a method fully functioning for only little more than a year.

The experimental work is described in a paper to be published in the Sept. 24 Physical Review Letters (PRL) online. A theoretical PRL paper to be published on the same date helps explain why the experimental method worked. The combined work demonstrates the viability of the novel approach.

Says senior manager Dan Sinars (1680), “We are committed to shaking this [fusion] tree until either we get some good apples or a branch falls down and hits us on the head.” He expects the project, dubbed MagLIF, for magnetized liner inertial fusion, will be “a key piece of Sandia’s submission for a July 2015 NNSA review of the national Inertial Confinement Fusion Program.”

MagLIF uses a laser to preheat hydrogen fuel, a large magnetic field to squeeze the fuel, and another separate magnetic field to keep charged atomic particles from leaving the scene.

Inertial confinement fusion creates nanosecond bursts of neutrons, ideal for creating data to plug into supercomputer codes that test the safety, security, and effectiveness of the US nuclear stockpile. Down the road, if the individual fusion pulses can be sequenced like an automobile’s cylinders firing, the method could be useful as an energy source.

It only took two magnetic fields and a laser, focused on a small amount of fusible material called deuterium (hydrogen with a neutron added to its nucleus), to produce a trillion fusion neutrons (neutrons created by the fusing of atomic nuclei). Had tritium (which carries two neutrons) been included in the fuel, scientific rule-of-thumb says that 100 times more fusion neutrons would have been released. (That is, the actual release of 10 to the 12th neutrons would be upgraded, by the more reactive nature of the fuel, to 10 to the 14th neutrons.)

Technique is still a toddler

 Even with this larger output, to achieve break-even fusion — as much power out of the fuel as placed into it — 100 times more neutrons (10 to the 16th) still would have to be produced. The gap is sizable, but the technique is a toddler, with researchers still involved in figuring out the simplest measures: how thick or thin key structural elements of the design should be, and the relation between the three key aspects of the approach — the two magnetic fields and the laser.

The first Sandia paper, “Experimental Demonstration of Fusion-Relevant Conditions in Magnetized Liner Inertial Fusion,” [MagLIF] by lead authors Matt Gomez (1683), Steve Slutz, and Adam Sefkow (both 1684), describes a fusion experiment remarkably simple to visualize. The deuterium target atoms are placed in a long thin tube called a liner. A magnetic field from two pancake-shaped Helmholtz coils above and below the liner creates an electromagnetic curtain that prevents charged particles, both electrons and ions, from escaping. The extraordinarily powerful magnetic field of Sandia’s Z machine then crushes the liner like an athlete crushing a soda can, forcefully shoving atoms in the container into more direct contact. As the liner begins to be crushed, a laser beam preheats these deuterium atoms, infusing them with energy to increase the chance of them fusing at the end of the implosion. (A nuclear reaction occurs when an atom’s core is combined with that of another atom, releasing large amounts of energy from a small amount of source material. That outcome is important in stockpile stewardship and, eventually, in civilian energy production.) Trapped energized particles including fusion-produced alpha particles (two neutrons, two protons) also help maintain the temperature of the reaction at a high level.

 “On a future facility, trapped alpha particles would further self-heat the plasma and increase the fusion rate, a process required for break-even fusion or better,” says Adam.

The actual MagLIF procedure follows this order: The Helmholtz coils are turned on for a few thousandths of a second. Within that relatively large amount of time, a 19-megaAmpere electrical pulse from Z, with its attendant huge magnetic field, fires for about 100 nanoseconds (less than a millionth of a second), with a power curve that rises to a peak and then falls in intensity. Just after the 50-nanosecond mark, near the current pulse’s peak intensity, the laser, called Z-Beamlet, fires for several nanoseconds, warming the fuel.

Slower pace = More fusible reactions

According to the paper’s authors, the unusual arrangement of using magnetic forces both to collapse the tube and simultaneously insulate the fuel, keeping it hot, means that researchers could slow down the process of creating fusion neutrons. What had been a precipitous process using X-rays or lasers to collapse a small unmagnetized sphere at enormous velocities of 300 kilometers per second, can happen at about one-quarter speed at a much more “modest” 70 km/sec. (Modest only comparatively; the speed is about six times greater than that needed to put a satellite in orbit.) The slower pace allows more time for fusible reactions to take place. The more benign implosion also means fewer unwanted materials from the collapsing liner mix into the fusion fuel, which would cool it and prevent fusion from occurring. By analogy, a child walking slowly in the ocean’s shallows stirs less mud than a vigorously running child.

Commenting on these phenomena, Sandia senior scientist Mike Campbell (1200) says, “This experiment showed that fusion will still occur if a plasma is heated by slow, rather than rapid, compression. With rapid compression, if you mix materials emitted from the tube’s restraining walls into the fuel, the fusion process won’t work; also, increased acceleration increases the growth of instabilities. A thicker can [tube] is less likely to be destroyed when contracted, which would dump unwanted material into the deuterium mix, and you also reduce instabilities, so you win twice.”

Says Matt more technically, “We demonstrated that the requirements for inertial confinement fusion can be dramatically reduced using insulating magnetic fields and laser pre-heating of the fuel. This allows us to substantially reduce the required implosion velocity of the target, which allows us to use targets that are more robust to instabilities. The magnetic field also reduces the fuel density requirement by several orders of magnitude.”

Besides the primary deuterium fusion neutron yields, the team’s measurements also found a smaller secondary deuterium-tritium neutron signal that was about a hundred-fold larger than what would have been expected without magnetization, providing a smoking gun for the existence of extreme magnetic fields.

The question had remained whether it was indeed the secondary magnetic field that caused the 100-fold increase in this additional neutron pulse, or some other,

still unknown cause. Fortunately, the pulse has a distinct nuclear signature arising from the interaction of tritium nuclei as they slow down and react with the primary deuterium fuel. This interaction, carrying with it a fingerprint of the influence of the magnetic field on the fusion process, was detected by the sensors of Sandia researchers.

A path to ‘high gain’ fusion conditions

 That is the subject of the theoretical paper “Understanding fuel magnetization and mix using secondary nuclear reactions in magneto-inertial fusion.” Using simulations, Paul Schmit (1684), Patrick Knapp (1688), et al confirmed the existence and effect of extreme magnetic fields via simulations. Their calculations showed that the tritium nuclei would be encouraged by these magnetic fields to move along tight helical paths. This confinement increased the probability of subsequently fusing with the main deuterium fuel.

Says Paul, “This dramatically increases the probability of fusion. That it happened validates a critical component of the MagLIF concept as a viable pathway forward for fusion. Our work has helped show that MagLIF experiments are already beginning to explore conditions that will be essential to achieving high yield and/or ignition in the future.” 

The foundation of Sandia’s MagLIF work is based on work led by Steve.

In a 2010 Physics of Plasmas article, Steve showed that a tube enclosing preheated deuterium and tritium, crushed by the large magnetic fields of the 25-million-ampere Z machine and a secondary magnetic field, would yield slightly more energy than is inserted into it.

A later simulation, published January 2012 in Physical Review Letters by Steve and Roger Vesey (1684), showed that a more powerful accelerator generating 60 million amperes or more could reach “high-gain” fusion conditions, where the fusion energy released greatly exceeds (by more than 1,000 times) the energy supplied to the fuel.

A paper led by Adam et al, published July 24 in Physics of Plasmas, further explicated and designed the experiments based on predictions made in Steve’s earlier paper.

But, says Mike Campbell, “There is still a long way to go.”

-- Neal Singer

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Evaluating powerful batteries for modular grid energy storage

Sandia has begun lab-based characterization of Transpower’s GridSaver, the largest grid energy storage system analyzed at Sandia’s Energy Storage Test Pad in Albuquerque.

by Stephanie Holinka

Sandia has begun lab-based characterization of Transpower’s GridSaver, the largest grid energy storage system analyzed at Sandia’s Energy Storage Test Pad in Albuquerque.

Project lead David Rosewater (6111) says Sandia will evaluate the 1 megawatt lithium-ion grid energy storage system for capacity, power, safety, and reliability. The Labs also will investigate the system’s frequency regulation, which grid operators need to manage the moment-to-moment differences between electrical supply and demand.

“Independent evaluations provide valuable feedback for industry efforts to standardize metrics for characterizing and reporting reliability, safety and performance. Companies need the standards to develop large procurement goals for grid energy storage because they must be able to compare performance and cost,” says David.

The data generated from characterizing a large system like GridSaver will improve operational models, identify technology or research gaps, and provide feedback to manufacturers to improve system performance, reliability, and safety. Additional specific tests will help validate Sandia’s grid energy storage characterization protocols, which have been developed jointly by industry and the national labs, as pre-standards to measure and express energy storage system performance.

“Industry needs these standards and they don’t yet have them. The protocol will give us critical information that can be used to compare flow battery systems, lead-acid battery systems, lithium-ion systems, and flywheel systems on an even field, apples to apples,” David says.

Utilities and other electricity and transmission providers and regulators often require that equipment be proved safe and reliable before it is permitted to operate on the electric grid. However, energy storage manufacturers and integrators are often unable to afford or provide the logistics necessary for this long-term testing and monitoring.

A complex integration of components

Sandia’s Energy Storage Test Pad and Energy Storage Analysis Laboratory test facilities validate manufacturers’ specifications of energy storage devices through characterization and application-specific cycle testing. They can also help users evaluate system parameters, including storage device efficiency, performance to specifications, reliability, and balance of plant operation.

David says national, state, and local policies that push for a cleaner, more secure electric grid are driving significant increases in variable renewable generation, but that makes the job of operators much more difficult. Storage helps mitigate that variability, when it’s safe, reliable, sustainable, and cost-effective. “Developing an energy storage system involves the complex integration of many components beyond just the battery, including sophisticated power electronics and controls — often communications. Sandia is assessing the entire system,” says Imre Gyuk, program manager in DOE’s Office of Electricity Delivery and Energy Reliability. The office has identified four challenges to the widespread deployment of energy storage: the cost of energy storage technologies (including manufacturing and grid integration), validated reliability and safety documentation, an equitable regulatory environment, and industry acceptance.

“Third-party evaluation of large systems like TransPower’s GridSaver can help break down the barriers to grid energy storage proliferation,” David says.

GridSaver was commissioned by the California Energy Commission’s Public Interest Energy Research (PIER) electric program. Sandia’s work is funded by DOE’s Office of Electricity Delivery and Energy Reliability.

-- Stephanie Holinka

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