Studying materials at the breaking point

LASER WELDS — Researcher Brad Salzbrenner (1831) tests laser-welded objects in pure tension, shear, and mixed tension-shear loading. One pilot study in Sandia's long-term Predicting Performance Margins program involves laser welds, which are widely used in engineered systems. (Photo by Randy Montoya)

by Sue Major Holmes

Humans have been using metals for thousands of years, but there’s still a lot about them that isn’t fully understood. Just how much stretching, bending, or compression a particular metal will take is determined by mechanical properties that can vary widely, even within parts made of the same material.

Sandia is working to fill gaps in the fundamental understanding of materials science through an ambitious long-term, multidisciplinary project called Predicting Performance Margins or PPM. From the atomic level to full-scale components, the research links variability in materials’ atomic configurations and microstructures with how actual parts perform.

PPM aims to identify how material variability affects performance margins for an engineering component or machine part. The goal is a science-based foundation for materials design and analysis — predicting how a material will behave in specific applications and how it might fail compared with its requirements, then using that knowledge to design high-reliability components and systems. Materials include such things as alloys, polymers, or composites; components are switches, engines, or aircraft wings, for example, while systems can be entire airplanes, appliances, or even bridges.

Safer, more reliable vehicles, machines hinge on how materials perform

Understanding materials reliability and performance at the fundamental materials science level isn’t important just to Sandia’s national security missions. Performance is crucial to safety and reliability in spacecraft, bridges, power grids, automobiles, nuclear power plants, and other complex engineered systems.

The PPM approach has become a prototype for tackling other difficult materials issues. Materials science researchers recently used the approach in a proposal to understand brittle materials, establishing a multidisciplinary project that develops the fundamental science while delivering improvements to those who use these materials during the life of the project. That way, they don’t have to wait years to reap benefits from the fundamental work. Future studies that could benefit from the approach include the aging of polymers and foams, friction between electrical contacts, and failures in glass-to-metal seals and in solders and interconnects.

“Too often, we are unable to predict precisely how a material will behave, and instead we must rely on expensive performance tests,” says program manager Amy Sun (1814). “Capturing variability by tests alone is too expensive and not predictive.”

PPM simultaneously tackles fundamental materials science issues at the atomistic and microstructural scale and engineering problems at the macroscopic scale.

Success requires connecting the two extremes. “The research focuses on where the scales connect — where the atomistic level and a single crystal intersect and where the crystal level and the component level intersect — to predict collective behavior,” says lead investigator Brad Boyce (1831).

Researchers examine how and why metals deform so they can predict that behavior and ultimately make metals stronger. Better understanding could lead both to better materials and improvements in processing materials. “It’s one thing to predict failure. It’s another to make metals better so they don’t fail,” Brad says.

PPM draws on expertise Labs-wide

Sandia’s core mission of nuclear weapons stewardship and national security requires it to meet the highest standards. “Few places in the world are asked to guarantee lifetime performance of complex engineering systems,” Amy says. But at Sandia, “we have to put a label on, ‘Best used by,’ and when we do, what are the scientific data that back our claims? We have to support our results with sound, quantitative evidence.”

PPM draws on expertise across Sandia’s campuses in New Mexico and California to study materials’ behavior at different scales, applying materials science, engineering science, and physical, chemical, and nanoscience. PPM researchers use such advanced characterization techniques as 3-D microscopy and focused ion beam and digital image correlation, as well as quantum and atomistic simulations and mesoscale material mechanics.

From the bottom up, the program studies how atoms undergo rearrangement that initiates defects in response to mechanical stresses and strains (nanoscale); how these crystalline defects evolve, multiply, and interact (mesoscale); and finally, how an ensemble of polycrystals works in concert to govern deformation and failure of a component (macroscale).

At the top end, PPM looks at how manufacturing processes determine the microstructure of a material and examines test data and failure statistics to better understand the relationship between microstructures and how engineering materials perform.

Laser welds better understood through pilot study

One pilot study involves laser welds, widely used in engineered systems. Weld performance can be unpredictable because a weld’s microstructure isn’t homogenous and geometric imperfections such as cracks and pores can be introduced in the welding process. The aim is to understand a basic engineering question: how the microscopic variability of a weld impacts the mechanical reliability of a welded component.

“We could say, ‘If you weld it with this margin of overdesign, you’re probably OK, you’re probably safe.’” Amy says. “But as a materials scientist, you’re not going to be happy with that answer.”

Instead, material scientists and system designers want to predict the effects of porosity on such weld properties as strength, ductility, and toughness. “How do these little microscopic pores or any kind of imperfections in your microstructure affect the actual material properties?” Amy asks. “How do we measure that? What is that fundamental mechanism for the pore to start failing?”

PPM studies pure metal. Knowing how atoms in crystals interact allows researchers to calculate inter-atomic forces. The calculations are used to predict how single crystals of the metal will respond to external stresses, and in turn, the how engineering materials, such as aggregates of small single crystal grains, will perform. While it’s not possible to model every atom in engineering-scale codes, “if you understand the atomistic effects well, you can approximate it very accurately in the high-level codes,” says senior manager Rick McCormick (1110).

Researchers use such tools as transmission electron microscopy to view individual atoms while bending and breaking tiny components or parts. “They can see what’s happening to these grains and these boundaries, how the stresses build up. They can see it at a microscale, where they’re looking at aggregates of grains,” Rick says.

Atomic-size crystals to large-scale testing covered by PPM

Then they scale up, running experiments on bigger components at Sandia’s large engineering science test facilities.

When PPM began in 2010, it didn’t take long to assemble a core multidisciplinary team of about 20 staff members and postdocs augmented by a large network of people doing interrelated projects, the “friends of PPM.”

“Everyone wanted to work on this problem,” Amy says. “It was so interesting and relevant to pretty much everything we do.”

The work is part of Sandia’s science-based nuclear stockpile stewardship mission, Rick says.

PPM already has had an impact. On the engineering side, nuclear weapons programs are using PPM expertise to help component engineers understand how to better design and qualify structural components and modify material processes such as recrystallization to achieve improved performance. In fundamental science, 20 PPM articles have been published in peer-reviewed scientific journals. PPM team members have given more than 100 presentations and international conferences have had sessions on PPM-related topics.


-- Sue Major Holmes

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Sandia, Hawaii Hydrogen Carriers partner on hydrogen storage system for forklifts

Adrian Narvaez of Hawaii Hydrogen Carriers (HHC) observes a metal hydride storage tank, part of a project led by Sandia. The tank will be combined with a fuel cell system to make a fuel cell power pack to power a forklift. Hydrogen fuel cell technology could have a significant advantage over batteries for a large slice of the $33 billion US forklift market.  (Photo by Dino Vournas)

by Holly Larsen

Zero-emission hydrogen fuel cell systems soon could be powering the forklifts used in warehouses and other industrial settings at lower costs and with faster refueling times than ever before, courtesy of a partnership between Sandia and Hawaii Hydrogen Carriers (HHC).

The goal of the project is to design a solid-state hydrogen storage system that can refuel at low pressure four to five times faster than it takes to charge a battery-powered forklift, giving hydrogen a competitive advantage over batteries for a big slice of the clean forklift market. The entire US forklift market was nearly $33 billion in 2013, according to Pell Research.

“Once you understand how these forklifts operate, the fuel cell advantage is clear,” says Joe Pratt (8366), who is leading the project for Sandia. Refueling hydrogen fuel cell-powered forklifts takes less than three minutes compared to the hours of recharging needed for battery-powered forklifts. Consequently, forklifts are able to operate continuously for eight or more hours between fills.

“If hydrogen refueling is short enough to occur during normal downtimes, such as during operator breaks, then a single hydrogen forklift can do the work of three battery packs over the course of 24 hours. That translates into a direct cost savings,” Joe adds.

Currently, companies using battery-powered forklifts need to purchase three battery packs for each forklift to ensure continuous operation. They also need to set aside warehouse space for battery recharging.

Sandia has worked with the fuel cell forklift industry for several years to help get clean, efficient, and cost-effective fuel cell systems to market faster. Standards developed by Sandia soon will be published so industry can develop new, high-performing hydrogen fuel systems for industrial trucks.

Department of Energy grant leads to collaboration

Intrigued by the potential benefits of fuel cells over the electric batteries that now power most forklifts, HHC obtained a grant from the Energy Department’s Fuel Cell Technologies office and asked Joe to help improve the design of a hydrogen storage system for fuel cells.

Joe has spearheaded other Sandia efforts to introduce hydrogen systems into the marketplace. He served as technical lead, for instance, for studies on the use of fuel cells to power construction equipment, personal electronic devices, auxiliary equipment, and portable generators. Most recently, he led a study and subsequent demonstration project on commercial use of hydrogen fuel cells to provide power at ports (see the Feb. 21, 2014, issue of Sandia Lab News).

For its part, HHC is developing technologies for the fuel cell forklift market and expects cost reductions and performance improvements that will help the market grow. The company is developing a low-pressure hydrogen storage system that can be refueled at standard industrial gas pressures. That should reduce fuel system cost and expand the market to facilities that can’t accommodate conventional high-pressure fueling systems.

To solidify the forklift collaboration, HHC sent Adrian Narvaez to Sandia’s Combustion Research Facility in California for several months. “Joe and I work together every day on the design, so it’s a huge advantage to be able to work on site at Sandia,” says Narvaez.

Technical, economic barriers to overcome

Today’s hydrogen storage units require high pressure (5,000 pounds per square inch, or psi) to achieve a short refueling time — and high-pressure refueling requires an on-site compression system. “That can be a big expense, especially for a small company,” Narvaez explained. “If we can provide a storage system that meets the target refueling time at, say, 500 psi, companies can get a break in the up-front costs. Plus, they no longer have to purchase battery rechargers or dedicate space for recharging. Instead, companies can simply purchase and store hydrogen tanks as needed.”

Designing a storage system that meets HHC’s specifications and can be integrated into a fuel cell power pack requires overcoming some key challenges. Among these are identifying optimal metal hydride materials, determining an optimal shape and size for the storage tank and ensuring thermal management to achieve and maintain the temperatures required for fast refueling and supply of the hydrogen.

Work to identify the right metal hydride for the system has focused on Hy-Stor 208, a misch metal-nickel-aluminum alloy that meets targets for hydrogen storage capacity, density, and thermal conductivity. The material also provides sufficient hydrogen pressure for refueling at an operating temperature of 60 degrees Celsius.

While this type of metal hydride is heavy, the weight acts as needed ballast and thus is a benefit in forklifts. To increase thermal conductivity, the team also explored adding to the metal hydride two forms of expanded natural graphite, flakes and so-called “worms” because of their tubular shape.

Joe and Narvaez drew on modeling and simulation results from an earlier project led by Sandia engineer Terry Johnson (8366) to identify a small-diameter tube as the best design for storing the metal hydride (see the May 22, 2009 issue of Sandia Lab News). They then varied several tube characteristics, such as the hydrogen distribution channel and the amount and type of thermal enhancement material used. Next, they conducted experiments to evaluate the effects of these variations on a range of performance parameters, including hydrogen storage capability, refill time, durability,  discharge ability and residual capacity at a minimum discharge point.

“As the models predicted, we saw only minor differences in performance when we varied the graphite types. Likewise, the presence or absence of the hydrogen distribution channel had little effect on performance,” says Narvaez. “These findings show that this application is not aggressively pushing the performance of the metal hydride storage to the point where these variations would make a difference. In fact, this is good, because it means we can use the lowest-cost solution and still expect good performance.”

Using findings from their experiments, Joe and Narvaez developed an optimized storage-system design.

More incentives to switch to fuel cell technology

During this time, the team also began to conceive of a tube array that would allow efficient thermal management, to be achieved via water flows around the tubes.

With Sandia’s and HHC’s design complete, project activity will transfer to Hawaii, where HHC will produce the first prototype metal hydride storage system. HHC will work with Canadian fuel cell company Hydrogenics, which will integrate the new storage system into its proton exchange membrane (PEM) fuel cell power pack, designed to fit into a forklift.

“DOE catalyzed the market for fuel cell forklifts, using industry cost-sharing to deploy more than 500 units through the American Reinvestment and Recovery Act,” says Joe. “The private sector recognized the advantages of fuel cell forklifts, and deployed more than 5,000 additional units since then without government funding. If successful, the HHC project will lead to lower cost, improved-performance fuel cell forklift systems that will lead to even greater market growth.”



-- Holly Larsen

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