Sandia LabNews

Failure the only option at the Mechanics Lab


Helena Jin and Kevin Nelson

BREAK TIME — Helena Jin and Kevin Nelson (both 8526) inspect the test setup for upcoming experiments to determine the breaking strength of weapon case lugs. (Photo by Dino Vournas)

For most people, breaking something is unplanned and unwelcome. But for Bonnie Antoun (8256) and the rest of the Micromechanics & Materials Mechanics Experimental Facilities staff, also known as the Mechanics Lab, it’s all in a day’s work.

Bonnie and the rest of the staff — Wei-Yang Lu, Bo Song, Helena Jin, Kevin Connelly, Andy Kung, and Kevin Nelson (8256) — will stretch, squeeze, torque, heat, cool, and pound any material to failure. Material systems of interest include metals, ceramics, structural foams, polymers, and composites.

They do this using equipment that can apply from 2 million pounds of load to less than 1 micronewton (µN) of load or 1 micrometer (µm) displacement; apply load at a rate of 220 inches per second; enforce strain rates of 1,000-5,000 per second; and capture it all with high speed and ultra-high speed optical cameras, and high-speed, high-resolution thermal imaging cameras.

To study the effects of complex stress history, Bonnie might subject a specific material to stress and twisting on the MTS 100 Kip axial and torsional test frame. “To see the effects of deformation history, we would apply tension and torsion to failure, then reverse the order on another sample, and continue repeating the experiment with different configurations,” she says.

Other equipment includes Hopkinson Bar test systems for testing materials at very high strain rates, flexible Endura Tech axial/torsional systems, atomic force microscope (AFM), scanning electronic microscope (SEM) loading stage for in-situ experiments under microscopes, and an extensive variety of loading frames and diagnostic equipment.

“All of our work is toward development of constitutive models that describe materials as a function of loading, rates, temperatures, environments, and other conditions,” Bonnie says.

The Mechanics Lab works closely with modeling and simulation in designing experiments. The physical experiment must exactly match the modeling boundary conditions so that the data can be used to then validate the model. 

“Our end-users are the material modelers, finite element analysts, and, finally, the weapons component and systems engineers,” she says. “Ultimately, we’re always trying to improve our understanding and modeling of complex events. As modeling and simulation capabilities have grown, they can handle more information. We are moving toward more volume measurements to study what goes on beneath the surface.”

Assessing the accuracy of models

Lu and Helena are developing X-ray computed tomography (XCT) techniques to understand what happens inside a material as it is loaded to failure. This new experimental capability is necessary to keep pace with advances in modeling and simulation. The Mechanics Lab hopes to acquire a high-resolution XCT device to gain more data for the modelers from these experiments.

Arthur Brown (8259) often turns to the Mechanics Lab for projects with experimental needs for material characterization, validation, or both. “They have helped me populate constitutive models for metals and composites over various loading rates and temperatures.  They have also performed complex experiments that I have used to assess the accuracy of model predictions,” he explains.

The Mechanics Lab may be physically located at the California site, but the group works equally with New Mexico organizations. The lab has teamed with the solid mechanics staff of the Engineering Sciences Center (1500) for many years.

“We have formed a uniquely integrated team of analysts and experimentalists, all working toward a common goal to better understand, design, assess, and predict physical response characteristics of our components and systems for many environmental threats,” says Frank Dempsey (1526).

“With computers getting larger and faster, enhancements to our predictive physics code capabilities require experimental validation. The Mechanics Lab staff is now an essential part of the analyst community with increasingly more challenges to predict, assess, design, and capture the physics correctly.”

About 75 percent of the group’s work is nuclear weapons-related. This has led to development of specific diagnostics and capabilities.

“There are few industries where you need to test to completely quantify materials to failure,” says Bonnie. “We need to understand not just when and how materials fail, but every step along the way.”

Much of the rest of the group’s work is Work for Others. In 2003, the Mechanics Lab played a key role in the space shuttle Columbia accident investigation (See the Sept. 5, 2003, issue of Sandia Lab News.)

Lu, Bonnie, and John Korellis (8254-1) led studies on the reinforced carbon-carbon (RCC) panels, thermal protection system (TPS) tiles, and foam impacting materials that provided data for the material response models critical to the computational studies. This led to a follow-on project with NASA for return-to-flight testing.

The Mechanics Lab has also worked on projects for the US Army, the US Navy, and DOE (non-nuclear weapons), and has been involved in several CRADAs, including Goodyear. In 2009, the group was involved in a DOE project to study liquid natural gas (LNG) cascading damage. “One concern is what would happen to the integrity of the tankers if there was a fire followed by rupturing of the tanks. The LNG is extremely cold, so it would quench the hot tanker steel immediately,” Bonnie says.

Currently, the group is studying potential materials for tubes on solar receivers for DOE. This project is unusual because of the duration of each experiment.

“Many of our experiments are finished in less than a minute,” says Bonnie. “The material in the solar receivers would be used for 30 years, so this is a test that will be going on for weeks.” To speed the aging process, researchers are imposing day and night cycles on materials of about a minute each.

The experiments may be fast, but the design and set-up are not. A new experiment can easily be more than a year in the making. Setting up experiments is a painstaking process as special fixtures often have to be designed to handle unusually shaped components.

Still, Bonnie says there is never a dull day in the Mechanics Lab.

“There is a balance between developing our own capabilities for the future and working on experiments happening right away,” she says. “Our work is all about helping others do their jobs better.”