Scientists make discovery that may help predict damage precursors in future Army vehicles

February 10, 2015

By Joyce M. Conant, ARL Public Affairs

Story Highlights

  • Discovery would not only impact future Army aircraft, it would also help reduce DOD operations and sustainment costs and the time it takes for maintenance crews to perform mandatory inspections
  • One of the most promising techniques is the materials microcompliance
  • ARL aims at maturing these technologies to enable VRAMS capability and make it a reality for sustaining Army Future Vertical Lift aircraft in 2048

Researchers from the U.S. Army Research Laboratory made an important discovery during testing on metals that could impact the way "the structural health of critical components" on current and future military vehicles are deemed healthy and more efficient—saving operation and maintenance time and cost and prolonging the life of critical platforms.

The ultimate objective is to provide operators with economical and novel health-monitoring technologies for the next generation of Army Future Vertical Lift. ARL's approach utilizes preventative diagnostics in conjunction with innovative techniques to evolve current maintenance processes from a manual to an automated approach.

While vibration and nanomechanical testing on the materials' structures continues to be conducted, this discovery would not only impact future Army aircraft but would also help reduce the Department of Defense's operations and sustainment costs and the time it takes for maintenance crews to perform mandatory inspections.

Ed Habtour, an ARL researcher who specializes in nonlinear structural dynamics, and Dr. Dan Cole, a materials science researcher, share their approach to developing the basic science behind damage precursor detection. Currently, they are developing novel techniques and methodologies to monitor progression of structural property degradation that can become damage. Some of the approaches utilized at ARL's Vehicle Technology Directorate (VTD) to extract precursors to fatigue crack formation include changes in the microstructure, electrical resistivity, acoustic response, localized thermal response, and materials microcompliance.

According to the team, one of the most promising techniques is the materials microcompliance, which provides explanations to complex mechanical behavior encountered in the field.

"I was studying ways to simulate the dynamic response of structures exposed to harsh vibration loading seen in the battlefield," said Habtour. "Military structures operating in such environments respond in a nonlinear matter (they shake intensely, especially in rotorcrafts). Thus, I was using classical nonlinear dynamic theory to simulate the structural response. There was a small drop in the resonance frequency and an increase in the structural softening over time, but conventional sensors such as strain gages were telling us the structure was healthy."

Habtour studied the structure's fatigue using conventional ways (vibration) for more than a year. He said he knew the material was undergoing fatigue, but he could not see any microcracks—something was changing in the material. That is when he recruited the expertise of Cole, who used a new and different approach to test the strength of the material.

"Reaching out to Dan helped us to connect the world of systems dynamic to the micromechanics world. The outcome was the birth of a powerful technique that utilizes the interplay between the micromaterial properties and the nonlinear dynamic parameters. The missing piece in the dynamic model was the fatigue-damage-precursor component that was captured through micromechanics and detected by adding precursor to the nonlinear dynamic model," said Habtour.

Cole's research involves the study of the structure of materials down to the micro and nano levels.

"Ed approached me about the shift in the structural resonance and the increase in the structural compliance when the structure is exposed to harsh vibratory loads. We began discussing the possibility of changes in the state of the material at the microlevel that couldn't be picked up through conventional mechanical tests," Cole said. "We decided to explore the local properties of the structure in areas that were expected to have experienced relatively high stresses. We used a technique known as instrumented indentation or 'nanoindentation,' which mechanically samples very small volumes of material; indents were performed approximately 100 nanometers into the structure surface.

"We noticed a very clear trend that showed a more compliant response (material softening) near these highly stressed locations. The indentation elastic modulus dropped by approximately 40 percent with respect the rest of the structure, which was a huge surprise. In fact, we were pretty skeptical at the initial results and ran hundreds of additional tests to confirm this effect," said Cole.

Dr. Volker Weiss, a senior research scientist at ARL's VTD and professor emeritus at Syracuse University, finds the discovery promising.

"Among several damage and damage-precursor indicators, the structural-stiffness effects observed in this study appear most promising. Softening (or compliance) measurement may well become one of the methods of choice for remaining service-life prediction of critical structural components," said Weiss.

The next step is to better understand the changes in the microstructure that are leading to the compliance effect at the structure's surface.

"Ultimately, we want to use the local mechanical tests to fully understand the material state before, during, and after loading," said Habtour. "From there, we want to see if this technique can be applied to more complicated aerospace materials, such as current and future composite materials."

Dy D. Le, chief of the mechanic's division in VTD, expands more on the importance of this discovery and how it is expected to enable the Virtual Risk-informed Agile Sustainment (VRAMS) concept in the future military platforms. VRAMS aims at providing self-diagnostics and inspections; multiscale modeling to project the evolution of damage and maneuver capability; and reconfigurable maneuvers to keep the operating stress level at or below the stress-level threshold to avoid fatigue failure of critical aircraft components.

"The ability to identify and capture material-damage precursors is one of the key technologies included in the VRAMS core engine," Le said. "Once a damage precursor is identified and its characteristics captured, the aircraft's on-board intelligent system will provide information on how long it may take for a precursor to evolve through various stages of damage and, most importantly, when it may grow to catastrophic failure.

"ARL aims at maturing these technologies to enable VRAMS capability and make it a reality for sustaining Army Future Vertical Lift aircraft in 2048," said Le.

"The envisioned technology, VRAMS, powered by the ability to identify and capture material-damage precursors, when integrated into Army and commercial aircraft, not only can enable the ability to detect indication of impending failure at very early stage—potentially saving Soldier and passenger lives—but can also be used to sustain Army Future Vertical Lift and commercial aircraft at substantially lower maintenance costs," concluded Le.

A manuscript that details this research was recently submitted to the journal Structural Health Monitoring.

 

Last Update / Reviewed: February 10, 2015