Researchers from the U.S. Army Research Laboratory have demonstrated the possibility of predicting early fatigue damage behavior in structures via the study of the microscale mechanical behavior of the material.
The findings are considered to be a vital result for the structural health monitoring (SHM) community and could result in the development of new sensing techniques for predicting the service life of critical components.
ARL Researchers Daniel Cole, Ed Habtour, Tomoko Sano, Sean Fudger and Scott Grendahl collaborated with the University of Maryland for the study, which will be published in an upcoming issue in the journal Experimental Mechanics (presently available online). The work was invited to be published in a special issue of the journal focused on "Recent Advances in Nanoindentation".
The research may permit the SHM community to better understand precursors to damage in high value mechanical, civil and aerospace systems. The team employs an analogy to the primary care doctor performing a check-up on a patient: A doctor may be able to identify early warning signs of an illness by running a quick exam, like blood pressure tests or bloodwork. They may be able to prevent costly and more serious treatment down the road if the condition is diagnosed in time.
Many current SHM techniques are limited to detecting very late stage damage; in our doctor-patient analogy, the patient would already be in a critical state. Therefore, our group is interested in fundamental material behavior prior to damage in the conventional sense, such as a macro-scale fatigue crack.
Daniel Cole, Key Author
He explained that if the behavior of the material before the damage is better understood, this could result in vehicle structures that behave as sensors themselves, with the potential to report their health state and adapt to different conditions.
The Researchers exposed slender cantilever beams to vibratory loads in order to develop controlled fatigue damage precursors. In the rotorcraft community, the vibratory loading is considered to be a common design consideration, however, the effects on materials are yet to be well understood and frequently result in over-designed structures and also high maintenance costs. Subtle differences in the dynamic behavior of the beams were identified by the Researchers as the fatigue cycles increased, such as shifts in the natural frequency.
The Researchers characterized the material behavior of the early stage damage by using a technique called Depth Sensing Indentation (DSI) or "nanoindentation", whereby a submicron probe is forced into the sample while the displacement and load are monitored. The DSI tests allowed the team to target minimal volumes of material in particular locations on the structure and determine differences in the micro-/nanoscale mechanical behavior at varied stages of fatigue life.
The DSI tests showed that the material had become more compliant in specific areas of the structure exposed to comparatively high stress. Additionally, the Researchers discovered that the material response to the DSI test may offer an indication of service life before crack formation.
"The structure has inherent energy from the way in which it was processed. This energy gets consumed as the structure is repeatedly loaded: through dislocation motion, slip band formation, grain reorientation, etc. The DSI tests provide insight into the remaining useful life of the structure."
The Researchers accept that the DSI tests are not realistic as a direct sensing method in the field, but the basic understanding obtained from the study could help bring about new SHM techniques. Specifically, monitoring and analyzing the dynamic behavior of structures could help in providing a straightforward way for detecting early stage damage.
If we understand the linkage between the microstructural evolution and the effect on structural dynamics, we can let the structure report its health state. In some cases, the sensors to perform these measurements are onboard current Army vehicles; we may just need to analyze the data in a different way.
Ed Habtour, Co-author
The team employed an analytical model in order to relate the variation in local mechanical properties to the altering dynamics of the structure, which agreed in a much better manner with the experimental dynamics tests.
We are encouraged by these results because they provide a framework for linking the materials science and structural dynamics communities, which typically are not well connected fields.
Daniel Cole, Key Author
The team hopes that the multidisciplinary approach will be able to attract more Researchers in the SHM community in order to study early fatigue damage and eventually decrease the high costs for the DoD to sustain high value aerospace and mechanical systems.
Going forward, the team intends to leverage the research in order to enable self-responsive, maintenance-free, damage adaptive maneuver for ground and air vehicles of the future.