Broad Agency Announcement (BAA)

FY06 MURI Topic #11

This BAA was announced on May 17, 2005.

Submit white papers and proposals to the Air Force Office of Scientific Research.

Health Monitoring And Materials Damage Prognosis For Metallic Aerospace Propulsion And Structural Systems

Background: Sustainment and life-cycle engineering of aircraft and their propulsion systems represent major and growing challenges for the Air Force. The aging of the legacy aircraft fleet threatens to drive maintenance costs to unprecedented levels and to consume budget that would otherwise be available for operation and modernization. For example, approximately two-thirds of the current Air Force budget for turbine engines is consumed by the cost of sustainment. In addition, safety and reliability are constant challenges, as exemplified by the danger of high cycle fatigue of rotor blades in turbine engines and cracking of airframes, which currently prompt frequent and expensive inspections. For some time, operators have called for health monitoring for aircraft and propulsion systems that can query the integrity of structural materials and components, enabling real-time monitoring, reducing the need for costly tear-down inspections, and enabling much more efficient operation and maintenance practices. More recently, it has been proposed that health monitoring can be made sensitive to small changes (e.g. cracking and creep) in the state of a structure and that it should be possible to forecast the useful life remaining in a structure, rather than being limited to a diagnostic system that warns only when the structure is no longer reliable.

There are significant technical challenges to realizing this vision, however. Even large cracks are difficult to detect in complex structures in noisy environments, and there are major uncertainties in the capability to predict the growth of critical material damage that evolves under realistic loading and environmental conditions. For example, when using Lamb-wave or modal-based vibration techniques, there are tradeoffs in damage-detection sensitivity between the area of coverage (better at low frequencies) and the crack-size sensitivity (better at high frequencies). It is known that aircraft have certain “hot spots” for failure, and that great cost savings could already be realized by monitoring only these locations. The proper identification of a sufficient set of these critical spots is a problem in and of itself. Signal processing for proper damage characterization in a noisy environment is another challenge, as are the problems posed by complex geometries where a query signal will be reflected and refracted in complex ways. Some have proposed schemes where baseline signals are measured so that only changes from the baseline are considered. Aging, environmental conditions, and loading will effect changes in that baseline – how does one distinguish these from damage?

True prognosis poses further challenges. Early identification of problems requires that one record and process large volumes of data regarding incipient cracks, as well as histories on loading and environment and to integrate this information with materials damage models and autonomic logistics methods. Crucially, one must have the physical and mechanistic understanding to predict future structural integrity based on the present component condition and projected loading and environmental conditions.

Objective: The objective of this program is to develop basic science needed to enable metallic material and structural health prognosis for turbine engines and aircraft, and thereby facilitate continual assessment and prediction of the current and future health of the flight systems. The ultimate goal is the development of quantitative and probabilistic models that relate material-level microstructural and damage events to system-level structural performance.

Research Concentration Areas: The achievement of the program objectives will require a highly integrated approach linking three basic elements:
(1) methods for in situ interrogation of the damage state of a material, such as that from fatigue and/or creep, in a complex structure with the presence of noise;
(2) physically-based models of the formation
and growth of material damage under realistic loading; and (3) coupled state-awareness and life models, including probabilistic and uncertainty approaches.
A successful effort will require basic research into fatigue crack initiation and growth, as well as the multiscale modeling required to interpret the effects of atomistic processes from a system level viewpoint. Not only do we need better deterministic models, but we also require models that can account for uncertainties in loading state, material properties, and modeling accuracy. The tractability of large computational models also must be addressed. The combined resources of the fields of materials science, mechanical/aeronautical engineering, and applied mathematics will be required to address these problems. This will not be a sensor development program.

Impact: The basic science and technology produced by this initiative will provide revolutionary understanding, capability, and models for damage-state awareness and life prediction of materials and complex structures. This will enable major reductions in the cost of sustainment of current and future aircraft and turbine engines, while providing new capabilities to improve safety and reliability. This effort will result in fundamental scientific understanding that will enable the prognosis of failure in any complex structure that can fail from cyclic loading, from aircraft to space vehicles.

Research Topic Chief:
Capt Clark Allred, AFOSR,
Phone: (703) 696-7259

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