N14A-T002 TITLE: Innovative Unified Damage Mechanisms-Based Model to Predict Remaining Useful Life for Rotorcraft Structures

TECHNOLOGY AREAS: Air Platform

OBJECTIVE: Develop an innovative damage accumulative model that quantifies structural damage on rotorcraft structures and predicts remaining useful life.

DESCRIPTION: Naval rotorcraft operate in environments that include harsh loading conditions and corrosive elements, both of which factor into the accumulation of structural damage on rotorcraft components. Due to the limited amount of procurement options for new naval rotorcraft, legacy platforms are required to operate beyond their original design life, incurring costs for fleet maintenance, operation, and support. Maintainers are faced with getting the most usage out of rotorcraft components but in many cases are required to retire components early because the calculated usage life has been expended. Presently, a safe-life approach is utilized to manage structural component life and is determined using S-N curves, flight regime loads from the mission spectrum, and the rate of occurrence for each flight regime. Therefore an innovative solution is required to accurately characterize the actual fatigue damage which has accumulated on rotorcraft components in order to ensure optimum use of structural components while assuring safety. Of particular interest is determining high cycle, low amplitude fatigue damage commonly encountered in rotorcraft operating environments and not easily captured by current rotorcraft damage modeling methods. The factors examined would be the predicted number of cycles before the onset (initiation) of a crack (for metals) or delamination (for composites) would be the metrics. Then the factors would be the predicted number of cycles for the propagation of the crack/delamination. These numbers will be validated through test.

Current work to move away from maintenance based on flight time to conditional maintenance such as the Integrated Hybrid Structural Monitoring System (IHSMS) require load tracking while in flight, which require sensors to be located on specific components or areas. Recent developments in fatigue damage prognosis reveal an intimate connection between accumulated fatigue damage and measured changes in surface temperature and acoustic emission signature. Temperature sensors and piezoelectric acoustic sensors may be leveraged to measure these changes, and it could alleviate the need to track loads for each component. Magnetic properties, electrical resistivity, and thermal conductivity have also been shown to change with the accumulation of fatigue damage. New analysis techniques offer the potential to bridge thermodynamic and mechanical principles for damage accumulation at the micro scale to macro scale crack formation.

An innovative unified damage model is therefore desired that can be used to predict remaining useful life. In the long term, the model should be able to take into account progressive damage, multiple damage mechanisms, modes of failure and multiple loadings including mechanical, electro-chemical and thermal. The model should be physics based vs. phenomenological and rely on measureable and quantifiable parameters. The model should measure one or a combination of these parameters such as temperature changes of acoustic emission readings, and relate them to accumulated damage and remaining useful life through sound analytical methods without requiring prior usage history or data.

PHASE I: Develop an innovative unified damage accumulation model with sound physical basis for estimating remaining useful life. Demonstrate the application of this model to constant amplitude loading for an aircraft structural material. Constant amplitude is an input. It is a fixed interval of loading that is pre-determined by the tester.

PHASE II: Demonstrate the application of the unified damage accumulation model to variable amplitude loading. Compare the model predictions with experimental measurements.

PHASE III: Develop a prototype model for field applications. Conduct field studies to improve sensitivity of the model, and develop user-friendly software using the proposed model for remaining life predictions. Then transition software to the Naval platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: All aircraft have dynamic components which are subject to fatigue damage although the rate of damage may differ depending on the loading. The model can be used on any aircraft, whether it be military or commercial, to determine the remaining life of a specific component to have it serviced before a failure occurs.

REFERENCES:
1. Amiri M, et al., 2011, January. "An experimental approach to evaluate the critical damage", International Journal of Damage Mechanics, Vol. 20. http://ijd.sagepub.com/content/20/1/89.abstract.

2. Amiri M, et al., 2010, "Rapid determination of fatigue failure based on temperature evolution: fully reversed bending load", International Journal of Fatigue 32, pp. 382�389. http://www.researchgate.net/publication/228453827_On_the_Role_of_Entropy_Generation_in_Processes_Involving_Fatigue.

3. Bourchak M, et al., 2007, "Acoustic emission energy as a fatigue damage parameter for CFRP composites", International Journal of Fatigue 29, pp. 457-470. http://scholar.google.com/citations?user=G09SkVgAAAAJ&hl=en.

4. Fatigue Mechanisms: "Advances in quantitative measurement of physical damage", 1983, ASTM Spec. Tech. Publication # 811. http://www.astm.org/DIGITAL_LIBRARY/STP/SOURCE_PAGES/STP811.htm.

5. Naderi M, et.al., 2010, "On the thermodynamic entropy of fatigue fracture", Proceeding of the Royal Society, A, p. 466, pp. 423�438. http://rspa.royalsocietypublishing.org/content/466/2114/423.full.

6. Yong Z, et al., 1993, "Slowing down metal fatigue damage with a magnetic field", Engineering Fracture Mechanics" Vol. 46, No. 2, pp. 347-352.http://www.sciencedirect.com/science/article/pii/0013794493902954