N102-113 TITLE: Innovative Capability to Quantify Fatigue Damage and Assessment of Endurance Limit in Spectrum Load Histories

TECHNOLOGY AREAS: Air Platform, Materials/Processes

ACQUISITION PROGRAM: PMA-261; H-53 Heavy Lift Helicopters Program

RESTRICTION ON PERFORMANCE BY FOREIGN CITIZENS (i.e., those holding non-U.S. Passports): This topic is "ITAR Restricted." The information and materials provided pursuant to or resulting from this topic are restricted under the International Traffic in Arms Regulations (ITAR), 22 CFR Parts 120 - 130, which control the export of defense-related material and services, including the export of sensitive technical data. Foreign Citizens may perform work under an award resulting from this topic only if they hold the "Permanent Resident Card", or are designated as "Protected Individuals" as defined by 8 U.S.C. 1324b(a)(3). If a proposal for this topic contains participation by a foreign citizen who is not in one of the above two categories, the proposal will be rejected.

OBJECTIVE: Develop and demonstrate an innovative approach to quantify damage from high frequency load content in long spectrum load histories through an analytical model.

DESCRIPTION: The extent of damage contributed by small amplitude cycles in the spectrum loading histories of aircraft and rotorcraft structures is not fully understood; it is either ignored completely or a conservative damage is assigned based on projected extensions of stress (strain)-life curve. Recently it has been shown that small amplitude cycles below the endurance limit cause surface and subsurface damages in aluminum and steel, respectively. Furthermore, crack growth has also been shown to grow below the traditional (delta) Kth levels. There is usually a discrepancy observed in analytical damage predictions and service findings of such cracks. This is partly due to ignoring damage contributions from random frequency load cycles that appear in service load histories such as buffet, gust cycles, or vibratory and acoustic phenomenon.

As the analytical life prediction capabilities and laboratory testing have improved significantly over the last two decades, it is important to assess and accurately quantify damage for both high frequency and small amplitude loads in service spectrum for better predicting service cracking and remaining life. Recent advances in very high frequency (Giga)cycle fatigue test capability, and NAVAIR efforts to develop testing capabilities for high frequency random loading may make it possible to develop analytical approaches for predicting fatigue life under very high frequency random loading that are based on representative test data. Furthermore, the effect of surface conditions on fatigue and crack growth life predictions under these long spectrum load histories with the above content needs to be further investigated in detail. Therefore, there is a need for an analytical method of predicting and modeling the fatigue lives of aluminum and steel used for airframe structural components under high frequency random vibratory loading.

PHASE I: Develop an innovative approach to quantify fatigue damage and crack growth in spectrum loads containing loads of high and low frequency and magnitude. Demonstrate the feasibility of such an approach to show how a program including fatigue tests, model and algorithm development efforts will be implemented.

PHASE II: Provide practical implementation of the recommended approach developed under Phase I. Evaluate the approach through verification and validation of the analytical model by performing a rigorous series of both laboratory and service tests. This testing must encompass major aluminum, steel, and titanium alloys used in aircraft industry.

PHASE III: Transition the analytical methodology to platforms that would benefit from modeling of high cycle loaded fatigue.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The analytical methodology to calculate and predict remaining lives will apply to aluminum, steel and titanium alloys; all of which are used in the public and private sector. This process will increase the safety of the aircrafts flown for commercial and military use. Accurate predictions will also reduce material cost for manufacturing aircraft components.

REFERENCES:
1. Bathias, C. (2000) "Gigacycle fatigue of high strength steels prediction and mechanisms," European Structural Integrity Society, Volume 26, pp. 163-171. Available at www.sciencedirect.com.

2. Marines, I., Bin, X. & Bathias C. (2003) "An understanding of very high cycle fatigue of metals," International Journal of Fatigue, Volume 25, Issues 9-11, September-November, pp. 1101-1107. Available at www.sciencedirect.com.

3. Murakami, Y., Yokoyama, N.N. & Nagati, J., (2001) "Mechanism of fatigue failure in ultra long life regime", Fatigue in the very high cycle regime, Stanzl-Tschegg S, Mayer H, Eds. Institute of Meteorology and Physics, Austria.

4. Sadananda K. and Vasudevan A.K. (1997) "Short Crack Growth Behavior," Fatigue and Fracture Mechanics: 27th Volume, ASTM STP1296, r.S. Piasik, J.C. Newman, and N.E. Dowling, Eds., American Society for Testing and Materials, 1997, pp. 301-316.

5. Sakai, T., Sato, Y. & Nagano, Y. (2002) Fatigue & Fracture of Engineering Materials & Structures, Vol.25, pp.765-773.

KEYWORDS: Fatigue Strength; Endurance Limit; Crack Propagation; Stress; Strain; High Cycle

** TOPIC AUTHOR (TPOC) **

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