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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
        • 0. Introduction
        • 1. Historical Perspective on Structural Integrity in the USAF
        • 2. Overview of MIL-HDBK-1530 ASIP Guidance
        • 3. Summary of Damage Tolerance Design Guidelines
        • 4. Sustainment/Aging Aircraft
          • 0. Sustainment/Aging Aircraft
          • 1. Widespread Fatigue Damage
          • 2. The Effect of Environment and Corrosion
        • 5. References
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
      • 6. Examples of Damage Tolerant Analyses
      • 7. Damage Tolerance Testing
      • 8. Force Management and Sustainment Engineering
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 8.2.1. Widespread Fatigue Damage

The Technical Oversight Group for Aging Aircraft (TOGAA) of the Federal Aviation Administration adopted the following definition of widespread fatigue damage (WFD) for aging aircraft [Lincoln, 2000]:

“The simultaneous presence of cracks at multiple structural details characterizes the onset of WFD.  These cracks are of sufficient size and density whereby the structure will no longer meet its damage tolerance requirement (e.g. maintaining required residual strength after partial structural failure).

Where damage tolerance is defined as follows:

Damage tolerance is the attribute of a structure that permits it to retain its required residual strength for a period of unrepaired usage.  It must be able to do this after it has sustained specified levels of fatigue, corrosion, accidental, or discrete source damage.  Examples of such damage are (a) unstable propagation of fatigue cracks, (b) unstable propagation of initial or service induced damage, and /or (c) impact damage from a discrete source.”

Current critical aircraft structures are designed to be damage tolerant.  The structure is designed to withstand failures or discrete source damage for a defined period of operation during which the damage will be detected.  For fail-safe designed structures, the analyses and tests for demonstrating fail-safety are based on the redundant or crack-stopping component to be essentially undamaged.  However, if an aging airframe is experiencing WFD, the remaining structure in the load path may not be capable of stopping the propagation of the damage.  Thus, WFD considerations shift the emphasis from the growth of a dominant, monolithic crack to the loss of fail-safety due to many small cracks.  This shift in emphasis has major ramifications with respect to the application of the ASIP damage tolerance process.

A damage tolerance criterion for scheduling inspections for WFD would need to be based both on the size of the cracks to be reliably detected and on the number and location of the cracks in the crack-stopping structure.  It has been shown that cracks on the order of 0.040 in. in the crack stopper can compromise fail-safety [Swift, 1987, 1992a, 1992b].  At present, the reliable detection of such small cracks, while possible, is cost prohibitive for the many details over the broad expanse of structure that would need inspection.  Further, the damage tolerance analysis process is essentially deterministic.  The loss of fail-safety can occur as a result of many combinations of crack sizes and locations in the crack stopper of the propagating damage.  The use of conservative, fixed-crack sizes in all of the crack stopper details would permit a deterministic analysis but would lead to unacceptably short inspection intervals.  Therefore, maintenance planning for WFD cannot be done with the ASIP damage tolerance process.

Since the aircraft can perform normal flight operations with WFD, its presence can easily be overlooked.  The problem for maintenance planning is to predict to onset of WFD so that repair, replacement, or retirement decisions can be made.  At present, there is no standard method for predicting the onset of WFD but structural risk analysis has been used in the decision making process.  The risk analysis objective is to determine the number of flight hours at which the probability of structural failure given a discrete source damage event exceeds a defined level.  For example, in a risk analysis of the C-5A, probability of failure given the discrete source damage greater than 10-4 was judged to be an unacceptable level of fail safety [Lincoln, 2000].  Risk analysis is discussed in Section 8.2.3, but it might be noted that predicting the growth of small cracks can play an integral part of risk analyses.

There are two general scenarios for WFD that affect fail-safety.  These are referred to as multiple-site damage (MSD) and multiple-element damage (MED).  MSD is usually considered to be fatigue cracking in multiple details of the same structural element.  A discrete source damage event (i.e. failure of an integral detail of an element) would raise stress levels in the remainder of the structural element.  The discrete source damage event could be caused by an external disturbance or by the sudden linking of cracks in the element.  An example of MSD leading to the loss of fail safety is provided by the failure in an Aloha Airlines Boeing 737 in April 1988.  The failure occurred after the airframe had experienced 89,960 flights.  Subsequent analyses have shown that the airframe had lost fail-safety at about 40,000 flights due to MSD (see NTSB [1989] and Lincoln [2000]).

In the MED scenario, fatigue cracking occurs in two or more multiple elements that support the same load path.  Failure of selected combinations of the elements may not lead to system failure, but the effects of the failures may well lead to load and geometry effects that do influence the integrity of the remaining structure.  An example of MED is provided by the fatigue cracking at WS-405 of the C-141 aircraft [Alford, et al., 1992].