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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
        • 0. Analysis Of Damage Growth
        • 2. Variable-Amplitude Loading
        • 3. Small Crack Behavior
        • 4. Stress Sequence Development
          • 0. Stress Sequence Development
          • 1. Service Life Description and Mission Profiles
          • 2. Sequence Development Techniques
          • 3. Application of Simplified Stress Sequences for Design Studies
        • 5. Crack Growth Prediction
        • 6. References
      • 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 5.4.1. Service Life Description and Mission Profiles

The load sequence developer works from the service life requirement summary and the mission profiles as given by the aircraft procurement documents.  The service life data contains the total flight hours, expected calendar year life, number of missions to be flown, identification of mission types, and number of touch and go and full stop landings.  The mission profile description provides the time variation of the airspeed, altitude, and gross weight such as illustrated in Figure 5.4.1.  Each mission is divided into segments, as shown, which can be easily characterized by the type and frequency of the various load sources.

Figure 5.4.1.  Mission Profile and Mission Segments

The load spectrum for each mission segment is characterized by a table of occurrences of a load parameter.  The commonly used parameter is the normal load factor at the aircraft center-of-gravity, nz.  Such a table can be presented as an exceedance plot, which shows the number of occurrences that exceed specified values during a specified time period. 

MIL-A-8866 presents tabular exceedance data for six classes of aircraft, broken out by mission segment.  The number of identified segments varies from three to seven.  These tables give the number of exceedances per 1,000 mission hours.  The total number of exceedances is on the order of 105 - 5 x 105.  Figure 5.4.2 shows a plot of the composite maneuver spectrum for the six classes of aircraft.  This composite was made by summing the exceedances of the mission segments for each class of aircraft.


Figure 5.4.2.  Maneuver Spectra According to MIL-A-8866

These three basic pieces of information, the service life summary, the mission profiles, and the load factor spectra are converted into the loads history and the stress history at critical locations on the aircraft.  This procedure is briefly described in the next section.