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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
      • 6. Examples of Damage Tolerant Analyses
      • 7. Damage Tolerance Testing
      • 8. Force Management and Sustainment Engineering
        • 0. Force Management and Sustainment Engineering
        • 1. Force Structural Management
          • 0. Force Structural Management
          • 1. Force Structural Management Plan (FSMP)
          • 2. Loads/Environment Spectra Survey (L/ESS)
          • 3. Individual Aircraft Tracking (IAT)
        • 2. Sustainment Engineering
        • 3. References
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 8.1.2. Loads/Environment Spectra Survey (L/ESS)

As noted earlier, the initial FSMP is based on the design load spectrum with its corresponding stress sequences at the critical locations.  Experience has shown that the actual usage spectrum usually differs significantly from that anticipated at the design stage of development.  Accordingly, ASIP calls for a data collection and analysis program to ascertain the baseline usage spectrum of the fleet.  The results of L/ESS provide the data for checking design load assumptions and for updating the FSMP through new crack growth curves of updated damage tolerance analyses.  L/ESS does not directly impact decision making in the development of the FSMP.

The L/ESS objectives are met through the collection of time histories of sufficient parameters to characterize the magnitude, frequency, and order of the stresses being encountered at the monitored structural locations.  MIL-HDBK-1530 recommends that 100 percent of operational aircraft be instrumented to measure relevant load parameters but that the data from 10 to 20 percent of the fleet be used to capture valid operational loads spectra.  The assumption is made that the monitored flights are representative of all flights in a known stratification of usage such as type of mission or mission segment.  The collected data are compared to the design spectrum and analyses are updated as needed.  The L/ESS process continues through the life of the fleet so that data are available when change in usage dictates the need to update the damage tolerance analyses.

The L/ESS influences the FSMP through the damage tolerance analyses and analyses that feed the crack growth curves of the IAT.  When sufficient data have been collected from the L/ESS to begin to define a spectrum, it can be compared with the design data that were used to generate the IAT analyses.  Variations in the usage parameter distributions can be determined. Various comparisons can be made depending on the parameters being measured and analyzed in the tracking function.  It is noted that the IAT parameters typically comprise a subset of the L/ESS parameters. Exceptions occur when strains are used as IAT parameters but not used in the L/ESS.  Commonly, comparisons are made on the basis of differences in the load factor spectra.  If the L/ESS is representative of the force usage, then the comparisons should be within sampling variation.  If the spectra are significantly different, the L/ESS methods should be examined and possibly modified or the IAT methods should be examined and modified.