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DTD Handbook

Handbook for Damage Tolerant Design

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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 8.1.1. Force Structural Management Plan (FSMP)

The FSMP is a schedule for performing the maintenance actions necessary to maintain structural integrity throughout the life of a fleet.  In principle, the FSMP provides the Office of Primary Responsibility (OPR) sufficient detail for the establishment of budgetary, structural integrity and maintenance plans.  The FSMP is initially based on design usage and is updated whenever significant changes occur in the fleet environment/stress histories.  Such changes are detected through the data of the Loads/Environment Spectra Survey (L/ESS) elements.  To maintain the airworthiness of the individual aircraft, the FSMP is keyed to the data generated under the Individual Aircraft Tracking (IAT) element.  Figure 8.1.3 is a schematic from Berens, et al. [1981] depicting the relation between the damage tolerance analyses, the operational data collection and analysis programs and FSMP.

 

Figure 8.1.3   Relation of FSMP to IAT and L/ESS Elements of ASIP

The FSMP should contain:

1)      all of the anticipated inspection, repair, and modification actions,

2)      the critical locations and the crack sizes that trigger the required maintenance actions for individual airframes, and

3)      supporting data required for the procedures of the Air Force Technical Order System.

The critical locations and critical crack sizes are the key items of the damage tolerance approach to structural integrity.  Figure 8.1.4 is a generic schematic for the process of determining inspection intervals for a monitored location for three or more inspection cycles.

Figure 8.1.4   Schematic of Inspection Interval Determination

Inspections for safety are scheduled at one half of the flight hours for an assumed initial crack to grow to critical in the anticipated stress environment.  For pristine structure, the initial crack size, a0, is representative of flaws that might be in a structural detail as a result of manufacturing (see Section 1.3.4.1).  After an inspection, the initiating flaw size, aNDI, is the reliably detected crack size of the NDI method for the location. See Section 3.1. The crack size versus time curves are adjusted to account for variations in usage severity that are experienced by individual airframes.

The FSMP is based initially on the design loads spectrum, but as data is obtained from the L/ESS program a new operational baseline loads spectrum is developed and the FSMP is updated to reflect the operational usage.

The IAT program, also based originally on the design loads spectrum, is updated to reflect the L/ESS data.  This update may involve changes in the IAT method but usually only includes changes in the crack growth rate in terms of the usage parameters being recorded by the IAT program.

Figure 8.1.5 from Berens, et al. [1981] shows the time sequence relation of these Force Management activities.  The final activity is the airplane maintenance and the accumulation of these records.

 

Figure 8.1.5   Sequence of Force Management Elements [Berens, et al., 1981]

The final FSM plan and all of the test results and analysis conducted during the design, manufacture, and testing of the aircraft form the final data package which is delivered to the Air Force.  It substantiates the damage tolerance characteristics of the structure and describes how it may be maintained during the life of the aircraft.

A transition period normally occurs during which the contractor trains the user in all stages of the L/ESS, IAT, and FSM plan.  It is essential that the user assume the same regard for the treatment of damage critical parts that was practiced during manufacture.  The damage tolerance analysis is highly dependent on the size of the initial quality flaw.  Manufacture processes and handling were watched so that quality was preserved.  It is now the responsibility of the user to handle the aircraft in the same manner.  Disregard for the structure could result in complete loss of all the previous efforts and could invalidate all of the tracking efforts.

It is the responsibility of the Air Force user to obtain the data from the L/ESS to be used in the baseline analysis update.  Early collection of L/ESS data will lead to the most accurate use of the IAT data.  Recognition of this operation as part of the fracture control plan should aid in the proper conduct of the task.  Keeping the equipment in service and striving for the maximum amount of data return will lead to the most accurate final results.  (This is, in part, also dependent on a selection of parameters that are easy to record.)  Recording equipment and transducers should have a high reliability and be easy to use.