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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
          • 0. Summary of Damage Tolerance Design Guidelines
          • 1. Summary of Guidelines
          • 2. Design Category
          • 3. Inspection Categories and Inspection Intervals
          • 4. Initial Damage Assumptions
          • 5. Residual Strength Guidelines
            • 0. Residual Strength Guidelines
            • 1. Fail-Safe Structure at Time of Load Path Failure
            • 2. Determining the Residual Strength Load for Remaining Structure
          • 6. Required Periods Of Safe Damage Growth
          • 7. Illustrative Example Of Guidelines
        • 4. Sustainment/Aging Aircraft
        • 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 Determining the Residual Strength Load for Remaining Structure

The magnitude of the required residual strength load depends upon the exposure time in service because the longer the exposure time, the greater the probability of encountering a high load.  Accordingly, the value of required Pxx load increases with an increase in the inspection interval or period of unrepaired service usage (allowable crack growth period).  For the short service exposure times between inspections for the In-Flight Evident, Ground Evident and Walk Around Visual categories, the probability of encountering limit load conditions is low and thus the required Pxx may be significantly below design limit load.  For the longer exposure times between depot or base level inspections, the probability of encountering limit load is much higher, and therefore for Depot Level and Non-Inspectable categories, the minimum required Pxx must be at least limit load, but Pxx need not be greater than 1.2 times the maximum load in one lifetime.

The value of Pxx is established from load spectra data derived from a mission analysis of the particular aircraft considering average usage within each mission segment.  Unless otherwise stated, MIL-A-8866 is the basic source of load factor data for the various classes of aircraft.  Since safe operation depends upon the residual strength capability and since any individual aircraft may encounter loads in excess of the average expected during the particular exposure time, the Pxx load required is larger than the average derived value. 

One way to determine the level of Pxx required is to hypothetically increase the service exposure time for the aircraft between inspections by a factor of M.  This is the method used in JSSG-2006.  The values of M are specified in JSSG-2006 Table X, and summarized in Table 1.3.4.  For example, under the ground-evident level inspectability category, the Pxx load is the maximum load expected to occur once in 100 flights (M x inspection interval = one flight x 100).

Table 1.3.4.  Inspection Interval Magnification Factors from JSSG-2006 Table X


Degree of Inspectability

Typical Inspection Interval

Magnification Factor


In-Flight Evident

One Flight



Ground Evident

One Flight



Walk-Around Visual

Ten Flights



Special Visual

One Year



Depot or Base Level

¼ Lifetime




One Lifetime


* Pxx = Minimum average interval member load that will occur once in M times the inspection interval.  Where PDM or PLT is determined to be less than the design limit load, the design limit load shall be the required residual strength load level. Pxx need not be greater than 1.2 times the maximum load in one lifetime if greater than design limit load.


The basis for the specified M values is somewhat arbitrary although it is felt that the loads derived by this method are not unreasonably conservative.  The basis for M = 100 is exceedance data for transport type aircraft, where it has been observed that shifting exceedances by approximately two decades (i.e., M = 100) magnifies the value of load factor (or stress) by approximately 1.5 (Figure 1.3.10).  It was recognized that for fighter data, exceedances approaching or exceeding design limit values are probable but that extrapolation of the basic exceedances curve very far beyond limit load factor (nz) is often meaningless and unwarranted due to physical limitations of the vehicle and crew.  Furthermore, in most cases actual service data is somewhat sparse for this region of the curve.  Therefore, (1) an upper limit was required on Pxx for fighter aircraft and (2) the value of M should be less for longer inspection intervals in order that unreasonable factors would not be imposed should the actual derived Pxx be less than the specified upper limit.  The values of M equal to 20 and 50 are arbitrary but probably not unreasonable.  Where the derived Pxx is larger that that associated with the design limit conditions, Pxx can be taken as 1.2 times the maximum load expected to occur in one design lifetime.

Figure 1.3.10.  Illustration of Procedure to Derive M Factor to Apply to Exceedance Curve


EXAMPLE 1.3.3         Derivation of Pxx From Exceedance Data for Non-Inspectable Structure

The procedure for obtaining Pxx is illustrated using the exceedance plot shown here.  This figure presents the average exceedance data for one design lifetime.  The point A represents the maximum load expected in one lifetime; this is shown to be larger than the limit load (Point E).  For the core of a non-inspectable structure, the twenty lifetime (Mx inspection interval) exceedance curve is obtained by shifting the exceedance curve from point A to point B and extrapolating to point C.  The twenty lifetime exceedance curve yields Pxx (derived) at C.  The required load Pxx then is either the value derived at C or 1.2 ´ (load at point A) i.e., the load at point D, whichever is smaller.  In this case, Pxx (= PLT ) is the load at point C.