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DTD Handbook

Handbook for Damage Tolerant Design

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    • About
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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
          • 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 1.3.2. Design Category

Slow Crack Growth structure consists of those design concepts where flaws or defects are not allowed to attain the critical size required for unstable rapid crack propagation.  Safety is assured through slow crack growth for specified periods of usage depending upon the degree of inspectability.  The strength of slow crack growth structure with sub-critical damage present shall not be degraded below a specified limit for the period of unrepaired service usage.

Fail Safe structure is designed and fabricated such that unstable rapid propagation will be stopped within a continuous area of the structure prior to complete failure.  Safety is assured through slow crack growth of the remaining structure and detection of the damage at subsequent inspections.  Strength of the remaining undamaged structure will not be degraded below a specified level for the period of unrepaired service usage.

In the development of the guidelines, it was recognized that multiple load path and crack arrest type structure have inherent potential for tolerating damage by virtue of geometric design features.  On the other hand, it is not always possible to avoid primary structure with only one major load path, and therefore some provisions are necessary to ensure that these situations can be designed to be damage tolerant.  It is the intent of the guidelines to encourage the exploration of the potentials for damage tolerance in each type of structure.  Single load path or monolithic structures must rely on the slow rate of growth of damage for safety and thus, the design stress level and material selection become the controlling factors.

Single load path “monolithic” structures must be qualified as Slow Crack Growth.  However, the guidelines allow flexibility for qualification of multiple load path cases.  The decision may be made to qualify multiple load path structure as Slow Crack Growth, instead of Fail Safe, if sufficient performance and life cycle costs advantages are identified to offset the burdens of the inspectability levels for Fail Safe structure.  Therefore, the method of construction may not agree with the design category selected, i.e. all multiple load path structure is not Fail Safe.  When deciding on the design category option, the most important factor to consider is that once a design category is chosen, the structure must meet all the guidelines in the guidelines that cover that category.

The mere fact that a structure has alternate load paths (local redundancy) in some locations does not necessarily qualify it as Fail Safe.  Examples are helpful in illustrating this point.  Examples 1.3.1 and 1.3.2 illustrate the fact that a structure is often locally redundant (usually good design practice), but in an overall sense may have some restriction such that one is not able to take advantage of the localized redundancy in order to qualify the structure as Fail Safe.

Considerable judgment is required for the selection of potential initial damage locations for the assessment of damage growth patterns and the selection of major load paths.  The qualification as Fail Safe is thus a complex procedure entailing judgment and analysis.  Because of this, the choice is often made to qualify the design as Slow Crack Growth regardless of the type of construction.  As stated in JSSG-2006 A3.12.2.3 Requirement Lessons Learned "There are currently no aircraft in the Air Force inventory which have been qualified as fail-safe crack arrest structure under Air Force criteria".



EXAMPLE 1.3.1         Identifying Non-Redundant Structure – Lug Example of Slow Crack Growth Structure

The lug fitting illustrated here has multiple lug ends at the pinned connection.  Failure or partial failure of one of the lugs (A) would allow the load to be redistributed to the remaining sound structure.  Localized redundancy is often beneficial, and in this case is good design practice.  However, the fitting cannot be qualified as Fail Safe Multiple Load Path structure since the occurrence and growth of damage at a typical location (B) would render the structure inoperative.  The only means of protecting the safety of this structural element would be to qualify it as Slow Crack Growth.




EXAMPLE 1.3.2         Choice Options for Redundant Structure – Wing Box Example

As shown here, a wing box is attached to the fuselage carry through structure by multiple fittings.  The upper and lower skin is one piece for manufacturing and cost reduction.  The substructure consists of multiple spars spaced to attach to the individual attachment fittings.  A case could be made to qualify this structure as Fail Safe Multiple Load Path.  Depending upon the amount of bending carried by the spars, it would be possible to design the structure such that damage in the skin would be arrested at a spar prior to becoming critical.  The design might also tolerate failure of one spar cap and a portion of the skin, prior to catastrophic failure.  The attachment system could be designed to satisfy Fail Safe guidelines with one fitting failed.

On the other hand, if the skin was the major bending member with a design stress of sufficient magnitude to result in a relatively short critical crack length, then the skin and spar structure could only be qualified as Slow Crack Growth structure.