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DTD Handbook

Handbook for Damage Tolerant Design

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    • About
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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
        • 0. Damage Size Characterizations
        • 1. NDI Demonstration of Crack Detection Capability
        • 2. Equivalent Initial Quality
        • 3. Proof Test Determinations
          • 0. Proof Test Determinations
          • 1. Description of the Proof Test Method
          • 2. Examples
        • 4. References
      • 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 3.3.0. Proof Test Determinations

Tiffany and Masters [1965] first suggested that the conventional structural proof test could be used to inspect for crack damage that would eventually lead to catastrophic failure.  These techniques were first applied to rocket motor cases and tankage as a result of numerous missile launch failure at Cape Canaveral.  Air Force acceptance of this proof test philosophy has been stimulated by the inability of alternate non-destructive inspection tools to reliably detect cracks of near-critical size.  The Air Force in the recent past has employed the proof test as a means of determining the maximum possible initial flaw that could exist in the structural subsystems identified in Table 3.3.1.  Note that almost all of the examples cited represent the application of the proof stress techniques as an In-service Inspection.  Buntin [1971], Cowie [1975], Horsley, et al. [1976], Gunderson [1974] and Albrectsen & Aitken-Case [1976] document the Table 3.3.1 and other Air Force uses of the crack-inspection proof test. White, et al. [1979] documents the recent Navy proof test of an A-7 arresting hook; this proof test is periodically repeated to ensure the continuing structural integrity of the component.

The proof test concept for all applications has been to size or eliminate the life degrading damage so that the structure would maintain its required level of structural integrity throughout a defined period of usage. However, due to substantially different technical requirements, the proof testing techniques employed in each case were different. The technical requirements that establish the type of tests conducted have been structural geometry, material properties, type of crack damage present in the structure, as well as the crack growth mechanism.

Table 3.3.1.  Proof Testing of Aircraft Structures

System

Subsystem

Damage

Special Techniques

F-111

Lower surface of inner wings and pivot fittings

Potential forging defects propagated due to fatigue in D6AC steel

Upwing bending at -40° F after every 1,000 hours of flight

B-1A

F-101 (Development) engine combustor case

Pores and inclusion stringers in circumferential butt welds in Inconel 901 alloy

Internal pressure to 200% operating pressure

B-52D

Center and inner wing structure

Fatigue and stress corrosion cracks nucleated during southeast Asia service in 7075-T6 and 7079-T6 aluminum alloy structure

Down and up-wing bending at ambient temperature

C-141

Main Landing gear (cylinder)

Hydrogen entrapped during refurbishment

500 hours of continuous static loading to initiate and propagate cracks to failure

A-7

Carrier arresting hook (Navy)

Fatigue cracking initiated during service

Repeat periodically