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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
    • 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
        • 4. Sustainment/Aging Aircraft
          • 0. Sustainment/Aging Aircraft
          • 1. Widespread Fatigue Damage
          • 2. The Effect of Environment and Corrosion
        • 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 1.4.2. The Effect of Environment and Corrosion

Typically, the environment and choice of the structural material will change the rates at which cracks nucleate and grow, and can cause cracks to nucleate in locations where the risk for cracking damage without the environment is negligible.   As a result of using military aircraft past their initial planned design life (about 20-25 years), new categories of structural integrity problems caused by environmental attack have been identified.  Developing a damage tolerant design guidelines handbook that covers corrosion damage and environmental attack requires a more systematic approach for presenting approaches and methods that engineers can use to control the risk of structural failure.

Stress corrosion cracking (SCC) is a particularly deleterious  form of environmental attack that will create opportunities for cracks to nucleate and grow to failure, even under limited fatigue loading conditions (mechanism requires constant tensile stress conditions, and low material resistance to this kind of attack).   SCC has caused extensive (and expensive) problems due to the limited resistance of older forging alloys initially used in the C-141, KC-135, B-52, C-130 and C-5 aircraft. These problems have been recognized and, as materials have been developed for service in the newer KC-10 and C-17 aircraft, the problem has been controlled.  Besides potentially causing SCC problems, the environment frequently will accelerate or enhance the fatigue process by creating corrosion sites (pits, exfoliation damage, surface roughness, etc.) where fatigue cracks will develop, accelerate the crack nucleation process, and then accelerate the rate at which these fatigue cracks grow.  In fuselage lap joints, the crevice corrosion that occurs will result in pressure build up between the layers, sometimes to the point where rivet heads will pop off and the joint will look pillowed, such as shown in Figure 1.4.1.

Figure 1.4.1.   Photo of Lap Joint Illustrating the Localized Pillowing Caused by Crevice Corrosion Occurring between the Two Layers

There are several different features of corrosion that can be used to characterize the severity and extent of damage.  These corrosion metrics include thickness loss, pitting, surface roughness and pillowing, deformation in the metal caused by the excess corrosion by product produced between layers (see Figure 1.4.1).  Corrosion can grow in exposed areas, under paint, around fasteners, between layers of skin, and inside structural components.  Depending on the type of corrosion, it can grow in depth and area.  It can grow along grain boundaries.  Growth rates are influenced by environmental and load factors.  The impact that each of these characteristics have on structural integrity continues to be the subject of current research, but will depend upon the structure within which the corrosion is located.

Stress corrosion cracking is another form of corrosion damage found in aircraft structures.  As stated in Tiffany, et al. [1996], “Stress corrosion cracking (SCC) is an environmentally induced, sustained-stress cracking mechanism.”  Of particular interest is the identification of the operational need to reevaluate, possibly during ASIP durability and damage tolerant assessments, SCC-susceptible components to look for potential safety risks.

Currently the ALCs are operating under a maintenance philosophy that has been termed “find it – fix it”.  Under this mode of operation, when corrosion is detected, it must be dealt with by either repairing the damage, or replacing the component with the damage.  Corrosion is considered an economic issue at this time, but the costs associated with maintaining the aircraft in accordance with this philosophy are escalating.  Correspondingly, the readiness of the fleet is adversely affected.  To respond to this trend, the Air Force is pursuing the technologies necessary to implement a corrosion management maintenance philosophy.  This so-called “Anticipate and Manage” mode of operation attempts to make disposition decisions based on the impact of the damage to structural integrity.  This requires knowing the condition of the corrosion damage through nondestructive inspection, understanding the corrosion growth rates as affected by the environment, and predicting the future corrosion condition using models of corrosion growth.  The present and future states of corrosion can then be used in structural integrity calculations to determine remaining strength and life.  Disposition may now include flying the aircraft with known corrosion present, among other alternatives.  Economical disposition can then be made while maintaining safety of the aircraft.

Other areas of ongoing research include understanding corrosion growth mechanisms, corrosion inhibition and arrest, coating technology and the replacement of chromate in coating systems.