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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
        • 0. Damage Size Characterizations
        • 1. NDI Demonstration of Crack Detection Capability
          • 0. NDI Demonstration of Crack Detection Capability
          • 2. NDI Capability Evaluation for Cracks
          • 3. NDI Capability Evaluation for Corrosion
            • 0. NDI Capability Evaluation for Corrosion
            • 1. Corrosion Metrics
            • 2. Corrosion Specimen Selection and Design
            • 3. Example of Evaluating the Capability of an Eddy Current Inspection to Detect Hidden Corrosion in Lap Joints
        • 2. Equivalent Initial Quality
        • 3. Proof Test Determinations
        • 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 3.1.3.1. Corrosion Metrics

When characterizing the NDI capability for detecting cracks, the natural metric for measuring crack damage was the linear crack dimension used in damage tolerance analyses.  The selection of the appropriate metric for corrosion damage, however, is not immediately apparent.  There are different types of corrosion damage and different metrics can be used to quantify the damages.  For example, in hidden corrosion in lap joints and doublers on fuselage structures there are several possible metrics: thickness loss, pit depth and/or frequency, surface roughness, and joint pillowing.  When inspecting for intergranular and exfoliation corrosion, around fasteners, useful metrics might be the maximum radial distance that the corrosion extends from the fastener hole or the corrosion area about each fastener.  In some sense, each metric plays a role in the effect that the corrosion defect has on the structure.  Consequently, it is important to consider all of the metrics for a given application.  Each corrosion type must be considered separately, but the important aspect of the metric is that it measures corrosion severity.  Ideally, the metric should be based on an “effect of defects” study; however, in practice the important metrics are generally known, and, in order to keep the assessment focused, it becomes necessary to select only one metric at a time for detection assessment.  If it is absolutely essential to include an evaluation of more than one metric, then multiple evaluations must be performed (one evaluation per metric).

There is a necessary relation between the corrosion metric and the NDI technique.  Obviously, the NDI technique must be responsive to changes in the corrosion damage metric.  For example, in inspecting for hidden corrosion in lap joints, eddy current is responsive to thickness loss but may not be sufficiently responsive to pit depth.  If pit depth is a critical parameter, a different NDI technique would be needed.