The committee concludes that, with increasing age and with changes in operation (or aircraft configuration) that increase the severity of the operational stress spectrum, the primary threats to structural safety arise from

  • the onset of widespread fatigue damage (WFD) in fail-safe-designed structures

  • the inexorable increase in the number of fatigue-critical areas in safe-crack-growth-designed structures and the potential for missing new areas as they develop

The primary technical needs for fail-safe designs are

  • improved methods of predicting the onset of WFD in an accurate and timely manner. This involves the prediction of initiation and growth of small fatigue cracks (or the interpretation of full-scale fatigue test data and service fatigue data), the prediction of fail-safe residual strength, and the evaluation of the potential effects of environmentally induced corrosion on crack initiation and growth and residual strength.

  • development and implementation of nondestructive evaluation (NDE) techniques that can rapidly detect small fatigue cracks over large areas of the structure prior to the onset of WFD. Methods to detect second-or inner-layer cracks and hidden corrosion that could lead to the initiation of cracks would be included.

The primary technical needs for safe crack growth structural designs are

  • to identify the next most probable fatigue-critical areas in the structure through careful evaluation of past full-scale fatigue test results, service experience, service loading data (including dynamic loads), design details (including potential areas for hidden corrosion), and the results of stress analyses and strain surveys

  • to perform simulative testing and crack growth analyses to establish safety limits and safety inspection requirements for all critical areas

  • to investigate the potential effects of corrosion on those factors that could affect safety limits and safety inspection requirements

  • to continue to improve methods of identifying fatigue-critical areas and flight load conditions to continue to improve NDE techniques that are sensitive enough to detect small cracks in multilayered and hidden structures to support safety inspections

Economics and Readiness

The economic burden associated with the inspection and repair of fatigue cracks can be expected to increase with age until the task of maintaining aircraft safety could become so overwhelming and the aircraft availability so poor that the continued operation of the aircraft is no longer viable. In addition, corrosion detection, repair, and component replacement can add significantly to or, in some cases, dominate the total structural maintenance burden.

The committee concludes that the major emphasis of the Air Force's technical and force management with regard to corrosion and stress corrosion cracking (SCC) should be focused on the early detection of corrosion and the implementation of effective corrosion control and mitigation practices so as to drastically reduce unscheduled repairs and replacement costs and aircraft downtime. Key technical issues and operational needs include

  • the development of improved NDE techniques for the detection and rough quantification of hidden corrosion

  • the classification of corrosion severity to provide guidance for maintenance

  • the generalized application of corrosion-preventive compounds and the development of corrosion-preventive compounds that can be applied on external surfaces to protect unsealed joints and fasteners

  • the development of a material and process substitution handbook and engineering guidelines for the replacement of components exhibiting corrosion and SCC with more-resistant materials and processes

  • the development and application of materials and processes to inhibit SCC

  • the development of technologies for the removal, surface preparation, and reapplication of surface finishes with improved corrosion-resistant finishes on existing aircraft

  • the assessment of the potential use of the dehumidified storage of aircraft, where practical

The committee believes that fatigue cracking will occur eventually on all aging aircraft as flight hours increase. From an economic standpoint, the major impact for a fail-safe-designed structure occurs with the onset of WFD. For safe-crack-growth-designed structures, the major impact occurs when the structure exhibits a rapid increase in the number of fracture-critical areas. In both cases, a choice must be made to undertake major modifications, structural replacement, or retirement. Although it may not be possible to avoid reaching this point for any given aircraft, operational changes such as fuel management, gust avoidance, active or passive load alleviation systems, reduced pressurization, and flight restrictions to minimize flight in severe mission segments can reduce the rate of fatigue damage and delay expensive repair-replace-retire decisions. For aircraft that are approaching their economic service limit, these options should be considered to allow time for modification or replacement acquisition programs.

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