Small Business Innovation Research to Support Aging Aircraft Priority Technical Areas and Process Improvements (2001) / Chapter Skim
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Air Force Aging Aircraft Program
Air Force Aging Aircraft Program
Pages 13-46
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From page 13... ...
Air Force Aircraft that the Air Force "appoint a single knowledgeable and experienced technical leader responsible for the oversight of the aging aircraft activities" (NRC, 1997, p.
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meeting the needs of Air Force aircraft; (2) improving flight safety, reducing maintenance costs, and enhancing availability of aircraft; (3)
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The structural-assessment too] sets include structural integrity analysis techniques and supporting technologies for the prevention, identification, repair, and maintenance of structural degradation caused by cracking and corrosion.
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Note that some PE 6.5 programming is being initiated in high-priority subsystems areas such as electrical wiring and landing gear. Future programs may focus on NDE/NDI, repair, corrosion control, and structural integrity (see Table 2-2~.
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TABLE 2-2 Future Technology Programs 19 Nondestructive investigation UNDO Repair Corrosion control/suppression technologies Structural integrity Corrosion-focused tools Multilayer inspection Hidden damage Health monitoring NDI through paint Smart patch repair Advanced mechanical repairs Composite patch total transition Surface preparation for field/depot Materials substitution Cadmium/chromium replacement Corrosion prediction/structural integrity modeling Paint-for-life corrosion system Selective stepping Piece part counting/repair technologies Add corrosion prediction to the structural integrity code Affordability framework Table courtesy of Air Force Aeronautical Systems Center.
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From page 20... ...
is implemented, SBIR funding used to support the development of innovations needed can be accorded attention when a new research or development focus is being planned or is just · egmmng. Resources The AFRL baseline funding for R&D on aging aircraft includes funding for projects focused on structural integrity, repair, NDE/NDI, and corrosion.
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· prevention and tracking of corrosion and incorporation of the effects of corrosion into structural integrity analyses · high-reliability repairs adequacy, completeness, and retention of flight data and field and depot maintenance information flight beyond design life ability to make repair, replacement, and retirement decisions: support of cost-of-ownership and economic-service-life determinations These issues, and the issue of structural dynamics and aeroelasticity, are discussed below.
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The procedure for handling the structural integrity of aircraft structures is described in the 1997 NRC report, which also references the detailed military standards that are followed. Damage-tolerance assessments are the basis for maintaining flight safety.
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Many groups have all-encompassing, finite-element capabilities for calculating stresses. Primary tools for the implementation of stresses into structural integrity methodology are Air Force Grow (AFGROW, a software code)
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The AFRL Corrosion Fatigue Structural Demonstration Program and companion ASC Corrosion Management Program, the core efforts in the corrosion fatigue strategy, are focused on adding corrosion effects to the baseline structural integrity analyses that have been the basis for the ASIP durability and damage-tolerance approach (and championed by the current Air Force technical leader for aging aircraft, Jack Lincoln)
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set and the longer term will include the following: the development of more complete corrosion growth rate models the development of improved NDE/ND} corrosion inspection techniques, especially for hidden corrosion, and the quantification of the effects of corrosion degradation on mechanical properties the incorporation of cost parameters into structural integrity too! sets the incorporation of new developments into multiple-phase, nextgeneration corrosion fatigue analysis tools
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From page 29... ...
The key technical issues include: identification, reduction, or elimination of sources of dynamic excitation passive and active methods to reduce the response of aircraft structures measurement and characterization of the threshold for fatigue propagation values for airframe materials, including the applicability of long crack thresholds to small crack behavior · in-flight monitoring of changes in dynamic behavior. The first two issues are discussed next.
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From page 30... ...
That is, the structural response changes the aerodynamic forces on the structure, giving rise to aerodynamic damping. It is well known that the aerodynamic forces caused by structural motion can lead to a dynamic instability called "flutter" when the damping in an aeroelastic mode sinks to zero at some critical flight speed, the flutter speed.
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It has been suggested that impact damage due to discrete sources, such as landing loads or bird stnkes, is often a more critical design condition for composite structures than fatigue induced by crack propagation (NRC, 1996~. Improved modeling and measurement of structural damage due to impact loads offer attractive opportunities for near-term and long-term R&D to predict and reduce structural response using modern computational and experimental methodologies.
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From page 32... ...
Unfortunately, a substantial amount of corrosion damage sustained by older Air Force aircraft is hidden from direct view; thus, a significant amount of material degradation can remain undetected. More importantly, the extent and severity of corrosion damage in similar aircraft can vary widely because of differences in mission cycle, environmental exposures, and the extent and type of maintenance.
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From page 33... ...
Corrosion prevention should begin during the acquisition stage with the selection of appropriate materials and manufacturing processes. The commercial aircraft industry has developed, as part of its structural maintenance programs, provisions to upgrade corrosion resistance through the use of substitute materials and heat treatments; improved protective finishes and corrosion prevention compounds (CPCs)
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· generalized use of CPCs that can be applied on external surfaces and that will penetrate and protect unsealed joints and around fasteners · guidance for the application of upgraded alloys and processes offering improved corrosion protection improved NDE/NDI techniques to reveal and estimate hidden corrosion without requiring disassembly of the aircraft classification of corrosion severity, similar to current commercial aircraft practice, to provide guidance for maintenance Detection of corrosion is necessary for assessing the damage tolerance of affected structures and taking appropriate corrective actions. Current inspection methods require component disassembly, which increases the probability of maintenance-induced damage.
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From page 35... ...
Prediction of the onset of cracks would then be a complement to damage-tolerance analyses, which do not currently predict the occurrence of~SCC. With continued and consistent improvements in prevention and control procedures, upgrading of susceptible materials with more corrosion-resistant alloys, and minimization of residual stress, SCC problems in aging Air Force aircraft should remain manageable and need not be a life-limiting damage mechanism.
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. SMALL BUSINESS INNO VA TION RESEARCH TO SUPPORTS GING AIRCRAFT Coatings Coatings are an obvious operational requirement for implementing a costeffective strategy to prevent and control corrosion damage to airframe structures.
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Also, some fonns of corrosion damage, such as SCC, are not readily detectable visually, even at an advanced stage.
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To the surprise of many researchers at the time, the enhanced visual method was the most effective of the portable, fieldlevel methods surveyed for the detection of hidden corrosion. However, not all variations were represented in the study, and none of the techniques were performed at the levels desired by the Air Force.
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From page 39... ...
In addition, the report recommended a long-term top priority for an "integrated quantitative NDE capability," indicating that the detection sensitivity requirements (i.e., percent corrosion, crack length) should be denved from structural analyses, including corrosion and crack geometry and local airframe structures, and that the NDE methods must have consistent, reliable POD and flaw sizing.
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Without a doubt, the development and validation of NDE/ND] methods for aging aircraft could benefit from SBIR programs, particularly those focused on implementation at the field depot level.
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However, as certain aircraft systems age such as the KC-135, which is more than 40 years old—corrosion is becoming a major maintenance item, and significant sums of money are being spent on the detection and repair of corrosion damage. Consequently, future health monitoring should include the tracking of corrosion damage as well as fatigue damage.
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From page 42... ...
The most pressing problem for aging aircraft is bonded repair of metal structures. The current Air Force R&D program includes design and analysis techniques for composite patch repairs, repair procedures, design guidelines, and surface preparation for bonding.
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From page 43... ...
and certification of bonded repairs; repair material management; and smart patch technology. Future repair technologies should include standard repairs for corrosion damage; self-monitoring bonded repair patches; repair of aging composite structures; and incremental improvements in existing capabilities.
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From page 44... ...
As funding allows, existing tape recorder systems could be replaced with microprocessor systems that can record information on aging aircraft. Summary The 1997 NRC report presented a list of recommendations for near-term and long-term research in the following categories: fatigue; corrosion prevention and control; SCC; NDE/NDI; and maintenance and repair (NRC, 1997~.
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Applique technology is currently being investigated jointly by the Air Force and the Navy. Common structural integrity issues were widespread fatigue damage, corrosion, unitized structures, and dynamics (e.g., sonic fatigue, buffet, and vibration)
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A joint program between the FAA and the Air Force Commercial Aircraft Composite Repair Committee is addressing composite reference standards. The Air Force and NASA have a coordinated program on enhanced laser-generated ultrasound.
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