Aging of U.S. Air Force Aircraft Final Report (1997) / Chapter Skim
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Part II Recommended Strategy and Opportunities for Near-Term and Long-Term Research
Part II Recommended Strategy and Opportunities for Near-Term and Long-Term Research
Pages 35-78
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From page 35... ...
Likewise the primary focus 35 of the engineering tasks of updating durability and damage tolerance assessments, force structural maintenance plans, and tracking programs is to protect the structural safety, but they also impact maintenance costs and force readiness. The task of estimating the economic service life of an aircraft weapon system involves both engineering and management; engineering predictions of structural deterioration need to be coupled with a number of cost and operational considerations to arrive at the most probable time that the Air Force should plan on replacing the system.
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From page 36... ...
Included in Part II are descriptions of recommended nearterm engineering and management tasks; assessments of current and planned research administered by the aging aircraft research program (detailed assessments are contained in the committee's interim report [NRC, 19971~; identification of near-term and long-term research opportunities in the areas of fatigue (low-cycle fatigue, high-cycle fatigue, and environmental effects) , corrosion and stress corrosion cracking, and inspection and maintenance technology (nondestructive evaluation and maintenance and repair)
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From page 37... ...
manage aging force Nondestructive Evaluation - Detection of small cracks, secondlayer cracks, hidden corrosion - Improve speed and reliability - Automation FIGURE II-2 Basic elements of the recommended near-term and long-term R&D programs. 37 Stress Corrosion Cracking - Susceptibility assessment - Prevention and mitigation - Component repair and replacement \ Maintenance and Repair - Design and analysis methods - Materials and processes - Guidelines for component replacement
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From page 39... ...
the determination of the safety limits, inspection intervals, and, for fail-safe designs, the estimated onset of WED. The results were then used to update the individual aircraft tracking programs and the force structural maintenance plans for the aircraft, both of which are key elements of ASIP.
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From page 40... ...
Force Structural Maintenance Plan TABLE 5-1 Prioritization of DADTA Update Needs for Air Force-Supported Aircraft AGING OF U.S. AIR FORCE AIRCRAFT Planned Use Fatigue Test and Inspection Data / Initial QualityAssessment 1: Upgraded Individual Aircraft Tracking Program Fail-SafeAdditional Years Replacement Current Fatigue Recent Structural Review Actions Aircraft Designin Inventory Aircraft Identified Cracking Reviewa Under Way Priority KC-135 yes25+ no limited yes yes 3 C-SA yes10-25 no no report no no 2 C-141B yes0-8 yes (C-17)
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From page 41... ...
For other aircraft it will require further identification of critical areas, stress spectra development, crack growth calculations and tests, and perhaps some tear-down inspections and/or fullscale fatigue testing. Contractor [ogistics-Supported Commercial Derivative Aircraft In a similar manner to the criteria for Air Force-supported aircraft (previous section)
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From page 42... ...
In particular, the Air Force should push for the force-wide use of the microprocessor-based recorders because of their improved reliability and the expanded data capture. STRESS CORROSION CRACKING ASSESSMENTS Although the environmental protection measures and material substitutions to eliminate corrosion-susceptible materials that take place as part of an aircraft's corrosion prevention and control program (CPCP)
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From page 43... ...
For safe crack growth designs, which are generally associated with high-performance combat aircraft, it is important that the stress corrosion cracks be prevented from occurring or that they be detected before failure, since failure of the parts or components may lead to the loss of the aircraft. As an aircraft ages and protective finishes and coatings break down, concern over part failure caused by SCC becomes more acute.
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From page 44... ...
(1) Corrosion damage occurring between successive inspections that is local and can be re-worked/blended-out within allowable limits as defined by the manufacturer; or (2)
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From page 45... ...
If not substantially diminished in the future through improved prevention and mitigation measures, corrosion damage, either by itself or in combination with fatigue cracking, could cause the Air Force to undertake major modifications, major component replacements, or perhaps aircraft retirement. Clearly, as was pointed out in Chapter 2, there is a need for an overall economic service life estimation model that integrates the estimates of structural deterioration caused by fatigue, corrosion, and SCC with all other operating cost elements.
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From page 46... ...
Ideally, the service life estimation model should utilize the best possible technical estimates of the major structural modification and/or component replacement times, account for the cost and operational metrics listed above, and balance and weigh their relative importance. The committee recommends that the Air Force make a concerted effort to develop a credible service life estimation model or methodology that would be accepted by the Air Force senior management and the Department of Defense decision makers (e.g., the Defense Acquisition Board ~ as the authoritative guide for supporting replacement decisions and budget inputs.
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From page 47... ...
organizations include specialists in nondestructive inspection, stress analysis, design of structural repairs, fracture mechanics analysis, failure analysis, and corrosion control. However, these groups also draw on the expertise of AFMC/ASC engineering and the Wright Laboratories for assistance on specific problems.
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From page 48... ...
TECHNOLOGY TRANSITION INTO AGING AIRCRAFT One of the most effective ways for increasing the reliability and speed of nondestructive evaluation and reducing the costs of repairing aircraft with structural cracking and corrosion problems is through the transition of improved technologies into application. In the past, this has been difficult for the system program directors because the links with technology development activities (e.g., labs, industry, other services)
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From page 49... ...
evaluate the effect of WFD on crack growth, (4) upgrade the Damage Tolerance Handbook, (5)
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From page 50... ...
A number of investigations have established that the use of elastic-plastic fracture mechanics is essential to determining the residual strength of an airframe structure with WFD (Harris et al., 1995; Atluri, 1997~. Improved methods are required to treat the effects of plasticity on the fatigue crack growth and fracture behavior typically exhibited by the ductile alloys used in aircraft construction.
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From page 51... ...
The quantification of the principal parameters aircraft use spectra, initial quality, stress level, and structural geometry needed to provide analytical estimates of the timeand use-dependent crack populations and the associated fatigue life and critical crack sizes requires an extension of the existing analytical methods and approaches. The committee suggests that the most promising approach is to combine existing deterministic tools for the prediction of stress levels, residual strength, and crack growth with existing risk analysis tools to account for statistical variability of the situations that might lead to failure of the aircraft.
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From page 52... ...
AIR FORCE AIRCRAFT evaluation of the sensitivity of crack growth thresholds to aggressive environments, such as humidity, saltwater, fuel, or hydraulic fluids modification of current test methods or development of new low-cost methods to develop crack growth threshold stress intensity factors estimation of fatigue life under high-cycle and mixed high-cycle-low-cycle regimes for intact and repaired structural components Dynamic fatigue failures are very sensitive to the threshold crack growth rates of the materials involved, which are related to the time to initiate cracks from inherent defects within the material or to the surface finish or roughness of the finished part (Bucci et al., 1996~. Maintaining sufficient fatigue life in the presence of dynamic loading requires either maintaining very low vibratory stress levels or increasing controls on material defects and design details (stress concentrations)
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From page 53... ...
Methods to analytically predict structural dynamic response, validated through the laboratory or flight tests, are required to assess aging structures and repairs. In some cases, structural repairs to address high-cycle fatigue serve to exacerbate the dynamic loading problems because local repairs of dynamically loaded structures can move the failure to a new location defined by the repair itself.
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From page 54... ...
the influence of corrosion on applied stresses resulting from material thinning and local bulging or pillowing of thin sheet due to buildup of corrosion products · the potential influence of corrosion on material mechanical properties (i.e., toughness, strength, elongation) resulting, for example, from the absorption of hydrogen by the metal during the corrosion process · the potential influence of corrosion and corrosive environments on crack growth rates below the threshold for stress corrosion cracking In the current Air Force program, corrosion and environmental effects on fatigue are part of a category of projects AGING OF U.S.
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From page 55... ...
However, this issue should be revisited. Specifically, this work should determine if prior corrosion affects the fatigue crack growth rates over the range of crack sizes (and stress intensity values)
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From page 56... ...
For example, it has been shown in laboratory tests of a high-purity Al-Zn-Mg alloy that preexposure to humid air causes reductions in fatigue resistance that are comparable to those resulting from exposure to water vapor during fatigue testing, an effect that was completely reversible by vacuum storage timed to permit hydrogen diffusion out of the samples (Ricker and Duquette, 1988~. The committee recommends fundamental research to determine the extent to which hydrogen governs the growth of small fatigue cracks relevant to the onset of WFD, as well as high-cycle fatigue crack growth.
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From page 57... ...
. basic research involving characterization and analysis of corrosion, fatigue damage development, and environmental and corrosion effects; also included is a new initiative to investigate the development of pitting corrosion in aluminum alloys applied research to develop in-service and experimental corrosion and fatigue data and efforts involving evaluation and characterization of improved corrosionresistant materials and corrosion chemistry · exploratory research that investigates fine-grain processing to improve corrosion resistance The committee supports efforts to improve the definition of corrosion damage metrics and the associated test protocols, 57 to characterize in-service corrosion damage to provide data for severity assessments, and to characterize and analyze corrosion.
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From page 58... ...
Examples of specific tasks related to aging aircraft include · characterization of the role of stress, both static and cyclic, as a source of initial defects in coatings · evaluation of coating durability in a fretting environment and in a crevice corrosion environment · evaluation of the effects of chemical and physical heterogeneity within coatings on the long-term performance · characterization of the effects of new paint removal techniques such as sodium bicarbonate, wheat starch blasting, and pulsed cold plasmas on corrosion resistance and the performance of subsequently applied coatings · determination of the effects of thermal and physical aging on the adhesion characteristics of primer coats and conversion coatings The Air Force has long recognized that the durability of protective finish systems is the most important factor, other than resistance to discrete mechanical damage, in the development of corrosion for aging aircraft (Miller, 1987~. Aircraft coatings must meet a demanding set of criteria, including (1)
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From page 59... ...
corrosion, and mended that the corrosion protection mechanisms provided stress corrosion cracking, will provide valuable information by chromates used as conversion coatings and as corrosion- to aircraft operators to support repair-replace decisions and inhibiting pigments be established. A multi-university re- toestablishinspectionandmaintenanceintervals.Inaddition,
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From page 60... ...
, coating resin chemistry, and the substrate surface chemistry on factors that lead to stable growth of coating defects investigation of adhesion mechanisms between coatings and relevant substrate materials to determine the role of coating adhesion in the long-term performance of coatings on metal substrates investigation of environmental effects on surface chemistry and morphology of new conversion coatings and subsequent adhesion of organic coatings development of analytical models to predict long-term coating performance based on materials and interracial characterization following short-term exposures . STRESS CORROSION CRACKING As described in Chapter 4, stress corrosion cracking (SCC)
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From page 61... ...
Quantitatively evaluate the SCC susceptibility of current Air Force materials (alloy and product forms) , based on experimental SCC threshold stress data, fracture mechanics threshold stress intensity data, and crack growth kinetics.
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From page 62... ...
the large number of variables known to affect SCC behavior; (3) relatively poor correlation between laboratory test results and service experience; (4)
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From page 63... ...
, most notably Oklahoma City ALC efforts on NDE for corrosion detection. Elements of these 63 basic research to investigate and demonstrate innovative NDE techniques that have the potential to produce significantly improved accuracy of defect detection and characterization and reliability for detection of corrosion and small fatigue cracks applied research to evaluate various NDE approaches for corrosion detection, including neutron radiography, optical fiber sensors, and neutron activation analysis; ribbon x-ray sensors; x-ray spectroscopy; nonlinear electromagnetic methods; and enhanced methods to detect incipient corrosion an exploratory research project to conduct depot-level demonstrations of successful methods from applied research efforts, evaluate data fusion and image analysis methods for NDE data evaluation, and demonstrate and validate high-resolution real-time radioscopy systems · a limited manufacturing research effort to evaluate NDE methods for corrosion in aging airframes The widespread fatigue damage (WFD)
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From page 64... ...
stress corrosion cracking in thick sections. As pointed out in Chapter 4, current NDE methods qualified for a given aircraft application for the detection and characterization of a particular flaw will not necessarily be applicable directly to another application, even though detectability requirements are the same, because of variations in geometries and materials.
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From page 65... ...
The committee recommends that the Air Force use a life-cycle approach to evaluate and validate methods that Candidate NDE Potential Cntical Need Methods Potential Techniques Validation Aircraft Fatigue cracks under B- 1, F- 15 fasteners Electromagnetic Magneto-optic imaging Pulsed eddy current Eddy current arrays Thermal Time-resolved thermography Ultrasonic Laser ultrasonics Scanning UT probes EMAT transducers Small cracks associated E-8, VC-137, C-18 with WED Ultrasonic Guided waves EMATs Laser ultrasonics Electromagnetic Scanning pulsed eddy current Microwave imaging (6~90 GHz) Large-area magneto-optic Thermal Time resolved, scanning Hidden corrosion Electromagnetic Pulsed eddy current KC-135, A-10, C-5, C-130 Thermal Ultrasonic Optical Multifrequency eddy current SQUID technology, eddy current Time-resolved thermography Energy-sensitive detectors Microfocus real-time radiography Neutron Bubbler/scanning methods Boroscope Cracks or corrosion in multilayer structures Electromagnetic Radiography Ultrasonic Pulsed eddy current Multifrequency eddy current Real-time imaging In-motion film Scanning (if gaps can be bridged)
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From page 66... ...
AIR FORCE AIRCRAFT development of probes and techniques based on accept -reject requirements dictated by the performance and materials requirements of the aircraft structure development and application of predictive reliability models that consider part geometry, performance requirements, NDE capabilities, failure modes, and lifecycle cost predictions development and application of validation and quali fication techniques for NDE probes and systems using simulation models with confirmation on service components development of inspection standards, including reference standards using simulation techniques, to aid implementation across the entire force structure explore and develop the use of NDE simulation capabilities coupled with new synthetic environment (virtual reality) technology for method development and operator training An integrated NDE program must recognize the interdisciplinary nature of NDE and the aging aircraft problem.
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From page 67... ...
, damage growth rates (e.g., fatigue crack growth) under expected operating conditions, performance requirements, and material fracture properties (e.g., critical crack sizes)
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From page 68... ...
In many cases bolted repairs are expected to perform well beyond their original intent, making the repair an aging structure much like the airframe itself. The current Air Force R&D program on repairs includes · basic research involving modeling of composite patch repairs as crack arrestors in aircraft and design and analysis techniques for composite patch repairs · a large amount of applied research, including projects related to bonded composite patch repairs to investigate repair procedures, analysis methods, and design considerations along with efforts to develop repair methods and design guide for composite structures; development of advanced life-extension techniques; development of structural life enhancement, force management, and internal and external loads handbooks;
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From page 69... ...
; an example would tee to reduce susceptibility to stress corrosion cracking through the use of improved aluminum alloys, tempers, and processes to reduce residual stresses repair and analysis methods for maintenance of structures susceptible to high-cycle fatigue maintenance and repair methods and guidelines for advanced composite structures Near-Term Research and Development Much has been learned in the past ten years concerning methods to analyze and repair damage in primary metallic and composite structures. Although the focus of much of the early work was on designing repairs for battle damage, the focus more recently has been on repairs for durability and life extension for current aircraft.
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From page 70... ...
Develop and validate guidelines for the relative lives of bolted repairs. Specific tasks include · testing to evaluate crack stopping by cold working, peening, laser shock treatment, stop drilling, or hole filling - testing to evaluate repair designs, including issues such as protection systems, taper ratios, fastener patterns, and fastener types - testing to evaluate innovative fastener concepts such as single-shank blind fasteners and hole-expanding blind fasteners Bolted repairs are the most common repair applied to aircraft structures.
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From page 71... ...
Examples of specific tasks would include . documentation of repair materials and processes and design considerations based on an understanding of root causes, dynamic load conditions, and environmental factors 71 develop and validate damped repair concepts based on currently available adhesive and composite repair technology Repairs for dynamically loaded structures offer the unique potential to significantly reduce load magnitudes or change the critical load frequencies while they serve to recover the integrity of the structure.
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From page 72... ...
Develop, characterize, and evaluate improved damping materials with improved environmental resistance and low-temperature performance for repair and modification of dynamically loaded structures. Examples of specific tasks include development of accelerated environmental test methods and criteria to evaluate resistance to aircraft service conditions, including thermal and fluid exposures development and validation of repair concepts that include improved damping materials Damping material systems currently in use have shown inadequate durability.
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From page 73... ...
Priority 2: important to improved flight safety (Objective A) or reduced maintenance costs and improved force readiness (Objective B)
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From page 74... ...
Priority 1 recommendations include (1) research to develop a fundamental understanding of corrosion and stress corrosion cracking to support the development of improved corrosion prevention and control procedures and (2)
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From page 75... ...
Effect of flaw morphology on crack growth Page 56 A moderate-high 2 (16) Effect of hydrogen on fatigue crack growth Page 56 A moderate 3 Corrosion Prevention and Control (20)
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From page 76... ...
The Air Force has only recently made significant use of composite primary structure on the B-2 and will continue on the upcoming F-22. The Navy and the commercial aircraft industry have limited service experience for primary composite structures on the Navy F/A-18 and AV-8B and on the Airbus A320.
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From page 77... ...
Fatigue is not generally a significant damage mechanism in fiber-dominated composite structures that meet impact damage tolerance requirements described above (Jeans et al., 1980~. Components that experience significant interlaminar or outof-plane loading can be susceptible to fatigue damage.
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From page 78... ...
Develop tools and methods for bolted repairs of composite primary structures. The thicker laminate construction used in composite primary structures, as well as the size and nature of discrete damage from typical aircraft service (e.g., impact damage, lightning attachment damage, delaminations)
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