15:30
Session 3: Fatigue life enhancement methods and repair solutions I
Chair: Ben Main
15:30
20 mins
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The A-10 Warthog damage tolerance and residual stresses in transition
Jacob Warner, Dallen Andrew
Abstract: For over 15 years, the A-10 Aircraft Structural Integrity Program (ASIP) team within the US Air Force (USAF) has facilitated the advancement of fatigue crack growth analytical methods for residual stresses at cold expanded holes through numerous research studies and test programs. These many years of research and test validation have consistently shown that fatigue crack growth analyses that incorporate residual stress can dramatically improve aircraft availability by extending inspection intervals, simultaneously increasing safety by accurately predicting scenarios that legacy methods (such as using a reduced initial crack size) have shown to overpredict.
In a breakthrough this last year, the USAF has released new guidance on analytical methods to account for residual stresses in fatigue crack growth analyses in the form of a Structures Bulletin, permitting the use of residual stresses in a Damage Tolerance Analysis (DTA) to justify an extension (or elimination) of recurring inspections. The development and completion of this bulletin was a culmination of many contributions from A-10 ASIP, the Engineered Residual Stress Implementation (ERSI) working group, and Air Force Research Laboratories partners.
Leveraging the guidance provided in the new structures bulletin, the A-10 ASIP team has now analyzed more than 10 fatigue critical locations, explicitly incorporating residual stresses in fatigue crack growth analyses with multi-point fracture mechanics models. Analyses are validated by spectrum crack growth test data.
A review of major milestones that have contributed to the introduction of residual stresses in A-10 DTAs is provided, including advancements in applying residual stresses, multi-point fracture mechanics, shortfalls of reduced initial crack size methods, and test validation. Test validation results are compared with multi-point and traditional two-point linear elastic fracture mechanics predictions. For comparison purposes, reduced initial crack size predictions are also shown. Including residual stresses directly in DTAs empowers more accurate predictions and inspection intervals while simultaneously increasing aircraft safety and airworthiness.
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15:50
20 mins
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Enhanced technology repair for corrosion and fatigue damage in hybrid aerostructure
Matthew Kokaly, Jude Restis
Abstract: Partworks is conducting a test and development program on a new repair for aerospace structures, including hybrid composite and metallic designs. The repair incorporates adding beneficial engineering residual stresses and the use of new technologies to advance beyond the status quo. The overall goal is to drastically reduce the cost and time to make in-service repairs while also providing a longer life and higher reliability compared to what is used today.
The use of hybrid, mixed material aerostructures utilizing carbon skins over aluminum substructures in aircraft today has driven higher levels of corrosion than all metal designs of the past. This is largely attributable to the significant galvanic potential difference between metal and carbon fiber composites. Existing repair methods for metal/carbon fiber composite skin bolt hole/fastener sites necessitate extensive and complete removal of corroded material, non-standard hole sizes and individual analysis and validation of each repair instance. Limited to no testing is done to demonstrate effectiveness of each unique repair configuration.
The proposed new repair method combines the use of a known engineering residual stress process with new technology and understanding including the use of thin wall bushings and/or rivetless nut plates, a new adhesive/sealant, a refined expansion process to better control the residual stresses developed during installation while maximizing post install compression, deliver standard hole sizes post treatment, incorporate technology to verify the level of residual strain and stresses and an ergonomic design of lightweight tooling. The repair method will capture installation parameters real-time in order to provide go/no-go feedback to the operator. The resulting repair is intended to restore or further enhance fatigue life at the bolt hole while minimizing the amount of material removal.
This presentation will describe the progress to date, including unique fatigue test results with the worst-case scenario of severe corrosion left in place during repairs. Early testing shows that the repair is capable of significantly extending the fatigue life, even for severely corroded material, over the standard open hole. We will also share the results from the application of a Digital Image Correlation (DIC) system to validate and quantify the residual strains from the repair and X-Ray Diffraction (XRD) measurements of the residual stress fields. Correlation of the measured DIC residual strains and XRD residual stresses will be shown along with a comparison with the results of a nonlinear finite element analysis (FEA) of the repair process. Future plans will be shared including the greater contribution of FEA to the development process.
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16:10
20 mins
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Evaluation of cold spray for aircraft repair
Sarah Galyon Dorman, Justin Rausch, Moriah Ausherman, Gregory Shoales
Abstract: Aircraft parts are subjected to stringent regulations to maintain airworthiness. As a result, minor corrosion or mechanical damage can disqualify a part and require replacement even though the majority of the part is acceptable. For complex parts that are difficult and expensive to manufacture in low production runs, it can be more economical to use an additive manufacturing repair process such as cold spray (CS) repair to restore the original part geometry without manufacturing an entire part. This extends the life of existing parts and thereby, extends the usable life of the aircraft. Substantiation is the first step to achieving certification for the repair on certified aircraft.
A logical and conservative analysis approach to substantiate the repair of non-structural and structural parts has been developed using stress analysis and material property generation [1]. Often prior to the substantiation, a comprehensive test matrix is carried out to determine the material allowables of the cold spray material system. The substantiation analysis incorporates this new information in a logical progression. The result is a process that is useful for non-structural and ultimately structural part repair.
Cold spray is being use in both civil aviation and military aircraft fleets as a method for repairing obsolete or damaged parts mainly for dimensional repair. There is ongoing research examining the corrosion and mechanical property equivalency of CS repairs on aluminium alloys for structural applications on aircraft. Coupon testing has shown that CS repairs of fatigue sample geometries with 15-30% blend outs depths are able to improve fatigue life to near that of a standard fatigue coupons. Tensile coupons with 15% depth CS repairs have also shown tensile properties within 90% of wrought material for two alloy systems. Various coupon geometries have been used to evaluate how cold spray repairs would perform, all have shown promising results. Bending and pin-bearing testing has also been conducted, both show excellent bond performance of the cold spray and acceptable mechanical properties. All the work developing coupon data is critical to building the material database which will allow for additively manufactured or repaired parts to be installed on aircraft [2].
[1] Sarah Galyon Dorman, and Scott Fawaz (2019). FAA Certification of Cold Spray Dimensional Repair. 2019 Cold Spray Action Team (CSAT) Workshop, Worcester, MA.
[2] Kabbara, J. and Gorelik, M., 2016, “FAA Perspectives on Additive Manufacturing,” from http://www.nianet.org/ODM/ODM%20Wednesday%20presentations%20Final/7%20Kabbara%20On-Demand%20Workshop%20AM%20Presentation(03-09-2016.pdf.
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16:30
20 mins
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Retardation of fatigue cracks in welded structures through laser shock peening
Nikolai Kashaev, Sören Keller, Uceu Fuad Hasan Suhuddin, Volker Ventzke, Benjamin Klusemann
Abstract: Aeronautical structures are often subjected to cyclic loading and can therefore fail due to fatigue. In most cases, fatigue cracks develop and propagate from critical areas, so-called stress concentrators, where the highest tensile stresses are present. When a fatigue crack has developed during operation, it has a significant impact on reducing fatigue life. To extend fatigue life, compressive residual stresses can be introduced in the critical areas to reduce the crack-driving tensile stresses and retard fatigue crack growth or even stop existing cracks. In this study, laser shock peening (LSP) is investigated as a promising technique to introduce deep compressive residual stresses in metallic aerospace materials. One application scenario of LSP is demonstrated on a welded stiffened panel representing a part of a fuselage structure, where the technique was successfully applied for the retardation of skin cracks. The skin-stringer AA2024-AA7050 T-joints were realized through stationary shoulder friction stir welding, a variant of the conventional friction stir welding process. It was shown, that application of LSP led to a 400 % increase in fatigue life.
Another positive application scenario of LSP to restore the fatigue life of laser-welded AA6056-T6 butt welds with already existing surface fatigue cracks is discussed. By applying LSP to surfaces of specimens with fatigue cracks, the fatigue life could be restored to the level of specimens in the as-welded condition. A similar positive effect of LSP was demonstrated on AA2024-T3 specimens with a fatigue crack originating from overlap joints manufactured using a solid-state joining process. The results of the study show that LSP is an efficient method for extending the fatigue life of structural components with small surface cracks. In this context, LSP can be used to improve the fatigue performance of components where fatigue cracks may occur in critical areas such as welds. Therefore, LSP can be used as a prophylactic residual stress engineering technique to extend the fatigue life of critical structures in aging aircraft where fatigue cracks have not yet reached the detectable size. In this regard, LSP could reduce the required safety margins (safety factors) of the fatigue-critical component or structure, thereby reducing its weight.
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