Administration of the FAA Center for Excellence for Advanced Materials in Transport Aircraft Structures (AMTAS)
In December 2003 the Federal Aviation Administration (FAA) announced a joint award to the University of Washington and Wichita State University to create a new Air Transportation Center of Excellence for Advanced Materials (JAMSCOE). This award established a center led by the UW and named the FAA Center for Advanced Materials in Transport Aircraft Structures (AMTAS). Academic members of AMTAS include the UW, Washington State University, Oregon State University, and Edmonds Community College. As lead institution of AMTAS, the UW is responsible for overall administration of AMTAS, oversight of AMTAS projects conducted by all academic members, and hosting of an annual AMTAS Conference. This proposal requests FAA funding necessary to administer AMTAS. As in all other AMTAS proposals, the FAA funding requested will be matched on a 1:1 basis from non-federal sources.
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Improving Adhesive Bonding of Composites through Surface Characterization
The purpose of this research is to determine the effect of atmospheric pressure plasma treatment on Mode I strain energy release rate (GIC) and failure mode of bonded peel ply prepared carbon fiber reinforced polymer composites. Previous research showed that Toray T800/3900 carbon fiber reinforced epoxy composites prepared with Precision Fabrics Group 52006 nylon peel ply and bonded with MetlBond 1515-3M structural film adhesive failed in adhesion at low fracture energies when tested in the double cantilever beam (DCB) configuration. Other research suggested that plasma treatment could be able to activate these “un-bondable” surfaces and result in good adhesive bonds. Nylon peel ply prepared 177°C cure carbon fiber reinforced epoxy laminates were treated with atmospheric pressure plasma after peel ply removal prior to bonding. Surface characterization methods, including contact angle (CA), Fourier transform infrared (FTIR) spectroscopy, and x-ray photoelectron spectroscopy (XPS) were used to determine how plasma treatment changed nylon peel ply prepared surfaces. CA can be used to measure surface energy of a composite prepared for adhesive bonding. This information can help understand one requirement of adhesion: surface wetting of the adherend by the adhesive. FTIR and XPS can be used to measure composite surface chemistry, which can help understand another requirement of adhesion: the formation of chemical bonds between the adherend and adhesive. FTIR and XPS can also be used for the identification of contaminants, which can inhibit adhesive bonding. DCB specimens were bonded with MetlBond 1515-3M and tested to determine failure mode and GIC. Plasma treated samples had acceptable failure modes and fracture energies that were triple that of peel ply only samples. It was demonstrated that atmospheric pressure plasma was able transform poor bonding surfaces to good bonding surfaces.
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Failure of Notched Laminates under Out-of-plane Bending
The design of aircraft structures made of composite materials is heavily influenced by damage tolerance requirements. The problem of predicting failure in notched laminates has been the subjected of numerous studies. In general, these investigations have focused on the response of laminates to in-plane tension, compression or shear. In spite of the fact that out-of-plane bending, twisting, or shear can be an important load situation, very little research has been devoted to this topic. The overall goal of this research is to develop analysis techniques that are useful for the design of composite aircraft structure subjected to general out-of-plane loading. For this project we will limit ourselves to the out-of-plane bending case and focus on some very basic experiments and modeling efforts involving simple structures (center-notched, unstiffened laminates) under pure bending. In partnership with the Boeing Commercial Airplane Company, we will determine the modes of failure of the laminates and evaluate the capability of some currently existing analysis techniques for predicting these failures. Accomplishing our objective will require both experimental and computational efforts.
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Effect of Surface Contamination on Composite Bond Integrity and Durability
Previous research has shown that surface contamination plays a key role in the initial bond strength of adhesively composite joints (ABCJ’s). Advancements in experimental methods to capture changes in bond strength are of significant relevance in predicting ABCJ’s performance. This study presents an experimental method to evaluate the effects of contamination on the durability of ABCJ’s. Bonded double cantilever beam (DCB) specimens were manufactured with laminates that were contaminated prior to bonding. Surface characterization of the laminates prior to bonding was conducted using ECS, FTIR and water contact angle measurements techniques. After manufacturing, specimens were environmentally exposed to a high humidity environment or cyclically loaded to challenge the bond with an accelerated aging process. DCB testing of the specimens provided the fracture toughness and mode of failure for each group of conditioned specimens. Results were compared with previous tests where no contamination was introduced but a similar accelerated aging process was utilized. Additionally, the moisture absorption of contaminated and non-contaminated specimens in unstressed conditions was used to obtain the equilibrium saturation point over a period of time. Future studies will include evaluating contaminated specimens that have been subjected to a high moisture environment and cyclic loading simultaneously.
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Development and Evaluation of Fracture Mechanics Test Methods for Sandwich Composites
Whereas the development of test methods for fracture mechanics of composite laminates has reached a high level of maturity in recent years, relatively little attention has been given to the development of fracture mechanics test methods for sandwich composites. Of the limited number of investigations performed to date, a majority have emphasized a particular sandwich material or the effects of specific environmental conditions. In general, the test methods proposed for fracture mechanics of sandwich composites have been found to be problematic due to problems in testing, crack propagation, and the analysis of test data. Emphasis in this research project grant will be focused on the development of fracture mechanics test methods for sandwich composites. The ultimate goal of the proposed research is to establish draft ASTM standards for both Mode I and Mode II fracture toughness of sandwich composite materials.
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Durability of Adhesively Bonded Joints for Aircraft Structures
The objective of this recently-initiated research investigation is to revisit and revise the ASTM D 3762 metal wedge crack durability test. While considered to be a reliable method for investigating adhesive bond durability, the existing standard provides little guidance regarding the conditions and requirements that constitute an acceptable metal bonded joint. Of particular concern is the reduction in strength of the bonded metal joint over time due to hydration. Thus a need exists to revise the existing test standard such that it provides specific guidance on how to successfully develop criteria for the wedge crack durability test. Possible revisions to the standard include proposed exposure environments, and pass/fail criteria with regards to both crack growth extension and failure modes. This research project will initially focus on reviewing the literature and identifying stakeholders associated with the test method. Test results and proposed additions and revisions to the ASTM D 3762 standard will be communicated regularly to ASTM Committee D14 on adhesives. In addition to proposing revisions to this standardized test method, research results from this investigation will be disseminated through an FAA technical report and journal publications. Expected benefits to aviation include an improved adhesive bond durability test method for use in assessing the reliability of adhesively bonded aircraft structures.
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Durability of Bonded Aerospace Structures
This work is concerned with understanding mechanisms contributing to the mechanical durability of adhesively bonded joints. To that end, the response of coupons bonded with brittle and toughened adhesives are compared under quasi-static and repeated load environments. Since this phase of the work considers adhesive response (rather than adherend failure), the adherends are made of 2024-T3 aluminum. The testing regime is designed to study the effects of peel (double cantilever beam coupons) and shear (wide area lap shear coupons and scarf joints) stress on bond strength and durability. The coupons are bonded using brittle and toughened adhesives (FM-300, FM73, EA9696) so that effects of adhesive strength and toughness may be independently considered.
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Effects of Moisture Diffusion in Sandwich Composites
Many structural elements external to the fuselage of transport aircraft are produced using polymer sandwich honeycomb composites; composite rudders or ailerons, for example. These components experience the environmental extremes typical of Class A airspace. In particular, temperatures at altitude often fall to -50ºC or below. Meanwhile, it has been shown that the humidity within the core region of a polymer honeycomb panel will slowly increase with time due to exposure to typical terrestrial humidity levels. Although immediately after production the internal core humidity level of sandwich composites is normally low (perhaps approaching zero humidity), over time water molecules diffuse through otherwise undamaged gelcoat and composite facesheets, slowly increasing internal humidity levels. If internal humidity becomes high enough then internal water vapor may condense and freeze within the core while the aircraft is at altitude. This implies that a condense-freeze-thaw-evaporate cycle may occur within a sandwich structure during the normal duty cycle of a commercial transport aircraft. The objective of this study is to determine if this condense-freeze-thaw-evaporate cycle leads to internal damage that may be detrimental to the mechanical performance of honeycomb sandwich structures. Specifically, the bending stiffness and mode I strain energy release rate (GIc) of honeycomb composites will be measured, under four conditions:
- as-produced (dry),
- following thermal cycling from room temperatures to -50 ºC under dry conditions,
- following exposure to 70%RH and 50%C for several months, and
- following exposure to 70%RH and 50%C for several months as well as thermal cycling from room temperatures to -50 ºC.
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Certification of Discontinuous Fiber Composite Material Forms for Aircraft Structures
The overarching goals of the proposed study are a) to characterize, in-situ, the main failure mechanisms occurring in Discontinuous Fiber Composites (DFCs), b) to develop a physically-based modeling approach that will ultimately lead to a certification process for DFC materials and structures based primarily on analysis, supported by relatively modest experimental verification, and c) to develop technical guidance suitable for inclusion in the CMH-17 Handbook.
To characterize the failure mechanisms in DFCs and to observe the damage evolution in DFCs, PIs will take advantage of a state-of-the-art dual source X-ray CT scanner recently acquired by PI Yang and other researchers at UW. This will allow in-situ inspection of DFC’s micro-structures at a resolution of microns, potentially unveiling the unique mechanisms of damage formation in DFCs. Based on the information provided by the proposed experimental tasks, numerical analysis of the main failure mechanisms of DFCs will be conducted. This will ultimately lead to the development of a physically-based failure model for intra-laminar and inter-laminar damage in DFCs. Once validated, the proposed computational and experimental framework can be used to provide insights and guidelines for the efficient non-destructive inspection of DFC structures.
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