AMTAS

Analysis of Disbond and Delamination Arrest Features in Laminated Composite Structures

An accurate and efficient analysis method is critically needed for the damage tolerant design of bonded composite structures.  In this proposal, sponsored by Boeing research and Technology, a technical approach to the disbond/delamination arrest problem has been presented. Major tasks to be accomplished during the six months period have been identified and discussed. These tasks include: development of analytical capability for the analysis of disbonds and delaminations as well as performing sensitivity studies on the effects of stacking sequence and the fastener dimensions and properties.  Results of this research will contribute directly to the integrity and safety of aircraft structures made of bonded materials.

Development of an Active Flutter Suppression Research Plan

The utilization of active control systems for gust alleviation, load redistribution, flight control, and ride comfort improvements has matured over the last two decades to levels of reliability and safety that allow implementation and certification on both military and civil aircraft. Active Flutter Suppression (AFS) – the reliance on active control systems to stabilize flutter-unstable vehicles throughout their flight envelopes – while studied extensively and demonstrated in theoretical, numerical, wind tunnel tests, and flight tests, is not approved for wide spread implementation, mainly because of the catastrophic nature of vehicle response to the failure of such systems. This paper and talk will present a general introduction to active flutter suppression in the context of flight vehicle aeroelasticity and aeroservoelasticity. First steps taken in the effort to assess the state of the art and the implementation level of readiness of the technology will be described.

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Certification of Discontinuous Composite Material Forms for Aircraft Structures

Polymeric composites based on discontinuous fiber reinforcement are being used in primary and secondary structures in modern transport aircraft. A prime example is HexMC®, a composite material system developed by Hexcel that is being used to produce several structural components within the Boeing 787, for example the window frames. Some of the major advantages of the HexMC material system include ease of manufacturing, near in-plane quasi-isotropic mechanical properties and low notch sensitivities.

The overall objective of the proposed project is to study the technical issues important to the development, certification and production of discontinuous composite material parts for primary aircraft structures. HexMC will be used as the model material form throughout the study.

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Composite Thermal Damage Measurement with Hand Held FT-IR

The purpose of this research is to determine if Fourier transform infrared (FTIR) spectroscopy can be used to quantify incipient thermal damage (ITD) in aerospace composites. ITD is the early stages of thermal damage that cannot be detected by visual or ultrasonic methods but can lead to significant reductions in matrix-dominated properties of the composite. This project involves the detection of thermal damage with FTIR and performing a simulated repair guided by FTIR. The first task is the development of calibration curves for FTIR data on short-beam strength (SBS) samples with varying levels of ITD. This effort focuses on constructing calibration curves from SBS samples that have varying levels of ITD using an ExoScan FITR configured with diffuse reflectance. The SBS samples are 177 °C cure carbon fiber reinforced epoxy laminates that were carefully heat soaked to achieve uniform ITD in the samples. FTIR spectra and calibration curves relating the FTIR spectra to levels of ITD are reported and compared to previous results. These calibration curves will be used to guide mapping of thermal damage both on the surface and during the scarfing repair process with localized heat damage.

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Damage Tolerance Test Method Development for Sandwich Composites

The objective of this research project was to investigate candidate damage tolerance test methodologies for sandwich composites and to propose specific methodologies and configurations for standardization.  Three candidate test configurations are currently under evaluation.  The first methodology utilizes an end-loaded Compression After Impact (CAI) test configuration. Initial evaluations have been performed using sandwich configurations with both carbon/epoxy and glass/epoxy facesheets and Nomex honeycomb core.  Research has focused on establishing the required specimen size to prevent interactions between the damage present and the boundary conditions during loading as well as special requirements to promote proper alignment and end loading for a variety of sandwich configurations.  Second, a four-point flexure test methodology has been identified for evaluating post-impact performance.  The composite sandwich panel may be oriented such that the damaged facesheet is loaded in compression or tension.  Similar to the CAI test configuration, research has focused on the required size of the gage section between the inner loading points as well as additional sandwich panel requirements to prevent outer-span core shear as well as loading-point compression failures. The third candidate methodolgy, the “hydromat” test configuration, utilizes a water-filled bladder to apply a distributed load to the surface of the composite sandwich panel while supporting the panel along its edges.  Initial evaluation using composite sandwich panels with carbon/epoxy and glass/epoxy facesheets and Nomex honeycomb core has exposed difficulties when using this test configuration for assessing strength reductions associated with damage.  All three candidate test methodologies are being examined for their limits of applicability and to establish recommended testing procedures.  Expected benefits to aviation include standardized test methodologies for use in assessing the damage tolerance of sandwich composites used in aircraft structures.

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Standardization of Analytical and Experimental Methods for Crashworthiness Energy Absorption of Composite Materials

The proposed research will develop a set of standardized procedures for the experimental and numerical characterization of the crush behavior of composite materials. Recent findings have identified the key factors preventing the introduction of polymer composites in primary crash structures as the lack of adequate design guidelines, accurate simulation tools, specialized test methods for energy absorption, and an available material database. The proposed research plans to address all of these factors in a uniquely integrated fashion. Initially the research aims to develop a test standard with which to characterize the Specific Energy Absorption (SEA), featuring a corrugated web coupon. The results using this specimen will be compared systematically against the values measured using a flat plate specimen with dedicated anti-buckling fixture, C-channels, and square tubes using identical material and processing conditions. The method will then be used as benchmark to compare the accuracy of material models and progressive failure criteria within mainstream commercially available finite element codes (LS-DYNA, ABAQUS Explicit and possibly PAM-CRASH). This unified and integrated investigation will be used to generate a set of accessible numerical guidelines for the industry to build on. Lastly, the standard will be used to generate design guidelines and to systematically characterize the material systems and forms. This effort provides direct support to the current standardization efforts of CMH-17 (former MIL-HDBK-17) and will aim to result in a test method for standardization by ASTM Committee D30.

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Inverse/Optimal Thermal Repair of Composites

The goal of this project is to develop a software tool that can be used in the field to optimize the repair of composites by ensuring that the temperature at the repair can be maintained at a specified level for the required duration. Producing the desired temperature field requires the specification of the heating intensity as a function of position and time consistent with the boundary conditions that exist at the time of repair. Since the boundary conditions are rarely known with sufficient accuracy, one of the project goals is to develop a method to estimate them from a diagnostic thermal analysis/test.

We aim to produce a tool that will tell the technician where to place the heating blankets, what the intensity of heating should be as a function of position, and how long the heating is to be applied. All of these results will come from a dynamic simulation of the heating process and subsequent optimization. The tool will be tested on repairs in the laboratory and in the field.

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Identification and Validation of Analytical Chemistry Methods for Detecting Composite Surface Contamination and Moisture

The primary objective of the proposed work is to verify the reliability and sensitivity of solid-state electrochemical sensors for detecting surface contaminants and moisture on pre-bond surfaces. Sensors, made using the novel concepts applied to solid-state electrochemical materials, were found to be very sensitive in experiments with polyester peel ply samples. However, the method has not been tested for other peel ply samples that may have more or less contamination and/or moisture levels. In addition, the correlation between the features of the signals and surface chemistry condition has not been established. The proposed work will be focused on comprehensive testing and evaluation of the sensors. The proposed work will also be focused on atomic force microscopy (AFM) and chemical force microscopy (CFM) analysis of the sample surfaces prepared with peel ply.

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Combined Global/Local Variability and Uncertainty in Integrated Aeroservoelasticity of Composite Aircraft

We propose to the FAA to develop analytical, computational and experimental capabilities to address “Combined Global/Local Variability and Uncertainty in Integrated Aeroservoelasticity of Composite Aircraft”. Computational capability development will focus on quantification of effects on stiffness of key local effects in composite structures, global aeroelastic/aeroservoelastic analyses capable of evaluating variations and uncertainity to such local effects, and integrated local/global modeling capability of uncertain composite structures. Capabilities for simulation of the effects of control surface nonlinearities on aeroelastic and aeroservoelastic behavior of full scale airplanes will be developed and used to study effects of nonlinearity and uncertainty mechanisms and guide maintenance practices. Simultaneously, an experimental structural dynamic/aeroelastic testing capability will be developed at UW, and tests will be planned & conducted to study the effects of damage on stiffness of components and models.

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Development of Reliability-Based Damage Tolerant Structural Design Methodology

The purpose of this proposal is to develop a new design approach to quantify the reliability of aerospace structures. In this approach, the “Level of Safety (LOS) of an existing structural component is determined based on a probabilistic assessment of in-service accumulated damage and the ability of non-destructive inspection methods to detect such damage. Specifically, the discrete LOS for a single inspection event is defined as the compliment of the probability that a single flaw size larger than the critical flaw size for residual strength of structure exists, and that the flaw will not be detected. The cumulative LOS for the entire structure is the product of the discrete LOS values for each damage type Present at each location in the structure. This approach can be utilized to develop a design process which evaluated the equivalent LOS of an existing structure, and use this value in the design of a new structure which matches or exceeds the existing LOS value. The LOS method enables the characterization of uncertainty associated with damage accumulation, inspection reliability and residual strength of the structure.

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The Effect of Surface Treatment on the Degradation of Composite Adhesives

To ensure the longevity of the commercial aircraft fleet, the long term durability of primary aircraft structure must be understood. The degradation of metals and their attachments (mechanical and adhesive) has been rigorously studied over the years. The introduction of composite materials in aerospace applications has presented challenges as methodologies that have successfully been used for metals do not always produce reliable results with new materials. This project will consider the effect of surface treatments on composite adherends and accelerated test methods that may be used to reliably compare their long term degradation. Follow-on projects will consider improving durability using nano-reinforced adhesives.

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AF555 Hot/Wet Creep Response

This is a Boeing-funded project though the AMTAS center, involving shear lap coupons exposed to hot water and creep stress. We have examined the failure surfaces of the adhesive using SEM. The failure was dominated by adherend failure at the fiber interface, the portion is which increased slightly with moisture content.

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Course Development: Maintenance of Composite Aircraft Structures

The goal of this proposal is to develop, in conjunction with AMTAS academic and industry partners, a syllabus and course material for a short course addressing the maintenance of composite aircraft structures.

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Training Strategy Development—Composite Materials Education for Aircraft Practitioners

The objectives of this project are to:

• Work with industry, other government agencies, and academia to ensure safe and efficient deployment of composite technologies used in existing and future aircraft
• Update policies, advisory circulars, training, and detailed background used to support standardized composite practices

Practitioners will be educated through technology knowledge transfer. The technology knowledge base consists of:

• Databases. Handbooks, Manuals
• Industry practices
• Practical insights
• Training that is relevant to industry practices and common understandings
• Research that is relevant to industry practices

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Advanced Materials & Manufacturing Training Innovation Center (AMMTIC)

AMMTIC's mission is to develop and implement a strategic business plan that supports a self-sustaining advanced materials and manufacturing innovation facility and organization in Snohomish County and in Washington State that integrates public and private research and training initiatives designed to transform the aerospace and other advanced manufacturing workforce and keeps the United States globally competitive. This project will continue to build upon the current composite, aerospace and other manufacturing training programs currently offered through Edmonds Community College and its affiliates through the joint FAA Center of Excellence partnership.

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