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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites 6 A Path Forward A number of key issues have been found to affect the durability and lifetimes of polymer composites in extreme environments. The most important finding is the need for data at extreme conditions, given that models without data to verify and validate them will not be used. It is also true that although modelers want their models to be accurate, they often do not have the expertise, time, or funding to run experiments for such correlation. Accessible data would accelerate the development of accurate models, particularly coupled models. Experiments have been done to examine coupled aging effects (for example, temperature and stress, or moisture and ultraviolet radiation), but lacking data, modelers are not motivated to test and improve models that couple such effects. Yet this is the most challenging area for modeling and needs more effort. The second finding is that while data are scarce in the open literature, large amounts exist but are not available to modelers. A lot of money was spent to create data under a number of programs, including the NASA High Speed Research (HSR) program, but these data have not been made available. Much of the HSR data currently exists in an spreadsheet file on a few personal computers.1 In addition, large amounts of data funded through government contracts are under the proprietary control of technology development companies. Currently, there is no easy way for modelers to know what data exist or to access the data for modeling verification.2 This is true for researchers inside these government programs as well as researchers in industries other than aerospace or those engaged in purely academic pursuits. It is clear that tasks such as materials informatics, roadmapping, or integrated team design will not be carried out without a change in overarching processes. For example, a roadmap will not be created unless someone is officially responsible for it and his or her job performance depends on its successful implementation. A database will not be created or maintained unless a curator is assigned to house, validate, distribute, update, and run the database. The links between chemical kinetics and mechanics and structural design will not be modeled unless a program is funded to carry out this research. It has also become clear over time that commercial companies do not invest in these types of activities on their own. Similarly, academicians lack sufficient knowledge of the application requirements to be successful on their own, nor are government program managers able to accomplish these goals in isolation. Because the success of any federal initiative will depend on the appropriate review of collaborative work, expertise from all sides—academic and industrial experts across a number of disciplines—will be needed to assess all elements of the work. 1 One might wish to compare the cost of generating the data in the HSR program with the cost of creating a database and maintaining the data there. 2 “Accessing” data for model verification or validation might not entail actually seeing the data or using it in ways that would violate either defense or proprietary protocols.
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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites Finally, the advantages of materials development in the context of verified models are becoming clear. An effective approach would link models of chemical kinetics to property evolution and incorporate environmental exposure. This would allow using the predictive ability of models to design new materials with modified chemistries that would provide the resistance required for a particular application. Models of damage at a structural level based on component property information can allow designers to come up with composite structures (fibers plus resin plus morphology) appropriate for a given application. Accurate and reliable modeling, coupled with accelerated testing, is key to expanding the use of PMCs in extreme environments. Currently, haphazard testing on a shortened timescale is used to derive simple knockdown factors that are used to sanction PMC materials and allow their application in structural components. This testing does not illuminate the underlying mechanisms of property evolution. The use of weight loss measured after exposure to high temperature to rank materials without understanding what caused the weight loss or how it is related to failure demonstrates how accelerated testing can be used with no couple to modeling. To establish such a connection and to link chemical kinetics to mechanical properties, modeling must mature. Only then can a coupled modeling approach be used to design experiments specifically to accelerate a mechanism that is critical for the lifetime of the given component. A PMC steering committee such as that recommended in Chapter 5, which would oversee the composition of teams, the development of roadmaps, and the establishment of database parameters, could have the following duties: Define initial baseline application(s) and their extreme environments; Establish real temperature limits for material systems; Select master pedigree composite materials (resin and fiber); Identify anticipated damage mechanisms; Establish security criteria that would respect export controls and proprietary databases; Establish a standardized list of critical material properties; Coordinate the critical properties list with the national testing standards of ASTM subcommittee D-30; Identify sources of reliable data, assuring its pedigree and suitability; Coordinate government and commercial activities to increase database acceptance and use; Provide feedback loops to end users to assure database accuracy and relevance; Organize and coordinate roadmapping and database workshops with national societies at their venues; Estimate the costs of database development and maintenance; Develop strategies for funding database upkeep through user fees, dues to a newly created oversight organization, or coordination with an existing national PMC working organization; and Ensure that experts in the appropriate disciplines participate in all of these activities. PMC PHASE I ACTIVITIES It is recommended that a steering committee gather low-hanging fruit by immediately developing a format and contracting for the assembly of a database utilizing existing PMC databases available from industry and government. Serious consideration should be given to including PMC resins in current practice in extreme environments, including, for example, PMR-15, 5250-4, and 977-3. The selection process needs to be closely coordinated with the roadmapping and teaming recommendations in this report. This first database effort should be targeted at utilizing data from existing PMC databases from industry and government, with emphasis on creating an architecture that will be robust and allow the insertion of new material systems and new properties. Initial efforts should also begin to examine the creation of tools for mining and analyzing the data. Connection to the modeling community must also be included at the outset, with modelers involved in the process, and initial efforts to use the data in model development should be targeted.
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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites PMC PHASE II ACTIVITIES Phase II activities will begin with roadmap development, concentrating on the context of data as part of materials informatics. Each master pedigree PMC system identified through this process should represent a PMC class (for example, brittle/ductile, brittle/brittle, epoxy or resin) that has all the salient characteristics (for example, thermal, environmental, mechanical) representing that class of PMC. Likewise, each master pedigree PMC system should be mature enough to have sufficient manufacturing consistency that there is no significant batch-to-batch variability. A steering committee should work to define the PMC classes and master pedigrees and should be involved in the creation of the requests for proposals to fund the work for each material. Working with PMC material classes will provide end users with qualitative, albeit not quantitative, material behavior. This can provide a cost savings for the whole database generation activity. Each PMC class will require only one specific PMC material system. The end user can investigate the salient characteristics of a specific material class to formulate the general form of a model. The model can be further calibrated using a series of specialized tests that are model dependent. PMC PHASE III ACTIVITIES A mechanism to fund curators and informaticists to collect, verify, evaluate, maintain, update, warehouse, and distribute such database concepts will also be needed. It is imperative that a curator's sense of ownership in this activity be considered in funding decisions and that such efforts be intimately connected to PMC roadmapping groups linked to existing community-wide efforts in professional societies and others.3 Curators would also be tasked to coordinate with the national testing standards developed through ASTM D-30 and related groups. Considerations for such a roadmapping and data activity include the following: Determination of which properties are critical versus which are simply nice to have. Evaluation of PMCs in current practice. Availability of data restricted by export controls (such as ITAR) or by proprietary limits. Development of a method to demonstrate how processing changes the properties of a sample material. Establishment of master pedigrees for a few key materials, such as epoxy resin. Establishment of real limits on materials systems imposed by temperature and other factors. A number of organizational issues will also need serious consideration. The need for sustained funding is primary, and examples should be studied from bioinformatics and other fields to learn how consortium funding can be organized. Feedback must also be solicited from groups working to develop an understanding of underlying mechanisms to ensure that the data will meet their needs and also to determine what is needed to test models against existing data. Cost is an important consideration, but is difficult to gauge. This issue will best be considered when the benefits of this effort are more fully understood. In summary, the use of PMCs in increasingly extreme environments will depend on a considered, consolidated, and sustained effort by government, industry, and academia to achieve the goal of durability—and trust in that durability. (Figure 6-1 shows one looming reason for this trust.) Such an effort will require sufficient federal funding focused on changing the paradigms for teaming, roadmapping, and informatics. 3 The potential curators for some of these functions include academic research collaboratives. Some private enterprises may also be useful, such as the current curator for MIL-HDBK-17: Composite Handbook, Materials Sciences, Inc. Other potentially relevant organizations include ASM International, the National Institute for Aviation Research, and the Aerospace Materials Technology Consortium.
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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites FIGURE 6-1 The Boeing 787 as designed will utilize composite materials and new manufacturing techniques, including building full barrel fuselage sections with integrated stringers, as shown here. This application of composite materials could result in fewer parts and improved aerodynamic performance and fuel efficiency.
Representative terms from entire chapter: