Executive Summary

Polymer matrix composites (PMCs) have been manufactured for over 50 years in the United States. The composites industry utilizes various resins (typically epoxies, vinyl esters, or polyesters), curing agents, and fiber reinforcements (typically, glass fibers). It produces a wide spectrum of industrial components and consumer goods, ranging from boats, bathtubs, and auto bodies to a variety of other parts and components.

Advanced PMCs are a sector of the composites industry that is characterized by the use of expensive, high-performance resin systems and high-strength, high-stiffness fiber reinforcement. While the aerospace industry, including military and commercial aircraft of all types, is the largest customer for advanced PMCs, these materials have also been adopted by sporting goods manufacturers, which sell high-performance equipment to the golf, tennis, fishing, and skiing and boarding markets.

Advanced PMCs have been extolled for their many advantages, including light weight, high specific strength and stiffness, property tailorability, and increased flexibility of design. However, in environmental conditions differing greatly from the ambient—that is, in extreme environments—the inherently complex material response of PMCs over time and the resulting evolution of their structural and functional properties have limited their effectiveness.

In particular, because of the complex nonequilibrium thermodynamic state of the polymer matrix, enormous uncertainties exist when predicting changes in the properties of PMCs as they are exposed over their lifetime to complex stress, moisture, and temperature conditions. Because such exposures can be coupled with exposure to chemical corrosives, ultraviolet radiation, or other degrading environments, the development and validation of predictive tools becomes ever more challenging. The inability to predict material performance under severe operating conditions is brought about by an inadequate understanding of the underlying physical mechanisms for material degradation, damage evolution, and failure in the hierarchical and highly heterogeneous material/structure. Taken together, these limitations on understanding have caused the design of composite components to be based on a relatively crude knockdown factor1 or similar approaches in an attempt to account for the long-term evolution of the material’s properties.

This inability to predict the long-term durability of PMCs—and the consequent overdesign of structures necessitated by this uncertainty—has limited their use. In some cases, PMCs are not used because the overdesigned part does not result in any design advantage. These issues are most serious for PMCs used in aggressive environments, where the stresses on the materials are high and numerous, leading to even larger uncertainties in polymer response over time. At the same time, it is in many such extreme environment applications that the advantages of lightweight, tailorable materials would be most beneficial.

1  

A simple way to proportionally adjust all properties of a material at an elevated temperature is to multiply them by a factor based on the ratio of one property (yield strength, for example) at that temperature to the same property at ambient conditions.



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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites Executive Summary Polymer matrix composites (PMCs) have been manufactured for over 50 years in the United States. The composites industry utilizes various resins (typically epoxies, vinyl esters, or polyesters), curing agents, and fiber reinforcements (typically, glass fibers). It produces a wide spectrum of industrial components and consumer goods, ranging from boats, bathtubs, and auto bodies to a variety of other parts and components. Advanced PMCs are a sector of the composites industry that is characterized by the use of expensive, high-performance resin systems and high-strength, high-stiffness fiber reinforcement. While the aerospace industry, including military and commercial aircraft of all types, is the largest customer for advanced PMCs, these materials have also been adopted by sporting goods manufacturers, which sell high-performance equipment to the golf, tennis, fishing, and skiing and boarding markets. Advanced PMCs have been extolled for their many advantages, including light weight, high specific strength and stiffness, property tailorability, and increased flexibility of design. However, in environmental conditions differing greatly from the ambient—that is, in extreme environments—the inherently complex material response of PMCs over time and the resulting evolution of their structural and functional properties have limited their effectiveness. In particular, because of the complex nonequilibrium thermodynamic state of the polymer matrix, enormous uncertainties exist when predicting changes in the properties of PMCs as they are exposed over their lifetime to complex stress, moisture, and temperature conditions. Because such exposures can be coupled with exposure to chemical corrosives, ultraviolet radiation, or other degrading environments, the development and validation of predictive tools becomes ever more challenging. The inability to predict material performance under severe operating conditions is brought about by an inadequate understanding of the underlying physical mechanisms for material degradation, damage evolution, and failure in the hierarchical and highly heterogeneous material/structure. Taken together, these limitations on understanding have caused the design of composite components to be based on a relatively crude knockdown factor1 or similar approaches in an attempt to account for the long-term evolution of the material’s properties. This inability to predict the long-term durability of PMCs—and the consequent overdesign of structures necessitated by this uncertainty—has limited their use. In some cases, PMCs are not used because the overdesigned part does not result in any design advantage. These issues are most serious for PMCs used in aggressive environments, where the stresses on the materials are high and numerous, leading to even larger uncertainties in polymer response over time. At the same time, it is in many such extreme environment applications that the advantages of lightweight, tailorable materials would be most beneficial. 1   A simple way to proportionally adjust all properties of a material at an elevated temperature is to multiply them by a factor based on the ratio of one property (yield strength, for example) at that temperature to the same property at ambient conditions.

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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites For example, the combination of light weight and stiffness offered by PMCs makes them attractive candidates for structural components in both military and civilian applications. In aerospace applications, where thin structures and complex shapes are needed to save weight, the ability to create a unitized structure complete with integral stiffeners and skin significantly reduces the number of parts and fasteners in an aircraft structure. Potentially more important than specific strength, though, is the ability to make polymer composites multifunctional. In addition to providing structure, multifunctional materials may provide sensing, moving, analyzing, communicating, and acting capabilities. The flexibility in processing composite materials could enable many of these advances. A number of innovative designs planned for deployment in extreme environments may only be possible with the use of advanced PMCs. In mission-critical applications, however, these new materials cannot be used without knowing their reliability. The current use of PMCs could increase dramatically if methods could be developed to reliably predict long-term durability and performance of polymer composites subjected to aggressive environmental conditions. To improve this situation, several factors must be considered, including the current state of predictive modeling and design practice for PMC components, the variety of accelerated testing techniques used to qualify material components, the influence of processing parameters, existing long-term databases, and target applications. To significantly advance the predictive capability, which will in turn enable reliable and tighter design of PMCs and dramatically increase insertion opportunities, the following recommendations are offered. Recommendation: To successfully address the complexities of predicting the performance of PMCs, organizations should assemble interdisciplinary teams with experience in chemistry, polymer physics, materials, processing, mechanics, testing, component and system design, and application of PMCs in extreme environments. While “interdisciplinary” has long since become a buzzword in scientific circles, actual interdisciplinary research is rare. Truly integrated interdisciplinary research is characterized by the synergistic fundamental contributions of researchers in different disciplines leading to advances that would not be possible otherwise. One reason for the lack of follow-through in many joint projects is that working across disciplines is difficult. It takes much more time and effort to communicate across disciplines and to focus on the connection between data and theories that originate from different length scales and different physics. Another reason for the failure to integrate research is that an individual researcher can be very successful while staying in one field, acquiring funding and publishing papers. However, the challenges of PMCs are too great for this situation to be allowed to persist. To enable design of PMCs to their potential for use in extreme environments, simplistic empirical approaches (such as knockdown factors) must be replaced by mechanism-based models for a range of behaviors, from chemical kinetics to delamination, which can predict long-term durability and performance. Researchers and organizations must team along nontraditional lines and form sustained collaborations to develop the models that are needed. Nowadays, teams are generally assembled by commercial technology integrators, so this recommendation is in many ways aimed at those teams. However, there are other ways to ensure the required teaming, including (1) wording federal program solicitations such that only sufficiently integrated teams will be able to compete successfully and (2) populating review panels for academic proposals with multidisciplinary experts. Recommendation: A steering committee should be created across the PMC community, initiated and supported across a number of federal organizations, to oversee the development and maintenance of a roadmap (or roadmaps) for PMCs in extreme environments. Technology roadmaps provide an effective framework for focused product development by highlighting the technology gaps that limit the transition of concepts to production. The preparation of a useful technology roadmap requires an understanding of a range of technical and nontechnical issues, including the current science and technology, future performance targets, technical, institutional, and market barriers, and R&D needs. While individuals, companies, and government agencies could all draft their own roadmaps, a team effort with an effective oversight mechanism is needed for success. The implementation of such a roadmap will require sustained attention, frequent updating, and effective integration of responsibilities and results.

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Going to Extremes: Meeting the Emerging Demand for Durable Polymer Matrix Composites The relevant PMC community includes the agencies interested in the efficacious use of PMCs to accomplish their missions—for example, National Aeronautics and Space Administration (NASA) for aeronautics and space exploration; Department of Defense (DOD) for defense systems; Federal Aviation Administration (FAA) for civil aviation; and National Oceanic and Atmospheric Administration (NOAA) for undersea exploration. As critical are the prime contractors responsible for designing and building the vehicles and structures that will operate in the extreme environments. The industrial technology companies are the main developers of the components, and the vehicle operators (the commercial airlines) are also stakeholders. Finally, the academic community that strives to understand and model the behavior of these advanced materials is also an essential element. Any oversight mechanism for roadmap stewardship must include members from all of these communities, but it should be initiated by the mission agencies. Recommendation: Mission agencies should offer sustained support to develop and maintain comprehensive information on PMC properties in a new materials informatics initiative. A steering committee on the needs of the PMC community should be formed to ensure that this effort is effectively targeted. This committee should be responsible for developing guidelines and overseeing operations for a national PMC informatics initiative. With one eye on the fine example set by the bioinformatics field, a materials informatics effort for PMCs should be pursued. Informatics has evolved in recent years as a branch of computer science and information technology that is concerned with the structure, creation, management, storage, retrieval, verification, validation, dissemination, and transfer of information. Informatics also innovates and optimizes the ways people generate, use, and find information.2 A number of official databases in bioinformatics and other fields are actively operating. The types of data sought for materials could be established from roadmaps and from lists of critical material properties. Data could be acquired automatically or through a manual curation process, or a combination of these. For instance, papers are added as they are published by automatic abstraction from a central journal database, and tools to visualize, extract, and analyze the data are constantly being updated. Such a model would be ideal for implementation of a database structure for PMCs. The materials database(s) will need to house critical data ranging from chemical structure and microstructural lay-up, to processing conditions and environmental profiles, to data utilization in designs, macroscopic properties over time, and failure modes and images. Curators and programmers will be essential to collect and add verified data, maintain and distribute data, and generate extraction, visualization, and analysis tools. The need for the centralized database is overwhelming given the vastness of the problem—namely, to systematically relate PMC response mechanistically by examining behaviors from chemical kinetics through microcracking. Readily available detailed, complex data are essential for researchers to develop reliable performance models for coupled response that can be validated and then incorporated into structure design. 2   The definitions and discussions of informatics in the Wikipedia are recommended. Available at <http://en.wikipedia.org/wiki/Informatics>. Accessed May 2005.