National Academies Press: OpenBook

High Performance Synthetic Fibers for Composites (1992)

Chapter: 4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY

« Previous: 3 Fiber-Forming Processes: Current and Potential Methods
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 103
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 104
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 105
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 106
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 107
Suggested Citation:"4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY." National Research Council. 1992. High Performance Synthetic Fibers for Composites. Washington, DC: The National Academies Press. doi: 10.17226/1858.
×
Page 108

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 103 4 IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY INTRODUCTION: OVERVIEW OF NEEDED RESEARCH AND DEVELOPMENT Previous chapters of this report have provided descriptions of the state of the art of high-performance synthetic fibers and have called attention to further development needs and application opportunities. This chapter discusses important factors that must be addressed in the effort to improve existing fiber systems and develop new ones for composite reinforcement. These factors, encompassing the scientific and engineering fundamentals underlying fiber preparation and property evaluation, can have a major impact on the rate of progress and success of fiber research efforts. Attention is required to the following technical issues concerning fiber characteristics and fiber-formation processes. • Fiber science begins with systems selection. A systematic approach must be developed to identify fiber compositions and structures that ensure compatibility with the matrix material in specific applications. Inasmuch as a reinforcing fiber will be incorporated in a matrix to form a composite, the characteristics of the matrix as well as the composition of the fiber must be carefully considered. Once these compositional issues are determined, fiber formation or processing routes must be devised that will produce a uniform and consistent product for evaluation and that will lead to the lowest-cost commercial product. Fiber selection normally involves physical property extrapolations from bulk materials data supplemented by thermodynamic calculations, followed by generation and evaluation of small samples of the fiber. The problems associated with production-level scale-up must be kept in mind in evaluating the performance of small samples. • A scientific understanding of the mechanisms involved in the formation of the fiber and of the microstructural factors affecting the fiber's properties is extremely important. The following factors must be understood: the chemical and physical processes involved in the formation of the fiber, their rates, and how these rates can be controlled for optimum throughput and

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 104 properties. Factors governing the ultimate strength of the fiber may include: surface flaws, internal stresses, internal composition gradients, substrate quality for chemical vapor deposition (CVD) fibers, or precursor quality for pyrolyzed fibers. Studies must answer these questions. Research must also reveal the optimum morphology throughout the fiber and at its surface and show how these morphologies can be achieved. • Of particular importance is an understanding and optimization of the chemical composition and morphology of the fiber surface, given the nature of the matrix that the fiber will be used to reinforce and the conditions for incorporating the fiber into that matrix. The type of bonding required at the fiber-matrix interface will often dictate the characteristics that must be designed into the fiber surface. In addition, surface coating layers may be required to enhance the fiber strength, to act as a diffusion barrier preventing chemical attack by the matrix, or to accommodate any thermal expansion coefficient mismatch between fiber and matrix. This important issue is addressed further in the next section. • In the development of new fibers for high-temperature applications, there is a growing need for predictive tools to guide and aid the fiber scientist in this task. Many of the envisioned applications require the materials system to meet several requirements simultaneously, such as high-temperature creep, minimum strength and modulus, and environmental resistance. However, at the present time there are no specific properties or indicators that can be reliably used to judge the suitability of a material for the proposed application. • A critical issue for processing science is the matter of fiber evaluation. Normally, room-temperature physical properties such as strength, modulus, diameter, and density of the fiber are of major interest. Methods for determining these properties must be better defined. However, it must be recognized that there is a statistical distribution of fiber strengths instead of a single value and that strength is strongly influenced by test gage length. In addition, for high-temperature applications, the fiber properties at elevated temperatures must be optimized; therefore, in principle, tests and evaluations of high- temperature behavior should be carried out during process development. Moreover, the ultimate physical properties of the fiber must be optimized with regard for its performance in the final composite. Absence of adequate fiber test methods encompassing all these requirements obviously can have significant impact on the results and interpretations of the initial fiber-screening evaluations. • To withstand long-term thermal cycling conditions, the composition of the reinforcing fiber must be in chemical equilibrium with the surrounding matrix. This means that the reinforcing fiber may need to contain certain chemical constituents of a given matrix to ensure the integrity of the composite or that its surface must be coated or modified to make the fiber compatible with the matrix.

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 105 • Current research indicates a direct link between the microstructure of the fiber and the ultimate fiber properties, such as compressive strength and even sensitivity of the fiber to flaws. In the case of fibers with diameters frequently smaller than the dimensions of grains in traditional macro-sized articles, microstructures necessarily comprise relatively minuscule features, and direct comparison of their effects with those in relatively large entities may not be significant. Nevertheless, there is ample evidence that a fiber's acceptability may be determined by its microstructure. Moreover, the role of processing conditions on the development of this microstructure is poorly understood. If the full potential of high-performance fibers is to be realized, future research should be directed toward a fundamental understanding of this role as well as the relationship between microstructure and fiber properties. The Fiber-Matrix Interface ("Interphase") and Fiber Coatings In all types of fiber-reinforced composites, the region located between the fiber and the matrix is extremely important from the early stages of composite processing to the ultimate application. While often referred to as the fiber-matrix interface, this region may be of considerable thickness and volume and may be more appropriately referred to as the phase or "interphase" between fiber and matrix. Important considerations for this region are as follows: • The nature of the fiber surface is important in controlling initial fiber properties. In many cases it can determine handleability and control fiber strength. A classic example is the complex carbon-rich surface preparation of chemical vapor deposition-silicon carbide (CVD-SiC) monofilaments, which significantly improves fiber abrasion resistance and handleability. • The surface chemistry and features of the fiber are important in determining composite fabricability. In both polymer-matrix and metal-matrix composites produced by casting, the ability of the matrix to "wet out" on the fiber can be determined by the fiber surface. Well-known examples of this include the epoxy-compatible "sizings" applied to carbon and glass fibers and the Ti-B surface preparation of carbon fibers in preparation for aluminum and magnesium metal infiltration. • Both fabrication and service conditions are important in providing driving forces for reactions between fiber, interphase, and matrix. In general, any reaction between fiber and matrix will cause changes (usually a decrease) in the fiber's properties. This behavior has been controlled by the following: 1. Applying a coating to the fiber to prevent or minimize any reaction. 2. Creating a complex coating on the fiber that minimizes the effect of any reaction with the matrix on fiber strength and yet provides a region to stop crack propagation. A fiber of this type is the large- diameter SiC monofilament produced by Avco-Textron in the United States and shown in Figure 4.1.1

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 106 3. Allowing the fiber-matrix reaction to create an interphase region, which is helpful to the composite. With nicalon-fiber reinforced glass-ceramic composite, for example, the reaction that occurs during processing can cause an interphase that is desirable in stopping cracks (see Figure 4.2).2 The reactions which create this interphase also are accompanied by a reduction in fiber strength, but this has been shown to vary based on the choice of matrix chemistry. The link between fiber and matrix is very important in determining the properties of composites. Load transfer is accomplished from the generally lower-stiffness matrix to the high-modulus fibers through the interphase. Mechanical fiber-matrix bonding is sufficient to accomplish this transfer for axial composite properties, but highly bonded regions are necessary to achieve high levels of off-axis and shear properties. In both polymeric matrix composites (PMC) and metal matrix composites (MMC) systems it has generally been desirable to maximize the integrity and strength of the interphase region for this purpose. Fracture control presents a more complex issue. The interphase region provides the opportunity to control the fracture process in several ways. First, strong fiber-matrix bonding can cause the dissipation of large amounts of energy during crack growth by causing extensive shear deformation in the interphase and matrix surrounding the fiber. In MMC systems this has been extremely effective. The fracture of well-bonded boron-reinforced aluminum composites has been characterized by large amounts of matrix shear (see Figure 4.3).3 Second, weak fiber-matrix bonding can also be important in the fracture control process by diverting cracks as they reach the fiber-matrix interface and during fiber pullout. While highly prominent in the discussion of CMC systems, this can also be true for MMC composites, such as that shown in Figure 4.4.4 Finally, with CMCs the region separating the fiber and the matrix becomes even more important, because for maximum performance it must prevent crack propagation. In these systems the matrix may crack at rather low strains. If these cracks are diverted or interfered with, composite toughness will be enhanced and the fibers will be able to reach higher stress levels before fracturing. Thus, with CMCs the region between fiber and matrix must exhibit sufficient strength to transfer load and be weak enough to divert cracks. SUMMARY OF TECHNICAL ISSUES Progress in high-performance synthetic fibers of all types demands greater emphasis on fundamental research directed at four interdependent technical issues: • Fiber-formation processes and mechanisms. • The effect of fiber processing on microstructure.

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 107 Figure 4.1. Structure and composition of Sic Figure 4.2. Interphase region produced during fiber produced by Chemical Vapor Deposition. fabrication of Nicalon fiber reinforced glass- ceramics. Figure 4.3. Fracture surface of boron Figure 4.4. Fracture surface of Borsic reinforced reinforced 6061 aluminum with prenotched titanium with prenotched region also shown. region also shown.

IMPORTANT ISSUES IN FIBER SCIENCE/TECHNOLOGY 108 • The relationship of microstructure to fiber properties. • Fiber-matrix interfacial interactions during fabrication and in service. A systematic approach needs to be developed, starting from the known properties of bulk material, to identify and select candidate materials for fiber development. This approach should incorporate early consideration of the matrix and potential fiber-matrix interactions during processing as well as in the service life and environment of the final composite. Standardization of fiber characterization and testing procedures for comparative evaluation of fiber performance is highly desirable. REFERENCES 1. Brennan, J. J., ''Proc. Conf. on Tailoring of Multiphase and Composite Ceramics,'' Penn State Univ., R. Tressler, ed. New York: Plenum Press. pp. 549-560, 1985. 2. Prewo, K. M. 1986. J. Mat. Sci. 21:3590. 3. Prewo, K. M. 1985. "Fiber Reinforced Metal and Glass Matrix Composites," in Frontiers in Materials Technologies, M. Meyers and O. T. Inal, eds. Elsevier. 4. Prewo, K. M., B. Johnson, and S. Starrett. April 1989. J. Mat. Sci., 24:1373.

Next: 5 IMPORTANT POLICY ISSUES »
High Performance Synthetic Fibers for Composites Get This Book
×
Buy Paperback | $50.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

High performance synthetic fibers are key components of composite materials—a class of materials vital for U.S. military technology and for the civilian economy. This book addresses the major research and development opportunities for present and future structural composite applications and identifies steps that could be taken to accelerate the commercialization of this critical fiber technology in the United States.

The book stresses the need for redesigning university curricula to reflect the interdisciplinary nature of fiber science and technology. It also urges much greater government and industry cooperation in support of academic instruction and research and development in fiber-related disciplines.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!