MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS 3.1 INTRODUCTION This chapter describes techniques and test methods that are generally used to characterize the chemical,physical,and mechanical properties of reinforcement fibers for application in organic matrix composite materials.Reinforcements in the form of unidirectional yarns,strands,or tows,and bidirec- tional fabrics are covered.Sophisticated experimental techniques generally are required for fiber charac- terization,and test laboratories must be well-equipped and experienced for measuring fiber properties.It is also recognized that in many cases the measurement of a fiber property that manifests itself in the rein- forced composite can best be accomplished with the composite.Sections 3.2 through 3.5 recommend general techniques and test methods for evaluating carbon,glass,organic (polymeric),and other spe- cialty reinforcement fibers.Section 3.6 contains examples of test methods that can be used for evaluating fibers. Most reinforcement fibers are surface treated or have a surface treatment(e.g.,sizing)applied during their production to improve handleability and/or promote fiber-resin bonding.Surface treatments affect wettability of the fiber during impregnation as well as the dry strength and hydrolytic stability of the fiber- matrix bond during use.Because of the direct relation to composite properties,the effectiveness of any treatments to modify surface chemistry is generally measured on the composite itself by means of me- chanical tests.The amount of sizing and its compositional consistency are significant in quality control of the fiber and measurement of these parameters is part of the fiber evaluation. 3.2 CHEMICAL TECHNIQUES A wide variety of chemical and spectroscopic techniques and test methods are available to character- ize the chemical structures and compositions of reinforcement fibers.Carbon fibers are found to range from 90-100%carbon.Typically,standard and intermediate modulus PAN carbon fibers are 90-95%car- bon,with most of the remaining material being nitrogen.Minor constituents and trace elements can be extremely important when composites containing these fibers are considered for use at elevated tempera- tures(above 500F or 260C).Organic fibers usually contain significant amounts of hydrogen and one or more additional elements (e.g.,oxygen,nitrogen,and sulfur)which can be identified by spectroscopic analysis.Glass fibers contain sulfur dioxide and usually aluminum and iron oxide.Depending upon the type of glass.calcium oxide,sodium oxide,and oxides of potassium,boron,barium,titanium,zirconium, sulfur,and arsenic may be found. 3.2.1 Elemental analysis A variety of quantitative wet gravimetric and spectroscopic chemical analysis techniques may be ap- plied to analyze the compositions and trace elements in fibers.ASTM Test Method C 169 may be used to determine the chemical compositions of borosilicate glass fibers(Reference 3.2.1(a)). A suitable standardized method for carbon and hydrogen analysis,modified to handle carbon and polymeric fibers is provided by ASTM D 3178(Reference 3.2.1(b)).Carbon and hydrogen concentrations are determined by burning a weighed quantity of sample in a closed system and fixing the products of combustion in an absorption train after complete oxidation and purification from interfering substances. Carbon and hydrogen concentrations are expressed as percentages of the total dry weight of the fiber. ASTM Method D 3174(Reference 3.2.1(c)describes a related test in which metallic impurities may be determined by the analysis of ash residue. Alternatively,a variety of commercial analytical instruments are available which can quickly analyze carbon,hydrogen,nitrogen,silicon,sodium,aluminum,calcium,magnesium and other elements in rein- forcement fibers.X-ray fluorescence,atomic absorption (AA),flame emission,and inductively coupled plasma emission(ICAP)spectroscopic techniques may be employed for elemental analysis.Operating instructions and method details are available from the instrument manufacturers. 3-1MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-1 CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS 3.1 INTRODUCTION This chapter describes techniques and test methods that are generally used to characterize the chemical, physical, and mechanical properties of reinforcement fibers for application in organic matrix composite materials. Reinforcements in the form of unidirectional yarns, strands, or tows, and bidirec￾tional fabrics are covered. Sophisticated experimental techniques generally are required for fiber charac￾terization, and test laboratories must be well-equipped and experienced for measuring fiber properties. It is also recognized that in many cases the measurement of a fiber property that manifests itself in the rein￾forced composite can best be accomplished with the composite. Sections 3.2 through 3.5 recommend general techniques and test methods for evaluating carbon, glass, organic (polymeric), and other spe￾cialty reinforcement fibers. Section 3.6 contains examples of test methods that can be used for evaluating fibers. Most reinforcement fibers are surface treated or have a surface treatment (e.g., sizing) applied during their production to improve handleability and/or promote fiber-resin bonding. Surface treatments affect wettability of the fiber during impregnation as well as the dry strength and hydrolytic stability of the fiber￾matrix bond during use. Because of the direct relation to composite properties, the effectiveness of any treatments to modify surface chemistry is generally measured on the composite itself by means of me￾chanical tests. The amount of sizing and its compositional consistency are significant in quality control of the fiber and measurement of these parameters is part of the fiber evaluation. 3.2 CHEMICAL TECHNIQUES A wide variety of chemical and spectroscopic techniques and test methods are available to character￾ize the chemical structures and compositions of reinforcement fibers. Carbon fibers are found to range from 90-100% carbon. Typically, standard and intermediate modulus PAN carbon fibers are 90-95% car￾bon, with most of the remaining material being nitrogen. Minor constituents and trace elements can be extremely important when composites containing these fibers are considered for use at elevated tempera￾tures (above 500°F or 260°C). Organic fibers usually contain significant amounts of hydrogen and one or more additional elements (e.g., oxygen, nitrogen, and sulfur) which can be identified by spectroscopic analysis. Glass fibers contain sulfur dioxide and usually aluminum and iron oxide. Depending upon the type of glass, calcium oxide, sodium oxide, and oxides of potassium, boron, barium, titanium, zirconium, sulfur, and arsenic may be found. 3.2.1 Elemental analysis A variety of quantitative wet gravimetric and spectroscopic chemical analysis techniques may be ap￾plied to analyze the compositions and trace elements in fibers. ASTM Test Method C 169 may be used to determine the chemical compositions of borosilicate glass fibers (Reference 3.2.1(a)). A suitable standardized method for carbon and hydrogen analysis, modified to handle carbon and polymeric fibers is provided by ASTM D 3178 (Reference 3.2.1(b)). Carbon and hydrogen concentrations are determined by burning a weighed quantity of sample in a closed system and fixing the products of combustion in an absorption train after complete oxidation and purification from interfering substances. Carbon and hydrogen concentrations are expressed as percentages of the total dry weight of the fiber. ASTM Method D 3174 (Reference 3.2.1(c) describes a related test in which metallic impurities may be determined by the analysis of ash residue. Alternatively, a variety of commercial analytical instruments are available which can quickly analyze carbon, hydrogen, nitrogen, silicon, sodium, aluminum, calcium, magnesium and other elements in rein￾forcement fibers. X-ray fluorescence, atomic absorption (AA), flame emission, and inductively coupled plasma emission (ICAP) spectroscopic techniques may be employed for elemental analysis. Operating instructions and method details are available from the instrument manufacturers