MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers Trace metallic constituents are significant in carbon and polymeric fibers because of their possible effect on the rate of fiber oxidation.The presence of metals is usually expressed as parts per million in the original dry fiber and can be determined by analyzing the ash residue.Semi-quantitative determina- tions are generally made using flame emission spectroscopy.When quantitative values are desired, atomic absorption methods are used.With respect to oxidation of carbon fibers,sodium is usually of most concern because of its tendency to catalyze the oxidation of carbon. 3.2.2 Titration The potential chemical activity of surface groups on fibers may be determined by titration techniques. For example,the relative concentration of hydrolyzable groups introduced during the manufacture or post treatment of carbon fibers may be determined by measuring the pH (section 3.6.1).However,titration techniques are typically not used on commercial carbon fibers due to the low levels of surface functional- ity. 3.2.3 Fiber structure X-Ray diffraction spectroscopy may be used to characterize the overall structure of crystalline or semi-crystalline fibers.The degree of crystallinity and orientation of crystallites have a direct effect on the modulus and other critical properties of carbon and polymeric fibers X-ray powder diffraction using commercial power supplies and diffractometer units is used to charac- terize the structure of carbon fibers.The fiber is ground into a fine powder and then the X-ray powder diffraction pattern is taken using CuK radiation.The patterns generally undergo computer analysis to de- termine the following parameters: (a)Average graphite layer spacing:from the 002 peak position. (b)Average crystal size L:from the 002 peak width (c)Average crystal size L:from the 100 peak width. (d)Average lattice dimension a-axis:from the 100 peak position. (e)The ratio of peak area to the diffused area. (f)The 002 peak area to the total diffraction area. (g)The 100 peak area to the total diffraction area. (h)The ratio of the 100 to 002 peak areas. (i)Crystallinity index:from a comparison of the X-ray diffraction of known crystallized and amor- phous carbons. X-ray scattering of crystalline fibrous materials shows the presence of sharp and diffuse diffraction patterns which are indicative of crystal phases interdispersed with amorphous regions.The concept of the crystallinity index is derived from the fact that a portion of the scattering from a fiber is diffuse and thereby contributes to the so-called amorphous background.Thus,a simple method of estimating crystal- linity is obtained by separating the diffraction pattern into crystalline (sharp)and amorphous (diffuse) components.The crystallinity index is a relative measure of crystallinity,and not an absolute numerical result,useful for correlating with physical properties of fibers. Wide angle X-ray spectroscopy and infrared spectroscopy techniques have also been developed to determine the crystallinity and orientation of molecules in polymeric fibers.Testing and interpretation of results requires specialized equipment,sophisticated computer models,and a high level of technical ex- pertise. 3.2.4 Fiber surface chemistry Fibers generally are given a surface treatment to improve the adhesion between the fibers and resin matrix materials.Gases,plasmas,liquid chemical or electrolytic treatments are employed to modify fiber 3-2MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-2 Trace metallic constituents are significant in carbon and polymeric fibers because of their possible effect on the rate of fiber oxidation. The presence of metals is usually expressed as parts per million in the original dry fiber and can be determined by analyzing the ash residue. Semi-quantitative determina￾tions are generally made using flame emission spectroscopy. When quantitative values are desired, atomic absorption methods are used. With respect to oxidation of carbon fibers, sodium is usually of most concern because of its tendency to catalyze the oxidation of carbon. 3.2.2 Titration The potential chemical activity of surface groups on fibers may be determined by titration techniques. For example, the relative concentration of hydrolyzable groups introduced during the manufacture or post treatment of carbon fibers may be determined by measuring the pH (section 3.6.1). However, titration techniques are typically not used on commercial carbon fibers due to the low levels of surface functional￾ity. 3.2.3 Fiber structure X-Ray diffraction spectroscopy may be used to characterize the overall structure of crystalline or semi-crystalline fibers. The degree of crystallinity and orientation of crystallites have a direct effect on the modulus and other critical properties of carbon and polymeric fibers. X-ray powder diffraction using commercial power supplies and diffractometer units is used to charac￾terize the structure of carbon fibers. The fiber is ground into a fine powder and then the X-ray powder diffraction pattern is taken using CuK radiation. The patterns generally undergo computer analysis to de￾termine the following parameters: (a) Average graphite layer spacing: from the 002 peak position. (b) Average crystal size Lc: from the 002 peak width (c) Average crystal size La: from the 100 peak width. (d) Average lattice dimension a-axis: from the 100 peak position. (e) The ratio of peak area to the diffused area. (f) The 002 peak area to the total diffraction area. (g) The 100 peak area to the total diffraction area. (h) The ratio of the 100 to 002 peak areas. (i) Crystallinity index: from a comparison of the X-ray diffraction of known crystallized and amor￾phous carbons. X-ray scattering of crystalline fibrous materials shows the presence of sharp and diffuse diffraction patterns which are indicative of crystal phases interdispersed with amorphous regions. The concept of the crystallinity index is derived from the fact that a portion of the scattering from a fiber is diffuse and thereby contributes to the so-called amorphous background. Thus, a simple method of estimating crystal￾linity is obtained by separating the diffraction pattern into crystalline (sharp) and amorphous (diffuse) components. The crystallinity index is a relative measure of crystallinity, and not an absolute numerical result, useful for correlating with physical properties of fibers. Wide angle X-ray spectroscopy and infrared spectroscopy techniques have also been developed to determine the crystallinity and orientation of molecules in polymeric fibers. Testing and interpretation of results requires specialized equipment, sophisticated computer models, and a high level of technical ex￾pertise. 3.2.4 Fiber surface chemistry Fibers generally are given a surface treatment to improve the adhesion between the fibers and resin matrix materials. Gases, plasmas, liquid chemical or electrolytic treatments are employed to modify fiber