MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers 3.3 PHYSICAL TECHNIQUES(INTRINSIC) The physical properties of fibers of importance in their applications in polymer matrix composites fall into two categories-those inherent in the filament itself (intrinsic).and those derived from the construc. tion of filaments into yarns,tows,or fabrics (extrinsic).The former includes density,diameter,and electri- cal resistivity;the latter includes yield,cross-sectional area,twist,fabric construction and areal weight. Density and the derived properties are used in the calculations required for the construction and analysis of the composite products.Density and yield are useful measures of quality assurance.Filament diame- ter and electrical resistivity are important for the nonstructural aspects of aerospace and aircraft applica- tions. 3.3.1 Filament diameter The average diameter of fibers may be determined by using an indexing microscope fitted with an image splitting eyepiece or from a photomicrograph of the cross-sectional view of a group of mounted fibers.Since fibers are not always true cylinders,effective diameters may be calculated from the total cross-sectional area of the yarn or tow and dividing by the number of filaments in the bundle.The cross- sectional area may also be estimated from the ratio of mass per unit length to density.For irregular,but characteristically-shaped,fibers an area factor may be required in calculating the average fiber diameter. Optical microscopy can provide information about fiber diameter and variation in diameter with length. The upper limit of resolution of the optical microscope is about one-tenth of a micron;hence features less than one micron can not be well-characterized by optical microscopy.A detailed procedure for the deter- mination of fiber diameter is described in Section 3.6.4. Other techniques,such as scanning electron microscopy (SEM),provide much higher resolution than optical microscopy for determining fiber diameter and cross-sectional characteristics.Features of fiber surfaces down to the 5 nanometer level can be observed.In addition,the large depth of field provided by SEM helps defined three-dimensional characteristics on fiber surfaces and define fiber topography. 3.3.2 Density of fibers 3.3.2.1 Overview Fiber density is not only an important quality control parameter in fiber manufacture,it is required for determination of the void content of the fibrous composite,as described in ASTM D 2734,"Void Content of Reinforced Plastics"(Reference 3.3.2.1(a)).Fiber density can also be used as a distinguishing pa- rameter to identify a fiber.For example,fiber density results can readily differentiate between E and S-2 glass(E glass is 2.54 g/cm(0.092 Ib/in),S-2 is 2.485 g/cm(0.090 Ib/in )) With few exceptions,the determination of density is accomplished indirectly by measuring the volume and weight of a representative sample of the fiber,and then combining these values to calculate density. The weight measurement is most easily obtained by using a quality analytical balance.To determine vol- ume,however,there are several approaches used.The most common approach uses simple Ar- chemedes methods involving displacement of liquids of known density.Direct measurement of density can be made by observation of the level to which the test material sinks in a density-graded liquid(Refer- ence3.3.2.1(b). Liquids are used almost exclusively in displacement techniques for the determination of volume. However,there are advantages to using a gas medium in place of liquid to determine the volume of fiber. One advantage is minimization of errors associated with liquid surface tension.The gas displacement approach is often referred to as helium pycnometry.When a gas displacement approach is used,the test specimen volume is determined by measuring pressure changes of a confined amount of a gas that be- haves as an ideal gas at room temperature(preferably high purity helium).Helium pycnometry is not a recognized test method for measuring the volume and density of fibers,yet it has been demonstrated to be a viable technique (References 3.3.2.1(c)and (d)).As no test standard or guidelines exist for this 3-6MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-6 3.3 PHYSICAL TECHNIQUES (INTRINSIC) The physical properties of fibers of importance in their applications in polymer matrix composites fall into two categories - those inherent in the filament itself (intrinsic), and those derived from the construc￾tion of filaments into yarns, tows, or fabrics (extrinsic). The former includes density, diameter, and electri￾cal resistivity; the latter includes yield, cross-sectional area, twist, fabric construction and areal weight. Density and the derived properties are used in the calculations required for the construction and analysis of the composite products. Density and yield are useful measures of quality assurance. Filament diame￾ter and electrical resistivity are important for the nonstructural aspects of aerospace and aircraft applica￾tions. 3.3.1 Filament diameter The average diameter of fibers may be determined by using an indexing microscope fitted with an image splitting eyepiece or from a photomicrograph of the cross-sectional view of a group of mounted fibers. Since fibers are not always true cylinders, effective diameters may be calculated from the total cross-sectional area of the yarn or tow and dividing by the number of filaments in the bundle. The cross￾sectional area may also be estimated from the ratio of mass per unit length to density. For irregular, but characteristically-shaped, fibers an area factor may be required in calculating the average fiber diameter. Optical microscopy can provide information about fiber diameter and variation in diameter with length. The upper limit of resolution of the optical microscope is about one-tenth of a micron; hence features less than one micron can not be well-characterized by optical microscopy. A detailed procedure for the deter￾mination of fiber diameter is described in Section 3.6.4. Other techniques, such as scanning electron microscopy (SEM), provide much higher resolution than optical microscopy for determining fiber diameter and cross-sectional characteristics. Features of fiber surfaces down to the 5 nanometer level can be observed. In addition, the large depth of field provided by SEM helps defined three-dimensional characteristics on fiber surfaces and define fiber topography. 3.3.2 Density of fibers 3.3.2.1 Overview Fiber density is not only an important quality control parameter in fiber manufacture, it is required for determination of the void content of the fibrous composite, as described in ASTM D 2734, "Void Content of Reinforced Plastics" (Reference 3.3.2.1(a)). Fiber density can also be used as a distinguishing pa￾rameter to identify a fiber. For example, fiber density results can readily differentiate between E and S-2 glass (E glass is 2.54 g/cm3 (0.092 lb/in3 ), S-2 is 2.485 g/cm3 (0.090 lb/in3 )). With few exceptions, the determination of density is accomplished indirectly by measuring the volume and weight of a representative sample of the fiber, and then combining these values to calculate density. The weight measurement is most easily obtained by using a quality analytical balance. To determine vol￾ume, however, there are several approaches used. The most common approach uses simple Ar￾chemedes methods involving displacement of liquids of known density. Direct measurement of density can be made by observation of the level to which the test material sinks in a density-graded liquid (Refer￾ence 3.3.2.1(b)). Liquids are used almost exclusively in displacement techniques for the determination of volume. However, there are advantages to using a gas medium in place of liquid to determine the volume of fiber. One advantage is minimization of errors associated with liquid surface tension. The gas displacement approach is often referred to as helium pycnometry. When a gas displacement approach is used, the test specimen volume is determined by measuring pressure changes of a confined amount of a gas that be￾haves as an ideal gas at room temperature (preferably high purity helium). Helium pycnometry is not a recognized test method for measuring the volume and density of fibers, yet it has been demonstrated to be a viable technique (References 3.3.2.1(c) and (d)). As no test standard or guidelines exist for this