《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 1）CHAPTER 3 EVALUATION OF REINFORCEMENT FIBERS

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 3-6

MIL-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

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers method as applied to fiber,a test procedure has been developed within the MIL-HDBK-17 Testing Work- ing Group (see Section 6.4.4.4.1). ASTM Test Method D 3800(Reference 3.3.2.1(e))deals specifically with obtaining the density of fi- bers.This standard covers three different liquid displacement procedures:Procedure A,which is very similar to the D 792 liquid displacement method (Reference 3.3.2.1(f);Procedure B,in which a low- density liquid is slowly mixed with a high-density liquid(containing the fibers)until the fibers become sus- pended;and Procedure C,which simply references D 1505,which is a density-gradient method. For detailed guidance on D 1505 and helium pycnometry,the reader is referred to Sections 6.4.4.3 through 6.4.4.5 of this volume of the Handbook.Note that Section 6.4.4 refers specifically to composites, but the methods discussed are fully applicable to fiber measurement except as noted below in Sections 3.3.2.2 through3.3.2.3. 3.3.2.2 ASTM D 3800,Standard Test Method for Density of High-Modulus Fibers The approach taken in ASTM D 3800 is threefold.Procedure A is identical to D 792 except that the immersion fluids recommended have only fibers in mind.The concern is complete fiber wetting and avoiding entrapped microbubbles.Procedure B relies on careful mixing of two liquids of different densi- ties (with the fiber immersed).When the fibers are suspended in the mixed liquid a hydrometer or liquid pycnometer is used to determine the density of the liquid.The density of the suspended fiber is equal to that of the liquid.Procedure C is D 1505 inserted as a part of D 3800 by reference. Given that apparatus and procedures are identical to D 792 for the liquid displacement procedure (Procedure A),and that Procedures B and C have much in common with D 1505,the reader is referred to Sections 6.4.4.2 through 6.4.4.5.Here,only those test aspects peculiar to fibers are discussed. The experimenter needs to be mindful to avoid entrapped bubbles,liquid absorption,and problems involving the fiber sizing coating (if any).Common sense immediately flags roving as a difficult fiber form to wet out,yet complete wetout is required to produce meaningful data.Pay close attention to the inter- filament regions.In D 1505 the problem is not as severe because the fibers can be cut and/or spread out prior to insertion.Since the measurement is direct the size of the fiber sample is irrelevant.Immersing many small fiber fragments allows for direct verification of density variations of the fiber,keeping in mind that small fragments may take hours to sink to their equilibrium density level.It can not be emphasized enough that complete wetout must be achieved.Use of high-wetting,vacuum-degassed liquids go a long way to this end.Remember that the fibers are a prime geometry for nucleation of gas bubbles out of so- lution.If the liquid is not fully degassed a bubble-free roving can quickly form new bubbles. The surface area to volume ratio of composite fibers is extremely high.For cylindrical shapes, S.A./V=2/R,where R,the radius,is only several microns.For a 0.028 mil (7 micron)fiber the ratio is 143,000 to 1.It is,therefore,very important to ensure compatibility between the fiber and liquid.Glass and polyethylene fibers are fairly immune in this regard;however,aramid,for example,is certainly not. The liquid immersion time should be kept to a minimum to avoid liquid diffusion into the fiber. The mistake is often made of thinking of the fiber by itself,when in reality it is usually coated with an interfacial sizing agent(to promote improved bonding with the matrix resin).It is good practice to re- search the sizing agent,as it is a completely different material than the fiber(with different absorption and chemical characteristics).Since the sizing is applied to the outer surface of the fiber even the volume of a thin coat quickly becomes significant.For example,a 0.028 mil(7 micron)diameter carbon fiber with a typical coating of 1%sizing agent on a weight basis (with assumed density of 1.2 g/cm(0.043 Ib/in ) gives a final product which is 98.5%fiber and 1.5%sizing on a volume basis.For precision work,strip the sizing agent off the fiber before measuring fiber density. 3-7

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-7 method as applied to fiber, a test procedure has been developed within the MIL-HDBK-17 Testing Work￾ing Group (see Section 6.4.4.4.1). ASTM Test Method D 3800 (Reference 3.3.2.1(e)) deals specifically with obtaining the density of fi￾bers. This standard covers three different liquid displacement procedures: Procedure A, which is very similar to the D 792 liquid displacement method (Reference 3.3.2.1(f)); Procedure B, in which a low￾density liquid is slowly mixed with a high-density liquid (containing the fibers) until the fibers become sus￾pended; and Procedure C, which simply references D 1505, which is a density-gradient method. For detailed guidance on D 1505 and helium pycnometry, the reader is referred to Sections 6.4.4.3 through 6.4.4.5 of this volume of the Handbook. Note that Section 6.4.4 refers specifically to composites, but the methods discussed are fully applicable to fiber measurement except as noted below in Sections 3.3.2.2 through 3.3.2.3. 3.3.2.2 ASTM D 3800, Standard Test Method for Density of High-Modulus Fibers The approach taken in ASTM D 3800 is threefold. Procedure A is identical to D 792 except that the immersion fluids recommended have only fibers in mind. The concern is complete fiber wetting and avoiding entrapped microbubbles. Procedure B relies on careful mixing of two liquids of different densi￾ties (with the fiber immersed). When the fibers are suspended in the mixed liquid a hydrometer or liquid pycnometer is used to determine the density of the liquid. The density of the suspended fiber is equal to that of the liquid. Procedure C is D 1505 inserted as a part of D 3800 by reference. Given that apparatus and procedures are identical to D 792 for the liquid displacement procedure (Procedure A), and that Procedures B and C have much in common with D 1505, the reader is referred to Sections 6.4.4.2 through 6.4.4.5. Here, only those test aspects peculiar to fibers are discussed. The experimenter needs to be mindful to avoid entrapped bubbles, liquid absorption, and problems involving the fiber sizing coating (if any). Common sense immediately flags roving as a difficult fiber form to wet out, yet complete wetout is required to produce meaningful data. Pay close attention to the inter￾filament regions. In D 1505 the problem is not as severe because the fibers can be cut and/or spread out prior to insertion. Since the measurement is direct the size of the fiber sample is irrelevant. Immersing many small fiber fragments allows for direct verification of density variations of the fiber, keeping in mind that small fragments may take hours to sink to their equilibrium density level. It can not be emphasized enough that complete wetout must be achieved. Use of high-wetting, vacuum-degassed liquids go a long way to this end. Remember that the fibers are a prime geometry for nucleation of gas bubbles out of so￾lution. If the liquid is not fully degassed a bubble-free roving can quickly form new bubbles. The surface area to volume ratio of composite fibers is extremely high. For cylindrical shapes, S.A./V=2/R, where R, the radius, is only several microns. For a 0.028 mil (7 micron) fiber the ratio is 143,000 to 1. It is, therefore, very important to ensure compatibility between the fiber and liquid. Glass and polyethylene fibers are fairly immune in this regard; however, aramid, for example, is certainly not. The liquid immersion time should be kept to a minimum to avoid liquid diffusion into the fiber. The mistake is often made of thinking of the fiber by itself, when in reality it is usually coated with an interfacial sizing agent (to promote improved bonding with the matrix resin). It is good practice to re￾search the sizing agent, as it is a completely different material than the fiber (with different absorption and chemical characteristics). Since the sizing is applied to the outer surface of the fiber even the volume of a thin coat quickly becomes significant. For example, a 0.028 mil (7 micron) diameter carbon fiber with a typical coating of 1% sizing agent on a weight basis (with assumed density of 1.2 g/cm3 (0.043 lb/in3 )) gives a final product which is 98.5% fiber and 1.5% sizing on a volume basis. For precision work, strip the sizing agent off the fiber before measuring fiber density

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers 3.3.2.3 Recommended procedure changes to Section 6.6.4.4.1(helium pycnometry)for use in measur- ing fiber density In general,it would seem that helium pycnometry lends itself to the measurement of fiber vol- ume/density(although this has yet to be rigorously tested).This is mainly due to the fact that the inert gas medium circumvents the issue of fiber wetout,which is a concern when using any of the liquid im- mersion methods.Recommended changes to the procedure in Section 6.6.4.4.1 are as follows: To prepare the fiber specimens,cut them to the height of the sample cell and stand them on end to get best packing. ● Fill the cell volume to a minimum of 30%of its full capacity. Precondition the fibers in the same manner as for immersion testing. Follow the instructions under step 2. 3.3.2.4 Density test methods for MIL-HDBK-17 data submittal Data produced by the following test methods (Table 3.3.2.4)are currently being accepted by MIL- HDBK-17 for consideration for inclusion in Volume 2. TABLE 3.3.2.4 Fiber density test methods for MIL-HDBK-17 data submittal. Property Symbol Fully Approved,Interim,and Screening Data Screening Data Only Density 0 D3800A,D3800C,D1505,3.3.2.3* D3800B* *When this method is used to generate data for subsequent determination of composite void volume,the test specimen must occupy at least 30%of the test cell volume. **Data from this method is not recommended for use in determining void volume of composites due to precision limitations. 3.3.3 Electrical resistivity The determination of electrical resistivity is recommended as a control measure for checking process- ing temperature and to determine compliance with specific resistance specifications,where required. Electrical resistivity is one of the properties dramatically affected by the structural anisotropy of carbon fibers.Measurements can be made on either a single filament or a yarn.The measured value is resis- tance per given length of fiber as read on an ohm meter or similar device.The contact resistance can be eliminated by obtaining the resistance for two different lengths of fiber and calculating the difference due to the longer length.This difference is then converted to resistance per unit length and then multiplied by the area of the fiber or yarn bundle expressed in consistent units.Resistivity is expressed as ohm- centimeter,ohm-meter,or ohm-inches and refers to the value in the axial direction.Transverse resistivity is seldom reported.A procedure for determining the electrical resistance of carbon cloth or felt is de- scribed in Section 3.6.5. 3-8

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-8 3.3.2.3 Recommended procedure changes to Section 6.6.4.4.1 (helium pycnometry) for use in measur￾ing fiber density In general, it would seem that helium pycnometry lends itself to the measurement of fiber vol￾ume/density (although this has yet to be rigorously tested). This is mainly due to the fact that the inert gas medium circumvents the issue of fiber wetout, which is a concern when using any of the liquid im￾mersion methods. Recommended changes to the procedure in Section 6.6.4.4.1 are as follows: • To prepare the fiber specimens, cut them to the height of the sample cell and stand them on end to get best packing. • Fill the cell volume to a minimum of 30% of its full capacity. • Precondition the fibers in the same manner as for immersion testing. • Follow the instructions under step 2. 3.3.2.4 Density test methods for MIL-HDBK-17 data submittal Data produced by the following test methods (Table 3.3.2.4) are currently being accepted by MIL￾HDBK-17 for consideration for inclusion in Volume 2. TABLE 3.3.2.4 Fiber density test methods for MIL-HDBK-17 data submittal. Property Symbol Fully Approved, Interim, and Screening Data Screening Data Only Density ρ D 3800A, D 3800C, D 1505, 3.3.2.3* D 3800B** *When this method is used to generate data for subsequent determination of composite void volume, the test specimen must occupy at least 30% of the test cell volume. **Data from this method is not recommended for use in determining void volume of composites due to precision limitations. 3.3.3 Electrical resistivity The determination of electrical resistivity is recommended as a control measure for checking process￾ing temperature and to determine compliance with specific resistance specifications, where required. Electrical resistivity is one of the properties dramatically affected by the structural anisotropy of carbon fibers. Measurements can be made on either a single filament or a yarn. The measured value is resis￾tance per given length of fiber as read on an ohm meter or similar device. The contact resistance can be eliminated by obtaining the resistance for two different lengths of fiber and calculating the difference due to the longer length. This difference is then converted to resistance per unit length and then multiplied by the area of the fiber or yarn bundle expressed in consistent units. Resistivity is expressed as ohm￾centimeter, ohm-meter, or ohm-inches and refers to the value in the axial direction. Transverse resistivity is seldom reported. A procedure for determining the electrical resistance of carbon cloth or felt is de￾scribed in Section 3.6.5