《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 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. 3-1

MIL-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

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 3-2

MIL-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

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers surfaces.Introducing surface oxidation is perhaps the most common approach to modifying fiber sur- faces. Fiber surface structure,the modifications which surfaces undergo as a result of the different fiber sur- face treatments,and the relative importance of these modifications for composite properties are not well understood.This arises because of the small surface areas involved(0.5 to 1.5 m /g)and the very low concentrations of functional groups.If 20%of the surface was covered by one particular species,this would only amount to 1 umole of chemical groups per gram of fiber.Surface characterization should be carried out on fibers which have not been sized.Residual size from solvent desized fiber can interfere with most techniques,while pyrolysis techniques may alter the fiber surface due to oxidation and char products. The following techniques have been used for characterizing fiber surfaces: (a)X-ray diffraction-provides information relating to crystallite size and orientation,degree of graph- itization,and micropore characteristics. (b)Electron diffraction-gives crystallite orientation,three-dimensional order,and degree of graph- itization.(better for surfaces since penetration is only 1000-). (c)Transmission Electron Microscopy (TEM)-provides the highest resolution of any of the micro- scopic techniques routinely available.Ultramicrotomy can be used to prepare specimens,typi- cally about 50 nanometers thick,for direct TEM analysis of the fiber surfaces.TEM provides in- formation about surface fine structure and show fibrils and needle-like pores. (d)Scanning Electron Microscopy(SEM)-Gives structural and surface features.SEM is a useful technique for determining fiber diameters and identifying morphological characteristics (scales, chips,deposits,pits)on fiber surfaces. (e)Electron Spin Resonance(ESR)Spectroscopy-gives crystallite orientation. (f)X-ray Photoelectron Spectroscopy(XPS)or Electron Spectroscopy for Chemical Analysis(ESCA) measures the binding energy of core electrons in atoms excited by low energy X-rays.Changes in the chemical environment of a surface region 10-15 nanometers thick(the first few atomic lay- ers)are revealed by slight shifts in the energy of these core electrons giving information on func- tional group types and concentrations.The surface sensitivity arises because the depth of the electrons is between 1 and 2 nanometers. The ratios of total oxygen to total carbon and of oxidized carbon(including hydroxyl,ether,ester,car- bonyl and carboxy functional groups)to total carbon may be determined in carbon fibers using XPS or ESCA. (g)Auger Electron Spectroscopy (AES)-directs high energy electrons (1-5 KeV)onto surfaces to create vacancies in the core levels of atoms.These vacancies represent excited ions which may undergo de-excitation and thereby create Auger electrons.By analyzing the characteristic ener- gies of all the back-scattered Auger electrons in the energy range 0-1 KeV,the elemental compo- sition of the first 30 or 40 atomic layers(about 30 nanometers)is possible and in some cases mo- lecular information can be obtained from analysis of data. (h)lon Scattering Spectroscopy(ISS)-uses an ion as a molecular probe to identify elements on the outermost surface layer.Only atomic information can be obtained and sensitivity depends upon the atomic element. 3-3

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-3 surfaces. Introducing surface oxidation is perhaps the most common approach to modifying fiber sur￾faces. Fiber surface structure, the modifications which surfaces undergo as a result of the different fiber sur￾face treatments, and the relative importance of these modifications for composite properties are not well understood. This arises because of the small surface areas involved (0.5 to 1.5 m2 /g) and the very low concentrations of functional groups. If 20% of the surface was covered by one particular species, this would only amount to 1 µmole of chemical groups per gram of fiber. Surface characterization should be carried out on fibers which have not been sized. Residual size from solvent desized fiber can interfere with most techniques, while pyrolysis techniques may alter the fiber surface due to oxidation and char products. The following techniques have been used for characterizing fiber surfaces: (a) X-ray diffraction - provides information relating to crystallite size and orientation, degree of graph￾itization, and micropore characteristics. (b) Electron diffraction - gives crystallite orientation, three-dimensional order, and degree of graph￾itization. (better for surfaces since penetration is only 1000S). (c) Transmission Electron Microscopy (TEM) - provides the highest resolution of any of the micro￾scopic techniques routinely available. Ultramicrotomy can be used to prepare specimens, typi￾cally about 50 nanometers thick, for direct TEM analysis of the fiber surfaces. TEM provides in￾formation about surface fine structure and show fibrils and needle-like pores. (d) Scanning Electron Microscopy (SEM) - Gives structural and surface features. SEM is a useful technique for determining fiber diameters and identifying morphological characteristics (scales, chips, deposits, pits) on fiber surfaces. (e) Electron Spin Resonance (ESR) Spectroscopy - gives crystallite orientation. (f) X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) - measures the binding energy of core electrons in atoms excited by low energy X-rays. Changes in the chemical environment of a surface region 10-15 nanometers thick (the first few atomic lay￾ers) are revealed by slight shifts in the energy of these core electrons giving information on func￾tional group types and concentrations. The surface sensitivity arises because the depth of the electrons is between 1 and 2 nanometers. The ratios of total oxygen to total carbon and of oxidized carbon (including hydroxyl, ether, ester, car￾bonyl and carboxy functional groups) to total carbon may be determined in carbon fibers using XPS or ESCA. (g) Auger Electron Spectroscopy (AES) - directs high energy electrons (1-5 KeV) onto surfaces to create vacancies in the core levels of atoms. These vacancies represent excited ions which may undergo de-excitation and thereby create Auger electrons. By analyzing the characteristic ener￾gies of all the back-scattered Auger electrons in the energy range 0-1 KeV, the elemental compo￾sition of the first 30 or 40 atomic layers (about 30 nanometers) is possible and in some cases mo￾lecular information can be obtained from analysis of data. (h) Ion Scattering Spectroscopy (ISS) - uses an ion as a molecular probe to identify elements on the outermost surface layer. Only atomic information can be obtained and sensitivity depends upon the atomic element

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers (i)Secondary lon Mass Spectroscopy(SIMS)-uses a controlled sputtering process with accelerated ions to remove surface atomic layers for direct analysis by mass spectroscopy.SIMS can be used to identify surface molecules and determine their concentrations. (j)Infrared Spectroscopy(IRS)or Fourier Transform IRS(FTIRS)-absorption vibrational spectros- copy technique to obtain molecular information about surface composition.IRS yields both quali- tative and quantitative information relating to the chemical composition of surface molecules.The quality of the IR analysis depends on the fiber composition and is directly related to the care taken during sample preparation. For fibers with diameters between 0.015 and 0.03 mm,no sample preparation is required if an IR mi- croscope is available to examine fibers directly.Organic fibers may be pressed (up to 1000/m) into a film of fiber grids. (k)Laser Raman spectroscopy-absorption/vibrational spectroscopic technique which complements IR and is relatively simple to apply.Little or no sample preparation is necessary.Fibers can be oriented in the path of the incident beam for direct analysis.Fiber sample must be stable to the high intensity incident light and should not contain species that fluoresce. (1)Contact angle and wetting measurements-provide an indirect measurement of fiber surface free energy for use in predicting interfacial compatibility and thermodynamic equilibrium with matrix materials.Contact angle and wetting measurement information can be obtained by direct meas- urement of contact angle,mass pick-up,or surface velocity.Measurement of contact angles on small diameter fibers(<10 microns)is difficult if done optically.If a fiber's dimensions are known, a simple force balance may be used to determine the contact angle by measuring the force in- duced by immersing the fiber into a liquid of known surface free energy.The apparatus usually employed for this test is the Wilhelmy balance(Reference 3.2.4(a)). Contact angles 0 also may be measured indirectly by the micro-Wilhelmy technique (References 3.2.4(b-e)).A single fiber is partially immersed in a liquid and the force exerted on the fiber due to the surface tension of the liquid is measured.The contact angle is determined from the relation- ship F CyLv cose where F is the force measured corrected for buoyancy,C is the circumference of the fiber,and YLv is the surface tension of the liquid.The results may be used to determine the fiber surface free energy and the contributions of polar and dispersive components to the free en- ergy (References 3.2.4(c)and(d)). (m)Physisorption and chemisorption measurements -adsorption of inert gas or organic molecules can be used to measure fiber surface area.To obtain accurate estimates of surface area,it is important that there is complete monolayer coverage of the surface,that the area occupied by the adsorbed gas is known and that significant amounts of the gas are not taken up in micropores. Additional complications arise when the adsorption of organic molecules is used in place of gas adsorption,since it may be necessary to know the orientation of the adsorbed molecules to calcu- late surface area.Adsorption may also occur only at specific active sites and,if solutions are used,solvent molecules may be adsorbed as well. The chemical reactivity of fiber surfaces can be determined by oxygen chemisorption and desorption measurements.Topographical changes (e.g.,pores,cracks and fissures)caused by surface treatments often can be readily detected by adsorption measurements.Flow microcalorimetry is a useful technique for directly measuring heats of adsorption(Reference 3.2.4(f)). (n)Thermal desorption measurements-desorption of volatile products from fibers by heat treatment in vacuo.Thermal gravimetric analysis (TGA),gas chromatography (GC),mass spectroscopy (MS).infrared spectroscopy(IRS)analysis or combinations of pyrolysis GC/MS or TGA/IRS may be used to identify components desorbed from fiber surfaces.Below 150C,CO,NH,CH and various organic molecules are observed depending upon the fiber type. 3-4

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-4 (i) Secondary Ion Mass Spectroscopy (SIMS) - uses a controlled sputtering process with accelerated ions to remove surface atomic layers for direct analysis by mass spectroscopy. SIMS can be used to identify surface molecules and determine their concentrations. (j) Infrared Spectroscopy (IRS) or Fourier Transform IRS (FTIRS) - absorption vibrational spectros￾copy technique to obtain molecular information about surface composition. IRS yields both quali￾tative and quantitative information relating to the chemical composition of surface molecules. The quality of the IR analysis depends on the fiber composition and is directly related to the care taken during sample preparation. For fibers with diameters between 0.015 and 0.03 mm, no sample preparation is required if an IR mi￾croscope is available to examine fibers directly. Organic fibers may be pressed (up to 1000/m2 ) into a film of fiber grids. (k) Laser Raman spectroscopy - absorption/vibrational spectroscopic technique which complements IR and is relatively simple to apply. Little or no sample preparation is necessary. Fibers can be oriented in the path of the incident beam for direct analysis. Fiber sample must be stable to the high intensity incident light and should not contain species that fluoresce. (l) Contact angle and wetting measurements - provide an indirect measurement of fiber surface free energy for use in predicting interfacial compatibility and thermodynamic equilibrium with matrix materials. Contact angle and wetting measurement information can be obtained by direct meas￾urement of contact angle, mass pick-up, or surface velocity. Measurement of contact angles on small diameter fibers (< 10 microns) is difficult if done optically. If a fiber's dimensions are known, a simple force balance may be used to determine the contact angle by measuring the force in￾duced by immersing the fiber into a liquid of known surface free energy. The apparatus usually employed for this test is the Wilhelmy balance (Reference 3.2.4(a)). Contact angles θ also may be measured indirectly by the micro-Wilhelmy technique (References 3.2.4(b-e)). A single fiber is partially immersed in a liquid and the force exerted on the fiber due to the surface tension of the liquid is measured. The contact angle is determined from the relation￾ship F = CγLV cosθ where F is the force measured corrected for buoyancy, C is the circumference of the fiber, and γLV is the surface tension of the liquid. The results may be used to determine the fiber surface free energy and the contributions of polar and dispersive components to the free en￾ergy (References 3.2.4(c) and (d)). (m) Physisorption and chemisorption measurements - adsorption of inert gas or organic molecules can be used to measure fiber surface area. To obtain accurate estimates of surface area, it is important that there is complete monolayer coverage of the surface, that the area occupied by the adsorbed gas is known and that significant amounts of the gas are not taken up in micropores. Additional complications arise when the adsorption of organic molecules is used in place of gas adsorption, since it may be necessary to know the orientation of the adsorbed molecules to calcu￾late surface area. Adsorption may also occur only at specific active sites and, if solutions are used, solvent molecules may be adsorbed as well. The chemical reactivity of fiber surfaces can be determined by oxygen chemisorption and desorption measurements. Topographical changes (e.g., pores, cracks and fissures) caused by surface treatments often can be readily detected by adsorption measurements. Flow microcalorimetry is a useful technique for directly measuring heats of adsorption (Reference 3.2.4(f)). (n) Thermal desorption measurements - desorption of volatile products from fibers by heat treatment in vacuo. Thermal gravimetric analysis (TGA), gas chromatography (GC), mass spectroscopy (MS), infrared spectroscopy (IRS) analysis or combinations of pyrolysis GC/MS or TGA/IRS may be used to identify components desorbed from fiber surfaces. Below 150°C, CO, NH , CH and various organic molecules are observed depending upon the fiber type

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers (o)Chemical identification of functional groups by titrimetric,coulometric and radiographic tech- niques. 3.2.5 Sizing content and composition The amount of sizing contained on fibers is expressed as a percentage of the dry sized fiber weight. It is generally determined by extracting the fibers with a heated solvent;then the cleaned fibers are washed,dried,and weighed.ASTM Test Method C 613(Reference 3.2.5)describes a suitable method utilizing Soxhlet extraction equipment;however,similar extractions using a laboratory hot plate and beaker are also common.The selection of a solvent which quantitatively removes all the sizing but not does dissolve the fiber is essential for accuracy in this determination. Thermal removal techniques are also utilized and are most practical for the more difficult soluble siz- ings.Time,temperature,and atmosphere conditions must be predetermined to ensure the sizing is re- moved with out seriously affecting the fiber.The precise amounts of residue from decomposition of the sizing and weight loss of the fibers due to oxidation must also be known from control tests for greatest accuracy.SACMA recommended test method SRM 14-90 "Determination of Sizing Content on Carbon Fibers"describes a pyrolysis technique for carbon fibers. Sizing compositions and lot-to-lot chemical consistency may be determined by spectroscopic and chromatographic analysis of materials isolated by extracting the fibers with a suitable solvent.Acetone tetrahydrofuran and methylene chloride are commonly used solvents for extraction.Liquid and gas chro- matography and diffuse infrared spectroscopy are used to analyze or"fingerprint"the chemical composi- tions of extracts 3.2.6 Moisture content The moisture content or moisture regain of fibers or textiles may be determined using the procedure shown in Section 3.6.3.Care must be taken when applying the procedure since volatile materials in addi- tion to moisture may be removed.If possible,tests should be performed on fibers that have not been sized.Moisture content is expressed as weight percentage moisture based upon the dry weight of the specimen. 3.2.7 Thermal stability and oxidative resistance The susceptibility of fibers and fiber surface to oxidation is measured as weight loss under given con- ditions of time,temperature,and atmosphere.This is especially important in the evaluation of carbon and organic fibers considered for use in plastics exposed to elevated temperatures since it contributes to the long term degradation of composite properties.Thermal gravimetric analysis(TGA)may be used to de- termine the thermal decomposition temperature Td of carbon and organic fibers and estimate the relative amounts of volatile,organic additives and inorganic residues. A standard method for determining the weight loss of carbon fibers is given in ASTM Test Method D 4102(Reference 3.2.7(a)).Variations in this test method regarding exposure of fibers have been stud- ied and give similar results(Reference 3.2.7(b)).In order to minimize variability in test results,proper control of gas flow rates and currents is critical when performing TGA analyses. 3.2.8 Chemical resistance This section reserved for future use. 3-5

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-5 (o) Chemical identification of functional groups by titrimetric, coulometric and radiographic tech￾niques. 3.2.5 Sizing content and composition The amount of sizing contained on fibers is expressed as a percentage of the dry sized fiber weight. It is generally determined by extracting the fibers with a heated solvent; then the cleaned fibers are washed, dried, and weighed. ASTM Test Method C 613 (Reference 3.2.5) describes a suitable method utilizing Soxhlet extraction equipment; however, similar extractions using a laboratory hot plate and beaker are also common. The selection of a solvent which quantitatively removes all the sizing but not does dissolve the fiber is essential for accuracy in this determination. Thermal removal techniques are also utilized and are most practical for the more difficult soluble siz￾ings. Time, temperature, and atmosphere conditions must be predetermined to ensure the sizing is re￾moved with out seriously affecting the fiber. The precise amounts of residue from decomposition of the sizing and weight loss of the fibers due to oxidation must also be known from control tests for greatest accuracy. SACMA recommended test method SRM 14-90 "Determination of Sizing Content on Carbon Fibers" describes a pyrolysis technique for carbon fibers. Sizing compositions and lot-to-lot chemical consistency may be determined by spectroscopic and chromatographic analysis of materials isolated by extracting the fibers with a suitable solvent. Acetone, tetrahydrofuran and methylene chloride are commonly used solvents for extraction. Liquid and gas chro￾matography and diffuse infrared spectroscopy are used to analyze or "fingerprint" the chemical composi￾tions of extracts. 3.2.6 Moisture content The moisture content or moisture regain of fibers or textiles may be determined using the procedure shown in Section 3.6.3. Care must be taken when applying the procedure since volatile materials in addi￾tion to moisture may be removed. If possible, tests should be performed on fibers that have not been sized. Moisture content is expressed as weight percentage moisture based upon the dry weight of the specimen. 3.2.7 Thermal stability and oxidative resistance The susceptibility of fibers and fiber surface to oxidation is measured as weight loss under given con￾ditions of time, temperature, and atmosphere. This is especially important in the evaluation of carbon and organic fibers considered for use in plastics exposed to elevated temperatures since it contributes to the long term degradation of composite properties. Thermal gravimetric analysis (TGA) may be used to de￾termine the thermal decomposition temperature Td of carbon and organic fibers and estimate the relative amounts of volatile, organic additives and inorganic residues. A standard method for determining the weight loss of carbon fibers is given in ASTM Test Method D 4102 (Reference 3.2.7(a)). Variations in this test method regarding exposure of fibers have been stud￾ied and give similar results (Reference 3.2.7(b)). In order to minimize variability in test results, proper control of gas flow rates and currents is critical when performing TGA analyses. 3.2.8 Chemical resistance This section reserved for future use

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

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers 3.3.4 Coefficient of thermal expansion Standardized methods for measuring the coefficient of thermal expansion(CTE)of the fibers are not generally available although good correlations between laboratories making these measurements do ex- ist.CTE's are directionally dependent,highly influenced by the anisotropy of fibers.Carbon fibers typi- cally have a negative axial CTE and a slightly positive transverse CTE.Commercial instruments (e.g., DuPont Model 943 Thermomechanical Analyzer,or equivalent)can be used directly or modified to meas- ure axial CTE. The CTE of the fiber can also be derived from measurements made on composites with unidirectional reinforcement.Laser interferometry and dilatometry are the techniques most frequently used.Other techniques,including some applied to the unimpregnated fiber,have also been found satisfactory.When testing the composite,the unidirectional fibers may be oriented parallel or perpendicular to the direction of measurement to obtain the axial or transverse CTE.To perform the analysis,the modulus of the fiber,the modulus and CTE of the matrix,and the fiber loading must be known.It may be desirable to perform the measurements on composites with different fiber loadings in order to check the results. 3.3.5 Thermal conductivity The thermal conductivity of fibers is generally determined analytically from measurements of axial thermal conductivity on unidirectional reinforced composites.However,some measurements have been made on both fiber bundles and single filaments.These have agreed quite well with values determined from composite measurements (Reference 3.3.5(a)).Both types of measurements require considerable operator skill and sophisticated equipment,and are perhaps best left to the thermophysics laboratory.A well defined relationship between axial thermal conductivity and axial electrical conductivity (or resistivity) has been developed for a wide range of carbon fibers.Since electrical resistivity is relatively easy to measure,reasonable estimates of thermal conductivity can be made for electrical resistivity measure- ments(Reference 3.3.5(b)).Transverse thermal conductivity can be determined for thin composites using a pulsed laser technique to measure thermal diffusivity.The thermal conductivity can then be calculated if the specific heat of the fiber is known. 3.3.6 Specific heat This property is measured in a calorimeter such as described in ASTM D 2766(Reference 3.3.6). This also is not a simple measurement and is best left to the experienced laboratory. 3.3.7 Thermal transition temperatures Differential scanning calorimetry (DSC),differential thermal analysis (DTA)or thermal mechanical analysis(TMA)instrumentation may be applied to measure the glass transition temperature T.and,if the fiber is semi-crystalline,its crystalline melting temperature Tm.General procedures for measuring T:and Tm of organic fibers are given in ASTM standards D 3417 and D 3418(References 3.3.7(a)and(b)). 3.4 PHYSICAL TECHNIQUES(EXTRINSIC) 3.4.1 Yield of yarn,strand,or roving Yield is generally expressed as length per unit weight,such as yards per Ib,or as its reciprocal,linear density,expressed as weight per unit length.The latter is normally the measured value and is determined by accurately weighing in air a precise length of yarn,strand,and roving. 3-9

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-9 3.3.4 Coefficient of thermal expansion Standardized methods for measuring the coefficient of thermal expansion (CTE) of the fibers are not generally available although good correlations between laboratories making these measurements do ex￾ist. CTE's are directionally dependent, highly influenced by the anisotropy of fibers. Carbon fibers typi￾cally have a negative axial CTE and a slightly positive transverse CTE. Commercial instruments (e.g., DuPont Model 943 Thermomechanical Analyzer, or equivalent) can be used directly or modified to meas￾ure axial CTE. The CTE of the fiber can also be derived from measurements made on composites with unidirectional reinforcement. Laser interferometry and dilatometry are the techniques most frequently used. Other techniques, including some applied to the unimpregnated fiber, have also been found satisfactory. When testing the composite, the unidirectional fibers may be oriented parallel or perpendicular to the direction of measurement to obtain the axial or transverse CTE. To perform the analysis, the modulus of the fiber, the modulus and CTE of the matrix, and the fiber loading must be known. It may be desirable to perform the measurements on composites with different fiber loadings in order to check the results. 3.3.5 Thermal conductivity The thermal conductivity of fibers is generally determined analytically from measurements of axial thermal conductivity on unidirectional reinforced composites. However, some measurements have been made on both fiber bundles and single filaments. These have agreed quite well with values determined from composite measurements (Reference 3.3.5(a)). Both types of measurements require considerable operator skill and sophisticated equipment, and are perhaps best left to the thermophysics laboratory. A well defined relationship between axial thermal conductivity and axial electrical conductivity (or resistivity) has been developed for a wide range of carbon fibers. Since electrical resistivity is relatively easy to measure, reasonable estimates of thermal conductivity can be made for electrical resistivity measure￾ments (Reference 3.3.5(b)). Transverse thermal conductivity can be determined for thin composites using a pulsed laser technique to measure thermal diffusivity. The thermal conductivity can then be calculated if the specific heat of the fiber is known. 3.3.6 Specific heat This property is measured in a calorimeter such as described in ASTM D 2766 (Reference 3.3.6). This also is not a simple measurement and is best left to the experienced laboratory. 3.3.7 Thermal transition temperatures Differential scanning calorimetry (DSC), differential thermal analysis (DTA) or thermal mechanical analysis (TMA) instrumentation may be applied to measure the glass transition temperature Tg and, if the fiber is semi-crystalline, its crystalline melting temperature Tm. General procedures for measuring Tg and Tm of organic fibers are given in ASTM standards D 3417 and D 3418 (References 3.3.7(a) and (b)). 3.4 PHYSICAL TECHNIQUES (EXTRINSIC) 3.4.1 Yield of yarn, strand, or roving Yield is generally expressed as length per unit weight, such as yards per lb, or as its reciprocal, linear density, expressed as weight per unit length. The latter is normally the measured value and is determined by accurately weighing in air a precise length of yarn, strand, and roving

MIL-HDBK-17-1F Volume 1,Chapter 3 Evaluation of Reinforcement Fibers 3.4.2 Cross-sectional area of yarn or tow This property is calculated rather than measured.However,it is very useful in subsequent calcula- tions of fiber loadings in prepregs and composites as well as in calculations for other physical and ther- mophysical properties.Often it is considered a quality assurance criterion for fiber manufacture.The cross-sectional area is determined by dividing the linear density,weight per unit length,by the volumetric density,weight per unit volume,using consistent units.It should be noted that this value includes only the cumulative total of the cross-sectional areas of all the individual filaments within the bundle.The cross- sectional area is not affected by any space between filaments nor related to any calculations based on yarn or bundle "diameter". 3.4.3 Twist of yarn Twist is defined as the number of turns about its axis per unit length in a yarn or other textile strand Twist is sometimes desirable to improve handleability and,at other times,undesirable because it restricts spreading of the yarn or tow.It can be measured according to the direct procedure described in ASTM D 1423(Reference 3.4.3). 3.4.4 Fabric construction Properties of fabrics such as handleability,drapability,physical stability,thickness,and the effective- ness of the translation of fiber properties to the fabric are all dependent on fabric construction.For the purpose of this document,fabric construction is defined according to the fiber used(by type and filament count),the weave style such as "plain"or "satin",and the number of yarns per inch of fabric in both warp and fill directions.The most common weave styles employed for carbon fabrics used in aircraft and aero- space applications are plain weave,crowfoot satin,five harness satin,and eight harness satin.For a given yarn,fabric physical stability decreases and drapability increases progressively from the plain weave to the eight harness satin weave.In order to maintain a satisfactory level of stability,more yarns per inch must be added progressively toward the 8-harness satin weave fabric,thus the lightest weight fabrics are of plain weave style.There are many construction-related tests applied in the textile industry which are beyond the scope of this document.Essential standards for measure of construction are De- termination of Yarn Count(ASTM D 3775),Length(ASTM D 3773),Width(ASTM D 3774)and Weight (ASTM D 3776)(References 3.4.4(a)-(d)).Additional information on weaves is provided in Volume 3, Section 2.5.1. 3.4.5 Fabric areal density This property,although related to the yarn count previously described,is itself useful in calculations for composite construction and analysis.Expressed as weight per unit area of fabric,fabric areal density along with the fiber density governs the thickness of a cured ply of impregnated fabric at a given fiber vol- ume loading.It is measured according to the method described in ASTM D 3776(Reference 3.4.4(d)). 3.5 MECHANICAL TESTING OF FIBERS 3.5.1 Tensile properties It is important to note that the fiber stress at specimen failure is test dependent.For example.Table 3.5.1 shows the difference in fiber tensile stress at failure for typical carbon fibers tested as a filament,an impregnated tow,and a unidirectional laminate.These data reflect the fact that composite tensile strength depends upon many factors,including interface characteristics,as well as fiber and matrix prop- erties.These data emphasize the need to define the objective of fiber testing.Thus,for acceptance test- ing,it is recommended that fiber strength be measured on a material form representative of composite behavior.For carbon fibers,an impregnated tow test is recommended;for boron fibers,single filament tests are recommended. 3-10

MIL-HDBK-17-1F Volume 1, Chapter 3 Evaluation of Reinforcement Fibers 3-10 3.4.2 Cross-sectional area of yarn or tow This property is calculated rather than measured. However, it is very useful in subsequent calcula￾tions of fiber loadings in prepregs and composites as well as in calculations for other physical and ther￾mophysical properties. Often it is considered a quality assurance criterion for fiber manufacture. The cross-sectional area is determined by dividing the linear density, weight per unit length, by the volumetric density, weight per unit volume, using consistent units. It should be noted that this value includes only the cumulative total of the cross-sectional areas of all the individual filaments within the bundle. The cross￾sectional area is not affected by any space between filaments nor related to any calculations based on yarn or bundle "diameter". 3.4.3 Twist of yarn Twist is defined as the number of turns about its axis per unit length in a yarn or other textile strand. Twist is sometimes desirable to improve handleability and, at other times, undesirable because it restricts spreading of the yarn or tow. It can be measured according to the direct procedure described in ASTM D 1423 (Reference 3.4.3). 3.4.4 Fabric construction Properties of fabrics such as handleability, drapability, physical stability, thickness, and the effective￾ness of the translation of fiber properties to the fabric are all dependent on fabric construction. For the purpose of this document, fabric construction is defined according to the fiber used (by type and filament count), the weave style such as "plain" or "satin", and the number of yarns per inch of fabric in both warp and fill directions. The most common weave styles employed for carbon fabrics used in aircraft and aero￾space applications are plain weave, crowfoot satin, five harness satin, and eight harness satin. For a given yarn, fabric physical stability decreases and drapability increases progressively from the plain weave to the eight harness satin weave. In order to maintain a satisfactory level of stability, more yarns per inch must be added progressively toward the 8-harness satin weave fabric, thus the lightest weight fabrics are of plain weave style. There are many construction-related tests applied in the textile industry which are beyond the scope of this document. Essential standards for measure of construction are De￾termination of Yarn Count (ASTM D 3775), Length (ASTM D 3773), Width (ASTM D 3774) and Weight (ASTM D 3776) (References 3.4.4(a) - (d)). Additional information on weaves is provided in Volume 3, Section 2.5.1. 3.4.5 Fabric areal density This property, although related to the yarn count previously described, is itself useful in calculations for composite construction and analysis. Expressed as weight per unit area of fabric, fabric areal density along with the fiber density governs the thickness of a cured ply of impregnated fabric at a given fiber vol￾ume loading. It is measured according to the method described in ASTM D 3776 (Reference 3.4.4(d)). 3.5 MECHANICAL TESTING OF FIBERS 3.5.1 Tensile properties It is important to note that the fiber stress at specimen failure is test dependent. For example, Table 3.5.1 shows the difference in fiber tensile stress at failure for typical carbon fibers tested as a filament, an impregnated tow, and a unidirectional laminate. These data reflect the fact that composite tensile strength depends upon many factors, including interface characteristics, as well as fiber and matrix prop￾erties. These data emphasize the need to define the objective of fiber testing. Thus, for acceptance test￾ing, it is recommended that fiber strength be measured on a material form representative of composite behavior. For carbon fibers, an impregnated tow test is recommended; for boron fibers, single filament tests are recommended