《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 2 MATERIALS AND PROCESSES - THE EFFECTS OF VARIABILITY ON COMPOSITE PROPERTIES

MIL-HDBK-17-3F Volume 3,Chapter 2 Materials and Processes-The Effects of Variability on Composite Properties CHAPTER 2 MATERIALS AND PROCESSES-THE EFFECTS OF VARIABILITY ON COMPOSITE PROPERTIES 2.1 INTRODUCTION The properties of organic matrix composites are,in general,cure and process dependent.This may result in variations of glass transition (service temperature),corrosion stability,susceptibility to micro- cracking,general strength,or fatigue and service life.In addition,in most cases these materials or struc- tural elements constructed from them are the products of complex multi-step materials processes.Fig- ures 2.1(a)and(b)illustrate the nature of the processing pipeline from raw materials to composite end item.Each rectangle in Figure 2.1(b)represents a process during which additional variability may be in- troduced into the material.Utilization of a standard composite material property database necessitates an understanding of the dependency of the measured material properties on the characteristics and variabil- ity associated with the constituent materials and the sequence of processes used to combine these mate- rials into end products.As a result,development and application of processing controls are essential to achieve the desired mechanical and physical properties for composite structures. 2.2 PURPOSE The purpose of this chapter is to provide an understanding of the origins and nature of proc- ess-induced variability in these materials in the context of an overview of types of composite materials and the associated material processing methodologies.It also seeks to addresses various approaches to minimizing variability,including implementation of process control,and the use of materials and process- ing specifications. 2.3 SCOPE This chapter includes descriptions of composite materials from the perspective of their introduction into the material pipeline as the constituent raw material,subsequent conversion of raw materials into intermediate product forms such as prepregs,and finally the utilization of these intermediate product forms by fabricators to process the materials further to form completed composite structures.Emphasis is placed on the cumulative effects that each processing phase in the pipeline contributes to the final products general quality as well as physical,chemical,and mechanical properties.Finally it includes an overview of common process control schemes and discusses preparation of materials and processing specifications. 2.4 CONSTITUENT MATERIALS 2.4.1 Fibers 2.4.1.1 Carbon and graphite fibers Carbon and graphite have substantial capability as reinforcing fibers,with great flexibility in the prop- erties that can be provided.Primary characteristics for reinforcing fibers in polymer matrix composites are high stiffness and strength.The fibers must maintain these characteristics in hostile environments such as elevated temperatures,exposure to common solvents and fluids,and environmental moisture.To be used as part of a primary structure material it should also be available as continuous fiber(Reference 2.4.1.1). These characteristics and requirements have substantial implications for the physical,chemical and me- chanical properties of the fiber,which in turn implies processing and acceptance parameters. 2-1

MIL-HDBK-17-3F Volume 3, Chapter 2 Materials and Processes - The Effects of Variability on Composite Properties 2-1 CHAPTER 2 MATERIALS AND PROCESSES - THE EFFECTS OF VARIABILITY ON COMPOSITE PROPERTIES 2.1 INTRODUCTION The properties of organic matrix composites are, in general, cure and process dependent. This may result in variations of glass transition (service temperature), corrosion stability, susceptibility to micro￾cracking, general strength, or fatigue and service life. In addition, in most cases these materials or struc￾tural elements constructed from them are the products of complex multi-step materials processes. Fig￾ures 2.1(a) and (b) illustrate the nature of the processing pipeline from raw materials to composite end item. Each rectangle in Figure 2.1(b) represents a process during which additional variability may be in￾troduced into the material. Utilization of a standard composite material property database necessitates an understanding of the dependency of the measured material properties on the characteristics and variabil￾ity associated with the constituent materials and the sequence of processes used to combine these mate￾rials into end products. As a result, development and application of processing controls are essential to achieve the desired mechanical and physical properties for composite structures. 2.2 PURPOSE The purpose of this chapter is to provide an understanding of the origins and nature of proc￾ess-induced variability in these materials in the context of an overview of types of composite materials and the associated material processing methodologies. It also seeks to addresses various approaches to minimizing variability, including implementation of process control, and the use of materials and process￾ing specifications. 2.3 SCOPE This chapter includes descriptions of composite materials from the perspective of their introduction into the material pipeline as the constituent raw material, subsequent conversion of raw materials into intermediate product forms such as prepregs, and finally the utilization of these intermediate product forms by fabricators to process the materials further to form completed composite structures. Emphasis is placed on the cumulative effects that each processing phase in the pipeline contributes to the final products general quality as well as physical, chemical, and mechanical properties. Finally it includes an overview of common process control schemes and discusses preparation of materials and processing specifications. 2.4 CONSTITUENT MATERIALS 2.4.1 Fibers 2.4.1.1 Carbon and graphite fibers Carbon and graphite have substantial capability as reinforcing fibers, with great flexibility in the prop￾erties that can be provided. Primary characteristics for reinforcing fibers in polymer matrix composites are high stiffness and strength. The fibers must maintain these characteristics in hostile environments such as elevated temperatures, exposure to common solvents and fluids, and environmental moisture. To be used as part of a primary structure material it should also be available as continuous fiber (Reference 2.4.1.1). These characteristics and requirements have substantial implications for the physical, chemical and me￾chanical properties of the fiber, which in turn implies processing and acceptance parameters

MIL-HDBK-17-3F Volume 3,Chapter 2 Materials and Processes-The Effects of Variability on Composite Properties 2.4.1.1.1 Carbon vs.graphite Interest in carbon fibers for structural materials was initiated in the late 1950s when synthesized ray- ons in textile form were carbonized to produce carbon fibers for high temperature missile applications (Reference 2.4.1.1.1).One of the first distinctions to be made is the difference between carbon and graphite fibers,although the terms are frequently used interchangeably.Background information for these differences is contained in the following sections.The primary purpose of making this distinction here is to alert the reader that users may mean different things when referring to graphite versus carbon fibers. ALL PRODUCTS,REGARDLESS TO STAGE OF WORK,ARE CONSID- ERED AS RAW MATERIALS.THESE MAY BE CHEMICALS FOR RAW RESINS,OR SAND TO PROCESS INTO GLASS PRODUCTS,OR PRECURSORS FOR FILAMENTS,OR WOVEN GOODS,OR FLAT MATERIAL PROCESSED SHEETS AND/OR MANY OTHER ARTICLES WHICH HAVE YET TO BE PROCESSED TO AN END ITEM. EACH RAW PRODUCT THEN IS PROCESSED,OR MIXED WITH OTHER RAW PRODUCTS,OR ALTERED TO BECOME STILL ANOTH- MANUFACTURE ER RAW ARTICLE TO EXPERIENCE YET ADDITIONAL PROCESS STEPS THROUGH THE PIPELINE.EACH AS RECEIVED RAW &/OR ARTICLE MUST BE PROCESSED IN SUCH A MANNER DURING ITS PROCESSING STEP(S)THAT VARIABILITY IS MINIMIZED AT THE PROCESSING NEXT PIPELINE FUNCTION.PROCESSING FUNCTIONS MAY BE COMPLEX;SUCH AS MATRIX IMPREGNATION,OR THEY MAY BE RELATIVELY SIMPLE;SUCH AS SHIPPING.REGARDLESS,EACH STEP MUST BE EFFECTIVE IN THAT IT DOES NOT INTRODUCE UNCONTROLLED CHANGES THAT ALTER THE PRODUCT FOR SUBSEQUENT USE OR END ITEM PERFORMANCE. FINISHED ARTICLES LEAVING ONE PROCESSING FUNCTION IS USUALLY STILL CONSIDERED A RAW PRODUCT WHEN DELIVERED FINISHED TO THE NEXT.THE ONUS FOR CONSISTENCY OF THIS PRODUCT MUST BE RECOGNIZED AND ATTENDED TO DURING THE JUST ARTICLE COMPLETED PROCESSING STEP(S).THE MATERIALS PIPELINE IS NOT COMPLETE UNTIL THE END ARTICLE IS FULLY FUNCTIONAL AS IS. FIGURE 2.1(a) Composite materials and processing,basic pipeline common to all materials and processes. 2-2

MIL-HDBK-17-3F Volume 3, Chapter 2 Materials and Processes - The Effects of Variability on Composite Properties 2-2 2.4.1.1.1 Carbon vs. graphite Interest in carbon fibers for structural materials was initiated in the late 1950s when synthesized ray￾ons in textile form were carbonized to produce carbon fibers for high temperature missile applications (Reference 2.4.1.1.1). One of the first distinctions to be made is the difference between carbon and graphite fibers, although the terms are frequently used interchangeably. Background information for these differences is contained in the following sections. The primary purpose of making this distinction here is to alert the reader that users may mean different things when referring to graphite versus carbon fibers. FIGURE 2.1(a) Composite materials and processing, basic pipeline common to all materials and processes

MIL-HDBK-17-3F Volume 3,Chapter 2 Materials and Processes-The Effects of Variability on Composite Properties RAW RESIN FIBER RAW MATERIAL MANUFACTURE MANUFACTURE MATERIAL PRODUCTS PRODUCT STRAND,TOW ROVING RAW RAW RAW MATERIAL MATERIAL MATERIAL RTM WET PREPREGER WEAVER MANUFACTURERS MANUFACTURE MANUFACTURE ARTICLE FINISHED FABRIC GOODS PRODUCTS PRODUCTS PRODUCTS FIGURE 2.1(b)Raw materials pipeline (example). Carbon and graphite fibers are both based on graphene(hexagonal)layer networks present in car- bon.If the graphene layers or planes stack with three dimensional order the material is defined as graph- ite (Reference 2.4.1.1.1).Usually extended time and temperature processing is required to form this or- der,making graphite fibers more expensive.Because the bonding between planes is weak,disorder fre- quently occurs such that only the two dimensional ordering within the layers is present.This material is 2-3

MIL-HDBK-17-3F Volume 3, Chapter 2 Materials and Processes - The Effects of Variability on Composite Properties 2-3 FIGURE 2.1(b) Raw materials pipeline (example). Carbon and graphite fibers are both based on graphene (hexagonal) layer networks present in car￾bon. If the graphene layers or planes stack with three dimensional order the material is defined as graph￾ite (Reference 2.4.1.1.1). Usually extended time and temperature processing is required to form this or￾der, making graphite fibers more expensive. Because the bonding between planes is weak, disorder fre￾quently occurs such that only the two dimensional ordering within the layers is present. This material is

MIL-HDBK-17-3F Volume 3,Chapter 2 Materials and Processes-The Effects of Variability on Composite Properties defined as carbon (Reference 2.4.1.1.1).With this distinction made,it should be understood that while some differences are implied,there is not a single condition which strictly separates carbon from graphite fibers,and even graphite fibers retain some disorder in their structure. 2.4.1.1.2 General material description Three different precursor materials are commonly used at present to produce carbon fibers:rayon, polyacrylonitrile(PAN),and isotropic and liquid crystalline pitches(Reference 2.4.1.1.1).Carbon fibers are made predominately from carbonization of PAN.The fibers consist of intermingled fibrils of turbostratic graphite with basal planes tending to align along the fiber axis.This forms an internal structure reminis- cent of an onion skin.Pitch fibers may have a different internal structure,more like sheafs or spokes(Ref- erence2.4.1.1). The highly anisotropic morphology gives rise to moduli in the range of 200-750 GPa parallel to the fiber long axis,and around 20 GPa in the normal direction.For comparison,single crystal (whisker)of graphite is about 1060 and 3 GPa,respectively,but these properties are not attainable in fiber form.Ultra high modulus fibers can be prepared from liquid-crystalline mesophase pitch;the higher degree of orien- tation in the precursor translates through to the final carbonized fiber leading to larger and more oriented graphite crystallites. 2.4.1.1.3 Processing High stiffness and strength implies strong interatomic and intermolecular bonds and few strength lim- iting flaws (Reference 2.4.1.1).Carbon fiber properties are dependent on the fiber microstructure,which is extremely process dependent,such that properties of fibers with the same precursor but different proc- essing can be dramatically different.The precursor itself can also change these properties.The process- ing may be optimized for high modulus or strength,or traded off with economics. 2.4.1.1.3.1 Manufacture The manufacturing process for carbon fiber described below is for the PAN variant,which is one of the most common.Some differences between PAN processing and the pitch and rayon precursors are then described afterwards.The manufacture of PAN based carbon fiber can be broken down into the white fiber and black fiber stages.Most manufacturers consider the details of these processes proprietary. 2.4.1.1.3.1.1 White fiber Production of PAN precursor,or white fiber,is a technology in itself.Fairly conventional fiber proc- esses are performed:polymerization,spinning,drawing,and washing.Additional drawing stages may be added in the process.Characteristics of the white fiber influence the processing and results for the black fiber processing. 2.4.1.1.3.1.2 Black fiber The black fiber process consists of several steps:oxidation(or thermosetting),pyrolysis(or carboniz- ing),surface treatment,and sizing.In the oxidation process the PAN fiber is converted to a thermoset from a thermoplastic.For this oxidation process the fiber diameter is limited by waste gas diffusion.In the pyrolysis process,which is performed under an inert atmosphere,most of the non-carbon material is ex- pelled,forming ribbons of carbon aligned with the fiber axis. In the surface treatment step the fiber may be etched in either gas or liquid phase by oxidizing agents such as chlorine,bromine,nitric acid or chlorates.This improves the wettability for the resin and encour- ages formation of a strong,durable bond.Some additional improvement through removal of surface flaws may also be realized.This process can be electrolytic.The carbon fibers are often treated with solution of unmodified epoxy resin and/or other products as a size.The sizing prevents fiber abrasion,improves han- dling,and can provide an epoxy matrix compatible surface. 2-4

MIL-HDBK-17-3F Volume 3, Chapter 2 Materials and Processes - The Effects of Variability on Composite Properties 2-4 defined as carbon (Reference 2.4.1.1.1). With this distinction made, it should be understood that while some differences are implied, there is not a single condition which strictly separates carbon from graphite fibers, and even graphite fibers retain some disorder in their structure. 2.4.1.1.2 General material description Three different precursor materials are commonly used at present to produce carbon fibers: rayon, polyacrylonitrile (PAN), and isotropic and liquid crystalline pitches (Reference 2.4.1.1.1). Carbon fibers are made predominately from carbonization of PAN. The fibers consist of intermingled fibrils of turbostratic graphite with basal planes tending to align along the fiber axis. This forms an internal structure reminis￾cent of an onion skin. Pitch fibers may have a different internal structure, more like sheafs or spokes (Ref￾erence 2.4.1.1). The highly anisotropic morphology gives rise to moduli in the range of 200-750 GPa parallel to the fiber long axis, and around 20 GPa in the normal direction. For comparison, single crystal (whisker) of graphite is about 1060 and 3 GPa, respectively, but these properties are not attainable in fiber form. Ultra high modulus fibers can be prepared from liquid-crystalline mesophase pitch; the higher degree of orien￾tation in the precursor translates through to the final carbonized fiber leading to larger and more oriented graphite crystallites. 2.4.1.1.3 Processing High stiffness and strength implies strong interatomic and intermolecular bonds and few strength lim￾iting flaws (Reference 2.4.1.1). Carbon fiber properties are dependent on the fiber microstructure, which is extremely process dependent, such that properties of fibers with the same precursor but different proc￾essing can be dramatically different. The precursor itself can also change these properties. The process￾ing may be optimized for high modulus or strength, or traded off with economics. 2.4.1.1.3.1 Manufacture The manufacturing process for carbon fiber described below is for the PAN variant, which is one of the most common. Some differences between PAN processing and the pitch and rayon precursors are then described afterwards. The manufacture of PAN based carbon fiber can be broken down into the white fiber and black fiber stages. Most manufacturers consider the details of these processes proprietary. 2.4.1.1.3.1.1 White fiber Production of PAN precursor, or white fiber, is a technology in itself. Fairly conventional fiber proc￾esses are performed: polymerization, spinning, drawing, and washing. Additional drawing stages may be added in the process. Characteristics of the white fiber influence the processing and results for the black fiber processing. 2.4.1.1.3.1.2 Black fiber The black fiber process consists of several steps: oxidation (or thermosetting), pyrolysis (or carboniz￾ing), surface treatment, and sizing. In the oxidation process the PAN fiber is converted to a thermoset from a thermoplastic. For this oxidation process the fiber diameter is limited by waste gas diffusion. In the pyrolysis process, which is performed under an inert atmosphere, most of the non-carbon material is ex￾pelled, forming ribbons of carbon aligned with the fiber axis. In the surface treatment step the fiber may be etched in either gas or liquid phase by oxidizing agents such as chlorine, bromine, nitric acid or chlorates. This improves the wettability for the resin and encour￾ages formation of a strong, durable bond. Some additional improvement through removal of surface flaws may also be realized. This process can be electrolytic. The carbon fibers are often treated with solution of unmodified epoxy resin and/or other products as a size. The sizing prevents fiber abrasion, improves han￾dling, and can provide an epoxy matrix compatible surface

MIL-HDBK-17-3F Volume 3,Chapter 2 Materials and Processes-The Effects of Variability on Composite Properties 2.4.1.1.3.1.3 Carbon fiber differences due to pitch/PAN/rayon precursors As a rule PAN precursor can provide higher strength carbon fibers,while pitch can provide higher moduli.Rayon based fibers tend to be less expensive but lower performance.Pitch fiber composites have been prepared with elastic moduli superior to steel,and electrical conductivity higher than copper conduc- tor.The shear strengths and impact resistance are degraded,however(Reference 2.4.1.1.3.1.3).Yield for PAN is approximately 50%,but for pitch can be as high as 90%. White Fiber Process PAN Acrylonitrile Spool Polymer/ PolymerizeSpinDraw Wash. Solvent Carbon/Graphite Fiber Process Pitch Melt Spin PAN ThermosetCarbonizeGraphitize Surface SizingDrying Spool (Oxidize) Treatment 250- 1500- Drawing 200-400°C Carbon/ 2500°C 3000°C Graphite Spool FIGURE 2.4.1.1.3.1.3 Carbon fiber typical process flow diagram. 2.4.1.1.3.2 Processing to microstructure Carbon fiber properties are driven by the type and extent of defects,orientation of the fiber,and the degree of crystallinity.The precursor makeup and heat treatment can affect the crystallinity and orienta- tion.The defect content can be driven by contaminants and processing.Orientation is also greatly af- fected by the drawing process which may be repeated many times in the processing of the fibers. 2.4.1.1.3.3 Microstructure to properties The strength of a brittle material is frequently controlled by presence of flaws,their number and mag- nitude.The probability of finding a flaw is volume dependent,thus a fiber with a lower volume per unit length appears stronger.Elimination of defects drives tensile strength up,and also improves thermal and electrical conductivity,and oxidation resistance.However,increasing crystallinity too far can degrade fiber strength and modulus. 2.4.1.1.3.4 Testing As with most composite material properties,the values obtained are greatly dependent on the testing performed.Determination of fiber modulus can be especially controversial.The stress/strain response can be nonlinear,so where and how measurements are taken can greatly influence the results.As a re- sult,fibers which may appear to be substantially different in the literature may have little or no difference in modulus.Reported differences may be entirely the result of test and calculation differences.Chapter 3 in Volume I can be referenced for more information of fiber test methods. 2-5

MIL-HDBK-17-3F Volume 3, Chapter 2 Materials and Processes - The Effects of Variability on Composite Properties 2-5 2.4.1.1.3.1.3 Carbon fiber differences due to pitch/PAN/rayon precursors As a rule PAN precursor can provide higher strength carbon fibers, while pitch can provide higher moduli. Rayon based fibers tend to be less expensive but lower performance. Pitch fiber composites have been prepared with elastic moduli superior to steel, and electrical conductivity higher than copper conduc￾tor. The shear strengths and impact resistance are degraded, however (Reference 2.4.1.1.3.1.3). Yield for PAN is approximately 50%, but for pitch can be as high as 90%. Thermoset (Oxidize) Drawing Carbonize Graphitize Surface Treatment PAN Sizing Drying Spool Pitch Melt Spin Carbon/ Graphite Spool Acrylonitrile Polymer/ Solvent Polymerize Spin Draw Wash PAN Spool White Fiber Process Carbon/Graphite Fiber Process 200-400°C 250- 2500°C 1500- 3000°C FIGURE 2.4.1.1.3.1.3 Carbon fiber typical process flow diagram. 2.4.1.1.3.2 Processing to microstructure Carbon fiber properties are driven by the type and extent of defects, orientation of the fiber, and the degree of crystallinity. The precursor makeup and heat treatment can affect the crystallinity and orienta￾tion. The defect content can be driven by contaminants and processing. Orientation is also greatly af￾fected by the drawing process which may be repeated many times in the processing of the fibers. 2.4.1.1.3.3 Microstructure to properties The strength of a brittle material is frequently controlled by presence of flaws, their number and mag￾nitude. The probability of finding a flaw is volume dependent, thus a fiber with a lower volume per unit length appears stronger. Elimination of defects drives tensile strength up, and also improves thermal and electrical conductivity, and oxidation resistance. However, increasing crystallinity too far can degrade fiber strength and modulus. 2.4.1.1.3.4 Testing As with most composite material properties, the values obtained are greatly dependent on the testing performed. Determination of fiber modulus can be especially controversial. The stress/strain response can be nonlinear, so where and how measurements are taken can greatly influence the results. As a re￾sult, fibers which may appear to be substantially different in the literature may have little or no difference in modulus. Reported differences may be entirely the result of test and calculation differences. Chapter 3 in Volume I can be referenced for more information of fiber test methods