《纺织复合材料》课程参考文献（Experimental Characterization of Advanced Composite Materials, Third Edition）01 Introduction

1 Introduction Most of the subject matter in this text has been taught for years to advanced undergraduate and first-year graduate students at the University of Delaware, Florida Atlantic University,and the University of Wyoming.During this time,the authors realized there was no textbook offering a concise treatment of the experimental characterization of composite materials.Most current text- books deal only with the analysis of composite materials.If the present text appears to emphasize advanced composite materials,it is only because these materials often present the greatest challenges to experimental characteriza- tion.These also are the materials most often used in structural applications, where accurate characterization is most important.Interestingly,today,many high-performance designs demand the use of advanced composite materials. The objective of this textbook is to present processing techniques,specimen preparation,analyses of test methods,test procedures,and data reduction schemes to determine mechanical properties,thermal expansion coefficients, and fracture and strength data for composite materials.Emphasis is placed on practical matters such as preparation and testing of specimens and data reduction methodology.Many of the test methods presented are American Society for Testing and Materials(ASTM)or other national or international standards.Others,although originating within an individual organization and sometimes continuing to be refined in terms of test specimen and fixture geometries,test procedures,and data reduction schemes,are being widely used within the composites testing community. No attempt is made to present a detailed review of composite mechanics or fracture mechanics.Such a treatment has been presented in many other textbooks to which references are made.Only a brief elementary outline of the theoretical background is provided in Chapter 2.Moreover,no attempt is made to present an overview of all test methods;such reviews are available elsewhere,as will be referenced.The methods presented here are deemed the most appropriate and widely accepted at present.Additional develop- ments can,however,be expected in this evolving field. This text was prepared for students who have an interest in experimental aspects of composite materials.It will also be useful for engineers in indus- trial or government laboratories who desire to extend their expertise into experimental characterization of anisotropic materials. ©2003 by CRC Press LLC

1 Introduction Most of the subject matter in this text has been taught for years to advanced undergraduate and first-year graduate students at the University of Delaware, Florida Atlantic University, and the University of Wyoming. During this time, the authors realized there was no textbook offering a concise treatment of the experimental characterization of composite materials. Most current text￾books deal only with the analysis of composite materials. If the present text appears to emphasize advanced composite materials, it is only because these materials often present the greatest challenges to experimental characteriza￾tion. These also are the materials most often used in structural applications, where accurate characterization is most important. Interestingly, today, many high-performance designs demand the use of advanced composite materials. The objective of this textbook is to present processing techniques, specimen preparation, analyses of test methods, test procedures, and data reduction schemes to determine mechanical properties, thermal expansion coefficients, and fracture and strength data for composite materials. Emphasis is placed on practical matters such as preparation and testing of specimens and data reduction methodology. Many of the test methods presented are American Society for Testing and Materials (ASTM) or other national or international standards. Others, although originating within an individual organization and sometimes continuing to be refined in terms of test specimen and fixture geometries, test procedures, and data reduction schemes, are being widely used within the composites testing community. No attempt is made to present a detailed review of composite mechanics or fracture mechanics. Such a treatment has been presented in many other textbooks to which references are made. Only a brief elementary outline of the theoretical background is provided in Chapter 2. Moreover, no attempt is made to present an overview of all test methods; such reviews are available elsewhere, as will be referenced. The methods presented here are deemed the most appropriate and widely accepted at present. Additional develop￾ments can, however, be expected in this evolving field. This text was prepared for students who have an interest in experimental aspects of composite materials. It will also be useful for engineers in indus￾trial or government laboratories who desire to extend their expertise into experimental characterization of anisotropic materials. TX001_ch01_Frame Page 1 Saturday, September 21, 2002 4:47 AM © 2003 by CRC Press LLC

1.1 Background Composite materials,in the context of high-performance materials for struc- tural applications,have been used increasingly since the early 1960s; although materials such as glass fiber-reinforced polymers were already being studied 20 years earlier.Initially,conventional test methods,originally developed for determining the physical and mechanical properties of metals and other homogeneous and isotropic construction materials,were used.It was soon recognized,however,that these new materials,which are non- homogeneous and anisotropic (orthotropic),require special consideration for determining physical and mechanical properties. During this initial period,composite materials technology was developed primarily within the aerospace community.Because composite material test methods were not standardized,each airframe manufacturer tended to develop its own procedures.Although these procedures were not usually proprietary,there was little incentive to adopt common test methods,par- ticularly because few methods had emerged as being clearly superior to others of the same type.The problem was further complicated by the con- tinuous emergence of new materials;e.g.,boron and carbon fibers in the mid-1960s and Kevlar fibers in the early 1970s,along with new epoxies, polyimides,and other matrix materials,including metals.A specific test method,which may have performed reasonably well for the types of com- posite materials being tested in the past,was not necessarily adequate for the material being evaluated at that time.That is,there was little possibility of standardization.As a result,many diverse test methods were developed for measuring the same properties.Some were easy to use but provided only limited results or data of questionable quality.Others were very complex, operator-dependent,and perhaps also of questionable quality. In the U.S.the federal government sponsored much of the early develop- ment work in composite materials,primarily through agencies such as the National Aeronautics and Space Administration(NASA),the Air Force,and the Navy.The problems associated with the lack of standards were recog- nized,and attempts were made to identify general test methods,to generate a database for comparison purposes,and to establish standards.These attempts were largely unsuccessful,primarily because newer composite materials did not necessarily behave in the same manner as the prior generation of materials around which the test methods had been established. Today,almost four decades after these initial attempts,general standards for testing composite materials still do not exist,and perhaps still for the same very practical reasons.That is,as new generations of composite mate- rials are developed,existing test methods have to be modified to accommo- date them.Rigid standardization would not permit this.On the other hand, consensus organizations such as ASTM have done much to maintain a degree of uniformity,and an awareness of the general problem of achieving standardization.As additional industries,e.g.,automotive,sporting goods, ©2003 by CRC Press LLC

1.1 Background Composite materials, in the context of high-performance materials for struc￾tural applications, have been used increasingly since the early 1960s; although materials such as glass fiber-reinforced polymers were already being studied 20 years earlier. Initially, conventional test methods, originally developed for determining the physical and mechanical properties of metals and other homogeneous and isotropic construction materials, were used. It was soon recognized, however, that these new materials, which are non￾homogeneous and anisotropic (orthotropic), require special consideration for determining physical and mechanical properties. During this initial period, composite materials technology was developed primarily within the aerospace community. Because composite material test methods were not standardized, each airframe manufacturer tended to develop its own procedures. Although these procedures were not usually proprietary, there was little incentive to adopt common test methods, par￾ticularly because few methods had emerged as being clearly superior to others of the same type. The problem was further complicated by the con￾tinuous emergence of new materials; e.g., boron and carbon fibers in the mid-1960s and Kevlar® fibers in the early 1970s, along with new epoxies, polyimides, and other matrix materials, including metals. A specific test method, which may have performed reasonably well for the types of com￾posite materials being tested in the past, was not necessarily adequate for the material being evaluated at that time. That is, there was little possibility of standardization. As a result, many diverse test methods were developed for measuring the same properties. Some were easy to use but provided only limited results or data of questionable quality. Others were very complex, operator-dependent, and perhaps also of questionable quality. In the U.S. the federal government sponsored much of the early develop￾ment work in composite materials, primarily through agencies such as the National Aeronautics and Space Administration (NASA), the Air Force, and the Navy. The problems associated with the lack of standards were recog￾nized, and attempts were made to identify general test methods, to generate a database for comparison purposes, and to establish standards. These attempts were largely unsuccessful, primarily because newer composite materials did not necessarily behave in the same manner as the prior generation of materials around which the test methods had been established. Today, almost four decades after these initial attempts, general standards for testing composite materials still do not exist, and perhaps still for the same very practical reasons. That is, as new generations of composite mate￾rials are developed, existing test methods have to be modified to accommo￾date them. Rigid standardization would not permit this. On the other hand, consensus organizations such as ASTM have done much to maintain a degree of uniformity, and an awareness of the general problem of achieving standardization. As additional industries, e.g., automotive, sporting goods, TX001_ch01_Frame Page 2 Saturday, September 21, 2002 4:47 AM © 2003 by CRC Press LLC

electronics,machine tool,and civil infrastructure,have moved toward the more extensive use of composite materials in their products,this general lack of standardization has become particularly disturbing to them.Most of the more traditional industries are accustomed to following specific design standards,purchasing materials to standards,and testing to standards.Thus, acceptance of the general lack of test method standards has become part of the indoctrination of newer industries into this relatively new technology. This lack of standardization in composite materials testing is not neces- sarily a negative aspect,although it may often be inconvenient for the new user,and it should not be unexpected.That is,the term composite material does not define a specific class of fabrication materials,but rather a broad spectrum of materials of widely varying properties.Thus,it can be expected that different test methods will be required for different classes of composite materials.This philosophy is no different than that associated with using a different test method for testing low carbon steel than for testing a ceramic. With this general background and philosophy in mind,current composite material characterization methods will be discussed and evaluated in the following chapters.Not every known method will be introduced,however. Some methods that were previously popular are now rapidly fading from use.Thus,although these names are familiar to many,and are frequently quoted in the literature,particularly in the older literature,they are becoming obsolete and need not be discussed here.Additional discussion can be found in References 1 and 2. 1.2 Laminate Orientation Code Typically,the basic building block of a composite material structural com- ponent is a unidirectional lamina,i.e.,a thin layer consisting of reinforcing fibers all oriented in the same direction and imbedded in a matrix such as a polymer.Alternatively,the reinforcement can be in the form of fibers woven to form a layer of fabric,a thin mat of randomly oriented fibers,or some similar form.All of these laminae are typically characterized experimentally using the test methods described in this text. However,in the actual structural design process,these individual laminae are stacked and processed together to form a laminate of the desired prop- erties.Such a laminate can be made as complex as required to satisfy the specified design criteria,by adding more and more plies of arbitrary orienta- tions,reinforcement forms,and material types.Until the early 1970s there was no unified system for defining the lay-up patterns of composite lami- nates.As composite materials moved from the research laboratory to the production shop,the need for a common terminology became obvious. The Air Force Structural Dynamics Laboratory included a Laminate Orienta- tion Code in the third edition of its Advanced Composites Design Guide published ©2003 by CRC Press LLC

electronics, machine tool, and civil infrastructure, have moved toward the more extensive use of composite materials in their products, this general lack of standardization has become particularly disturbing to them. Most of the more traditional industries are accustomed to following specific design standards, purchasing materials to standards, and testing to standards. Thus, acceptance of the general lack of test method standards has become part of the indoctrination of newer industries into this relatively new technology. This lack of standardization in composite materials testing is not neces￾sarily a negative aspect, although it may often be inconvenient for the new user, and it should not be unexpected. That is, the term composite material does not define a specific class of fabrication materials, but rather a broad spectrum of materials of widely varying properties. Thus, it can be expected that different test methods will be required for different classes of composite materials. This philosophy is no different than that associated with using a different test method for testing low carbon steel than for testing a ceramic. With this general background and philosophy in mind, current composite material characterization methods will be discussed and evaluated in the following chapters. Not every known method will be introduced, however. Some methods that were previously popular are now rapidly fading from use. Thus, although these names are familiar to many, and are frequently quoted in the literature, particularly in the older literature, they are becoming obsolete and need not be discussed here. Additional discussion can be found in References 1 and 2. 1.2 Laminate Orientation Code Typically, the basic building block of a composite material structural com￾ponent is a unidirectional lamina, i.e., a thin layer consisting of reinforcing fibers all oriented in the same direction and imbedded in a matrix such as a polymer. Alternatively, the reinforcement can be in the form of fibers woven to form a layer of fabric, a thin mat of randomly oriented fibers, or some similar form. All of these laminae are typically characterized experimentally using the test methods described in this text. However, in the actual structural design process, these individual laminae are stacked and processed together to form a laminate of the desired prop￾erties. Such a laminate can be made as complex as required to satisfy the specified design criteria, by adding more and more plies of arbitrary orienta￾tions, reinforcement forms, and material types. Until the early 1970s there was no unified system for defining the lay-up patterns of composite lami￾nates. As composite materials moved from the research laboratory to the production shop, the need for a common terminology became obvious. The Air Force Structural Dynamics Laboratory included a Laminate Orienta￾tion Code in the third edition of its Advanced Composites Design Guide published TX001_ch01_Frame Page 3 Saturday, September 21, 2002 4:47 AM © 2003 by CRC Press LLC

in January 1973 [3].This code,established by general consensus of the aerospace industry at the time,has survived to the present with minimal modification and continues to be used almost universally by the composites community. Thus,it is important for the reader to know at least its general features. The Laminate Orientation Code,as presented in the 1973 edition of the Advanced Composites Design Guide,is summarized in the following sections. 1.2.1 Standard Laminate Code The Standard Laminate Code,used to describe a specific laminate uniquely, is most simply defined by the following detailed descriptions of its features: 1.The plies are listed in sequence from one laminate face to the other, starting with the first ply laid up,with square brackets used to indicate the beginning and end of the code. 2.A subscript capital T following the closing square bracket should be used to indicate that the total laminate is shown.Although it is not good practice,as will be seen subsequently,the T is sometimes omitted.For a symmetric laminate(see Chapter 2),only the plies on one side of the midplane are shown,and a subscript capital S follows the closing bracket.A subscript capital Q is also defined in the code,to designate an antisymmetric laminate (improperly termed a quasi-symmetric laminate in Reference [3]).However, antisymmetric laminates are not commonly used. 3.Each ply within the laminate is denoted by a number representing its orientation in degrees as measured from the geometric x-axis of the laminate to the lamina principal material coordinate direction (1-axis).Material and geometric coordinate axis systems are described in Chapter 2.Positive angles are defined as clockwise when looking toward the lay-up tool surface.Note that this con- vention is consistent with the definition of a positive angle in Figure 2.3,because there the view is toward the surface of the laminate,i.e.,away from the lay-up tool surface. 4.When two or more plies of identical properties and orientation are adjacent to each other,a single number representing the angular orientation,with a numerical subscript indicating the number of identically oriented adjacent plies,is used. For example,a laminate consisting of just three-45 plies would be designated as [-45al.The notation [-45]3r is also acceptable, and in fact is more commonly used. 5.If the angles of otherwise identical adjacent plies are different,or if the angles are the same but the materials are different,the plies are separated in the code by a slash. ©2003 by CRC Press LLC

in January 1973 [3]. This code, established by general consensus of the aerospace industry at the time, has survived to the present with minimal modification and continues to be used almost universally by the composites community. Thus, it is important for the reader to know at least its general features. The Laminate Orientation Code, as presented in the 1973 edition of the Advanced Composites Design Guide, is summarized in the following sections. 1.2.1 Standard Laminate Code The Standard Laminate Code, used to describe a specific laminate uniquely, is most simply defined by the following detailed descriptions of its features: 1. The plies are listed in sequence from one laminate face to the other, starting with the first ply laid up, with square brackets used to indicate the beginning and end of the code. 2. A subscript capital T following the closing square bracket should be used to indicate that the total laminate is shown. Although it is not good practice, as will be seen subsequently, the T is sometimes omitted. For a symmetric laminate (see Chapter 2), only the plies on one side of the midplane are shown, and a subscript capital S follows the closing bracket. A subscript capital Q is also defined in the code, to designate an antisymmetric laminate (improperly termed a quasi-symmetric laminate in Reference [3]). However, antisymmetric laminates are not commonly used. 3. Each ply within the laminate is denoted by a number representing its orientation in degrees as measured from the geometric x-axis of the laminate to the lamina principal material coordinate direction (1-axis). Material and geometric coordinate axis systems are described in Chapter 2. Positive angles are defined as clockwise when looking toward the lay-up tool surface. Note that this con￾vention is consistent with the definition of a positive angle in Figure 2.3, because there the view is toward the surface of the laminate, i.e., away from the lay-up tool surface. 4. When two or more plies of identical properties and orientation are adjacent to each other, a single number representing the angular orientation, with a numerical subscript indicating the number of identically oriented adjacent plies, is used. For example, a laminate consisting of just three –45° plies would be designated as [–453]T. The notation [–45] 3T is also acceptable, and in fact is more commonly used. 5. If the angles of otherwise identical adjacent plies are different, or if the angles are the same but the materials are different, the plies are separated in the code by a slash. TX001_ch01_Frame Page 4 Saturday, September 21, 2002 4:47 AM © 2003 by CRC Press LLC

For example,a two-ply laminate consisting of a +45-ply and a -30-ply of the same material would be expressed as [45/-30]. Note that the first ply listed in the code is always the first ply to be laid up in the fabrication process.Note also that the plus sign is not used unless omitting it would create an ambiguity. Asix-ply symmetric laminate consisting of identical plies oriented at +45,0,-30,-30,0,and +45 would be expressed as [45/0/-30]s. When a symmetric laminate contains an odd number of plies of the same material,e.g.,-30,90,45,90,and-30,the center ply is designated with an overbar,i.e.,[-30/90/45]s. 6.When adjacent plies are at angles of the same magnitude but of opposite sign,the appropriate use of plus and minus signs is employed.Each plus or minus sign represents one ply and super- cedes the use of the numerical subscript,which is used only when the directions are identical (as in item 4,above). For example,a four-ply laminate consisting of plies oriented at +20,+20,-30,and +30 would be designated as [202/F30].Note that F and not is used here,to preserve the intended order. 7.Repeating sequences of plies are called sets and are enclosed in parentheses.A set is coded in accordance with the same rules that apply to a single ply. For example,a six-ply 45,0,90,45,0,and 90 laminate would be designated as [(45/0/90)2],or alternatively as [(45/0/90)]2T. As in item 4,above,this latter form is no more correct,but is more commonly used. 8.If a laminate contains plies of more than one type of material and/ or thickness,a distinguishing subscript(or superscript)is used with each ply angle,to define the characteristics of that ply.For example, [0/90x/45ls for a glass,Kevlar,and carbon/fiber laminate. 1.2.2 Basic Condensed Code When the exact number of plies need not be specified (as in preliminary design when the laminate in-plane properties but not the final laminate thickness are needed),the Basic Condensed Code can be used.The plies are written in the order of ascending angle,with only the relative proportions being expressed by whole number subscripts. For example an actual [302/0/-452/90]laminate would be expressed using the Basic Condensed Code as [03/30/-45/902].An actual 30-ply [90/+(0/45)]as laminate would be expressed as [02/+45/90].In both exam- ples,the lack of a subscript after the closing bracket indicates that it is a Basic Condensed Code. ©2003 by CRC Press LLC

For example, a two-ply laminate consisting of a +45°–ply and a –30°–ply of the same material would be expressed as [45/–30]T. Note that the first ply listed in the code is always the first ply to be laid up in the fabrication process. Note also that the plus sign is not used unless omitting it would create an ambiguity. A six-ply symmetric laminate consisting of identical plies oriented at +45, 0, –30, –30, 0, and +45° would be expressed as [45/0/–30]S. When a symmetric laminate contains an odd number of plies of the same material, e.g., –30, 90, 45, 90, and –30°, the center ply is designated with an overbar, i.e., [–30/90/45 ]S. 6. When adjacent plies are at angles of the same magnitude but of opposite sign, the appropriate use of plus and minus signs is employed. Each plus or minus sign represents one ply and super￾cedes the use of the numerical subscript, which is used only when the directions are identical (as in item 4, above). For example, a four-ply laminate consisting of plies oriented at +20, +20, –30, and +30° would be designated as [202/m30]T. Note that m and not ± is used here, to preserve the intended order. 7. Repeating sequences of plies are called sets and are enclosed in parentheses. A set is coded in accordance with the same rules that apply to a single ply. For example, a six-ply 45, 0, 90, 45, 0, and 90° laminate would be designated as [(45/0/90)2 ]T, or alternatively as [(45/0/90)]2T. As in item 4, above, this latter form is no more correct, but is more commonly used. 8. If a laminate contains plies of more than one type of material and/ or thickness, a distinguishing subscript (or superscript) is used with each ply angle, to define the characteristics of that ply. For example, [0g/90k/45c]S for a glass, Kevlar®, and carbon/fiber laminate. 1.2.2 Basic Condensed Code When the exact number of plies need not be specified (as in preliminary design when the laminate in-plane properties but not the final laminate thickness are needed), the Basic Condensed Code can be used. The plies are written in the order of ascending angle, with only the relative proportions being expressed by whole number subscripts. For example an actual [302/06/–452/904]T laminate would be expressed using the Basic Condensed Code as [03/30/–45/902]. An actual 30-ply [90/±(0/45)]3S laminate would be expressed as [02/±45/90]. In both exam￾ples, the lack of a subscript after the closing bracket indicates that it is a Basic Condensed Code. TX001_ch01_Frame Page 5 Saturday, September 21, 2002 4:47 AM © 2003 by CRC Press LLC