《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 1）CHAPTER 6 LAMINA, LAMINATE, AND SPECIAL FORM CHARACTERIZATION

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization CHAPTER 6 LAMINA,LAMINATE,AND SPECIAL FORM CHARACTERIZATION 6.1 INTRODUCTION The use of composite materials continues to increase as new performance,reliability,and durability requirements drive hardware designs to higher levels of structural efficiency.Additionally,government requirements are becoming more stringent to ensure proper levels of structural integrity are maintained. These design drivers,among others,have resulted in a growing recognition that certification or qualifica- tion of aerospace structure requires an extensive combination of analysis,testing,and documentation. Further,because of the large number of design variables inherent to composite structure,analytic models are even more necessary than for metallic structure to ensure completeness of the hardware qualification process.Inherent in all structural analysis models are material,physical,and mechanical property characterization data.Ideally,these analytic models would permit analysts to predict full-scale structural response (e.g.stability,deflections,strength,life)directly from a generic (lamina)material data- base.In truth,test data is required at design development(element,subcomponent,component)and full- scale article test levels as well as the generic(coupon)levels of evaluation. The purpose of Chapter 6 is to provide guidelines of testing procedures for characterization of physi- cal and mechanical lamina(ply)and laminate properties. A laminate is a product made by bonding together two or more layers of material or materials,and a lamina is a single ply or layer in a laminate.The material forming each layer typically consists of a car- bon,glass,or organic (polymeric)fiber reinforcement embedded in a thermoplastic or thermosetting resin matrix.While retaining their identities in the composite,the constituents combine to provide specific characteristics and properties. Many techniques are used to characterize the chemical,physical,and mechanical properties of com- posite materials.The purpose of this chapter is to provide information on techniques that may be used to analyze and evaluate these properties.The test methods discussed in each section may not be appro- priate for all types of composite materials.Currently,more studies are being conducted to investigate how variations in reinforcement and resin chemistry and morphology may affect the physical properties and long term performance of composites.Where possible,the limitations of existing test methods are discussed. 6.2 SPECIMEN PREPARATION 6.2.1 Introduction This section provides general recommendations for the fabrication and preparation of the test speci- mens detailed in this document.These recommendations cover specimen traceability,test article'fabri- cation,specimen location,configuration,and machining. The validity of material properties used in design of structure is dependent on the quality of the specimens being tested.If the objective of the testing is to provide comparative information of different materials,it is crucial that variability due to specimen preparation be kept to a minimum.If the data being generated are intended to be used to generate allowables,the goal is to reflect the interaction of the base material and processing which is expected to occur in production.In either case care must be taken in the specimen preparation process to minimize the variation which naturally occurs during the process. Specimen fabrication should be performed in compliance to ASTM D 5687(Standard Guide for Prepara- tion of Flat Composite Panels with Processing Guidelines for Specimen Preparation).Even test articles that are not flat can benefit from the ASTM guide. 1A test article is any construction from which individual specimens are extracted.Such a test article may be a flat panel fabricated specifically to develop material properties,or it may be a production part set aside for test purposes. 6-1

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-1 CHAPTER 6 LAMINA, LAMINATE, AND SPECIAL FORM CHARACTERIZATION 6.1 INTRODUCTION The use of composite materials continues to increase as new performance, reliability, and durability requirements drive hardware designs to higher levels of structural efficiency. Additionally, government requirements are becoming more stringent to ensure proper levels of structural integrity are maintained. These design drivers, among others, have resulted in a growing recognition that certification or qualifica￾tion of aerospace structure requires an extensive combination of analysis, testing, and documentation. Further, because of the large number of design variables inherent to composite structure, analytic models are even more necessary than for metallic structure to ensure completeness of the hardware qualification process. Inherent in all structural analysis models are material, physical, and mechanical property characterization data. Ideally, these analytic models would permit analysts to predict full-scale structural response (e.g. stability, deflections, strength, life) directly from a generic (lamina) material data￾base. In truth, test data is required at design development (element, subcomponent, component) and full￾scale article test levels as well as the generic (coupon) levels of evaluation. The purpose of Chapter 6 is to provide guidelines of testing procedures for characterization of physi￾cal and mechanical lamina (ply) and laminate properties. A laminate is a product made by bonding together two or more layers of material or materials, and a lamina is a single ply or layer in a laminate. The material forming each layer typically consists of a car￾bon, glass, or organic (polymeric) fiber reinforcement embedded in a thermoplastic or thermosetting resin matrix. While retaining their identities in the composite, the constituents combine to provide specific characteristics and properties. Many techniques are used to characterize the chemical, physical, and mechanical properties of com￾posite materials. The purpose of this chapter is to provide information on techniques that may be used to analyze and evaluate these properties. The test methods discussed in each section may not be appro￾priate for all types of composite materials. Currently, more studies are being conducted to investigate how variations in reinforcement and resin chemistry and morphology may affect the physical properties and long term performance of composites. Where possible, the limitations of existing test methods are discussed. 6.2 SPECIMEN PREPARATION 6.2.1 Introduction This section provides general recommendations for the fabrication and preparation of the test speci￾mens detailed in this document. These recommendations cover specimen traceability, test article1 fabri￾cation, specimen location, configuration, and machining. The validity of material properties used in design of structure is dependent on the quality of the specimens being tested. If the objective of the testing is to provide comparative information of different materials, it is crucial that variability due to specimen preparation be kept to a minimum. If the data being generated are intended to be used to generate allowables, the goal is to reflect the interaction of the base material and processing which is expected to occur in production. In either case care must be taken in the specimen preparation process to minimize the variation which naturally occurs during the process. Specimen fabrication should be performed in compliance to ASTM D 5687 (Standard Guide for Prepara￾tion of Flat Composite Panels with Processing Guidelines for Specimen Preparation). Even test articles that are not flat can benefit from the ASTM guide. 1 A test article is any construction from which individual specimens are extracted. Such a test article may be a flat panel fabricated specifically to develop material properties, or it may be a production part set aside for test purposes

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 6.2.2 Traceability All specimens should be traceable to the material batch,lot,roll,process and test article.The re- questing organization may choose to require traceability of each specimen to its location within the test article. The specification,or purchasing paperwork,should require batch,lot,roll traceability and lot accep- tance test information.It is recommended that when uncured material is purchased it be required that all available traceability information,including vendor certifications and material receiving inspection data of acceptance test results,be delivered with the material.The organization conducting the investigation should review the information to ensure there is enough traceability information to proceed with test arti- cle and specimen fabrication. All prepreg material that is stored before fabrication should have a storage history record.Information such as accumulated time in and out of refrigeration should be recorded. For the test article,the prepreg batch number,lot number,roll number,and processing information should be recorded.Another piece of information which needs to be maintained throughout specimen fabrication is ply orientation.One method by which this may be accomplished is through the use of a wit- ness line.as discussed in the next section. 6.2.3 Test article fabrication The following is a list of important items that should be considered when fabricating test articles: a.Test articles should be built according to engineering drawing requirements or sketches.The drawing requirements or sketches should specify:ply materials,test article reference orientation, ply orientation,material and process specifications or equivalent process document,and inspec- tion requirements. b.Vital material and process identification,such as prepreg batch number,lot number,roll number, autoclave run,press,or other consolidation method and lay-up stacking sequence,should be re- corded.This information is stored to maintain traceability of the test articles.This same traceabil- ity should be maintained on any excess material left after the specimens have been removed. c.The test article identification code and witness line should be permanently identified on each test article.A witness line should be established on the fabrication tool to act as a reference to the fi- ber orientation on the test article.For hand lay-up methods a witness line which will be main- tained during the lay-up and curing process must be identified as the reference orientation.The angular tolerance between the plies put down and this line depends on the processing specifica- tion by which the material is being processed.In automated processes some other method of es- tablishing the reference orientation must be established.Once established,the witness line should be transferred to the test article,and maintained throughout specimen extraction. d.It is generally recommended that for cured test articles at least 1 in.(25 mm)of material be trimmed from the edges.One of the machined edges of the test article may be used to perma- nently maintain the reference orientation on the article. e.The requesting organization (or if required,the appropriate quality assurance organization) should inspect test articles.This inspection should be done before the specimens are fabricated to ensure they meet all requirements in the controlling process specification or appropriate equivalent document.If the test article does not meet all requirements,the requesting organiza- tion and,when applicable,the customer representative,should provide the final disposition of the test article. 6-2

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-2 6.2.2 Traceability All specimens should be traceable to the material batch, lot, roll, process and test article. The re￾questing organization may choose to require traceability of each specimen to its location within the test article. The specification, or purchasing paperwork, should require batch, lot, roll traceability and lot accep￾tance test information. It is recommended that when uncured material is purchased it be required that all available traceability information, including vendor certifications and material receiving inspection data of acceptance test results, be delivered with the material. The organization conducting the investigation should review the information to ensure there is enough traceability information to proceed with test arti￾cle and specimen fabrication. All prepreg material that is stored before fabrication should have a storage history record. Information such as accumulated time in and out of refrigeration should be recorded. For the test article, the prepreg batch number, lot number, roll number, and processing information should be recorded. Another piece of information which needs to be maintained throughout specimen fabrication is ply orientation. One method by which this may be accomplished is through the use of a wit￾ness line, as discussed in the next section. 6.2.3 Test article fabrication The following is a list of important items that should be considered when fabricating test articles: a. Test articles should be built according to engineering drawing requirements or sketches. The drawing requirements or sketches should specify: ply materials, test article reference orientation, ply orientation, material and process specifications or equivalent process document, and inspec￾tion requirements. b. Vital material and process identification, such as prepreg batch number, lot number, roll number, autoclave run, press, or other consolidation method and lay-up stacking sequence, should be re￾corded. This information is stored to maintain traceability of the test articles. This same traceabil￾ity should be maintained on any excess material left after the specimens have been removed. c. The test article identification code and witness line should be permanently identified on each test article. A witness line should be established on the fabrication tool to act as a reference to the fi￾ber orientation on the test article. For hand lay-up methods a witness line which will be main￾tained during the lay-up and curing process must be identified as the reference orientation. The angular tolerance between the plies put down and this line depends on the processing specifica￾tion by which the material is being processed. In automated processes some other method of es￾tablishing the reference orientation must be established. Once established, the witness line should be transferred to the test article, and maintained throughout specimen extraction. d. It is generally recommended that for cured test articles at least 1 in. (25 mm) of material be trimmed from the edges. One of the machined edges of the test article may be used to perma￾nently maintain the reference orientation on the article. e. The requesting organization (or if required, the appropriate quality assurance organization) should inspect test articles. This inspection should be done before the specimens are fabricated to ensure they meet all requirements in the controlling process specification or appropriate equivalent document. If the test article does not meet all requirements, the requesting organiza￾tion and, when applicable, the customer representative, should provide the final disposition of the test article

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 6.2.4 Specimen fabrication The following is a list of important items that should be considered when fabricating specimens. a. Specimens should be extracted from test articles in the region that meets all process,engineering drawing,and specimen drawing requirements. b. Specimens should be located on the test article according to the cutting diagram provided by the re- questing organization.If a test article does not pass the inspection criteria,the requesting organiza- tion may choose to cut specimens relative to identified test article defects to make the effect of the de- fects on the specimen response representative of the full-scale item. NOTE:When defining specimen locations,allow for material removed in the cutting operation. c.A specimen identification code should be defined in the test plan,referenced in the test instructions, and recorded in the data sheets.The specimen identification code should be permanently marked on each specimen.Care should be taken to keep the code outside the failure area of the specimen. d.For specimens too small to mark the complete code,mark only the unique serial number on the specimen.It is recommended that care be taken to place small specimens in bags properly labeled with that specimen's full identification. e.If it is required that the location of the specimen on the test article be known,specimens should be labeled before being extracted.This labeling should allow all specimen and excess material locations to be known after cutting. f.The reference edge of the specimen should be aligned within the specified orientation using the wit- ness line.In instances where a smaller subtest article is machined and used to make several speci- mens at once,a reference line or edge should be transferred to this subtest article from the witness line.This transferred line should be orientated within t0.25 with respect to the witness line. g.Before cutting,the specimen location and orientation should be verified by the requesting organiza- tion or an independent reviewer. h.Specimens should be extracted from the fabricated test articles according to the appropriate machin- ing procedure as specified.Specimens may be machined with a variety of machining tools.In gen- eral the final cutting tool should have a fine grit,be hardened,and run at a high tool speed without wobble.The cut itself should be executed to minimize excess heating of the laminate. i.The added cost and manufacturing associated with tabbed specimens should be considered when selecting specimen type.The limitations and problems associated with the tabbing of specimens is stated in each individual test method.If bonded tabs are required.the cure of the adhesive should be evaluated to determine if it is compatible with the composite system and tab material(if different).If the tab configuration produced in the bonding process is not within the geometry requirements of the specimen configuration,further machining of the tabs may be required. j.Holes in specimens should be drilled in accordance to the applicable process specification. k.Any fasteners that are required should be installed in accordance to the applicable process specifica- tion. 1.Completed specimens should be inspected prior to testing to ensure conformance with the standards being used.Variations in individual specimen thickness should be within the applicable test method tolerances.Larger variations may cause improper loading when used with close tolerance test fix- tures.These variations may indicate that the specimen was fabricated improperly (e.g.,ply drop-off or resin bleed). 6-3

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-3 6.2.4 Specimen fabrication The following is a list of important items that should be considered when fabricating specimens. a. Specimens should be extracted from test articles in the region that meets all process, engineering drawing, and specimen drawing requirements. b. Specimens should be located on the test article according to the cutting diagram provided by the re￾questing organization. If a test article does not pass the inspection criteria, the requesting organiza￾tion may choose to cut specimens relative to identified test article defects to make the effect of the de￾fects on the specimen response representative of the full-scale item. NOTE: When defining specimen locations, allow for material removed in the cutting operation. c. A specimen identification code should be defined in the test plan, referenced in the test instructions, and recorded in the data sheets. The specimen identification code should be permanently marked on each specimen. Care should be taken to keep the code outside the failure area of the specimen. d. For specimens too small to mark the complete code, mark only the unique serial number on the specimen. It is recommended that care be taken to place small specimens in bags properly labeled with that specimen’s full identification. e. If it is required that the location of the specimen on the test article be known, specimens should be labeled before being extracted. This labeling should allow all specimen and excess material locations to be known after cutting. f. The reference edge of the specimen should be aligned within the specified orientation using the wit￾ness line. In instances where a smaller subtest article is machined and used to make several speci￾mens at once, a reference line or edge should be transferred to this subtest article from the witness line. This transferred line should be orientated within ±0.25° with respect to the witness line. g. Before cutting, the specimen location and orientation should be verified by the requesting organiza￾tion or an independent reviewer. h. Specimens should be extracted from the fabricated test articles according to the appropriate machin￾ing procedure as specified. Specimens may be machined with a variety of machining tools. In gen￾eral the final cutting tool should have a fine grit, be hardened, and run at a high tool speed without wobble. The cut itself should be executed to minimize excess heating of the laminate. i. The added cost and manufacturing associated with tabbed specimens should be considered when selecting specimen type. The limitations and problems associated with the tabbing of specimens is stated in each individual test method. If bonded tabs are required, the cure of the adhesive should be evaluated to determine if it is compatible with the composite system and tab material (if different). If the tab configuration produced in the bonding process is not within the geometry requirements of the specimen configuration, further machining of the tabs may be required. j. Holes in specimens should be drilled in accordance to the applicable process specification. k. Any fasteners that are required should be installed in accordance to the applicable process specifica￾tion. l. Completed specimens should be inspected prior to testing to ensure conformance with the standards being used. Variations in individual specimen thickness should be within the applicable test method tolerances. Larger variations may cause improper loading when used with close tolerance test fix￾tures. These variations may indicate that the specimen was fabricated improperly (e.g., ply drop-off or resin bleed)

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 6.3 CONDITIONING AND ENVIRONMENTAL EXPOSURE 6.3.1 Introduction Conditioning is the process of exposure of material to a potentially property-altering environment prior to subsequent test.'This section focuses on conditioning of materials subjected to moisture exposure (immersion in all types of fluids,but especially humid air).There are,of course,many other types of con- ditioning environments.An incomplete list includes:subambient(moderately low temperatures),cryo- genic(very low temperatures),elevated temperature(dry),oxidizing,low-Earth orbit simulation(including exposure to monatomic oxygen),and exposure to various types of radiation.Conditioning issues in these other environments will not be explicitly discussed in this section.A related,but much more difficult,ex- tension of material conditioning is associated with the issue of long-term aging(for example,10,000 to 80,000 or more hours of exposure).which for practical engineering purposes requires development of procedures for accelerated conditioning.While some very limited and restricted guidelines for accelera- tion of basic moisture conditioning are discussed in the following subsections,acceleration of long-term aging processes is a state-of-the-art topic that is beyond the scope of this section. Most polymeric materials,whether unreinforced resin,polymeric composite matrix,or a polymer- based fiber,are capable of absorbing relatively small but potentially significant amounts of moisture from the surrounding environment.The physical mechanism for moisture mass change,assuming there are no cracks or other wicking paths,is generally assumed to be mass diffusion following Fick's Law(the moisture analog to thermal diffusion is discussed in Section 6.4.8).Fickian moisture diffusion into or out of the interior occurs relatively slowly;many orders of magnitude slower than heat flow in thermal diffu- sion.Nevertheless,given enough exposure-time in a moist environment,a significant amount of moisture may be absorbed into the material.This absorbed moisture may cause material swelling,and,particularly at higher temperatures,may soften and weaken the matrix and matrix/fiber interface,which is deleterious to many mechanical properties that are often design drivers for structural applications.Absorbed mois- ture effectively lowers the maximum use temperature of the material(see Sections 2.2.7 and 2.2.8).The effect is demonstrated by a lowering of the glass transition temperature(thus the particular interest in Ts test results) The two main types of basic moisture conditioning of materials are:fixed-time conditioning,where a material specimen is exposed to a conditioning environment for a specified period of time;and equilibrium conditioning,where a specimen is exposed until the material reaches equilibrium with the conditioning environment.While fixed-time conditioning is still in common use when screening materials,it usually results in a material condition that is substantially non-uniform through the thickness;subsequent test re- sults are,therefore,considered only a qualitative assessment rather than a quantitative result.Except for certain screening-level purposes,or as part of application-specific structural-level tests.fixed-time condi- tioning as summarized in Section 6.3.2 is not considered sufficient or representative;only equilibrium conditioning as discussed in Section 6.3.3 provides a true assessment of comparable material response. When absorbed moisture is a potential design concern,a material testing program should evaluate both the moisture absorption material properties(diffusion rate and equilibrium content)and the effect of absorbed moisture on key design properties after equilibrium moisture exposure.An ASTM moisture ab- sorption conditioning/material property test method,ASTM D 5229/D 5229M(Reference 6.3.1),has been created to define the conditioning parameters and procedures needed to assure that uniform through- 'Nonambient testing is another subject,and,for mechanical testing,is covered in Section 6.5.3. While certain polymers,like polybutadiene,resist water vapor absorption to the point that humidity conditioning may not be re- quired,these materials are still considered rare exceptions.On the other hand,most reinforcements,including those of the carbon glass,metallic,and ceramic fiber families,are not hygroscopic.As a result,except for polymeric fibers like aramid,it is usually as- sumed that any water vapor absorption is limited to the polymer matrix. 6-4

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-4 6.3 CONDITIONING AND ENVIRONMENTAL EXPOSURE 6.3.1 Introduction Conditioning is the process of exposure of material to a potentially property-altering environment prior to subsequent test.1 This section focuses on conditioning of materials subjected to moisture exposure (immersion in all types of fluids, but especially humid air). There are, of course, many other types of con￾ditioning environments. An incomplete list includes: subambient (moderately low temperatures), cryo￾genic (very low temperatures), elevated temperature (dry), oxidizing, low-Earth orbit simulation (including exposure to monatomic oxygen), and exposure to various types of radiation. Conditioning issues in these other environments will not be explicitly discussed in this section. A related, but much more difficult, ex￾tension of material conditioning is associated with the issue of long-term aging (for example, 10,000 to 80,000 or more hours of exposure), which for practical engineering purposes requires development of procedures for accelerated conditioning. While some very limited and restricted guidelines for accelera￾tion of basic moisture conditioning are discussed in the following subsections, acceleration of long-term aging processes is a state-of-the-art topic that is beyond the scope of this section. Most polymeric materials, whether unreinforced resin, polymeric composite matrix, or a polymer￾based fiber, are capable of absorbing relatively small but potentially significant amounts of moisture from the surrounding environment.2 The physical mechanism for moisture mass change, assuming there are no cracks or other wicking paths, is generally assumed to be mass diffusion following Fick’s Law (the moisture analog to thermal diffusion is discussed in Section 6.4.8). Fickian moisture diffusion into or out of the interior occurs relatively slowly; many orders of magnitude slower than heat flow in thermal diffu￾sion. Nevertheless, given enough exposure-time in a moist environment, a significant amount of moisture may be absorbed into the material. This absorbed moisture may cause material swelling, and, particularly at higher temperatures, may soften and weaken the matrix and matrix/fiber interface, which is deleterious to many mechanical properties that are often design drivers for structural applications. Absorbed mois￾ture effectively lowers the maximum use temperature of the material (see Sections 2.2.7 and 2.2.8). The effect is demonstrated by a lowering of the glass transition temperature (thus the particular interest in Tg test results). The two main types of basic moisture conditioning of materials are: fixed-time conditioning, where a material specimen is exposed to a conditioning environment for a specified period of time; and equilibrium conditioning, where a specimen is exposed until the material reaches equilibrium with the conditioning environment. While fixed-time conditioning is still in common use when screening materials, it usually results in a material condition that is substantially non-uniform through the thickness; subsequent test re￾sults are, therefore, considered only a qualitative assessment rather than a quantitative result. Except for certain screening-level purposes, or as part of application-specific structural-level tests, fixed-time condi￾tioning as summarized in Section 6.3.2 is not considered sufficient or representative; only equilibrium conditioning as discussed in Section 6.3.3 provides a true assessment of comparable material response. When absorbed moisture is a potential design concern, a material testing program should evaluate both the moisture absorption material properties (diffusion rate and equilibrium content) and the effect of absorbed moisture on key design properties after equilibrium moisture exposure. An ASTM moisture ab￾sorption conditioning/material property test method, ASTM D 5229/D 5229M (Reference 6.3.1), has been created to define the conditioning parameters and procedures needed to assure that uniform through- 1 Nonambient testing is another subject, and, for mechanical testing, is covered in Section 6.5.3. 2 While certain polymers, like polybutadiene, resist water vapor absorption to the point that humidity conditioning may not be re￾quired, these materials are still considered rare exceptions. On the other hand, most reinforcements, including those of the carbon, glass, metallic, and ceramic fiber families, are not hygroscopic. As a result, except for polymeric fibers like aramid, it is usually as￾sumed that any water vapor absorption is limited to the polymer matrix

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization thickness equilibrium'is obtained during conditioning.ASTM D 5229/D 5229M also defines how to de- termine the moisture absorption properties,and its use for this purpose is discussed in more detail in Sec- tion6.6.8. 6.3.2 Fixed-time conditioning As stated above,fixed-time conditioning is only of limited usefulness2,it cannot generally provide the desired uniform moisture condition through the thickness of the material.The shortcomings of the fixed- time approach are illustrated in Figure 6.3.2 for a simulated 30-day exposure of IM6/3501-6 carbon/epoxy at 140F(60C)and 95%RH.Using known values for moisture diffusivity and moisture equilibrium con- tent,the calculated average moisture content of various laminate thicknesses is plotted and shown as a smooth curve.From this curve,it can be seen that the maximum laminate thickness that can reach equi- librium at this temperature during this fixed,though fairly lengthy,conditioning exposure,is 0.035 in.(0.89 mm).For greater thicknesses,the moisture distribution through the thickness will not be uniform,as the interior moisture levels will be below equilibrium moisture content.This is further illustrated by an exam- ple in Section 6.3.3. SPECIMEN THICKNESS (mm) 0.0 1.0 2.03.0 4.0 5.0 6.0 7.0 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 0.00 0.05 0.100.15 0.20 0.25 0.30 SPECIMEN THICKNESS (in) FIGURE 6.3.2 Two-sided moisture absorption of carbon/epoxy laminate after 30 days exposure at 140F(60C)/95%RH. As will be discussed in Section 6.3.3.1,with lower target relative humidity levels,it is common to try to accelerate conditioning by subjecting the material to a higher relative humidity level for a shorter period of time.The objective is to introduce the same average moisture content in the material as would be seen in equilibrium conditioning at the lower relative humidity level,although the distribution of moisture content distribution will be less uniform through the thickness.Using a single-humidity level,fixed-time condition- ing example,again illustrated by Figure 6.3.2,equilibrium at 78%RH(1.2%equilibrium moisture content for this material)can be approximated only at a thickness of 0.070 in.(1.8 mm).For a thickness greater than 0.070 in.(1.8 mm),the average moisture content will be insufficient,and for a thickness less than The discussion focuses on through the thickness moisture absorption;however,in-plane moisture absorption will locally dominate near edges,and may even dominate the overall absorption process in those cases where edge area is a substantial portion of the total exposed area. 2Examples of fixed-time conditioning methods that should specifically be avoided include:ASTM D 618(Reference 6.3.2(a)),ASTM D 570(Reference 6.3.2(b)),and SACMA RM 11-88 Method I(Reference 6.3.2(c)). 6-5

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-5 thickness equilibrium1 is obtained during conditioning. ASTM D 5229/D 5229M also defines how to de￾termine the moisture absorption properties, and its use for this purpose is discussed in more detail in Sec￾tion 6.6.8. 6.3.2 Fixed-time conditioning As stated above, fixed-time conditioning is only of limited usefulness2 , it cannot generally provide the desired uniform moisture condition through the thickness of the material. The shortcomings of the fixed￾time approach are illustrated in Figure 6.3.2 for a simulated 30-day exposure of IM6/3501-6 carbon/epoxy at 140°F (60°C) and 95% RH. Using known values for moisture diffusivity and moisture equilibrium con￾tent, the calculated average moisture content of various laminate thicknesses is plotted and shown as a smooth curve. From this curve, it can be seen that the maximum laminate thickness that can reach equi￾librium at this temperature during this fixed, though fairly lengthy, conditioning exposure, is 0.035 in. (0.89 mm). For greater thicknesses, the moisture distribution through the thickness will not be uniform, as the interior moisture levels will be below equilibrium moisture content. This is further illustrated by an exam￾ple in Section 6.3.3. FIGURE 6.3.2 Two-sided moisture absorption of carbon/epoxy laminate after 30 days exposure at 140°F (60°C)/95% RH. As will be discussed in Section 6.3.3.1, with lower target relative humidity levels, it is common to try to accelerate conditioning by subjecting the material to a higher relative humidity level for a shorter period of time. The objective is to introduce the same average moisture content in the material as would be seen in equilibrium conditioning at the lower relative humidity level, although the distribution of moisture content distribution will be less uniform through the thickness. Using a single-humidity level, fixed-time condition￾ing example, again illustrated by Figure 6.3.2, equilibrium at 78% RH (1.2% equilibrium moisture content for this material) can be approximated only at a thickness of 0.070 in. (1.8 mm). For a thickness greater than 0.070 in. (1.8 mm), the average moisture content will be insufficient, and for a thickness less than 1 The discussion focuses on through the thickness moisture absorption; however, in-plane moisture absorption will locally dominate near edges, and may even dominate the overall absorption process in those cases where edge area is a substantial portion of the total exposed area. 2 Examples of fixed-time conditioning methods that should specifically be avoided include: ASTM D 618 (Reference 6.3.2(a)), ASTM D 570 (Reference 6.3.2(b)), and SACMA RM 11-88 Method I (Reference 6.3.2(c))

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 0.070 in.(1.8 mm),the moisture content will be higher than desired.Again,the fixed-time conditioning approach is inadequate. As seen from the examples above,total moisture content resulting from fixed-time conditioning is thickness dependent.However,since fluids diffuse through different materials at different rates,fixed- time conditioning cannot produce a uniform material condition for all materials,even if thickness is held constant.Therefore,test results based on fixed-time conditioning should not be used for design values and generally should not even be used in qualitative comparisons between different materials.However, fixed-time conditioning can serve a purpose when combined with a flexure test(which is sensitive to sur- face exposure)for qualitative aerospace fluids assessment,as discussed in Section 2.3.1.3. 6.3.3 Equilibrium conditioning To evaluate worst-case effects of moisture content on material properties,tests are performed with specimens preconditioned to the design service (end-of-life)moisture content (hereinafter assumed equivalent to equilibrium at the design service relative humidity).The preferred conditioning methodology uses ASTM D 5229/D 5229M(Reference 6.3.1),a test method that includes procedures for conditioning as well as for determining the two Fickian moisture material properties:moisture diffusivity and moisture equilibrium content(weight percent moisture). ASTM D 5229/D 5229M is a gravimetric test method that exposes a specimen to a moisture environ- ment and plots moisture mass gain versus the square-root of elapsed time.The early portion of the mass/square-root-time relationship is linear,the slope of which is related to the moisture diffusivity.As the moisture content of the material near the surface begins to approach equilibrium,the slope of this curve becomes increasingly smaller.Eventually,as the interior of the material approaches equilibrium,the dif- ference between subsequent weighings will be very small and the slope will be nearly zero.At this point the material is said to be at equilibrium moisture content.This process is illustrated in Figures 6.3.3(a) and(b).Figure 6.3.3(a)shows the total mass gain versus square-root-time during specimen moisture exposure;the different curves illustrate the difference in response due to different temperatures.For the 150F condition(the diamonds in Figure 6.3.3(a)),Figure 6.3.3(b)shows the moisture profile through the thickness of the specimen for several early time periods,illustrating the rapid moisture uptake near the surface together with the relatively slow update of moisture in the middle of the specimen. A similar,but more limited and not fully equivalent,procedure for conditioning and equilibrium mois- ture content(but not diffusivity)is documented by SACMA RM 11R-94(Reference 6.3.3(b)),which first brings three specimens to moisture equilibrium at 85%RH.The actual SACMA conditioning process on test specimens is then subsequently conducted,and terminated when the weight gain of the conditioned specimens reaches 90%of the moisture equilibrium content,resulting in a lower moisture content in the test specimen as compared to that resulting from ASTM D 5229/D 5229M.As an example,a 0.1 in.(2.5 mm)thick laminate with a diffusivity of 1.6E-09 in /s(1.0E-06 mm /s)and a true (very long-term)equilib- rium moisture content of 1.50%,when evaluated by the two approaches,would reach effective equilibrium at 1.45%in 24 days (ASTM),or at 1.43%in 21 days (SACMA).In subsequent conditioning,the ASTM procedure would reproduce the same 1.45%moisture content in 24 days,while the SACMA conditioning procedure would produce a moisture content of 1.29%(0.9 x 1.43)in 13 days. "Including a specific material system produced at different resin contents While the 1988 version of SACMA RM 11 used a different definition of equilibrium,the 1994 edition adopted the ASTM definition with one difference:the reference time period(minimum weighing time interval for equilibrium)was fixed at 24 hours.For suffi- ciently high diffusion rates there is no difference.For example,for the SACMA RM 11R-94 preferred thickness of 0.040 in.(1 mm). the two definitions begin to deviate when the moisture diffusivity is slower(smaller in value)than 3.6E-10 in /s (2.3E-07 mm/s).As the rate of diffusion slows below 3.6E-10 in'/s(2.3E-07 mm2/s).the SACMA calculated equilibrium moisture content will begin to deviate from the ASTM value.This diffusivity crossover point is a function of thickness;for the maximum SACMA thickness of 0.080 in.(2 mm),the crossover point increases to a diffusivity of 1.4E-09 in'/s(9.3E-7 mm'/s).When determining the moisture equilibrium content of low diffusivity materials,the ASTM definition,which is sensitive to both diffusion rate and coupon thickness,should be used. 6-6

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-6 0.070 in. (1.8 mm), the moisture content will be higher than desired. Again, the fixed-time conditioning approach is inadequate. As seen from the examples above, total moisture content resulting from fixed-time conditioning is thickness dependent. However, since fluids diffuse through different materials at different rates, fixed￾time conditioning cannot produce a uniform material condition for all materials,1 even if thickness is held constant. Therefore, test results based on fixed-time conditioning should not be used for design values, and generally should not even be used in qualitative comparisons between different materials. However, fixed-time conditioning can serve a purpose when combined with a flexure test (which is sensitive to sur￾face exposure) for qualitative aerospace fluids assessment, as discussed in Section 2.3.1.3. 6.3.3 Equilibrium conditioning To evaluate worst-case effects of moisture content on material properties, tests are performed with specimens preconditioned to the design service (end-of-life) moisture content (hereinafter assumed equivalent to equilibrium at the design service relative humidity). The preferred conditioning methodology uses ASTM D 5229/D 5229M (Reference 6.3.1), a test method that includes procedures for conditioning as well as for determining the two Fickian moisture material properties: moisture diffusivity and moisture equilibrium content (weight percent moisture). ASTM D 5229/D 5229M is a gravimetric test method that exposes a specimen to a moisture environ￾ment and plots moisture mass gain versus the square-root of elapsed time. The early portion of the mass/square-root-time relationship is linear, the slope of which is related to the moisture diffusivity. As the moisture content of the material near the surface begins to approach equilibrium, the slope of this curve becomes increasingly smaller. Eventually, as the interior of the material approaches equilibrium, the dif￾ference between subsequent weighings will be very small and the slope will be nearly zero. At this point the material is said to be at equilibrium moisture content. This process is illustrated in Figures 6.3.3(a) and (b). Figure 6.3.3(a) shows the total mass gain versus square-root-time during specimen moisture exposure; the different curves illustrate the difference in response due to different temperatures. For the 150°F condition (the diamonds in Figure 6.3.3(a)), Figure 6.3.3(b) shows the moisture profile through the thickness of the specimen for several early time periods, illustrating the rapid moisture uptake near the surface together with the relatively slow update of moisture in the middle of the specimen. A similar, but more limited and not fully equivalent, procedure for conditioning and equilibrium mois￾ture content (but not diffusivity) is documented by SACMA RM 11R-94 (Reference 6.3.3(b)), which first brings three specimens to moisture equilibrium at 85% RH.2 The actual SACMA conditioning process on test specimens is then subsequently conducted, and terminated when the weight gain of the conditioned specimens reaches 90% of the moisture equilibrium content, resulting in a lower moisture content in the test specimen as compared to that resulting from ASTM D 5229/D 5229M. As an example, a 0.1 in. (2.5 mm) thick laminate with a diffusivity of 1.6E-09 in2 /s (1.0E-06 mm2 /s) and a true (very long-term) equilib￾rium moisture content of 1.50%, when evaluated by the two approaches, would reach effective equilibrium at 1.45% in 24 days (ASTM), or at 1.43% in 21 days (SACMA). In subsequent conditioning, the ASTM procedure would reproduce the same 1.45% moisture content in 24 days, while the SACMA conditioning procedure would produce a moisture content of 1.29% (0.9 x 1.43) in 13 days. 1 Including a specific material system produced at different resin contents. 2 While the 1988 version of SACMA RM 11 used a different definition of equilibrium, the 1994 edition adopted the ASTM definition, with one difference: the reference time period (minimum weighing time interval for equilibrium) was fixed at 24 hours. For suffi￾ciently high diffusion rates there is no difference. For example, for the SACMA RM 11R-94 preferred thickness of 0.040 in. (1 mm), the two definitions begin to deviate when the moisture diffusivity is slower (smaller in value) than 3.6E-10 in2 /s (2.3E-07 mm2 /s). As the rate of diffusion slows below 3.6E-10 in2 /s (2.3E-07 mm2 /s), the SACMA calculated equilibrium moisture content will begin to deviate from the ASTM value. This diffusivity crossover point is a function of thickness; for the maximum SACMA thickness of 0.080 in. (2 mm), the crossover point increases to a diffusivity of 1.4E-09 in2 /s (9.3E-7 mm2 /s). When determining the moisture equilibrium content of low diffusivity materials, the ASTM definition, which is sensitive to both diffusion rate and coupon thickness, should be used

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 1.6 ●180°F(82C) ◆150F(66C) 1.4 A1200F (49C) 图 5°F (24C) 1.2 8 1.0 8 HTS Carbon/Epoxy:B Ply Laminate:95%RH 1 6 9 10 Time (Days) FIGURE 6.3.3(a)Typical moisture absorption response(Reference 6.3.3(a)). The relative humidity level to be used when moisture conditioning is application dependent.As dis- cussed in more detail in Section 2.2.7.3,the MIL-HDBK-17 Coordination Group has agreed that a rea- sonable upper-bound value for aircraft design service relative humidity is 85%,and that this value may be used when a specific determination of design service moisture content has not been established for a specific aircraft application.Accepted design service moisture levels for other applications have not yet been established. 6.3.3.1 Accelerating conditioning times Because equilibrium moisture conditioning can take a very long time,there is a strong desire to at- tempt to accelerate the process.While certain two-step,accelerated conditioning cycles are considered acceptable,such as use of an initial high-humidity step(95+%RH)to speed up moisture gain,followed by completion to equilibrium at a lower final humidity level(85%RH),one must be careful not to select an accelerating environment that changes the material,alters the physics of diffusion,or both.Since the moisture diffusion rate is so strongly dependent on temperature,there is a temptation to accelerate the process by increasing the conditioning temperature.1 However,long exposures to high temperatures combined with moisture may alter the chemistry of the material.2 350F(177C)cure epoxy-based mate- rials are typically not conditioned above 180F(82C)in order to avoid this problem;materials that cure at lower-temperatures may need to be conditioned below 180F(82C).And while an initial high relative humidity step is acceptable,the extreme cases of exposure to pressurized steam or immersion in hot/boiling water are not accepted methods of accelerating humidity absorption,as they have been found to produce different results from that of 100%humidity.3 'As an example,for the material illustrated by Figure 2.2.7.1(a),increasing the temperature from 150F(65C)to 180F(82C)in- creased the moisture diffusivity of the material from 4.5E-10 in/s(2.9E-07 mm/s)to 9.8E-10 in/s (6.3E-07 mm/s).resulting in substantially reduced conditioning times. 2The definition of"high"temperature,is,of course,relative to the material system in question,and cannot properly be addressed here. The differences reported in the literature are probably due in part to excessively-high conditioning temperatures,but even at mod- erate temperatures water immersion appears to produce a different response in many polymers than water vapor.In some cases matrix components have been known to dissolve into the water. 6-7

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-7 FIGURE 6.3.3(a) Typical moisture absorption response (Reference 6.3.3(a)). The relative humidity level to be used when moisture conditioning is application dependent. As dis￾cussed in more detail in Section 2.2.7.3, the MIL-HDBK-17 Coordination Group has agreed that a rea￾sonable upper-bound value for aircraft design service relative humidity is 85%, and that this value may be used when a specific determination of design service moisture content has not been established for a specific aircraft application. Accepted design service moisture levels for other applications have not yet been established. 6.3.3.1 Accelerating conditioning times Because equilibrium moisture conditioning can take a very long time, there is a strong desire to at￾tempt to accelerate the process. While certain two-step, accelerated conditioning cycles are considered acceptable, such as use of an initial high-humidity step (95+% RH) to speed up moisture gain, followed by completion to equilibrium at a lower final humidity level (85% RH), one must be careful not to select an accelerating environment that changes the material, alters the physics of diffusion, or both. Since the moisture diffusion rate is so strongly dependent on temperature, there is a temptation to accelerate the process by increasing the conditioning temperature.1 However, long exposures to high temperatures combined with moisture may alter the chemistry of the material.2 350°F (177°C) cure epoxy-based mate￾rials are typically not conditioned above 180°F (82°C) in order to avoid this problem; materials that cure at lower-temperatures may need to be conditioned below 180°F (82°C). And while an initial high relative humidity step is acceptable, the extreme cases of exposure to pressurized steam or immersion in hot/boiling water are not accepted methods of accelerating humidity absorption, as they have been found to produce different results from that of 100% humidity.3 1 As an example, for the material illustrated by Figure 2.2.7.1(a), increasing the temperature from 150°F (65°C) to 180°F (82°C) in￾creased the moisture diffusivity of the material from 4.5E-10 in2 /s (2.9E-07 mm2 /s) to 9.8E-10 in2 /s (6.3E-07 mm2 /s), resulting in substantially reduced conditioning times. 2 The definition of "high" temperature, is, of course, relative to the material system in question, and cannot properly be addressed here. 3 The differences reported in the literature are probably due in part to excessively-high conditioning temperatures, but even at mod￾erate temperatures water immersion appears to produce a different response in many polymers than water vapor. In some cases, matrix components have been known to dissolve into the water

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 1.6 1.4 024hr A 72 hr o72 hr 1.0 0120hr -Predried at 200 F(93 C) ······Predried at150F(64Cl 0.8 0.6 0.4 HTS carbon/epoxy 150 F (64 C)and 95%RH 0.2 0.0 ● 50 50 Coupon Depth(%) FIGURE 6.3.3(b)Through the thickness moisture profile versus time (Reference 6.3.3(a)). 6.3.3.2 Procedural hints While the procedural description and requirements for ASTM D 5229/D 5229M are fairly complete, the following items justify emphasis: 1.It is highly recommended that some knowledge of the material moisture response be obtained prior to starting conditioning,either from the literature,or from prior test. 2. In moisture property measurement the actual specimen must be initially dry,and the precision and timing of early mass measurements are critical.But for material conditioning needs,knowl- edge of the initial moisture content may not be important,or may adequately be separately de- termined from other specimens in parallel.Therefore,it is common not to begin moisture condi- tioning with a material dry-out step.Moisture conditioning also does not require the repetitive, precise weighings early in the exposure process that are needed to determine the moisture diffu- sivity.Thus,conditioning without simultaneous determination of the moisture absorption proper- ties is faster and less labor intensive. 3.If the moisture properties are desired,it is faster and less labor intensive to create two other sets of specialized moisture property specimens:a"thin"set that will reach equilibrium quickly,and a "thick"set from which a stable slope to the moisture weight gain versus square-root-time curve can be reliably obtained with minimum test sensitivity.This process is discussed in more detail in Section 6.6.8. While the procedures for both moisture property determination and equilibrium moisture conditioning are similar,there are some practical reasons why simultaneous determination of moisture properties during a moisture conditioning phase is rarely desirable. 6-8

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-8 FIGURE 6.3.3(b) Through the thickness moisture profile versus time (Reference 6.3.3(a)). 6.3.3.2 Procedural hints While the procedural description and requirements for ASTM D 5229/D 5229M are fairly complete, the following items justify emphasis: 1. It is highly recommended that some knowledge of the material moisture response be obtained prior to starting conditioning, either from the literature, or from prior test. 2. In moisture property measurement the actual specimen must be initially dry, and the precision and timing of early mass measurements are critical. But for material conditioning needs, knowl￾edge of the initial moisture content may not be important, or may adequately be separately de￾termined from other specimens in parallel. Therefore, it is common not to begin moisture condi￾tioning with a material dry-out step. Moisture conditioning also does not require the repetitive, precise weighings early in the exposure process that are needed to determine the moisture diffu￾sivity. Thus, conditioning without simultaneous determination of the moisture absorption proper￾ties is faster and less labor intensive. 3. If the moisture properties are desired, it is faster and less labor intensive to create two other sets of specialized moisture property specimens: a “thin” set that will reach equilibrium quickly, and a “thick” set from which a stable slope to the moisture weight gain versus square-root-time curve can be reliably obtained with minimum test sensitivity. This process is discussed in more detail in Section 6.6.8. While the procedures for both moisture property determination and equilibrium moisture conditioning are similar, there are some practical reasons why simultaneous determination of moisture properties during a moisture conditioning phase is rarely desirable

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization Moisture content measurements are taken either by weighing the actual specimens,or by weighing in their place "travelers,"which are material conditioning specimens cut from the same panel and condi- tioned at the same time as the specimens.Travelers are required when the specimen is either too small. too large,or includes other materials,such as specimens with tabs,or sandwich specimens.A traveler, when used,accompanies the specimen,or group of related specimens,throughout all subsequent condi- tioning history. Because the weight gain of typical polymeric composites is relatively small (on the order of 1%),mass measurement equipment must be selected accordingly.For larger specimens(>50 g),a balance accu- rate to 0.001 g is generally adequate.For smaller specimens with mass down to 5 g,a precision analyti- cal balance capable of reading to 0.0001 g is required.Direct moisture mass monitoring of coupons weighing less than 5 g is not recommended;a traveler should be used instead. Near the end of conditioning,minor weighing errors or small relative humidity excursions of the envi- ronmental chamber,particularly slight depressions in relative humidity,may artificially cause the material to appear to have reached equilibrium,when,in fact,the material is still absorbing moisture.The lower the temperature (lower the diffusion rate),the more important these errors become.Despite the literal definition of equilibrium expressed by ASTM D 5229/D 5229M,in view of the likely possibility of these ex- perimental errors,the prudent engineer should do the following: 1.Even after the material satisfies the definition of equilibrium,review the chamber records to en- sure that a depression in chamber relative humidity did not occur during the reference time period (weighing time interval).If such a depression is found to have occurred,continue the exposure until the chamber has stabilized,then go to item 2. 2.Even after the material satisfies the definition of equilibrium,maintain the exposure,and show satisfaction of the criterion for several consecutive reference time periods. If the required reference time period does not match a reasonable human time schedule for weighing. then a more regular time interval may be adopted and the ASTM D 5229/D 5229M requirement(less than 0.01%mass change over the reference time period)pro rated to the adjusted time interval.For example, if a required reference time period for equilibrium is determined to be 115,000 s(32 hours),the coupons may be weighed at either 24 hour intervals or 48 hour intervals,with the mass change requirement ad- justed from 0.01%to either 0.0075%(24/32 x 0.01)or 0.015%(48/32 x 0.01),respectively. While many newer models have solid-state controls,a great many environmental chambers control the chamber humidity via monitoring of"dry-bulb"(actual)and "wet-bulb"(moisture depressed)tempera- tures,which are converted to equivalent relative humidity via a table or algorithm supplied by the manu- facturer.The ability of these chambers to control relative humidity is dependent on the accuracy of the thermometer readings.Particularly important in these chambers is regular cleaning of the water reservoir, replacement of the wick,and maintenance of a proper contact between the wick and the wet-bulb ther- mometer (Reference 6.3.3.2).Chambers that control the dry-bulb temperature and the differentia/be- tween the dry-bulb and wet-bulb temperatures generally have improved control of chamber relative hu- midity over those that control the dry-bulb and wet-bulb temperatures. If a drying step is included,whether as an initial step prior to moisture conditioning,or has part of an oven-dry experiment,care should be taken to avoid excessively high drying temperatures and high ther- mal excursions that may induce thermal cracking in the material. A variant of equilibrium conditioning uses equilibrium conditioning test data,for a specific material and relative humidity,to establish a table or plotted-curve of minimum exposure time required to achieve equi- librium versus laminate thickness.This approach requires some up-front testing and calculation,but eliminates much of the repetitive weighing otherwise required.A continuous record of the chamber envi- ronment must be maintained to prove that proper exposure was achieved. 6-9

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-9 Moisture content measurements are taken either by weighing the actual specimens, or by weighing in their place “travelers,” which are material conditioning specimens cut from the same panel and condi￾tioned at the same time as the specimens. Travelers are required when the specimen is either too small, too large, or includes other materials, such as specimens with tabs, or sandwich specimens. A traveler, when used, accompanies the specimen, or group of related specimens, throughout all subsequent condi￾tioning history. Because the weight gain of typical polymeric composites is relatively small (on the order of 1%), mass measurement equipment must be selected accordingly. For larger specimens (>50 g), a balance accu￾rate to 0.001 g is generally adequate. For smaller specimens with mass down to 5 g, a precision analyti￾cal balance capable of reading to 0.0001 g is required. Direct moisture mass monitoring of coupons weighing less than 5 g is not recommended; a traveler should be used instead. Near the end of conditioning, minor weighing errors or small relative humidity excursions of the envi￾ronmental chamber, particularly slight depressions in relative humidity, may artificially cause the material to appear to have reached equilibrium, when, in fact, the material is still absorbing moisture. The lower the temperature (lower the diffusion rate), the more important these errors become. Despite the literal definition of equilibrium expressed by ASTM D 5229/D 5229M, in view of the likely possibility of these ex￾perimental errors, the prudent engineer should do the following: 1. Even after the material satisfies the definition of equilibrium, review the chamber records to en￾sure that a depression in chamber relative humidity did not occur during the reference time period (weighing time interval). If such a depression is found to have occurred, continue the exposure until the chamber has stabilized, then go to item 2. 2. Even after the material satisfies the definition of equilibrium, maintain the exposure, and show satisfaction of the criterion for several consecutive reference time periods. If the required reference time period does not match a reasonable human time schedule for weighing, then a more regular time interval may be adopted and the ASTM D 5229/D 5229M requirement (less than 0.01% mass change over the reference time period) pro rated to the adjusted time interval. For example, if a required reference time period for equilibrium is determined to be 115,000 s (32 hours), the coupons may be weighed at either 24 hour intervals or 48 hour intervals, with the mass change requirement ad￾justed from 0.01% to either 0.0075% (24/32 x 0.01) or 0.015% (48/32 x 0.01), respectively. While many newer models have solid-state controls, a great many environmental chambers control the chamber humidity via monitoring of “dry-bulb” (actual) and “wet-bulb” (moisture depressed) tempera￾tures, which are converted to equivalent relative humidity via a table or algorithm supplied by the manu￾facturer. The ability of these chambers to control relative humidity is dependent on the accuracy of the thermometer readings. Particularly important in these chambers is regular cleaning of the water reservoir, replacement of the wick, and maintenance of a proper contact between the wick and the wet-bulb ther￾mometer (Reference 6.3.3.2). Chambers that control the dry-bulb temperature and the differential be￾tween the dry-bulb and wet-bulb temperatures generally have improved control of chamber relative hu￾midity over those that control the dry-bulb and wet-bulb temperatures. If a drying step is included, whether as an initial step prior to moisture conditioning, or has part of an oven-dry experiment, care should be taken to avoid excessively high drying temperatures and high ther￾mal excursions that may induce thermal cracking in the material. A variant of equilibrium conditioning uses equilibrium conditioning test data, for a specific material and relative humidity, to establish a table or plotted-curve of minimum exposure time required to achieve equi￾librium versus laminate thickness. This approach requires some up-front testing and calculation, but eliminates much of the repetitive weighing otherwise required. A continuous record of the chamber envi￾ronment must be maintained to prove that proper exposure was achieved

MIL-HDBK-17-1F Volume 1,Chapter 6 Lamina,Laminate,and Special Form Characterization 6.4 INSTRUMENTATION AND CALIBRATION 6.4.1 Introduction The ability to accurately and repeatably measure deformation and displacement is critical to the test- ing and characterization of composite materials.This section will discuss the various types of instrumen- tation used to make strain measurements,and provide guidelines to help determine the appropriate methods for various test types,material forms,test conditions,and data requirements.Only those exten- someters which can be classified as ASTM E 83 Class B-2 or better are acceptable for generating data to be included in MIL-HDBK-17(Reference 6.4.1). 6.4.2 Test specimen dimensional measurement 6.4.2.1 Introduction Virtually all mechanical property testing requires that dimensional measurements of the test specimen be made.The types of measurements vary depending upon the particular specimen geometry and test requirements,and may include specimen length,width,thickness,gage length,hole diameter,and fas- tener diameter.Required precision is usually specified by the test method or specification,but generally depends on how a measurement will be used.Some measurements are simply informational,while oth- ers are used in calculations (to convert load to stress,for example),and still others are needed to verify conformance to a required geometry.The following five sections discuss (in order of decreasing preci- sion)the various devices commonly used to measure specimen dimensions.Following this is a section on special hole diameter measuring devices.The final section discusses calibration of dimensional measurement devices. 6.4.2.2 Calibrated microscopes Microscopes with calibrated scales in their eyepieces can provide an extremely accurate means for measuring small specimen dimensions.Resolutions down to 0.0001 inch(2.5 um)can routinely be at- tained using magnifications in the range of 50x-200x.Although this technique is usually more time con- suming than micrometer measurement,there are some instances where optical methods may be the only practical option.For example,the thickness of a tabbed specimen may be in question after destruction of the gage section during test.Thickness may be measured and/or verified by optically measuring the thickness of the laminate remaining intact under the bonded tabs.Under the calibrated microscope the laminate thickness between the adhesive bondlines of the tabs can easily be seen and measured (al- though there is a bias on rough textured specimens).Except for such special cases,however,direct mi- crometer measurement is usually preferable. 6.4.2.3 Micrometers Micrometers are precision instruments that are most commonly used for measuring small dimensions. Although some models are available for measurements up to several inches,or even several feet,they generally can only measure continuously over a one inch(25 mm)interval,and require extension rods for different intervals.For this reason calipers are often more convenient for measuring dimensions larger than one inch. The standard one inch micrometer(25.4 mm)'is the most popular instrument for measuring speci- men thicknesses.For wide specimens,deep reach micrometers are available for making thickness measurements several inches or more from the specimen edges.The readout may be a scale engraved around the barrel (optionally with a vernier scale),a mechanical digital display,or an electronic digital dis- play.Most instruments indicate in 0.0001 inch graduations and digital models often estimate a fifth deci- mal place 'Note that the Sl equivalent dimensions provided in this section are Asoft=conversions,that is SI dimensions for measuring instru- ments and gradations are provided but sizes are not necessarily converted to SI standard sizes. 6-10

MIL-HDBK-17-1F Volume 1, Chapter 6 Lamina, Laminate, and Special Form Characterization 6-10 6.4 INSTRUMENTATION AND CALIBRATION 6.4.1 Introduction The ability to accurately and repeatably measure deformation and displacement is critical to the test￾ing and characterization of composite materials. This section will discuss the various types of instrumen￾tation used to make strain measurements, and provide guidelines to help determine the appropriate methods for various test types, material forms, test conditions, and data requirements. Only those exten￾someters which can be classified as ASTM E 83 Class B-2 or better are acceptable for generating data to be included in MIL-HDBK-17 (Reference 6.4.1). 6.4.2 Test specimen dimensional measurement 6.4.2.1 Introduction Virtually all mechanical property testing requires that dimensional measurements of the test specimen be made. The types of measurements vary depending upon the particular specimen geometry and test requirements, and may include specimen length, width, thickness, gage length, hole diameter, and fas￾tener diameter. Required precision is usually specified by the test method or specification, but generally depends on how a measurement will be used. Some measurements are simply informational, while oth￾ers are used in calculations (to convert load to stress, for example), and still others are needed to verify conformance to a required geometry. The following five sections discuss (in order of decreasing preci￾sion) the various devices commonly used to measure specimen dimensions. Following this is a section on special hole diameter measuring devices. The final section discusses calibration of dimensional measurement devices. 6.4.2.2 Calibrated microscopes Microscopes with calibrated scales in their eyepieces can provide an extremely accurate means for measuring small specimen dimensions. Resolutions down to 0.0001 inch (2.5 µm) can routinely be at￾tained using magnifications in the range of 50x - 200x. Although this technique is usually more time con￾suming than micrometer measurement, there are some instances where optical methods may be the only practical option. For example, the thickness of a tabbed specimen may be in question after destruction of the gage section during test. Thickness may be measured and/or verified by optically measuring the thickness of the laminate remaining intact under the bonded tabs. Under the calibrated microscope the laminate thickness between the adhesive bondlines of the tabs can easily be seen and measured (al￾though there is a bias on rough textured specimens). Except for such special cases, however, direct mi￾crometer measurement is usually preferable. 6.4.2.3 Micrometers Micrometers are precision instruments that are most commonly used for measuring small dimensions. Although some models are available for measurements up to several inches, or even several feet, they generally can only measure continuously over a one inch (25 mm) interval, and require extension rods for different intervals. For this reason calipers are often more convenient for measuring dimensions larger than one inch. The standard one inch micrometer (25.4 mm)1 is the most popular instrument for measuring speci￾men thicknesses. For wide specimens, deep reach micrometers are available for making thickness measurements several inches or more from the specimen edges. The readout may be a scale engraved around the barrel (optionally with a vernier scale), a mechanical digital display, or an electronic digital dis￾play. Most instruments indicate in 0.0001 inch graduations and digital models often estimate a fifth deci￾mal place. 1 Note that the SI equivalent dimensions provided in this section are Αsoft≅ conversions, that is SI dimensions for measuring instru￾ments and gradations are provided but sizes are not necessarily converted to SI standard sizes