《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 3）Chapter 12 Lessons Learned

MIL-HDBK-17-3F Volume 3,Chapter 12-Lessons Learned CHAPTER 12 LESSONS LEARNED 12.1 INTRODUCTION The focus of much of what is in this handbook concentrates on establishing proper techniques for development and utilization of composite material property data.The motivation prompting specific choices is not always evident.This chapter provides a depository of knowledge gained from a number of involved contractors,agencies,and businesses for the purpose of disseminating lessons learned to po- tential users who might otherwise repeat past mistakes.Many of the contractors involved in developing the lessons learned are aerospace oriented.Thus,the lessons learned may have a decidedly aerospace viewpoint. The chapter starts with a discussion of some of the characteristics of composite materials that makes them different from metals.These characteristics are the primary cause for establishing the methods and techniques contained in the handbook. Specific lessons learned are defined in later sections.They contain the specific "rule of thumb"and the reason for its creation or the possible consequence if it is not followed.The lessons learned are or- ganized into six different categories for convenience. 12.2 UNIQUE ISSUES FOR COMPOSITES Composites are different from metals in several ways.These include their largely elastic response, their ability to be tailored in strength and stiffness,their damage tolerance characteristics,and their sensi- tivity to environmental factors.These differences force a different approach to analysis and design,proc- essing,fabrication and assembly,quality control,testing,and certification. 12.2.1 Elastic properties The elastic properties of a material are a measure of its stiffness.This property is necessary to de- termine the deformations that are produced by loads.In composites,the stiffness is dominated by the fibers;the role of the matrix is to prevent lateral deflections of the fibers and to provide a mechanism for shearing load from one fiber to another.Continuous fiber composites are transversely isotropic and in a two-dimensional stress state require four elastic properties to characterize the material: Modulus of elasticity parallel to the fiber,E Modulus of elasticity perpendicular to the fiber,E2 Shear modulus,Gi2 Major Poisson's ratio,v12 In general,material characterization may require additional properties not defined above.A thorough dis- cussion of this subject is given in Section 5.3.1.Only two elastic properties are required for isotropic ma- terials,the modulus of elasticity and Poisson's ratio. The stress-strain response of commonly used fiber-dominated orientations of composite materials is almost linear to failure although some glasses and ceramics have nonlinear or bilinear behavior.This is contrasted to metals that exhibit nonlinear response above the proportional limit and eventual plastic de- formation above the yield point.Many composites exhibit very little,if any,yielding in fiber dominated be- havior.Toughened materials and thermoplastics can show considerable yielding,particularly in matrix dominated directions.This factor requires composites to be given special consideration in structural de- tails where there are stress risers (holes,cutouts,notches,radii,tapers,etc.).These types of stress ris- ers in metal are not a major concern for static strength analysis(they do play a big role in durability and damage tolerance analysis,however).In composites they must be considered in static strength analysis. 12-1

MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-1 CHAPTER 12 LESSONS LEARNED 12.1 INTRODUCTION The focus of much of what is in this handbook concentrates on establishing proper techniques for development and utilization of composite material property data. The motivation prompting specific choices is not always evident. This chapter provides a depository of knowledge gained from a number of involved contractors, agencies, and businesses for the purpose of disseminating lessons learned to po￾tential users who might otherwise repeat past mistakes. Many of the contractors involved in developing the lessons learned are aerospace oriented. Thus, the lessons learned may have a decidedly aerospace viewpoint. The chapter starts with a discussion of some of the characteristics of composite materials that makes them different from metals. These characteristics are the primary cause for establishing the methods and techniques contained in the handbook. Specific lessons learned are defined in later sections. They contain the specific "rule of thumb" and the reason for its creation or the possible consequence if it is not followed. The lessons learned are or￾ganized into six different categories for convenience. 12.2 UNIQUE ISSUES FOR COMPOSITES Composites are different from metals in several ways. These include their largely elastic response, their ability to be tailored in strength and stiffness, their damage tolerance characteristics, and their sensi￾tivity to environmental factors. These differences force a different approach to analysis and design, proc￾essing, fabrication and assembly, quality control, testing, and certification. 12.2.1 Elastic properties The elastic properties of a material are a measure of its stiffness. This property is necessary to de￾termine the deformations that are produced by loads. In composites, the stiffness is dominated by the fibers; the role of the matrix is to prevent lateral deflections of the fibers and to provide a mechanism for shearing load from one fiber to another. Continuous fiber composites are transversely isotropic and in a two-dimensional stress state require four elastic properties to characterize the material: Modulus of elasticity parallel to the fiber, E1 Modulus of elasticity perpendicular to the fiber, E2 Shear modulus, G12 Major Poisson's ratio, ν 12 In general, material characterization may require additional properties not defined above. A thorough dis￾cussion of this subject is given in Section 5.3.1. Only two elastic properties are required for isotropic ma￾terials, the modulus of elasticity and Poisson's ratio. The stress-strain response of commonly used fiber-dominated orientations of composite materials is almost linear to failure although some glasses and ceramics have nonlinear or bilinear behavior. This is contrasted to metals that exhibit nonlinear response above the proportional limit and eventual plastic de￾formation above the yield point. Many composites exhibit very little, if any, yielding in fiber dominated be￾havior. Toughened materials and thermoplastics can show considerable yielding, particularly in matrix dominated directions. This factor requires composites to be given special consideration in structural de￾tails where there are stress risers (holes, cutouts, notches, radii, tapers, etc.). These types of stress ris￾ers in metal are not a major concern for static strength analysis (they do play a big role in durability and damage tolerance analysis, however). In composites they must be considered in static strength analysis

MIL-HDBK-17-3F Volume 3,Chapter 12-Lessons Learned In general,if these stress risers are properly considered in design/analysis of laminated parts,fatigue loadings will not be critical. Another unique characteristic of composite material elastic response is its orthotropy.When metals are extended in one direction,they contract in the perpendicular direction in an amount equal to the Pois- son's ratio times the longitudinal strain.This is true regardless of which direction is extended.In compos- ites,an extension in the longitudinal(1 or x)direction produces a contraction in the transverse direction(2 or y)equal to the "major"Poisson's ratio,vxy,times the longitudinal extension.If this is reversed,an ex- tension in the transverse direction produces a much lower contraction in the longitudinal direction.In fiber dominated laminates,Poisson's ratio can vary from <0.1 to >0.5. The most unusual characteristic of composites is the response produced when the lay-up is unbal- anced and/or unsymmetric.Such a laminate exhibits anisotropic warping characteristics.In this condition an extension in one direction can produce an in-plane shear deformation.It can also cause an out-of-plane bending or torsional response.All these effects are sometimes observed in one laminate. This type of response is generally undesirable because of warping or built-in stresses that occur.Hence, most laminate configurations are balanced and symmetric. Classical lamination theory is used to combine the individual lamina properties to predict the linear elastic behavior of arbitrary laminates.Lamination theory requires the definition of lamina elastic proper- ties,their orientation within the laminate,and their stacking position.The process assumes plane sec- tions remain-plane and enforces equilibrium.Lamination theory will solve for the loads/stresses/strains for each lamina within the laminate at a given location for a given set of applied loads.This combined with appropriate failure theory will predict the strength of the laminate(empirically modified input ply prop- erties are often necessary). 12.2.2 Tailored properties and out-of-plane loads The properties of a composite laminate depend on the orientation of the individual plies.This pro- vides the engineer with the ability to tailor a laminate to fit a particular requirement.For high axial loads predominantly in one direction,the laminate should have a majority of its plies oriented parallel to that loading direction.If the laminate is loaded mostly in shear,there should be a high percent of t45 pairs. For loads in multi-directions,the laminate should be quasi-isotropic.An all 0 laminate represents the maximum strength and stiffness that can be attained in any given direction,but is impractical for most ap- plications since the transverse properties are so weak that machining and handling can cause damage. Fiber-dominated,balanced and symmetric,laminate designs that have a minimum of 10%of the plies in each of the0°，+45°，-45°，and90°directions are most commonly used. Tailoring also means an engineer is not able to cite a strength or stiffness value for a composite lami- nate until he knows the laminate's ply percentages in each direction.Carpet plots of various properties vs.the percent of plies in each direction are commonly used for balanced and symmetric laminates.An example for stiffness is shown in Figure 12.2.2.Similar plots for strength can also be developed. Out-of-plane loads can also be troublesome for composites.These loads cause interlaminar shear and tension in the laminate.Interlaminar shear stress can cause failure of the matrix or the fiber-matrix interphase region.Interlaminar shear and tensile stresses can delaminate or disbond a laminate.Such loading should be avoided if possible.Design situations that tend to create interlaminar shear loading include high out-of-plane loads(such as fuel pressure),buckling,abrupt changes in cross-section(such as stiffener terminations),ply drop-offs,and in some cases laminate ply orientations that cause unbal- anced or unsymmetric lay-ups.Interlaminar stresses will arise at any free edge.Interlaminar stresses will arise between plies of dissimilar orientation wherever there is a gradient in the components of in-plane stress. 12-2

MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-2 In general, if these stress risers are properly considered in design/analysis of laminated parts, fatigue loadings will not be critical. Another unique characteristic of composite material elastic response is its orthotropy. When metals are extended in one direction, they contract in the perpendicular direction in an amount equal to the Pois￾son's ratio times the longitudinal strain. This is true regardless of which direction is extended. In compos￾ites, an extension in the longitudinal (1 or x) direction produces a contraction in the transverse direction (2 or y) equal to the "major" Poisson's ratio, ν xy , times the longitudinal extension. If this is reversed, an ex￾tension in the transverse direction produces a much lower contraction in the longitudinal direction. In fiber dominated laminates, Poisson's ratio can vary from <0.1 to >0.5. The most unusual characteristic of composites is the response produced when the lay-up is unbal￾anced and/or unsymmetric. Such a laminate exhibits anisotropic warping characteristics. In this condition an extension in one direction can produce an in-plane shear deformation. It can also cause an out-of-plane bending or torsional response. All these effects are sometimes observed in one laminate. This type of response is generally undesirable because of warping or built-in stresses that occur. Hence, most laminate configurations are balanced and symmetric. Classical lamination theory is used to combine the individual lamina properties to predict the linear elastic behavior of arbitrary laminates. Lamination theory requires the definition of lamina elastic proper￾ties, their orientation within the laminate, and their stacking position. The process assumes plane sec￾tions remain-plane and enforces equilibrium. Lamination theory will solve for the loads/stresses/strains for each lamina within the laminate at a given location for a given set of applied loads. This combined with appropriate failure theory will predict the strength of the laminate (empirically modified input ply prop￾erties are often necessary). 12.2.2 Tailored properties and out-of-plane loads The properties of a composite laminate depend on the orientation of the individual plies. This pro￾vides the engineer with the ability to tailor a laminate to fit a particular requirement. For high axial loads predominantly in one direction, the laminate should have a majority of its plies oriented parallel to that loading direction. If the laminate is loaded mostly in shear, there should be a high percent of ±45° pairs. For loads in multi-directions, the laminate should be quasi-isotropic. An all 0° laminate represents the maximum strength and stiffness that can be attained in any given direction, but is impractical for most ap￾plications since the transverse properties are so weak that machining and handling can cause damage. Fiber-dominated, balanced and symmetric, laminate designs that have a minimum of 10% of the plies in each of the 0°, +45°, -45°, and 90° directions are most commonly used. Tailoring also means an engineer is not able to cite a strength or stiffness value for a composite lami￾nate until he knows the laminate's ply percentages in each direction. Carpet plots of various properties vs. the percent of plies in each direction are commonly used for balanced and symmetric laminates. An example for stiffness is shown in Figure 12.2.2. Similar plots for strength can also be developed. Out-of-plane loads can also be troublesome for composites. These loads cause interlaminar shear and tension in the laminate. Interlaminar shear stress can cause failure of the matrix or the fiber-matrix interphase region. Interlaminar shear and tensile stresses can delaminate or disbond a laminate. Such loading should be avoided if possible. Design situations that tend to create interlaminar shear loading include high out-of-plane loads (such as fuel pressure), buckling, abrupt changes in cross-section (such as stiffener terminations), ply drop-offs, and in some cases laminate ply orientations that cause unbal￾anced or unsymmetric lay-ups. Interlaminar stresses will arise at any free edge. Interlaminar stresses will arise between plies of dissimilar orientation wherever there is a gradient in the components of in-plane stress

MIL-HDBK-17-3F Volume 3,Chapter 12-Lessons Learned 140 20 18 120 80%- 0's 16 100 14 60%- 0's 0 80 0 202 90'5 (2) 05 40% 十 40% 90's 60 (4) 90 60 20%-0's} 6 90's 40 80% 90' 0%-0's 20 2 0 0 10 20 30 4050 60 70 80 90 100 号45 PLIES ELASTIC MODULI FIGURE 12.2.2 Sample carpet plot. 12-3

MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-3 FIGURE 12.2.2 Sample carpet plot

MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned 12.2.3 Damage tolerance Damage tolerance is the measure of the structure's ability to sustain a level of damage or presence of a defect and be able to perform its operating functions.The concern is with the damaged structure hav- ing adequate residual strength and stiffness to continue in service safely:1)until the damage can be de- tected by scheduled maintenance inspection and repaired,or 2)if the damage is undetected,for the re- mainder of the aircraft's life.Thus,safety is the primary goal of damage tolerance.Both static load and durability related damage tolerance must be interrogated experimentally because there are few,if any. accurate analytical methods. There are basically two types of damage that are categorized by their occurrence during the fabrica- tion and use of the part,i.e.,damage occurring during manufacturing or damage occurring in service.It is hoped that the occurrence of the majority of manufacturing associated damage,if beyond specification limits,will be detected by routine quality inspection.Nevertheless,some "rogue"defects or damage be- yond specification limits may go undetected.Consequently,their occurrence must be assumed in the design procedure and subsequent testing(static and fatigue)performed to verify the structural integrity. Service damage concerns are similar to those for manufacturing.Types of service damage include edge and surface gouges and cuts or foreign object collision and blunt object impact damage caused by dropped tools or contact with service equipment.A level of non-detectable damage should be established and verified by test that will not endanger the normal operation of the aircraft structure for two lifetimes.A certain level(maximum allowed)damage that can be found by inspection should be defined such that the vehicle can operate for a specified number of hours before repair or replacement at loads not exceeding design limit.This damage should also be tested(statically and in fatigue)to verify the structural integrity. Delaminations can also be critical defects.However,unless they are very large,historically more than 2 inches(50 mm)in diameter,the problem is mostly with thin laminates.Effects of manufacturing defects such as porosity and flawed fastener holes that are slightly in excess of the maximum allowable are usually less severe.They are generally accounted for by the use of design allowable properties that have been obtained by testing specimens with stress concentrations,e.g.,notches.Most commonly these are specimens with a centered hole.Open holes are typically used for compression specimens while either open or filled holes (holes with an installed fastener)are used for tension testing.(Open holes are more critical than filled holes for compression.Filled holes may be more critical in tension,es- pecially for laminates with ply orientations with a predominate number of plies in the load direction.)Con- sequently,the design allowables thus produced may be used to account for a nominal design stress con- centration caused by an installed or missing fastener,at least to a 0.25 inch(6.4 mm)diameter,as well as accounting for many other manufacturing defects.This is sometimes called the "rogue flaw"approach to laminate design,see Reference 12.2.3. 12.2.4 Durability Durability of a structure is its ability to maintain strength and stiffness throughout the service life of the structure.A structure must have adequate durability when subjected to the expected service loads and environment spectra to prevent excessive maintenance,repair,or modification costs over the service life. Thus,durability is primarily an economic consideration. Metallic structure can be very sensitive to durability issues;major factors limiting life are corrosion and fatigue.Metal fatigue is dictated by the number of load cycles required to start a crack(crack initia- tion)and the number of load cycles for the crack to grow to its critical length,reaching catastrophic failure (crack growth).Crack/damage growth rate is very dependent on the concentration of stress around the crack. In composites,it has been demonstrated that one of the most common damage growth mechanisms is intercracking(delamination).This makes composites most sensitive to compression-dominated fatigue loading.A second common fatigue failure mode is fastener hole wear caused by high bearing stresses. 12-4

MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-4 12.2.3 Damage tolerance Damage tolerance is the measure of the structure's ability to sustain a level of damage or presence of a defect and be able to perform its operating functions. The concern is with the damaged structure hav￾ing adequate residual strength and stiffness to continue in service safely: 1) until the damage can be de￾tected by scheduled maintenance inspection and repaired, or 2) if the damage is undetected, for the re￾mainder of the aircraft's life. Thus, safety is the primary goal of damage tolerance. Both static load and durability related damage tolerance must be interrogated experimentally because there are few, if any, accurate analytical methods. There are basically two types of damage that are categorized by their occurrence during the fabrica￾tion and use of the part, i.e., damage occurring during manufacturing or damage occurring in service. It is hoped that the occurrence of the majority of manufacturing associated damage, if beyond specification limits, will be detected by routine quality inspection. Nevertheless, some "rogue" defects or damage be￾yond specification limits may go undetected. Consequently, their occurrence must be assumed in the design procedure and subsequent testing (static and fatigue) performed to verify the structural integrity. Service damage concerns are similar to those for manufacturing. Types of service damage include edge and surface gouges and cuts or foreign object collision and blunt object impact damage caused by dropped tools or contact with service equipment. A level of non-detectable damage should be established and verified by test that will not endanger the normal operation of the aircraft structure for two lifetimes. A certain level (maximum allowed) damage that can be found by inspection should be defined such that the vehicle can operate for a specified number of hours before repair or replacement at loads not exceeding design limit. This damage should also be tested (statically and in fatigue) to verify the structural integrity. Delaminations can also be critical defects. However, unless they are very large, historically more than 2 inches (50 mm) in diameter, the problem is mostly with thin laminates. Effects of manufacturing defects such as porosity and flawed fastener holes that are slightly in excess of the maximum allowable are usually less severe. They are generally accounted for by the use of design allowable properties that have been obtained by testing specimens with stress concentrations, e.g., notches. Most commonly these are specimens with a centered hole. Open holes are typically used for compression specimens while either open or filled holes (holes with an installed fastener) are used for tension testing. (Open holes are more critical than filled holes for compression. Filled holes may be more critical in tension, es￾pecially for laminates with ply orientations with a predominate number of plies in the load direction.) Con￾sequently, the design allowables thus produced may be used to account for a nominal design stress con￾centration caused by an installed or missing fastener, at least to a 0.25 inch (6.4 mm) diameter, as well as accounting for many other manufacturing defects. This is sometimes called the "rogue flaw" approach to laminate design, see Reference 12.2.3. 12.2.4 Durability Durability of a structure is its ability to maintain strength and stiffness throughout the service life of the structure. A structure must have adequate durability when subjected to the expected service loads and environment spectra to prevent excessive maintenance, repair, or modification costs over the service life. Thus, durability is primarily an economic consideration. Metallic structure can be very sensitive to durability issues; major factors limiting life are corrosion and fatigue. Metal fatigue is dictated by the number of load cycles required to start a crack (crack initia￾tion) and the number of load cycles for the crack to grow to its critical length, reaching catastrophic failure (crack growth). Crack/damage growth rate is very dependent on the concentration of stress around the crack. In composites, it has been demonstrated that one of the most common damage growth mechanisms is intercracking (delamination). This makes composites most sensitive to compression-dominated fatigue loading. A second common fatigue failure mode is fastener hole wear caused by high bearing stresses

MIL-HDBK-17-3F Volume 3.Chapter 12-Lessons Learned In this failure mode the hole gradually elongates.The most serious damage to composite parts is low velocity impact damage which can reduce static strength,fatigue strength,or residual strength after fa- tigue.Again,testing is a must! The strain level of composites in most actual vehicle applications to date has been held to relatively low values.Composites under in-plane loads have relatively flat stress-life(S-N)curves with high fatigue thresholds (endurance limits).These two factors combined have resulted in insensitivity to fatigue for most load cases.However,the greater variability found with composites requires an engineer to still characterize the composite's fatigue life to failure to correctly characterize its fatigue scatter. 12.2.5 Environmental sensitivity When a composite with a polymeric matrix is placed in a wet environment,the matrix will absorb moisture.The moisture absorption of most fibers used in practice is negligible;however,aramid fibers (e.g.,Kevlar)absorb significant amounts of moisture when exposed to high humidity.The absorption of moisture at the interface of glass/quartz fibers is a well-known degrading phenomena. When a composite has been exposed to moisture and sufficient time has elapsed,the moisture con- centration throughout the matrix will be uniform.A typical equilibrium moisture content for severe humidity exposure of common epoxy composites is 1.1 to 1.3 percent weight gain.The principal strength degrad- ing effect is related to a change in the glass transition temperature of the matrix material.As moisture is absorbed,the temperature at which the matrix changes from a glassy state to a viscous state decreases. Thus,the strength properties decrease with increasing moisture content.Current data indicate this proc- ess is reversible.When the moisture content is decreased,the glass transition temperature increases and the original strength properties return.With glass/quartz fibers there is additional degradation at the interface with the matrix.For aramid fibers there is additional degradation at the interface with the matrix and.also.in the fibers. The same considerations also apply for a temperature rise.The matrix,and therefore the lamina, loses strength and stiffness when the temperature rises.This effect is primarily important for the ma- trix-dominated properties.Temperature rise also worsens the fiber/matrix interface degradation for glass/quartz fibers and aramid fibers.The aramid fiber properties are also degraded by a rise in tempera- ture The approach for design purposes is to assume a worst case.If the material is assumed to be fully saturated and at the maximum temperature,material allowables can be derived for this extreme.This is a conservative approach,since typical service environments do not generate full saturation for most com- plex structures.Once the diffusivity of a composite material is known,the moisture content and through the thickness distribution can be accurately predicted by Fickian equations.This depends on an accurate characterization of the temperature-humidity service environment. Thermal expansion characteristics of common composites,like carbon/epoxy,are quite different from metals.In the (0 or 1)longitudinal direction,the thermal expansion coefficient of carbon/epoxy is almost zero.Transverse to the fiber(90 or 2 direction),the thermal expansion is the same magnitude as alumi- num.This property gives composites the ability to provide a dimensionally stable structure throughout a wide range of temperatures. Another feature of composites that is related to environment is resistance to corrosion.Polymer ma- trix composites (with the exception of some carbon/bismaleimides)are immune to salt water and most chemical substances as far as corrosion sensitivity.One precaution in this regard is galvanic corrosion. Carbon fiber is cathodic (noble);aluminum and steel are anodic (least noble).Thus carbon in contact with aluminum or steel promotes galvanic action which results in corrosion of the metal.Corrosion barri- ers(such as fiberglass and sealants)are placed at interfaces between composites and metals to prevent metal corrosion.Another precaution regards the use of paint strippers around most polymers.Chemical paint strippers are very powerful and attack the matrix of composites very destructively.Thus,chemical paint stripping is forbidden on composite structure. 12-5

MIL-HDBK-17-3F Volume 3, Chapter 12 - Lessons Learned 12-5 In this failure mode the hole gradually elongates. The most serious damage to composite parts is low velocity impact damage which can reduce static strength, fatigue strength, or residual strength after fa￾tigue. Again, testing is a must! The strain level of composites in most actual vehicle applications to date has been held to relatively low values. Composites under in-plane loads have relatively flat stress-life (S-N) curves with high fatigue thresholds (endurance limits). These two factors combined have resulted in insensitivity to fatigue for most load cases. However, the greater variability found with composites requires an engineer to still characterize the composite's fatigue life to failure to correctly characterize its fatigue scatter. 12.2.5 Environmental sensitivity When a composite with a polymeric matrix is placed in a wet environment, the matrix will absorb moisture. The moisture absorption of most fibers used in practice is negligible; however, aramid fibers (e.g., Kevlar) absorb significant amounts of moisture when exposed to high humidity. The absorption of moisture at the interface of glass/quartz fibers is a well-known degrading phenomena. When a composite has been exposed to moisture and sufficient time has elapsed, the moisture con￾centration throughout the matrix will be uniform. A typical equilibrium moisture content for severe humidity exposure of common epoxy composites is 1.1 to 1.3 percent weight gain. The principal strength degrad￾ing effect is related to a change in the glass transition temperature of the matrix material. As moisture is absorbed, the temperature at which the matrix changes from a glassy state to a viscous state decreases. Thus, the strength properties decrease with increasing moisture content. Current data indicate this proc￾ess is reversible. When the moisture content is decreased, the glass transition temperature increases and the original strength properties return. With glass/quartz fibers there is additional degradation at the interface with the matrix. For aramid fibers there is additional degradation at the interface with the matrix and, also, in the fibers. The same considerations also apply for a temperature rise. The matrix, and therefore the lamina, loses strength and stiffness when the temperature rises. This effect is primarily important for the ma￾trix-dominated properties. Temperature rise also worsens the fiber/matrix interface degradation for glass/quartz fibers and aramid fibers. The aramid fiber properties are also degraded by a rise in tempera￾ture. The approach for design purposes is to assume a worst case. If the material is assumed to be fully saturated and at the maximum temperature, material allowables can be derived for this extreme. This is a conservative approach, since typical service environments do not generate full saturation for most com￾plex structures. Once the diffusivity of a composite material is known, the moisture content and through the thickness distribution can be accurately predicted by Fickian equations. This depends on an accurate characterization of the temperature-humidity service environment. Thermal expansion characteristics of common composites, like carbon/epoxy, are quite different from metals. In the (0 or 1) longitudinal direction, the thermal expansion coefficient of carbon/epoxy is almost zero. Transverse to the fiber (90 or 2 direction), the thermal expansion is the same magnitude as alumi￾num. This property gives composites the ability to provide a dimensionally stable structure throughout a wide range of temperatures. Another feature of composites that is related to environment is resistance to corrosion. Polymer ma￾trix composites (with the exception of some carbon/bismaleimides) are immune to salt water and most chemical substances as far as corrosion sensitivity. One precaution in this regard is galvanic corrosion. Carbon fiber is cathodic (noble); aluminum and steel are anodic (least noble). Thus carbon in contact with aluminum or steel promotes galvanic action which results in corrosion of the metal. Corrosion barri￾ers (such as fiberglass and sealants) are placed at interfaces between composites and metals to prevent metal corrosion. Another precaution regards the use of paint strippers around most polymers. Chemical paint strippers are very powerful and attack the matrix of composites very destructively. Thus, chemical paint stripping is forbidden on composite structure