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. 12-5MIL-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