《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 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