MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization Periodically remove a traveler and destructively remove the face sheets and adhesive quickly, cleanly,and without generating heat.Weigh the core portion,and then determine the mois- ture content of the core by desorption. Compare the traveler core moisture level to the previously determined equilibrium level. When the traveler core reaches the equilibrium level within a defined tolerance,the test arti- cle(s)is also at equilibrium. 9.As in Combinations 1 and 5,the metallic face sheets shield the adhesive from the conditioning medium.Therefore,even though the adhesive is expected to fail,there is no need to condition such articles(assuming that edge absorption into the bondline is not significant). 10.As in Combinations 2 and 6,the metallic face sheets shield the adhesive and core from the condi- tioning medium.Therefore,even though failure is expected in the adhesive,there is no need to condition such articles(assuming that edge absorption into the bondline and organic honeycomb is not significant). 11.In this combination the face sheets are composite,allowing moisture to reach the adhesive (which is expected to fail).Since the adhesive layer is relatively thin and in contact with the face sheets,it is reasonable to assume that the adhesive will be near equilibrium when the composite skins have reached equilibrium.Therefore,the approach of using solid laminate travelers that are twice the thickness of the face sheets can be used(as described for Combination 3 above). 12.See Combination 11. 7.4 NOTCHED LAMINATE TESTS 7.4.1 Overview and general considerations The most common method of assembling composite structure is by the use of mechanical fasteners, even though bolted joints are relatively inefficient.The stress concentration due to the hole will cause substantial reduction in both the notched tensile and compressive strength of a composite laminate.The magnitude of this reduction varies considerably with a multitude of factors.All composite materials that exhibit a linear elastic stress-strain relationship to failure will be very sensitive to notches.Unlike metallic materials,the effects of the notch on strength will vary with the size of the notch but are relatively inde- pendent of notch geometry.Under uniaxial load,large holes will produce a stress concentration factor approaching the theoretical factor for wide plates given by the relationship: 7.4.1(a Gxy For a quasi-isotropic laminate,the above relationship reduces to the well-known value k=3.0 for a circu- lar hole.This relationship also indicates that holes in high modulus laminates have a much greater effect on strength than holes in low modulus laminates.The stress concentration factor described by the above equation is reasonably proportional to the parameter E/G,the laminate axial modulus divided by the lami- nate shear modulus. Considerable research literature exists regarding the influence of holes on the strength of composite laminates.An excellent summary of this literature is given in Reference 7.4.1 which includes over 300 citations.While the influence of holes in composites has been researched and reported extensively,there are additional effects to be considered.Two of these effects relate to the influence a fastener has in"fill- ing"a hole in a laminate.The fastener,particularly in tight or interference holes,can induce a biaxial stress field by preventing ovalization of the hole under load.The factor tends to decrease the notch ten- sile strength of 0-ply dominated laminates and increase the strength of laminates with predominantly 45 7-6
MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-6 • Periodically remove a traveler and destructively remove the face sheets and adhesive quickly, cleanly, and without generating heat. Weigh the core portion, and then determine the moisture content of the core by desorption. • Compare the traveler core moisture level to the previously determined equilibrium level. • When the traveler core reaches the equilibrium level within a defined tolerance, the test article(s) is also at equilibrium. 9. As in Combinations 1 and 5, the metallic face sheets shield the adhesive from the conditioning medium. Therefore, even though the adhesive is expected to fail, there is no need to condition such articles (assuming that edge absorption into the bondline is not significant). 10. As in Combinations 2 and 6, the metallic face sheets shield the adhesive and core from the conditioning medium. Therefore, even though failure is expected in the adhesive, there is no need to condition such articles (assuming that edge absorption into the bondline and organic honeycomb is not significant). 11. In this combination the face sheets are composite, allowing moisture to reach the adhesive (which is expected to fail). Since the adhesive layer is relatively thin and in contact with the face sheets, it is reasonable to assume that the adhesive will be near equilibrium when the composite skins have reached equilibrium. Therefore, the approach of using solid laminate travelers that are twice the thickness of the face sheets can be used (as described for Combination 3 above). 12. See Combination 11. 7.4 NOTCHED LAMINATE TESTS 7.4.1 Overview and general considerations The most common method of assembling composite structure is by the use of mechanical fasteners, even though bolted joints are relatively inefficient. The stress concentration due to the hole will cause substantial reduction in both the notched tensile and compressive strength of a composite laminate. The magnitude of this reduction varies considerably with a multitude of factors. All composite materials that exhibit a linear elastic stress-strain relationship to failure will be very sensitive to notches. Unlike metallic materials, the effects of the notch on strength will vary with the size of the notch but are relatively independent of notch geometry. Under uniaxial load, large holes will produce a stress concentration factor approaching the theoretical factor for wide plates given by the relationship: 1 1 2 2 x x t xy y xy E E K12 v E G =+ − + 7.4.1(a) For a quasi-isotropic laminate, the above relationship reduces to the well-known value kt = 3.0 for a circular hole. This relationship also indicates that holes in high modulus laminates have a much greater effect on strength than holes in low modulus laminates. The stress concentration factor described by the above equation is reasonably proportional to the parameter E/G, the laminate axial modulus divided by the laminate shear modulus. Considerable research literature exists regarding the influence of holes on the strength of composite laminates. An excellent summary of this literature is given in Reference 7.4.1 which includes over 300 citations. While the influence of holes in composites has been researched and reported extensively, there are additional effects to be considered. Two of these effects relate to the influence a fastener has in "filling" a hole in a laminate. The fastener, particularly in tight or interference holes, can induce a biaxial stress field by preventing ovalization of the hole under load. The factor tends to decrease the notch tensile strength of 0°-ply dominated laminates and increase the strength of laminates with predominantly 45°
MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization plies.The second effect is when clamp-up of the fastener prevents damage in the form of longitudinal slits and delaminations from occurring around the hole.These delaminations are the result of "free edge" stresses and are very sensitive to stacking sequence.When damage is suppressed by the fastener,no stress concentration relief occurs and the notch sensitivity increases. Filled hole compressive strengths are significantly higher than open hole strengths and,in some cases,approach the unnotched strength.This is particularly true with close-fitting holes where load can be transferred through the hole by direct bearing through the fastener.Fabric laminates,because of the balanced nature of fabric materials,tend to have lower stress concentration factors and are less prone to free edge delaminations.The influence of free edge stresses and stacking sequence on delaminations are discussed in Volume 3.Sections 5.6.3 and 5.6.5. When holes are placed together as in a bolted joint,the stress concentrations at the holes start to interact and the notch strength of the composite laminate decreases.A finite width correction factor is used to account for this interaction effect.For isotropic materials the"finite width correction"factor(FWC) is given by: D W FWC. 7.4.1(b) where D=fastener diameter W=fastener spacing The correction factor for orthotropic materials cannot be expressed in a closed form.In most cases.the isotropic correction has been found to be reasonably accurate. When the hole diameter is significantly greater than the laminate thickness,the stress concentration is two-dimensional in nature.Most of the research on holes in laminates is for this case.The notch strength of composites is much more difficult to predict when the thickness of the laminate significantly exceeds the hole diameter.The stress concentration at the hole becomes three-dimensional in nature and stacking sequence effects become more dominant. There have been many failure models proposed for describing the notch strength of composite lami- nates.All of the models require some form of empirical "calibration"factor such as a "characteristic di- mension".Characteristic dimensions have been used as a measure of notch sensitivity.Once calibrated, all of the models are reasonably accurate in describing the notch strength of composites.The drawback to these models is that many parameters such as laminate composition,temperature,and even hole size require re-calibration of the failure model.Some of the calibration factors are reasonably consistent,over a wide range of application laminates,among various material systems of similar characteristics.Low strength or stiffness fibers,or highly nonlinear toughened resins are examples of material constituents which can produce widely different"calibration"factors.Progressive damage failure models have shown some promise in not being overly dependent on empirical factors.For more discussion on this topic see Volume 3,Chapter 7(bolted joints). 7.4.2 Notched laminate tension A uniaxial tension test of a balanced,symmetric laminate with a centrally located 0.250 inch (6.35 mm)diameter hole is performed to determine the notched laminate tensile strength.The test consists of loading an untabbed,straight-sided,1.5 inch(3.8 cm)wide,12 inch(30 cm)long laminate specimen in tension until two-part failure occurs.The head travel and load on the specimen are recorded during the test.The tensile load is applied to the specimen through a mechanical shear interface at the ends of the specimen,normally by either wedge or hydraulic grips.The test machine grip wedges must be at least the same width as the specimen,and must be able to grip at least 2.0 inch(5 cm)of each end of the speci- men.The recommended specimen configuration is shown in Figure 7.4.2.Both open hole and fastener filled hole specimens are tested.There is no need for tabbing or special gripping treatments unless ex- 7-7
MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-7 plies. The second effect is when clamp-up of the fastener prevents damage in the form of longitudinal slits and delaminations from occurring around the hole. These delaminations are the result of "free edge" stresses and are very sensitive to stacking sequence. When damage is suppressed by the fastener, no stress concentration relief occurs and the notch sensitivity increases. Filled hole compressive strengths are significantly higher than open hole strengths and, in some cases, approach the unnotched strength. This is particularly true with close-fitting holes where load can be transferred through the hole by direct bearing through the fastener. Fabric laminates, because of the balanced nature of fabric materials, tend to have lower stress concentration factors and are less prone to free edge delaminations. The influence of free edge stresses and stacking sequence on delaminations are discussed in Volume 3, Sections 5.6.3 and 5.6.5. When holes are placed together as in a bolted joint, the stress concentrations at the holes start to interact and the notch strength of the composite laminate decreases. A finite width correction factor is used to account for this interaction effect. For isotropic materials the "finite width correction" factor (FWC) is given by: 3 D 2 1 W FWC D 3 1 W + + = − 7.4.1(b) where D = fastener diameter W = fastener spacing The correction factor for orthotropic materials cannot be expressed in a closed form. In most cases, the isotropic correction has been found to be reasonably accurate. When the hole diameter is significantly greater than the laminate thickness, the stress concentration is two-dimensional in nature. Most of the research on holes in laminates is for this case. The notch strength of composites is much more difficult to predict when the thickness of the laminate significantly exceeds the hole diameter. The stress concentration at the hole becomes three-dimensional in nature and stacking sequence effects become more dominant. There have been many failure models proposed for describing the notch strength of composite laminates. All of the models require some form of empirical "calibration" factor such as a "characteristic dimension". Characteristic dimensions have been used as a measure of notch sensitivity. Once calibrated, all of the models are reasonably accurate in describing the notch strength of composites. The drawback to these models is that many parameters such as laminate composition, temperature, and even hole size require re-calibration of the failure model. Some of the calibration factors are reasonably consistent, over a wide range of application laminates, among various material systems of similar characteristics. Low strength or stiffness fibers, or highly nonlinear toughened resins are examples of material constituents which can produce widely different "calibration" factors. Progressive damage failure models have shown some promise in not being overly dependent on empirical factors. For more discussion on this topic see Volume 3, Chapter 7 (bolted joints). 7.4.2 Notched laminate tension A uniaxial tension test of a balanced, symmetric laminate with a centrally located 0.250 inch (6.35 mm) diameter hole is performed to determine the notched laminate tensile strength. The test consists of loading an untabbed, straight-sided, 1.5 inch (3.8 cm) wide, 12 inch (30 cm) long laminate specimen in tension until two-part failure occurs. The head travel and load on the specimen are recorded during the test. The tensile load is applied to the specimen through a mechanical shear interface at the ends of the specimen, normally by either wedge or hydraulic grips. The test machine grip wedges must be at least the same width as the specimen, and must be able to grip at least 2.0 inch (5 cm) of each end of the specimen. The recommended specimen configuration is shown in Figure 7.4.2. Both open hole and fastener filled hole specimens are tested. There is no need for tabbing or special gripping treatments unless ex-
MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization tremely coarse serrated grips or excessive pressure are used.Normally the large stress concentration at the hole will eliminate problems with grip failures.The test is normally run without instrumentation,re- cording only maximum load,specimen dimensions,and failure mode and location.The test methods are also applicable to specimens with different fastener types,width/diameter ratios,and hole sizes.The open-hole and filled-hole tensile strength is presented in terms of gross-area strength without any finite- width correction.The following equations are used to calculate the notched tensile strengths: (W0) and Ff W) Where Pmax= maximum tensile load W measured width at midsection t calculated nominal laminate thickness The calculated nominal thickness is calculated by summing the nominal per-ply thickness of the individual plies in the laminate. 7.4.2.1 Open-hole tensile test methods ASTM D 5766"Standard Test Method for Open Hole Tensile Strength of Polymer Matrix Composite Laminates".This test method determines the open hole tensile strength of polymer matrix composite laminates reinforced by high-modulus fibers.The composite material forms are limited to continuous-fiber or discontinuous-fiber reinforced composites in which the laminate is balanced and symmetric with re- spect to the test direction.The standard test laminate is of the [45/90/-45/0]ns stacking sequence family, where the sublaminate repeat index is adjusted to yield a laminate thickness within the range of 0.080 to 0.160 inch(2.03 to 4.06 mm).The standard specimen width is 1.5 inch(3.8 cm)and the length is 8.0 to 12.0 inches(20 to 30 cm).The notch consists of a 0.250 inch(6.35 mm)diameter centrally located hole. Other laminates may be tested provided the laminate configuration is reported with the results,however, the test method is unsatisfactory for unidirectional tape laminates containing only one ply orientation. 7.4.2.2 Filled-hole tensile test methods The filled-hole tensile test typically uses the open-hole tensile test method procedures to conduct the test.The standard specimen width is 1.5 inch(3.8 cm)and the length is 8.0 to 12.0 inches(20 to 30 cm). The notch consists of a 0.250 inch(6.35 mm)diameter centrally located hole.The standard specimen configuration for this test should have a protruding head,hex drive fastener installed in the hole prior to testing.Filled-hole tensile strength is dependent upon the amount of fastener clamp-up,with a higher clamp-up force generally producing a lower filled-hole tensile strength.Fastener clamp-up is a function of fastener type,nut or collar type,and installation torque.In general,the strengths obtained using this fas- tener should be conservative relative to most fastener installations in composite structure.The test method procedures are also applicable to specimens with different fastener types,width/diameter ratios, and fastener/hole sizes. 7.4.3 Notched laminate compression A uniaxial compressive test of a balanced,symmetric laminate with a centrally located 0.250 inch (6.35 mm)diameter hole is performed to determine the notched laminate compressive strength.The test involves loading an untabbed,straight-sided,1.5 inch (3.8 cm)wide,12 inch(30 cm)long laminate specimen in compression until two-part failure occurs.The head travel and load on the specimen are re- corded during the test.The recommended specimen is shown in Figure 7.4.2 with recommended thick- ness greater than 0.08 inch(2.0 mm)but less than 0.25 inch(6.3 mm).The multi-piece bolted compres- sive support fixture shown in Figure 7.4.3 is used to stabilize the specimen from general column buckling failures.The specimen/fixture assembly is clamped in the hydraulic grips and the load is sheared into the specimen.The grips must apply enough lateral pressure to prevent slippage without locally crushing the specimen.Boeing has recently updated the compressive support fixture configuration that has been in common use throughout industry for some time.This update was done to correct some errors and omis- 7-8
MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-8 tremely coarse serrated grips or excessive pressure are used. Normally the large stress concentration at the hole will eliminate problems with grip failures. The test is normally run without instrumentation, recording only maximum load, specimen dimensions, and failure mode and location. The test methods are also applicable to specimens with different fastener types, width/diameter ratios, and hole sizes. The open-hole and filled-hole tensile strength is presented in terms of gross-area strength without any finitewidth correction. The following equations are used to calculate the notched tensile strengths: ( )( ) W t P F oht max = and ( )( ) W t P F fht max = Where Pmax = maximum tensile load W = measured width at midsection t = calculated nominal laminate thickness The calculated nominal thickness is calculated by summing the nominal per-ply thickness of the individual plies in the laminate. 7.4.2.1 Open-hole tensile test methods ASTM D 5766 “Standard Test Method for Open Hole Tensile Strength of Polymer Matrix Composite Laminates”. This test method determines the open hole tensile strength of polymer matrix composite laminates reinforced by high-modulus fibers. The composite material forms are limited to continuous-fiber or discontinuous-fiber reinforced composites in which the laminate is balanced and symmetric with respect to the test direction. The standard test laminate is of the [45/90/-45/0]ns stacking sequence family, where the sublaminate repeat index is adjusted to yield a laminate thickness within the range of 0.080 to 0.160 inch (2.03 to 4.06 mm). The standard specimen width is 1.5 inch (3.8 cm) and the length is 8.0 to 12.0 inches (20 to 30 cm). The notch consists of a 0.250 inch (6.35 mm) diameter centrally located hole. Other laminates may be tested provided the laminate configuration is reported with the results, however, the test method is unsatisfactory for unidirectional tape laminates containing only one ply orientation. 7.4.2.2 Filled-hole tensile test methods The filled-hole tensile test typically uses the open-hole tensile test method procedures to conduct the test. The standard specimen width is 1.5 inch (3.8 cm) and the length is 8.0 to 12.0 inches (20 to 30 cm). The notch consists of a 0.250 inch (6.35 mm) diameter centrally located hole. The standard specimen configuration for this test should have a protruding head, hex drive fastener installed in the hole prior to testing. Filled-hole tensile strength is dependent upon the amount of fastener clamp-up, with a higher clamp-up force generally producing a lower filled-hole tensile strength. Fastener clamp-up is a function of fastener type, nut or collar type, and installation torque. In general, the strengths obtained using this fastener should be conservative relative to most fastener installations in composite structure. The test method procedures are also applicable to specimens with different fastener types, width/diameter ratios, and fastener/hole sizes. 7.4.3 Notched laminate compression A uniaxial compressive test of a balanced, symmetric laminate with a centrally located 0.250 inch (6.35 mm) diameter hole is performed to determine the notched laminate compressive strength. The test involves loading an untabbed, straight-sided, 1.5 inch (3.8 cm) wide, 12 inch (30 cm) long laminate specimen in compression until two-part failure occurs. The head travel and load on the specimen are recorded during the test. The recommended specimen is shown in Figure 7.4.2 with recommended thickness greater than 0.08 inch (2.0 mm) but less than 0.25 inch (6.3 mm). The multi-piece bolted compressive support fixture shown in Figure 7.4.3 is used to stabilize the specimen from general column buckling failures. The specimen/fixture assembly is clamped in the hydraulic grips and the load is sheared into the specimen. The grips must apply enough lateral pressure to prevent slippage without locally crushing the specimen. Boeing has recently updated the compressive support fixture configuration that has been in common use throughout industry for some time. This update was done to correct some errors and omis-
MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization sions that were found in the original Boeing drawings for these support fixtures.The updated details are contained in the proposed ASTM Open-Hole Compression Test Method and have been supplied to some vendors (MTS and Wyoming Test Fixture Inc.)and test laboratories(Intec and Delson)for incorporation into their fixtures.The open-hole and filled-hole compressive strength is presented in terms of gross-area strength without any finite-width correction.The following equations are used to calculate the notched compressive strengths: (ww 目 k 回 回 A0] C网 (0.3mm) 5 (0.3mm) 0 超〉 6.00 (152.4mm) ® 0.250+/-0.003 (6.35+-0.08mm) 12.00(305mm) 45 125 90 1.50 R 0.10(2.5mm) +/0.005 NOM. (38.1+/-0.1mm) NOTES: 1.UNLESS NOTED ALL TOLERANCES ARE 0.100 EDGE ROUGHNESS IN ACCORDANCE WITH ANSI B46.1 2.HOLE MUST NOT HAVE DELAMINATION OR OTHER DAMAGE 3.ALL DIMENSIONS IN INCHES.(MILLIMETERS IN PARENTHESES.) 4.CONFIGURATION SHOWN IS FOR 0.25 in.DIAMETER HOLE.FOR ALL OTHER HOLE SIZES,THE WIDTH WOULD CHANGE TO MAINTAIN W/D=6. FIGURE 7.4.2 Notched tensile/compressive strength specimen(based on Reference 7.4.1). 7-9
MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-9 sions that were found in the original Boeing drawings for these support fixtures. The updated details are contained in the proposed ASTM Open-Hole Compression Test Method and have been supplied to some vendors (MTS and Wyoming Test Fixture Inc.) and test laboratories (Intec and Delson) for incorporation into their fixtures. The open-hole and filled-hole compressive strength is presented in terms of gross-area strength without any finite-width correction. The following equations are used to calculate the notched compressive strengths: FIGURE 7.4.2 Notched tensile/compressive strength specimen (based on Reference 7.4.1)
MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization Pmax and Ffhe= Pmax (W)(t) (w)(t) METRIC HARDWARE US CUSTOMARY HARDWARE NA0036-060050BOLT(4) NAS660532B0LT(4) NA0179B-060 WASHER (8+) NAS 1587-5C WASHER (8+) (#AS REQ'D) (#AS REQD.) NA0033-060M NUT (4) NAS 1804-5 NUT (4) (OR EQUIVALENT) [OR EQUIVALENT] OR OR FOR THREADED PLATES SUPPORT PLATE FOR THREADED PLATES NA0036-060045BOLT(4) (2 PLACES) NAS6605-28 BOLT (4) NA0179B-060 WASHER(4) NAS 1587-5C WASHER(4) (OR EQUIVALENT) [OR EQUIVALENT] STAINLESS STEEL SHIM (AS REQUIRED) GRIP AREA (2 PLACES) SPECIMEN STAINLESS STEEL SHIM LONG GRIP (AS REQUIRED) (2 PLACES) SHORT GRIP (2 PLACES) FIGURE 7.4.3 Notched compressive strength support fixture. Where Pmax= maximum tensile load W= measured width at midsection t calculated nominal laminate thickness The calculated nominal thickness is calculated by summing the nominal per-ply thickness of the individual plies in the laminate. 7.4.3.1 Open-hole compressive test methods SACMA SRM 3 "Open-Hole Compression Properties of Oriented Fiber-Resin Composites".This method covers the procedure for the determination of the compressive properties of oriented fiber-resin composites laminates reinforced by continuous,high modulus,>3Msi(>20Gpa),fibers containing a circu- lar hole.The standard test laminate for unidirectional tape composites is of the [45/0/-45/90]2s stacking sequence.The standard specimen width is 1.5 inch(3.8 cm)and the length is 12.0 inches(30 cm).The notch consists of a 0.250 inch(6.35 mm)diameter centrally located hole.The commonly used compres- sive support fixture is used to stabilize the specimen from general column buckling failures.The preferred test method is to hydraulically grip the specimen/fixture assembly.but the test method allows the speci- 7-10
MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-10 ( )( ) ohc Pmax F W t = and ( )( ) fhc Pmax F W t = FIGURE 7.4.3 Notched compressive strength support fixture. Where Pmax = maximum tensile load W = measured width at midsection t = calculated nominal laminate thickness The calculated nominal thickness is calculated by summing the nominal per-ply thickness of the individual plies in the laminate. 7.4.3.1 Open-hole compressive test methods SACMA SRM 3 “Open-Hole Compression Properties of Oriented Fiber-Resin Composites”. This method covers the procedure for the determination of the compressive properties of oriented fiber-resin composites laminates reinforced by continuous, high modulus, >3Msi (>20Gpa), fibers containing a circular hole. The standard test laminate for unidirectional tape composites is of the [45/0/-45/90]2S stacking sequence. The standard specimen width is 1.5 inch (3.8 cm) and the length is 12.0 inches (30 cm). The notch consists of a 0.250 inch (6.35 mm) diameter centrally located hole. The commonly used compressive support fixture is used to stabilize the specimen from general column buckling failures. The preferred test method is to hydraulically grip the specimen/fixture assembly, but the test method allows the speci-