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《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 1）CHAPTER 7 STRUCTURAL ELEMENT CHARACTERIZATION

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MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization men to be ended loaded as an option.This option was required because many test laboratories did not have the very large hydraulic grips needed to handle the 3 inch(8 cm)wide support fixture.The new side-load hydraulic grips can easily handle the support fixture.The option to end-load the specimen re- quired the tolerances on the ends of the specimen to be much tighter and also required the fixture to be modified. ASTM D 6484 "Standard Test Method for Open-Hole Compressive Strength of Polymer Matrix Com- posite Laminates".This method determines the open hole compressive strength of multi-directional polymer matrix composite laminates reinforced by high-modulus fibers.The composite material forms are limited to continuous-fiber or discontinuous-fiber(tape and/or fabric)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 lami- nate thickness within the range of 0.125 to 0.200 inch(3.17 to 5.08 mm).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.Figure 7.4.3 compressive support fixture is used to stabilize the speci- men from general column buckling failures.The test method uses hydraulic wedge grips to load the specimen/fixture assembly.Other laminates may be tested provided the laminate configuration is re- ported with the results,however,the test method is unsatisfactory for unidirectional tape laminates con- taining only one ply orientation. 7.4.3.2 Filled-hole compressive test methods The filled-hole compression test typically uses the open-hole compressive test method procedures to conduct the test.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 standard specimen configuration for this test should have a protruding head,hex drive fastener installed in the hole prior to testing.Filled-hole compressive strength is dependent upon the amount of fastener hole clearance with tighter holes producing a higher filled-hole compressive strength.The test method procedures are also applicable to specimens with different fastener types,width/diameter ratios,and fastener/hole sizes. 7.4.4 Suggested notched laminate test matrix The minimum recommended test matrix for initial empirical assessment of "calibration"of the various theoretical models and determination of notch strength data for a range of laminates is given in Table 7.4.4.This matrix is just part of the overall development test plan.The matrix requires selective tests to be performed under tensile and compressive loadings in various environments applicable to the design of structural components.The test matrix is for open holes but bolted joint design criteria will also require filled hole test data to be generated.It is recommended that portions of the matrix in Table 7.4.4 be used to spot test for filled hole strengths,particularly in tension.For filled hole strengths,a reduction factor is applied to the open hole strength and the predictive model is not re-calibrated.The matrix represents the range of laminates commonly used in bolted joint designs.This assures that important interactions be- tween laminate stiffness,failure modes,and joint parameters are assessed.If the laminate of interest is significantly outside the range of behavior of the test laminates,open hole tests for that laminate should be added to that matrix. The procedure,often used to calibrate single-fastener-hole laminate strength methodology,such as for the "characteristic dimension"approaches,starts by evaluating the effect of hole-size on strength data for the isotropic(25/50/25)laminate,using a baseline specimen width/diameter ratio of six.Three fas- tener diameter sizes are selected for testing which will span the usual application range of fastener hard- ware.The trend of the effect of hole size on tensile and compressive strength data is established.The characteristic dimension that produces the trend line which best fits the test data is then selected.All other test case predictions now use that selected characteristic dimension. 7-11

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-11 men to be ended loaded as an option. This option was required because many test laboratories did not have the very large hydraulic grips needed to handle the 3 inch (8 cm) wide support fixture. The new side-load hydraulic grips can easily handle the support fixture. The option to end-load the specimen required the tolerances on the ends of the specimen to be much tighter and also required the fixture to be modified. ASTM D 6484 “Standard Test Method for Open-Hole Compressive Strength of Polymer Matrix Composite Laminates”. This method determines the open hole compressive strength of multi-directional polymer matrix composite laminates reinforced by high-modulus fibers. The composite material forms are limited to continuous-fiber or discontinuous-fiber (tape and/or fabric) 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 0.125 to 0.200 inch (3.17 to 5.08 mm). 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. Figure 7.4.3 compressive support fixture is used to stabilize the specimen from general column buckling failures. The test method uses hydraulic wedge grips to load the specimen/fixture assembly. 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. 7.4.3.2 Filled-hole compressive test methods The filled-hole compression test typically uses the open-hole compressive test method procedures to conduct the test. 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 standard specimen configuration for this test should have a protruding head, hex drive fastener installed in the hole prior to testing. Filled-hole compressive strength is dependent upon the amount of fastener hole clearance with tighter holes producing a higher filled-hole compressive strength. The test method procedures are also applicable to specimens with different fastener types, width/diameter ratios, and fastener/hole sizes. 7.4.4 Suggested notched laminate test matrix The minimum recommended test matrix for initial empirical assessment of "calibration" of the various theoretical models and determination of notch strength data for a range of laminates is given in Table 7.4.4. This matrix is just part of the overall development test plan. The matrix requires selective tests to be performed under tensile and compressive loadings in various environments applicable to the design of structural components. The test matrix is for open holes but bolted joint design criteria will also require filled hole test data to be generated. It is recommended that portions of the matrix in Table 7.4.4 be used to spot test for filled hole strengths, particularly in tension. For filled hole strengths, a reduction factor is applied to the open hole strength and the predictive model is not re-calibrated. The matrix represents the range of laminates commonly used in bolted joint designs. This assures that important interactions between laminate stiffness, failure modes, and joint parameters are assessed. If the laminate of interest is significantly outside the range of behavior of the test laminates, open hole tests for that laminate should be added to that matrix. The procedure, often used to calibrate single-fastener-hole laminate strength methodology, such as for the "characteristic dimension" approaches, starts by evaluating the effect of hole-size on strength data for the isotropic (25/50/25) laminate, using a baseline specimen width/diameter ratio of six. Three fastener diameter sizes are selected for testing which will span the usual application range of fastener hardware. The trend of the effect of hole size on tensile and compressive strength data is established. The characteristic dimension that produces the trend line which best fits the test data is then selected. All other test case predictions now use that selected characteristic dimension

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization Further correlations between the model and the data are then performed to assess the generality of this single characteristic dimension.Additional tests provide data for correlation with predicted effects of laminate composition,temperature variation,and finite width variations.The hole-size effect data,used initially to select a characteristic dimension,"builds in"a correlation for finite width of W/D=6.If subse- quent theory/test correlations are inconsistent or errors too large,further fitting of the "characteristic"di- mension may be required.If still unacceptable,for the application range of variables,the test data will be the basis for other analytical or purely empirical approaches,but significantly more testing may be re- quired to offset the loss of predictive methodology which provided an analytical bridge among the limited test conditions defined in Table 7.4.4. 7.4.5 Notched laminate test methods for MIL-HDBK-17 data submittal Data provided by the following test methods(Table 7.4.5)are currently being accepted by MIL-HDBK- 17 for consideration for inclusion in Volume 2. TABLE 7.4.5 Notched laminate test methods for MIL-HDBK-17 data submittal. Property Symbol All Data Classes Screening Data Only Open Hole Tension Strength F D5766 Filled Hole Tension Strength F D 5766 as modified by Section 7.4.2.2 Open Hole Compression Strength Fohe D6484 Filled Hole Compression Strength F D 6484 as modified by Section 7.4.3.2 7.5 MECHANICALLY-FASTENED JOINT TESTS 7.5.1 Overview 7.5.1.1 Definitions The following definitions are relevant to this section. Bearing Area--The diameter of the hole multiplied by the thickness of the specimen. Bearing Load--A compressive load on an interface. Bearing Strain--The ratio of the deformation of the bearing hole in the direction of the applied force to the pin diameter. Bearing Strength--The bearing stress value corresponding to total failure of the test specimen. Bearing Stress--The applied load divided by the bearing area. Bypass Strength--The load that transfers around a hole divided by the laminate gross section area. Edge Distance Ratio--The distance from the center of the bearing hole to the edge of the specimen in the direction of the applied load,divided by the diameter of the hole. 7-13

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-13 Further correlations between the model and the data are then performed to assess the generality of this single characteristic dimension. Additional tests provide data for correlation with predicted effects of laminate composition, temperature variation, and finite width variations. The hole-size effect data, used initially to select a characteristic dimension, "builds in" a correlation for finite width of W/D = 6. If subsequent theory/test correlations are inconsistent or errors too large, further fitting of the "characteristic" dimension may be required. If still unacceptable, for the application range of variables, the test data will be the basis for other analytical or purely empirical approaches, but significantly more testing may be required to offset the loss of predictive methodology which provided an analytical bridge among the limited test conditions defined in Table 7.4.4. 7.4.5 Notched laminate test methods for MIL-HDBK-17 data submittal Data provided by the following test methods (Table 7.4.5) are currently being accepted by MIL-HDBK- 17 for consideration for inclusion in Volume 2. TABLE 7.4.5 Notched laminate test methods for MIL-HDBK-17 data submittal. Property Symbol All Data Classes Screening Data Only Open Hole Tension Strength oht Fx D 5766 – Filled Hole Tension Strength fht Fx D 5766 as modified by Section 7.4.2.2 – Open Hole Compression Strength ohc Fx D 6484 – Filled Hole Compression Strength fhc Fx D 6484 as modified by Section 7.4.3.2 – 7.5 MECHANICALLY-FASTENED JOINT TESTS 7.5.1 Overview 7.5.1.1 Definitions The following definitions are relevant to this section. Bearing Area -- The diameter of the hole multiplied by the thickness of the specimen. Bearing Load -- A compressive load on an interface. Bearing Strain -- The ratio of the deformation of the bearing hole in the direction of the applied force to the pin diameter. Bearing Strength -- The bearing stress value corresponding to total failure of the test specimen. Bearing Stress -- The applied load divided by the bearing area. Bypass Strength -- The load that transfers around a hole divided by the laminate gross section area. Edge Distance Ratio -- The distance from the center of the bearing hole to the edge of the specimen in the direction of the applied load, divided by the diameter of the hole

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization Offset Bearing Strength--The bearing stress at the intersection of the bearing load-deformation curve with the tangent modulus drawn from a pre-selected offset value.Offset may be 1,2 or 4%of the nominal hole diameter. Proportional Limit Bearing Stress--The bearing stress value corresponding to the deviation from linearity of the bearing stress versus hole elongation curve. Ultimate Bearing Strength--The maximum bearing stress that can be sustained. 7.5.1.2 Failure modes An important consideration in joint testing and analysis is the selection of the type of test method with due attention to the failure mode which is likely to result with a specific joint design in a particular compos- ite system.A brief discussion on various failure modes is provided in this section.The occurrence of a particular failure mode is dependent primarily on joint geometry and laminate lay-up.Composite bolted joints may fail in various modes as shown in Figure 7.5.1.2.The likelihood of a particular failure mode is influenced by bolt diameter(D),laminate width(w),edge distance(e),and thickness(t).The type of fas- tener used can also influence the occurrence of a particular failure mode.A more detail classification of the failure modes is in Section 7.5.2.6. SHEAROUT FAILURE TENSION FAILURE CLEAVAGE-TENSION FAILURE BEARING FAILURE FIGURE 7.5.1.2 Typical failure modes for bolted joints in advanced composites. Net section tensile/compressive failures occur when the bolt diameter is a sufficiently large fraction of the strip width.This fraction is about one-quarter or more(w/D<=4)for near-isotropic lay-ups in graph- ite/epoxy systems.It is characterized by failure of the plies in the primary load direction.Cleavage failures occur because of the proximity of the end of the specimen.A cleavage failure can be triggered from a net- section tension failure.This type of failure often initiates at the end of the specimen rather than adjacent to the fastener.In some instances the bolt head may be pulled out through the laminate after the bolt is bent and deformed.This mode is frequently associated with countersunk fasteners and is highly depend- ent on the particular fastener used.Finally,it is important to note that for any given geometry,the failure mode may vary as a function of lay-up and stacking sequence. 7-14

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-14 Offset Bearing Strength -- The bearing stress at the intersection of the bearing load-deformation curve with the tangent modulus drawn from a pre-selected offset value. Offset may be 1, 2 or 4% of the nominal hole diameter. Proportional Limit Bearing Stress -- The bearing stress value corresponding to the deviation from linearity of the bearing stress versus hole elongation curve. Ultimate Bearing Strength -- The maximum bearing stress that can be sustained. 7.5.1.2 Failure modes An important consideration in joint testing and analysis is the selection of the type of test method with due attention to the failure mode which is likely to result with a specific joint design in a particular composite system. A brief discussion on various failure modes is provided in this section. The occurrence of a particular failure mode is dependent primarily on joint geometry and laminate lay-up. Composite bolted joints may fail in various modes as shown in Figure 7.5.1.2. The likelihood of a particular failure mode is influenced by bolt diameter (D), laminate width (w), edge distance (e), and thickness (t). The type of fastener used can also influence the occurrence of a particular failure mode. A more detail classification of the failure modes is in Section 7.5.2.6. FIGURE 7.5.1.2 Typical failure modes for bolted joints in advanced composites. Net section tensile/compressive failures occur when the bolt diameter is a sufficiently large fraction of the strip width. This fraction is about one-quarter or more (w/D<=4) for near-isotropic lay-ups in graphite/epoxy systems. It is characterized by failure of the plies in the primary load direction. Cleavage failures occur because of the proximity of the end of the specimen. A cleavage failure can be triggered from a netsection tension failure. This type of failure often initiates at the end of the specimen rather than adjacent to the fastener. In some instances the bolt head may be pulled out through the laminate after the bolt is bent and deformed. This mode is frequently associated with countersunk fasteners and is highly dependent on the particular fastener used. Finally, it is important to note that for any given geometry, the failure mode may vary as a function of lay-up and stacking sequence

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization 7.5.1.3 Design requirements In order to design against the different failure modes and the interactions between them,the capabil- ity of the composite has to be determined by test for: Notch/Net Tension/Compression Bearing Bearing/By-Pass Shear-Out These are described in Sections 7.4.2,7.4.3 and 7.5.2 to 7.5.4.The amount of testing will vary among manufacturers and certifying agencies depending on the confidence assigned to the analysis capability of each company.The philosophy of MIL-HDBK-17 is to provide guidance as to amount of testing that would be typical,but not necessarily the minimum or maximum.The bearing,net tension/compression,and bearing/by-pass failure mode criticality is best illustrated by a plot shown in Figure 7.5.1.3.This figure which is typically used by airframe designers,encompasses five failure possibilities as a function of bolt load and strain in the joining members.This plot is usually determined by tests that are described in Sec- tion 7.5.3 to 7.5.4.At zero bearing (no bolt load),the failure is in net tension or compression(points A and E).Open-hole or filled-hole specimens described in Sections 7.4.2 and 7.4.3 are used to determine this property.The line between A and C represents the reduction of net tension strength due to the bearing load.Similarly the line from E to C'represents the effect of bolt load on net compression strength.Points C and C'are the strengths of a single fastener joint where the load is reacted by the bolt.Section 7.5.3 describes the tests required to establish this design point for different joint variables.In practice,joints C and C are not much different so that a tension-bearing test is usually sufficient.Plots such as Figure 7.5.1.3 may be different for each distinct laminate,fastener type,and environmental condition,but many application ranges may be covered by one plot.The shape of the curves could also change depending on the percentage of 0,90 or t45 direction plies in the laminate.The intent of the sections that follow is to provide guidance on how to establish by test the critical points of Figure 7.5.1.3.The number of laminates to be tested is governed by analysis capability and degree of confidence in extrapolation.The shear-out mode of failure is usually avoided in design by providing sufficient edge distance between the holes or the free edge and balanced laminate configuration.However,in certain rework situations shear-out critical joints cannot be avoided.In those situations,a test program must be undertaken to establish design val- ues (see Section 7.5.4). 7.5.2 Bearing Tests 7.5.2.1 Overview Bearing tests are used to determine bearing response of composites.From the experimental load displacement curve,the bearing strength at maximum load and at some intermediate value(identified as yield or offset)are calculated using the following equation Fbr =P/tD 7.5.2.1 where Pbr bearing strength,psi(Pa) bearing load,Ibr (N) 0 bearing hole diameter,in.(m) t specimen thickness,in.(m) Superscripts bry and bru are commonly used to differentiate between yield and ultimate bearing strengths An offset bearing strength may be determined to represent the yield value.In that case,the subscript bro should be used. 7-15

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-15 7.5.1.3 Design requirements In order to design against the different failure modes and the interactions between them, the capability of the composite has to be determined by test for: − Notch/Net Tension/Compression − Bearing − Bearing/By-Pass − Shear-Out These are described in Sections 7.4.2, 7.4.3 and 7.5.2 to 7.5.4. The amount of testing will vary among manufacturers and certifying agencies depending on the confidence assigned to the analysis capability of each company. The philosophy of MIL-HDBK-17 is to provide guidance as to amount of testing that would be typical, but not necessarily the minimum or maximum. The bearing, net tension/compression, and bearing/by-pass failure mode criticality is best illustrated by a plot shown in Figure 7.5.1.3. This figure, which is typically used by airframe designers, encompasses five failure possibilities as a function of bolt load and strain in the joining members. This plot is usually determined by tests that are described in Section 7.5.3 to 7.5.4. At zero bearing (no bolt load), the failure is in net tension or compression (points A and E). Open-hole or filled-hole specimens described in Sections 7.4.2 and 7.4.3 are used to determine this property. The line between A and C represents the reduction of net tension strength due to the bearing load. Similarly the line from E to C1 represents the effect of bolt load on net compression strength. Points C and C1 are the strengths of a single fastener joint where the load is reacted by the bolt. Section 7.5.3 describes the tests required to establish this design point for different joint variables. In practice, joints C and C1 are not much different so that a tension-bearing test is usually sufficient. Plots such as Figure 7.5.1.3 may be different for each distinct laminate, fastener type, and environmental condition, but many application ranges may be covered by one plot. The shape of the curves could also change depending on the percentage of 0°, 90° or ±45° direction plies in the laminate. The intent of the sections that follow is to provide guidance on how to establish by test the critical points of Figure 7.5.1.3. The number of laminates to be tested is governed by analysis capability and degree of confidence in extrapolation. The shear-out mode of failure is usually avoided in design by providing sufficient edge distance between the holes or the free edge and balanced laminate configuration. However, in certain rework situations shear-out critical joints cannot be avoided. In those situations, a test program must be undertaken to establish design values (see Section 7.5.4). 7.5.2 Bearing Tests 7.5.2.1 Overview Bearing tests are used to determine bearing response of composites. From the experimental load displacement curve, the bearing strength at maximum load and at some intermediate value (identified as yield or offset) are calculated using the following equation br F = P/ tD 7.5.2.1 where Fbr = bearing strength, psi (Pa) P = bearing load, lbf (N) D = bearing hole diameter, in. (m) t = specimen thickness, in. (m) Superscripts bry and bru are commonly used to differentiate between yield and ultimate bearing strengths. An offset bearing strength may be determined to represent the yield value. In that case, the subscript bro should be used

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