《纺织复合材料》课程参考文献（Composite Materials Handbook，Volume 1）CHAPTER 7 STRUCTURAL ELEMENT CHARACTERIZATION.pdf_P16-P20

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization 80 60 40 6ujDeg 20 E 0 日 -6000 -4000 -2000 0 2000 4000 6000 By-Pass Strain,u in/in FIGURE 7.5.1.3 Example of bearing/by-pass interaction. The bearing test is conducted either in double or single shear with configurations that range from simple pin to a two bolt single shear load introduction,the latter being the closest to representing an ac- tual joint.A suggested test matrix is described in Section 7.5.2.4 that can be used to establish bearing design values.The bearing tests to be used in conjunction with the test matrix are the ASTM D 5961 Pro- cedure A,if the joints used in the application are in double shear,or ASTM D 5961 Procedure B two bolt specimen,if the joints are in single shear. 7.5.2.2 Double shear bearing tests The two tests described in this section introduce the bearing load in a double shear configuration.In actual applications,load transfer in a single shear configuration is more commonplace,resulting in larger stress concentrations in the thickness direction,and lowering the realizable bearing strength;these sin- gle-shear tests are discussed in Section 7.5.2.3.In other words,the bearing strength values measured by the double-shear tests cannot be applied to single shear joints. The main difference between the two test standards described below is how the bearing load is ap- plied.ASTM D 953 uses a pin,where ASTM D 5961,Procedure A uses a bolt with torque.As the clamp- up force is a significant factor for increasing the bearing strength,ASTM D 953 provides a lower bound on the bearing strength for the double shear configuration.Furthermore,as the pin is not representative of a bolted joint,the results of this test are usually not used for design but as a material property for compari- son purposes of different materials. 7.5.2.2.1 ASTM D 953 bearing strength of plastics This test method(Reference 7.5.2.2.1)is the oldest method to measure the bearing response of a composite material.It is the only method available to measure pure bearing strength of a material without the intrusion of bolt influences,such as clamping and washer.As such it is useful for comparison of bear- ing properties of different materials.The test can obtain bearing strength under tension and compression loading. Limitations of this test are: Pin Loading-Introduction of bearing load by a pin is not representative of most structural joints. 7-16

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-16 FIGURE 7.5.1.3 Example of bearing/by-pass interaction. The bearing test is conducted either in double or single shear with configurations that range from simple pin to a two bolt single shear load introduction, the latter being the closest to representing an actual joint. A suggested test matrix is described in Section 7.5.2.4 that can be used to establish bearing design values. The bearing tests to be used in conjunction with the test matrix are the ASTM D 5961 Procedure A, if the joints used in the application are in double shear, or ASTM D 5961 Procedure B two bolt specimen, if the joints are in single shear. 7.5.2.2 Double shear bearing tests The two tests described in this section introduce the bearing load in a double shear configuration. In actual applications, load transfer in a single shear configuration is more commonplace, resulting in larger stress concentrations in the thickness direction, and lowering the realizable bearing strength; these single-shear tests are discussed in Section 7.5.2.3. In other words, the bearing strength values measured by the double-shear tests cannot be applied to single shear joints. The main difference between the two test standards described below is how the bearing load is applied. ASTM D 953 uses a pin, where ASTM D 5961, Procedure A uses a bolt with torque. As the clampup force is a significant factor for increasing the bearing strength, ASTM D 953 provides a lower bound on the bearing strength for the double shear configuration. Furthermore, as the pin is not representative of a bolted joint, the results of this test are usually not used for design but as a material property for comparison purposes of different materials. 7.5.2.2.1 ASTM D 953 bearing strength of plastics This test method (Reference 7.5.2.2.1) is the oldest method to measure the bearing response of a composite material. It is the only method available to measure pure bearing strength of a material without the intrusion of bolt influences, such as clamping and washer. As such it is useful for comparison of bearing properties of different materials. The test can obtain bearing strength under tension and compression loading. Limitations of this test are: Pin Loading – Introduction of bearing load by a pin is not representative of most structural joints

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization Fixturing-The test apparatus is unnecessarily complicated.ASTM D 5961 has a much simpler arrangement. Specimen Geometry-The geometry of the specimen is inconsistent in e/D and W/D ratios for the two specimen thicknesses specified.As these ratios have a significant influence on bearing strength,a user may find differences in bearing strength for the two thickness where such differ- ence does not exist in the material. Specimen Configuration-The lay-up of the specimen is not specified and may lead users to test unidirectional material with disastrous results. Data Reduction-The data reduction mandated by the standard is specifically tied to a parabolic shape that does not reflect actual load-displacement curves.The use of template is antiquated in this computer age.The data reduction method of ASTM D 5961 is more general and useful. In summary,this test is useful to differentiate between materials as to their bearing strength,but the bearing properties,ultimate strength,yield strength,and the load-displacement response do not relate to the bearing properties of an actual double shear joint.Bearing strength,as measured by the test in this section,is considered a material property for relative evaluation and design.Furthermore D 5961 allows use of pins and hence can be used instead of D 953 and take advantage of simpler fixturing.In realistic structural joints,factors like geometry,fastener type,and load eccentricity will significantly influence the realizable fraction of the bearing strength measured in the proposed test.Bearing strength tests more appropriate in design of joints are discussed in Sections 7.5.2.2.2 and 7.5.2.3. 7.5.2.2.2 ASTM D 5961,Procedure A This recently developed standard has addressed all of the deficiencies of ASTM D 953 while still permitting a test with a load introduction by a close tolerance pin.ASTM is a standardized adaptation of, and taken in large part from previous MIL-HDBK-17 work.The flexibility built-in in the ASTM D 5961 al- lows for testing to a standard configuration or to a variation that may be representative of the particular user's application.The loading clevice is simple to make and the test procedures and data requirements are clearly described.Only a tensile loading condition is proposed for evaluating bearing failures;under compression,the larger edge distance (e>>3D)should only influence the bearing stress at failure mini- mally unless a shear-out mode of failure is possible(e.g.,a laminate with a large percent of 0 plies).The data generated by this standard is acceptable to be included in MIL-HDBK-17.Bearing and joint strength values are reported in MIL-HDBK-17 as typical or average values.Therefore,bearing and joint strength values that are available for each specific condition should be analyzed to produce typical property values as described in Chapter 8.Test data must include the data documentation required by Table 2.5.6 and will be published in property tables per Volume 2,Section 1.4.2.Bearing data developed at a specific fiber volume may not be applicable for fiber volumes that are much different because of failure mode changes The standard test specimen and the fixture assembly are reproduced here from ASTM D 5961 as Figures 7.5.2.2.2(a)and(b).For the standard test,bearing load is applied by the lightly torqued bolt.In this test it is mandatory to measure average displacement across the loaded hole as the function of load. An example of the resulting bearing stress/bearing strain curve is shown in Figure 7.5.2.2.2(c).The bear- ing strain was obtained by normalizing by bolt diameter.Thus,the 2%offset measurement,which is the default in this standard,is in actuality 2%of the bolt diameter.There is no general consensus as to what the value of the offset should be.The usage in the aerospace industry varies from 1%D,for stiff double shear joints to 4%D for single shear joints,the latter being a standard for metal bearing tests in MIL- HDBK-5.Before selecting an offset measurement,for both aerospace and non-aerospace applications, the user should decide how it would be used.If the goal is to use it to represent bearing yield strength. the offset value should be close to 0.67F,relating to the aircraft industry's safety factor of 1.5.Another measure of the offset value could be the amount of deformation a given design was limited to. 7-17

MIL-HDBK-17-1F Volume 1, Chapter 7 Structural Element Characterization 7-17 Fixturing – The test apparatus is unnecessarily complicated. ASTM D 5961 has a much simpler arrangement. Specimen Geometry – The geometry of the specimen is inconsistent in e/D and W/D ratios for the two specimen thicknesses specified. As these ratios have a significant influence on bearing strength, a user may find differences in bearing strength for the two thickness where such difference does not exist in the material. Specimen Configuration – The lay-up of the specimen is not specified and may lead users to test unidirectional material with disastrous results. Data Reduction – The data reduction mandated by the standard is specifically tied to a parabolic shape that does not reflect actual load-displacement curves. The use of template is antiquated in this computer age. The data reduction method of ASTM D 5961 is more general and useful. In summary, this test is useful to differentiate between materials as to their bearing strength, but the bearing properties, ultimate strength, yield strength, and the load-displacement response do not relate to the bearing properties of an actual double shear joint. Bearing strength, as measured by the test in this section, is considered a material property for relative evaluation and design. Furthermore D 5961 allows use of pins and hence can be used instead of D 953 and take advantage of simpler fixturing. In realistic structural joints, factors like geometry, fastener type, and load eccentricity will significantly influence the realizable fraction of the bearing strength measured in the proposed test. Bearing strength tests more appropriate in design of joints are discussed in Sections 7.5.2.2.2 and 7.5.2.3. 7.5.2.2.2 ASTM D 5961, Procedure A This recently developed standard has addressed all of the deficiencies of ASTM D 953 while still permitting a test with a load introduction by a close tolerance pin. ASTM is a standardized adaptation of, and taken in large part from previous MIL-HDBK-17 work. The flexibility built-in in the ASTM D 5961 allows for testing to a standard configuration or to a variation that may be representative of the particular user’s application. The loading clevice is simple to make and the test procedures and data requirements are clearly described. Only a tensile loading condition is proposed for evaluating bearing failures; under compression, the larger edge distance (e>>3D) should only influence the bearing stress at failure minimally unless a shear-out mode of failure is possible (e.g., a laminate with a large percent of 0° plies). The data generated by this standard is acceptable to be included in MIL-HDBK-17. Bearing and joint strength values are reported in MIL-HDBK-17 as typical or average values. Therefore, bearing and joint strength values that are available for each specific condition should be analyzed to produce typical property values as described in Chapter 8. Test data must include the data documentation required by Table 2.5.6 and will be published in property tables per Volume 2, Section 1.4.2. Bearing data developed at a specific fiber volume may not be applicable for fiber volumes that are much different because of failure mode changes. The standard test specimen and the fixture assembly are reproduced here from ASTM D 5961 as Figures 7.5.2.2.2(a) and (b). For the standard test, bearing load is applied by the lightly torqued bolt. In this test it is mandatory to measure average displacement across the loaded hole as the function of load. An example of the resulting bearing stress/bearing strain curve is shown in Figure 7.5.2.2.2(c). The bearing strain was obtained by normalizing by bolt diameter. Thus, the 2% offset measurement, which is the default in this standard, is in actuality 2% of the bolt diameter. There is no general consensus as to what the value of the offset should be. The usage in the aerospace industry varies from 1%D, for stiff double shear joints to 4%D for single shear joints, the latter being a standard for metal bearing tests in MILHDBK-5. Before selecting an offset measurement, for both aerospace and non-aerospace applications, the user should decide how it would be used. If the goal is to use it to represent bearing yield strength, the offset value should be close to 0.67Fbru, relating to the aircraft industry’s safety factor of 1.5. Another measure of the offset value could be the amount of deformation a given design was limited to

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization It should be noted that in laboratory practice,the bearing response is usually recorded in terms of bolt load versus average displacement and not as shown in Figure 7.5.2.2.2(c). 4D5961/D5961M 110 100 28 ￥Jltimate Strength 15) 00 0 Offset Strength 60 50 40 .25-40kH Chord 30 Offset Line 20 10 10 15 20 25 Bearing Scrain. FIGURE 7.5.2.2.2(c)Example of bearing stress/bearing strain curve. 7.5.2.3 Single shear bearing tests 7.5.2.3.1 Overview The single shear bearing test configuration is more representative of most aircraft bolted joint applica- tions than the double shear tests described in Section 7.5.2.2.The single lap induces both bending and shear loads on the fastener,while the double lap induces mostly shear loads.Two types of single shear specimens are used,one with one bolt and the second with two bolts.The latter being closer to replicat- ing a multi-fastener joint.Both specimens need to be tabbed to assure the load line alignment at the fay- ing surface of the two joining plates.As such the specimens are somewhat more complex than for the double shear configuration.On the other hand.there is no need to fabricate a clevice. 7.5.2.3.2 ASTM D 5961,Procedure B By developing Procedure B of ASTM D 5961.ASTM recognized the need for a bearing test that is representative of single lap joints found in realistic structures.Single bolt and two bolt configurations are allowed by the standard. The recommended single fastener joint configuration is shown in Figure 7.5.2.3.2(a).This is the same specimen specified in MIL-STD-1312-X (Reference 7.5.2.3.2).It should be recognized that this joint configuration is subject to high bending due to the load eccentricity transmitted through the bolt.The bending can be reduced by increasing the stiffness of the two laps,either through increased thickness, and/or material stiffness.It should also be noted that the single fastener joint is generally not representa- 7-19

MIL-HDBK-17-1F Volume 1,Chapter 7 Structural Element Characterization tive of multi-fastener joint applications because of excessive joint rotation and deflection.Therefore,it is generally used for screening purposes or for fastener development. D5961/D5961M DRAWING NOTES: ATNYSUDET TO TME FOLOWING ALDIMENSONSNNCHES WITH DECIMAL TOLERANCES AS FOLLOWS 3 PLY ORIENTATION DIRECTION TOLERANCE RELATIVE TO-A-WITHIN+59. FNI ON MACHINED EOGES NOT TO EXCEED 6(SYMBOLOGY IN ACCORDANCE WITH ASA B4G1.WITH ROUGHNESS 6. VALUES TO FOR THE FOLLOWING.SUBJECT TO ANY RANGES SHOWN ON THE FIELD OF DRAWVING:MATERIAL LAY-UP.PLY ORIENTATION REFERENCE RELATIVE TO-A OVERALL LENGTH.HOLE DIAMETER,COUNTERSINK DETAILS COUPON THICKNESS,DOUBLER MATERIAL,DOUBLER ADHESIVE. L 3.0 .003A +909 445 w/2 0 D±8一 Parameter Standard Dimension.n. fastener diameter,d 0,250+0.000/-0.T le diameter.D 0.250+0.001/-0,000 thickness range.h 0125-0208 length.L 5.5 width.w 1.50.03 edge distance.e 0.75±003 countersink none (optional) F1GURE7.5.2.3.2(a) Single-shear test specimen drawing(inch-pound)(see Figure 7.5.2.3.2(b) for details of double-fastener version) The two bolt lap configuration shown in Figure 7.5.2.3.2(b)may be used to generate both design and fastener screening data.When tested,the specimen geometry shown in Figure 7.5.2.3.2(b)is intended to result in composite bearing failures (as opposed to tension or cleavage failures).It should be noted that this specimen configuration is not pure bearing but has a by-pass load resulting in tensile strain in the two laps.The tensile bypass strain level will be low for the configuration specified in the standard,however, any configuration variations should be checked to make sure that the by-pass strain is not greater than 0.2%to prevent tensile failure of the laps.Fastener pull-thru's and fastener failures,although not accept- able as a measure of composite bearing strength,do provide a measure of joint strength for a particular fastener type. Both the single bolt specimen,Figure 7.5.2.3.2(a),and the two bolt specimen,Figure 7.5.2.3.2(b).can be adopted to test metal to composite joints.A one-piece metal tongue can be machined for one lap or the tab can be bonded to a metal strip with dimensions so as to align the load path along the interface between the two laps. Limitations of the test(s)are Shim Allowance-The standard does not discuss the use of shims between the composite laps to simulate mating gaps occurring in actual joints.The thickness of the shim has a large influence on the bearing strength as discussed in Section 7.5.2.5.A common aerospace practice is to place 7-20