ASTM D7248/D7248M-23

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7248/D7248M − 21 D7248/D7248M − 23
Standard Test Method for
High Bearing - Low Bypass Interaction Response of
Polymer Matrix Composite Laminates Using 2-Fastener
Specimens
This standard is issued under the fixed designation D7248/D7248M; the number immediately following the designation indicates the
year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last
reapproval. A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method determines the uniaxial bearing/bypass interaction response of multi-directional polymer matrix composite
laminates reinforced by high-modulus fibers by either double-shear tensile loading (Procedure A) or single-shear tensile or
compressive loading (Procedure B) of a two-fastener specimen. The scope of this test method is limited to net section (bypass)
failure modes. Standard specimen configurations using fixed values of test parameters are described for each procedure. A number
of test parameters may be varied within the scope of the standard, provided that the parameters are fully documented in the test
report. The composite material forms are limited to continuous-fiber or discontinuous-fiber (tape or fabric, or both) reinforced
composites for which the laminate is balanced and symmetric with respect to the test direction. The range of acceptable test
laminates and thicknesses are described in 8.2.1. Test methods for high bypass - low bearing response of polymer matrix composite
materials, previously published under Procedure C of this test method, are now published in Test Method D8387/D8387M.
1.2 This test method is consistent with the recommendations of Composite Materials Handbook, CMH-17, which describes the
desirable attributes of a bearing/bypass interaction response test method.
1.3 The two-fastener test configurations described in this test method are similar to those in Test Method D5961/D5961M as well
as those used by industry to investigate the bearing portion of the bearing/bypass interaction response for bolted joints, where the
specimen may produce either a bearing failure mode or a bypass failure mode. Should the test specimen fail in a bearing failure
mode rather than the desired bypass mode, then the test should be considered to be a bearing dominated bearing/bypass test, and
the data reduction and reporting procedures of Test Method D5961/D5961M should be used instead of those given in this test
method.
1.4 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in
each system are not necessarily exact equivalents; therefore, to ensure conformance with the standard, each system shall be used
independently of the other, and values from the two systems shall not be combined.
1.4.1 Within the text, the inch-pound units are shown in brackets.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
This test method is under the jurisdiction of ASTM Committee D30 on Composite Materials and is the direct responsibility of Subcommittee D30.05 on Structural Test
Methods.
Current edition approved May 15, 2021Sept. 1, 2023. Published June 2021September 2023. Originally approved in 2007. Last previous edition approved in 20172021 as
D7248/D7248M – 12D7248/D7248M – 21.(2017). DOI: 10.1520/D7248_D7248M-21.10.1520/D7248_D7248M-23.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7248/D7248M − 23
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D792 Test Methods for Density and Specific Gravity (Relative Density) of Plastics by Displacement
D883 Terminology Relating to Plastics
D2584 Test Method for Ignition Loss of Cured Reinforced Resins
D2734 Test Methods for Void Content of Reinforced Plastics
D3171 Test Methods for Constituent Content of Composite Materials
D3878 Terminology for Composite Materials
D5229/D5229M Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite
Materials
D5687/D5687M Guide for Preparation of Flat Composite Panels with Processing Guidelines for Specimen Preparation
D5766/D5766M Test Method for Open-Hole Tensile Strength of Polymer Matrix Composite Laminates
D5961/D5961M Test Method for Bearing Response of Polymer Matrix Composite Laminates
D6484/D6484M Test Method for Open-Hole Compressive Strength of Polymer Matrix Composite Laminates
D6742/D6742M Practice for Filled-Hole Tension and Compression Testing of Polymer Matrix Composite Laminates
D8387/D8387M Test Method for High Bypass – Low Bearing Interaction Response of Polymer Matrix Composite Laminates
D8509 Guide for Test Method Selection and Test Specimen Design for Bolted Joint Related Properties
E4 Practices for Force Calibration and Verification of Testing Machines
E6 Terminology Relating to Methods of Mechanical Testing
E83 Practice for Verification and Classification of Extensometer Systems
E122 Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot or
Process
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E251 Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages
E456 Terminology Relating to Quality and Statistics
E1237 Guide for Installing Bonded Resistance Strain Gages
2.2 Other Document:
Composite Materials Handbook, CMH-17 Polymer Matrix Composites, Volume 1, Chapter 7
3. Terminology
3.1 Definitions—Terminology D3878 defines terms relating to high-modulus fibers and their composites. Terminology D883
defines terms relating to plastics. Terminology E6 defines terms relating to mechanical testing. Terminology E456 and Practice
E177 define terms relating to statistics. In the event of a conflict between terms, Terminology D3878 shall have precedence over
the other documents.
NOTE 1—If the term represents a physical quantity, its analytical dimensions are stated immediately following the term (or letter symbol) in fundamental
dimension form, using the following ASTM standard symbology for fundamental dimensions, shown within square brackets: [M] for mass, [L] for length,
[T] for time, [θ] for thermodynamic temperature, and [nd] for non-dimensional quantities. Use of these symbols is restricted to analytical dimensions when
used with square brackets, as the symbols may have other definitions when used without the brackets.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 bearing area, [L ],n—the area of that portion of a specimen used to normalize applied loading into an effective bearing
stress; equal to the diameter of the fastener multiplied by the thickness of the specimen.
br -1 -2
3.2.2 bearing chord stiffness, E [ML T ],n—the chord stiffness between two specific bearing stress or bearing strain points in
the linear portion of the bearing stress/bearing strain curve.
-2
3.2.3 bearing force, P [MLT ],n—the in-plane force transmitted by a fastener to a specimen at the fastener hole.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from SAE International (SAE), 400 Commonwealth Dr., Warrendale, PA 15096, http://www.sae.org.
D7248/D7248M − 23
br
3.2.4 bearing strain, ε, [nd],n—the normalized hole deformation in a specimen, equal to the deformation of the bearing hole in
the direction of the bearing force, divided by the diameter of the hole.
br_byp -1 -2
3.2.5 bearing strength, F [ML T ],n—the value of bearing stress occurring at the point of bypass (net section) failure.
x
br -1 -2
3.2.6 bearing stress, σ [ML T ],n—the bearing force divided by the bearing area.
3.2.7 diameter to thickness ratio, D/h [nd],n—in a bearing specimen, the ratio of the hole diameter to the specimen thickness.
3.2.7.1 Discussion—
The diameter to thickness ratio may be either a nominal value determined from nominal dimensions or an actual value determined
from measured dimensions.
3.2.8 edge distance ratio, e/D [nd],n—in a bearing specimen, the ratio of the distance between the center of the hole and the
specimen end to the hole diameter.
3.2.8.1 Discussion—
The edge distance ratio may be either a nominal value determined from nominal dimensions or an actual value determined from
measured dimensions.
gr_byp -1 -2
3.2.9 gross bypass stress, f [ML T ],n—the gross bypass stress for tensile loadings is calculated from the total force
bypassing the fastener hole.
net_byp -1 -2
3.2.10 net bypass stress, f [ML T ],n—the net bypass stress for tensile loading is calculated from the force bypassing the
fastener hole minus the force reacted in bearing at the fastener.
NOTE 2—For compressive loadings, the gross and net bypass stresses are equal and are calculated using the force that bypasses the fastener hole (since
for the compressive loading case, the bearing stress reaction is on the same side of the fastener as the applied force, the force reacted in bearing does
not bypass the fastener hole).
NOTE 3—Several alternate definitions for gross and net bypass stress have been used historically in the aerospace industry. Comparison of data from tests
conforming to this test method with historical data may need to account for differences in the bypass definitions.
3.2.11 nominal value, n—a value, existing in name only, assigned to a measurable quantity for the purpose of convenient
designation. Tolerances may be applied to a nominal value to define an acceptable range for the quantity.
bro -1 -2
3.2.12 offset bearing strength, F [ML T ],n—the value of bearing stress, in the direction specified by the subscript, at the point
x
where a bearing chord stiffness line, offset along the bearing strain axis by a specified bearing strain value, intersects the bearing
stress/bearing strain curve.
3.2.12.1 Discussion—
Unless otherwise specified, an offset bearing strain of 2 % is to be used in this test method.
bru -1 -2
3.2.13 ultimate bearing strength, F [ML T ],n—the value of bearing stress, in the direction specified by the subscript, at the
x
maximum force capability of a bearing specimen.
gr_byp -1 -2
3.2.14 ultimate gross bypass strength, F [ML T ],n—the value of gross bypass stress, in the direction specified by the
x
subscript, at the maximum force capability of the specimen.
net_byp -1 -2
3.2.15 ultimate net bypass strength, F [ML T ],n—the value of net bypass stress, in the direction specified by the subscript,
x
at the maximum force capability of the specimen.
3.2.16 width to diameter ratio, w/D [nd],n—in a bearing specimen, the ratio of specimen width to hole diameter.
3.2.16.1 Discussion—
The width to diameter ratio may be either a nominal value determined from nominal dimensions or an actual value, determined
as the ratio of the actual specimen width to the actual hole diameter.
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3.2 Definitions of Terms Specific to This Standard—Refer to Guide D8509.
3.3 Symbols:
A = cross-sectional area of a specimen
CV = coefficient of variation statistic of a sample population for a given property (in percent)
d = fastener or pin diameter
D = specimen hole diameter
d = countersink depth
csk
d = countersink flushness (depth or protrusion of the fastener in a countersunk hole)
fl
e = distance, parallel to applied force, from hole center to end of specimen; the edge distance
br
E = bearing chord stiffness in the test direction specified by the subscript
x
br_byp
F = bearing stress at the ultimate bypass strength in the test direction specified by the subscript
x
gr_byp_c
F = ultimate compressive gross bypass strength in the test direction specified by the subscript
x
gr_byp_t
F = ultimate tensile gross bypass strength in the test direction specified by the subscript
x
net_byp_c
F = ultimate compressive net bypass strength in the test direction specified by the subscript
x
net_byp_t
F = ultimate tensile net bypass strength in the test direction specified by the subscript
x
g = distance, parallel to applied force, from hole edge to end of specimen
h = specimen thickness
k = calculation factor used in net bypass strength calculations to determine net force portion
L = extensometer gagegauge length
g
n = number of specimens per sample population
P = force carried by test specimen
f
P = force carried by test specimen at failure
max
P = maximum force carried by test specimen prior to failure
s = standard deviation statistic of a sample population for a given property
n-1
w = specimen width
x = test result for an individual specimen from the sample population for a given property
i
x¯ = mean or average (estimate of mean) of a sample population for a given property
δ = extensional displacement
ε = general symbol for strain, whether normal strain or shear strain
br
ε = bearing strain
br
σ = bearing stress
4. Summary of Test Method
4.1 Bearing/Bypass Procedures—Definition of the uniaxial bearing/bypass interaction response requires data for varying amounts
of bearing and bypass forces at a fastener hole. Refer to Guide Fig. 1 shows a typical composite laminate bearing/bypass interaction
diagram (Refs 1-3), along with illustrative data from various test types. Data from Practice D6742/D6742MD8509 and Test
Method D5961/D5961M define the 100 % bypass and bearing ends of the interaction diagram. Rationale for the baseline
bearing/bypass specimen geometry and fastener torques are given in for discussion 6.7 and 6.8. Procedures A and B of this test
method provide data in the bypass/high bearing region, while tests per Test Method of D8387/D8387M provide data in the
bypass/low bearing region. More complicated test setups have been used to develop data across the full range of bearing/bypass
interaction. This test method is limited to cases where the bearing and bypass loads are aligned in the same direction. It is also
limited to uniaxial tensile or compressive bypass loads. Test procedures for cases where the bearing and bypass loads act at
different directions, or cases with biaxial or shear bypass loads are outside the scope of this test method.bearing/bypass test
procedures.
4.1.1 Ultimate strength for all procedures is calculated based on the specimen gross cross-sectional area, disregarding the presence
of the hole. While the hole causes a stress concentration and reduced net section, it is common industry practice to develop notched
design allowable strengths based on gross section stress to account for various stress concentrations (fastener holes, free edges,
flaws, damage, and so forth) not explicitly modeled in the stress analysis. This is consistent with the ASTM D30 test methods for
open and filled hole tension and compression strength (Test Methods D5766/D5766M, D6484/D6484M, and Practice
D6742/D6742M).
4.2 Procedure A, Bypass/High Bearing Double Shear:
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4.2.1 A flat, constant rectangular cross-section test specimen with two centerline holes located near the end of the specimen, as
shown in the test specimen drawings of Figs. 21 and 32, is loaded at the hole in bearing. The bearing force is normally applied
through a close-tolerance, lightly torqued fastener (or pin) that is reacted in double shear by a fixture similar to that shown in Figs.
43 and 54. The bearing force is created by pulling the assembly in tension in a testing machine. The difference from a standard
“bearing” test is that the expected primary failure mode is net section tension, rather than a bearing mode.
FIG. 21 Double-Shear, Two-Fastener Test Specimen Drawing (SI)
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FIG. 32 Double-Shear, Two-Fastener Test Specimen Drawing (Inch-Pound)
4.2.2 Both the applied force and the associated deformation of the hole are monitored. The applied force is normalized by the
projected hole area to create an effective bearing stress. The specimen is loaded until a two part failure is achieved.
NOTE 4—Should the test specimen fail in a bearing failure mode rather than the desired bypass (net tension or compression) mode, then the test should
D7248/D7248M − 23
FIG. 43 Fixture Loading Plate for Procedure A (2 Required)
be considered to be a bearing dominated bearing/bypass test, and the data reduction and reporting procedures of Test Method D5961/D5961M should
be used instead of those given in this test method.
D7248/D7248M − 23
FIG. 54 Fixture Assembly for Procedure A
4.2.2 The standard test configurationRefer to Guide D8509 for this procedure has defined values for the major test parameters.
However, the following variations in configuration are allowed and can be considered as being in accordance with this test method
as long as the values of all variant test parameters are prominently documented with the results.additional test details and for the
standard test configuration.
Parameter Standard Variation
Loading condition double-shear none
Loading type tensile none
Mating material steel fixture any, if documented
Number of holes 2 3
Countersink none none
Hole fit tight any, if documented
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Parameter Standard Variation
Fastener torque 9.0-10.7 N·m [90-95 lbf-in.] any, if documented
Laminate quasi-isotropic any, if documented
Fastener diameter 6 mm [0.250 in.] any, if documented
Edge distance ratio 3 any, if documented
w/D ratio 5 any, if documented
D/h ratio 1.2-2 any, if documented
4.3 Procedure B, Bypass/High Bearing Single Shear:
4.3.1 The flat, constant rectangular cross-section test specimen is composed of two like halves fastened together through two
centerline holes located near one end of each half, as shown in the test specimen drawings of Figs. 65 and 76. The eccentricity
in applied force that would otherwise result is minimized by a doubler bonded to each grip end of the specimen, resulting in a force
line-of-action along the interface between the specimen halves, through the centerline of the hole(s).
FIG. 65 Single-Shear, Two-Fastener Test Specimen Drawing (SI)
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FIG. 76 Single-Shear, Two-Fastener Test Specimen Drawing (Inch-Pound)
4.3.1.1 Unstabilized Configuration (No Support Fixture)—The ends of the test specimen are gripped in the jaws of a test machine
and loaded in tension.
4.3.1.2 Stabilized Configuration (Using Support Fixture)—The test specimen is face-supported in a multi-piece bolted support
fixture, as shown in Fig. 87. The test specimen/fixture assembly is clamped in hydraulic wedge grips and the force is sheared into
the support fixture and then sheared into the specimen. Either tensile or compressive force may be applied. The stabilization fixture
is required for compressive loading. For tensile loading, the fixture is optional, but is often used to simulate actual stabilized joint
configurations.
4.3.2 Both the applied force and the associated deformation of the hole(s) are monitored. The applied force is normalized by the
projected hole area to yield an effective bearing stress. The specimen is loaded until a two part failure is achieved.
NOTE 5—Should the test specimen fail in a bearing failure mode rather than the desired net tension or compression, then the test should be considered
D7248/D7248M − 23
FIG. 87 Support Fixture Assembly for Procedure B (for details of the Support Fixture, see Test Method D5961/D5961M)
to be a bearing dominated bearing/bypass test, and the data reduction and reporting procedures of Test Method D5961/D5961M should be used instead
of those given in this test method.
4.3.2 The standard test configurationRefer to Guide D8509 for this procedure has defined values for the major test parameters.
However, the following variations in configuration are allowed and can be considered as being in accordance with this test method
as long as the values of all variant test parameters are prominently documented with the results.additional test details and for the
standard test configuration.
Parameter Standard Variation
Loading condition single-shear none
Loading type tensile compressive
Support fixture no for tensile load yes, if documented
yes for compressive load
Number of holes 2 3
Countersunk holes no yes, if documented
Grommets no yes, if documented
Mating material same laminate any, if documented
Hole fit tight any, if documented
Fastener torque 9.0-10.7 N·m [80-95 lbf-in.] any, if documented
for tensile load
2.2-3.4 N·m [20-30 lbf-in.]
for compressive load
Laminate quasi-isotropic any, if documented
Fastener diameter 6 mm [0.250 in.] any, if documented
Edge distance ratio 3 any, if documented
w/D ratio 5 any, if documented
D/h ratio 1.2-2 any, if documented
D7248/D7248M − 23
5. Significance and Use
5.1 This test method is designed to produceRefer to Guide D8509bearing/bypass interaction response data for research and
development, and for structural design and analysis. The standard configuration for each procedure is very specific and is intended
as a baseline configuration for developing structural design data.
5.1.1 Procedure A, the bypass/high bearing double-shear configuration is recommended for developing data for specific
applications which involve double shear joints.
5.1.2 Procedure B, the bypass/high bearing single-shear configuration is more useful in the evaluation of typical joint
configurations. The specimen may be tested in either an unstabilized (no support fixture) or stabilized configuration. The
unstabilized configuration is intended for tensile loading and the stabilized configuration is intended for compressive loading.
These configurations, particularly the stabilized configuration, have been extensively used in the development of design allowables
data. The variants of either procedure provide flexibility in the conduct of the test, allowing adaptation of the test setup to a specific
application. However, the flexibility of test parameters allowed by the variants makes meaningful comparison between datasets
difficult if the datasets were not tested using identical test parameters.
5.2 General factors that influence the mechanical response of composite laminates and should therefore be reported include the
following: material, methods of material preparation and lay-up, specimen stacking sequence, specimen preparation, specimen
conditioning, environment of testing, specimen alignment and gripping, speed of testing, time held at test temperature, void
content, and volume percent reinforcement.
5.3 Specific factors that influence the bearing/bypass interaction response of composite laminates and should therefore be reported
include not only the loading method (either Procedure A or B) and loading type (tension or compression) but the following (for
both procedures): edge distance ratio, width to diameter ratio, diameter to thickness ratio, fastener torque, fastener or pin material,
fastener or pin clearance; and (for Procedure B only) countersink angle and depth of countersink, type of grommet (if used), type
of mating material, and type of support fixture (if used). Properties, in the test direction, which may be obtained from this test
method include the following:
5.3.1 Filled hole tensile bearing/bypass strength.
5.3.2 Filled hole compressive bearing/bypass strength.
5.3.3 Bearing stress/bypass strain curve.
6. Interferences
6.1 Material and Specimen Preparation—Bearing/bypass response is sensitive to poor material fabrication practices (including
lack of control of fiber alignment), damage induced by improper specimen machining (hole preparation is especially critical), and
torqued fastener installation. Fiber alignment relative to the specimen coordinate axis should be maintained as carefully as possible,
although there is currently no standard procedure to ensure or determine this alignment. A practice that has been found satisfactory
for many materials is the addition of small amounts of tracer yarn to the prepreg parallel to the 0° direction, added either as part
of the prepreg production or as part of panel fabrication. See Refer to Guide D5687/D5687MD8509 for further information on
recommended specimen preparation practices.
6.2 Restraining Surfaces—The degree to which out-of-plane hole deformation is possible, due to lack of restraint by the fixture
or the fastener, has been shown to affect test results.
6.3 Cleanliness—The degree of cleanliness of the mating surfaces has been found to produce significant variations in test results.
6.4 Eccentricity (Procedure B Only)—A loading eccentricity is created in single-shear tests by the offset, in one plane, of the line
of action of force between each half of the test specimen. This eccentricity creates a moment that, particularly in clearance hole
tests, rotates the fastener, resulting in an uneven contact stress distribution through the thickness of the specimen. The effect of this
eccentricity upon test results is strongly dependent upon the degree of clearance in the hole, the size of the fastener head, the mating
area, the coefficient of friction between the specimen and the mating material, the thickness and stiffness of the specimen, the
D7248/D7248M − 23
thickness and stiffness of the mating material, and the configuration of the support fixture. Consequently, results obtained from this
procedure where the support fixture is used may not accurately replicate behavior in other structural configurations.
6.5 Hole Preparation—Due to the dominating presence of the filled hole(s), results from this test method are relatively insensitive
to parameters that would be of concern in an unnotched tensile or compressive property test. However, since the filled hole(s)
dominates the strength, consistent preparation of the hole(s) without damage to the laminate is important to meaningful results.
Damage due to hole preparation will affect strength results. Some types of damage, such as delaminations, can blunt the stress
concentration due to the hole, increasing the force carrying capacity of the coupon and the calculated strength. Other types of
damage can reduce the calculated strength.
6.6 Fastener-Hole Clearance—Compressive bearing/bypass results are affected by the clearance arising from the difference
between hole and fastener diameters. Clearance can change the observed specimen behavior by delaying the onset of bearing
damage. Tensile bearing/bypass results are also affected by clearance, but to a lesser degree than under compressive loads. Hole
clearance also effects the proportion of force transferred in each fastener, and the proportions can change as the force is increased
during the test. Damage due to insufficient clearance during fastener installation will affect strength results. Countersink flushness
(depth or protrusion of the fastener head in a countersunk hole) will affect strength results and may affect the observed failure
mode. For these reasons, both the hole and fastener diameters must be accurately measured and recorded. A typical aerospace
tolerance on fastener-hole clearance is +75/–0 μm [+0.003/–0.000 in.] for structural fastener holes.
6.7 Fastener Torque/Pre-load—Results are affected by the installed fastener pre-load (clamping pressure). Laminates can exhibit
significant differences in both failure force and failure mode due to changes in fastener pre-load under both tensile and compressive
loading. The critical pre-load condition (that is, either high or low clamping pressure) can vary depending upon the type of loading,
the laminate stacking sequence, and the desired failure mode. For compressive loaded bearing/bypass, the nominal test
configuration uses a relatively low level of fastener installation torque to give conservative results. For tensile loaded
bearing/bypass, the nominal test configuration uses a high level of fastener installation torque (full fastener installation torque)
since this usually gives conservative results. Fastener torque levels used for bearing/bypass test specimens should correspond to
the torque levels that give the most conservative results for corresponding filled hole tension and filled hole compression tests of
the same material and layup (see Practice D6742/D6742M).
6.8 Specimen Geometry—Results are affected by the ratio of specimen width to hole diameter (w/D); this ratio should be
maintained at 5 to avoid bearing failure modes, unless the experiment is investigating the influence of this ratio, or invalid (bearing)
failure modes occur. If bearing failures occur with w/D = 5 specimens, then the width should be reduced; with some layups having
low bearing capabilities, w/D values as low as 3 may be required to obtain a bypass failure mode. Results may also be affected
by the spacing distance between the two fasteners; the baseline distance is equal to 6D. Results may also be affected by the distance
between the end hole and the end of the specimen, with small end distances potentially resulting in invalid shear-out failure modes;
the baseline end distance to diameter ratio is 3D unless the experiment is investigating the influence of this ratio. Results may also
be affected by the ratio of hole diameter to thickness; the preferred ratio is the range from 1.5-3.0 unless the experiment is
investigating the influence of this ratio. Results may also be affected by the ratio of countersunk (flush) head depth to thickness;
the preferred ratio is the range from 0.0-0.7 unless the experiment is investigating the influence of this ratio. Results may also be
affected by the ratio of ungripped specimen length to specimen width; this ratio should be maintained as shown, unless the
experiment is investigating the influence of this ratio.
6.9 Material Orthotropy—The degree of laminate orthotropy strongly affects the failure mode and measured bearing/bypass
strengths. Bearing/bypass strength results should only be reported when appropriate and valid failure modes are observed, in
accordance with 11.4.
6.10 Thickness Scaling—Thick composite structures do not necessarily fail at the same strengths as thin structures with the same
laminate orientation (that is, strength does not always scale directly with thickness). Thus, data gathered using these procedures
may not translate directly into equivalent thick-structure properties.
6.11 Environment—Results are affected by the environmental conditions under which the tests are conducted. Laminates tested in
various environments can exhibit significant differences in both bearing/bypass strength and failure mode. Experience has
demonstrated that elevated temperature, humid environments are generally critical for bearing failure modes, while bypass
dominated failure modes can be critical at either cold or hot/wet conditions, depending on the material and layup. Therefore,
critical environments must be assessed independently for each material system, stacking sequence, and torque condition tested.
D7248/D7248M − 23
6.12 Fastener Force Ratios—The ratio of force in each of the two fasteners (Procedures A and B) may vary with the applied force
level during the test. This variation in load transfer can result from the onset of bearing damage, fastener bending, and frictional
effects.
7. Apparatus
7.1 Micrometers—A micrometer with a 44 mm to 8 mm [0.16[0.16 in. to 0.32 in.] nominal diameter ball interface shall be used
to measure the specimen thickness when at least one surface is irregular (such as the bag-side of a laminate). A micrometer with
a 44 mm to 8 mm [0.16[0.16 in. to 0.32 in.] nominal diameter ball interface or with a flat anvil interface shall be used to measure
the specimen thickness when both surfaces are smooth (such as tooled surfaces). A micrometer or caliper, with a flat anvil interface,
shall be used to measure the width of the specimen. The accuracy of the instruments shall be suitable for reading to within 1 %
of the sample dimensions. For typical specimen geometries, an instrument with an accuracy of 60.0025 mm [60.0001 in.] is
adequate for the thickness measurement, while an instrument with an accuracy of 60.025 mm [60.001 in.] is adequate for the
width measurement. Additionally, a micrometer or gagegauge capable of determining the hole diameters to 60.025 mm
[60.001 in.] shall be used.
NOTE 2—The accuracies given above are based on achieving measurements that are within 1 % of the sample width and thickness.
7.2 Loading Fasteners or Pins—The fastener (or pin) type shall be specified as an initial test parameter and reported. The assembly
torque (if applicable) shall be specified as an initial test parameter and reported. This value may be a measured torque or a
specification torque for fasteners with lock-setting features. If washers are utilized, the washer type, number of washers, and
washer location(s) shall be specified as initial test parameters and reported. The reuse of fasteners is not recommended due to
potential differences in through-thickness clamp-up for a given torque level, caused by wear of the threads or deformation of the
locking features.
7.3 Torque Wrench—If using a torqued fastener, a torque wrench used to tighten the fastener shall be capable of determining the
applied torque to within 610 % of the desired value.
7.4 Fixture:
7.4.1 Procedure A—The force shall be applied to the specimen by means of a double-shear clevis similar to that shown in Figs.
43 and 54, using the loading fasteners or pins. The fixture shall allow a bearing strain indicator to monitor the hole deformation
relative to the fixture as shown in Fig. 98.
NOTE 3—The double shear loading straps do not have the bosses around the hole as used for the Test Method D5961/D5961M bearing test method in
order to more closely simulate actual joint configurations and to simplify the fixture. With flat loading straps the through-thickness clamp-up force will
be distributed over a larger area and therefore the specimen is expected to experience greater bearing damage and lower (conservative) bearing/bypass
strengths.
7.4.2 Procedure B—The force shall be applied to the specimen by means of a mating single-shear attachment (normally identical
to the specimen) using two fasteners. The mating material, thickness, edge distance, length, and hole clearance shall be specified
as part of the test parameters. The line of action of the force shall be adjusted by specimen doublers to be coincident and parallel
to the interface between the test specimen and the joint mate. If the mating attachment is permanently deformed by the test, it shall
be replaced after each test, as required. The mating attachment and support fixture (if used) will allow a bearing strain indicator
to measure the required hole deformation relative to the mating attachment, as shown in Fig. 98.
7.5 Support Fixture (Procedure B)—If compressive loads are applied or if requested in the test plan, a support fixture shall be used
to stabilize the specimen. The fixture is a face-supported test fixture as shown in Fig. 87. The fixture consists of two
short-grip/long-grip assemblies, two support plates, and stainless steel shims as required to maintain a nominally zero
(0.00(0.00 mm to 0.12 mm [0.000[0.000 in. to 0.005 in.] tolerance) gap between the support plates and the long grips. If this gap
does not meet the minimum requirement, shim the contact area between the support plate and the short grip with stainless steel
shim stock. If the gap is too large, shim between the support plate and the long grip, holding the shim stock on the support plate
with tape. The fixture should be checked for conformity to engineering drawings. Each short-grip/long-grip assembly is line-drilled
and must be used as a matched set. The threading of the support plate is optional. The fixture is hydraulically gripped on each end
and the force is sheared by means of friction through the fixture and into the test specimen. A cutout exists on both faces of the
fixture for a thermocouple, fastener(s) and surface-mounted extensometer. The long and short fixtures have an undercut along the
D7248/D7248M − 23
FIG. 98 Transducer GageGauge Length and Location
D7248/D7248M − 23
corner of the specimen grip area so that specimens are not required to be chamfered and to avoid damage caused by the radius.
The fixtures also allow a slight clearance between the fixture and the gagegauge section of the specimen, in order to minimize grip
failures and friction effects. This fixture does not allow specimens to be end loaded.
7.5.1 Support Fixture Details—The detailed drawings for manufacturing the support fixture are contained in Test Method
D5961/D5961M. Other fixtures that meet the requirements of this section may be used. The following general notes apply to these
figures:
7.5.1.1 Machine surfaces to a 3.2 [125] finish unless otherwise specified.
7.5.1.2 Break all edges.
7.5.1.3 Specimen-gripping area shall be thermal sprayed using high-velocity oxygen fueled (HVOF), electro-spark deposition
(ESD), or equivalent process.
7.5.1.4 The test fixture may be made of low-carbon steel for ambient temperature testing. For non-ambient environmental
conditions, the recommended fixture material is a non-heat-treated ferritic or precipitation-hardened stainless steel (heat treatment
for improved durability is acceptable but not required).
NOTE 4—Experience has shown that fixtures may be damaged in use; thus, periodic re-inspection of the fixture dimensions and tolerances is important.
NOTE 5—The Test Method D5961/D5961M support fixture has been successfully used for 30 mm [1.25 in.] wide bearing/bypass tests provided careful
specimen alignment is achieved. Optional spacers should be added to the fixture to reduce the grip area width to 30.5 mm [1.27 in.]. Such spacers should
be thinner than the test sample (to ensure the fixture adequately seats against the specimen surfaces) and should be no longer than the short grips, to ensure
that they do not provide a contacting surface which could restrict the motion of the long grip and permit load to be transferred through the fixture.
Alternately, a reduced width fixture may be fabricated. Similarly, wider fixtures of the same basic design may be used for specimens that are wider than
36 mm [1.5 in.].
7.6 Testing Machine—The testing machine shall be in conformance with Practices E4, and shall satisfy the following
requirements:
7.6.1 Testing Machine Configuration—The testing machine shall have both an essentially stationary head and a movable head. A
short loading train and rigidly mounted hydraulic grips shall be used for Procedure B when using the support fixture.
7.6.2 Drive Mechanism—The testing machine drive mechanism shall be capable of imparting to the movable head a controlled
velocity with respect to the stationary head. The velocity of the movable head shall be capable of being regulated as specified in
11.3.
7.6.3 Force Indicator—The testing machine force-sensing device shall be capable of indicating the total force being carried by the
test specimen. This device shall be essentially free from inertia-lag at the specified rate of testing and shall indicate the force with
an accuracy over the force range(s) of interest of within 61 % of the indicated value.
7.6.4 Grips—Each head of the testing machine shall be capable of holding one end of the test assembly so that the direction of
force applied to the specimen is coincident with the longitudinal axis of the specimen. Wedge grips shall apply sufficient lateral
pressure to prevent slippage between the grip face and the test specimen or support fixture.
7.7 Bearing Deformation Indicator—Bearing deformation data shall be determined by an indicator device able to measure
longitudinal hole deformation simultaneously on opposite edges of the specimen, as shown in Fig. 98 (the average of which
corrects for in-plane joint rotation). The arms of the indicator device must fit within the stabilization fixture when a specimen with
a width less than 38 mm [1.5 in.] is tested in the standard fixture. Transducer gagegauge lengths on the order of 50 mm [2.0 in.]
are typically used. The transducers of the bearing deformation indicator may provide either individual signals to be externally
averaged or an electronically averaged signal. The indicator may consist of two matched strain-gagestrain-gauge extensometers or
displacement transducers such as LVDTs or DCDTs. Attachment of the bearing deformation indicator to the specimen shall not
cause damage to the specimen surface. Transducers shall satisfy, at a minimum, Practice E83, Class B-2 requirements for the
displacement range of interest, and shall be calibrated over that range in accordance with Practice E83. The transducers shall be
essentially free of inertia-lag at the specified speed of testing.
NOTE 6—A matched set of extensometers mounted on opposite faces would be required to quantify and correct for out-of-plane joint rotation, which is
the primary variable of concern in a single-shear loading configuration.
D7248/D7248M − 23
7.8 Conditioning Chamber—When conditioning materials at non-laboratory environments, a temperature-/vapor-level controlled
environmental conditioning chamber is required that shall be capable of maintaining the required temperature to within 63 °C
[65 °F] and the required relative humidity level to within 63 %. Chamber conditions shall be monitored either on an automated
continuous basis or on a manual basis at regular intervals.
7.9 Environmental Test Chamber—An environmental test chamber is required for test environments other than ambient testing
laboratory conditions. This chamber shall be capable of maintaining the gagegauge section of the test specimen at the required test
environment during the mechanical test within 63 °C [65 °F].
7.10 Strain-Indicating Device—Strain data, when required, shall be determined by means of bonded resistance strain
gages.gauges.
7.10.1 Bonded Resistance Strain GageGauge Selection—Strain gagegauge selection is based on the type of material to be tested.
A minimum active gagegauge length of 3 mm [0.125 in.] is recommended for composite laminates fabricated from unidirectional
layers. Larger strain gagegauge sizes may be more suitable for some textile fabrics. GageGauge calibration certification shall
comply with Test Method E251. Strain gagesgauges with a minimum normal strain range of approximately 3 % are recommended.
When testing textile fabric laminates, gagegauge selection should consider the use of an active gagegauge length that is at least
as great as the characteristic repeating unit of the fabric. Some guidelines on the use of strain gagesgauges on composite materials
follow.
7.10.1.1 Surface preparation of fiber-reinforced composites in accordance with Guide E1237 can penetrate the matrix material and
cause damage to the reinforcing fibers, resulting in improper coupon failures. Reinforcing fibers should not be exposed or damaged
during the surface preparation process. The strain gagegauge manufacturer should be consulted regarding surface preparation
guidelines and recommended bonding agents for composites, pending the development of a set of standard practices for strain
gagegauge installation surface preparation of fiber-reinforced composite ma
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