ASTM D6415/D6415M-06a(2013)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: D6415/D6415M − 06a (Reapproved 2013)
Standard Test Method for
Measuring the Curved Beam Strength of a Fiber-Reinforced
Polymer-Matrix Composite
This standard is issued under the fixed designation D6415/D6415M; 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 D3171Test Methods for Constituent Content of Composite
Materials
1.1 Thistestmethoddeterminesthecurvedbeamstrengthof
D3878Terminology for Composite Materials
a continuous fiber-reinforced composite material using a 90°
D5229/D5229MTestMethodforMoistureAbsorptionProp-
curved beam specimen (Fig. 1 and Fig. 2). The curved beam
erties and Equilibrium Conditioning of Polymer Matrix
consists of two straight legs connected by a 90° bend with a
Composite Materials
6.4-mm [0.25 in.] inner radius. An out-of-plane (through-the-
D5687/D5687MGuide for Preparation of Flat Composite
thickness) tensile stress is produced in the curved region of the
Panels with Processing Guidelines for Specimen Prepara-
specimen when force is applied. This test method is limited to
tion
use with composites consisting of layers of fabric or layers of
E4Practices for Force Verification of Testing Machines
unidirectional fibers.
E6Terminology Relating to Methods of MechanicalTesting
1.2 This test method may also be used to measure the
E122PracticeforCalculatingSampleSizetoEstimate,With
interlaminar tensile strength if a unidirectional specimen is
Specified Precision, the Average for a Characteristic of a
used where the fibers run continuously along the legs and
Lot or Process
around the bend.
E177Practice for Use of the Terms Precision and Bias in
1.3 This standard does not purport to address all of the ASTM Test Methods
safety concerns, if any, associated with its use. It is the
E456Terminology Relating to Quality and Statistics
responsibility of the user of this standard to establish appro- E1309 Guide for Identification of Fiber-Reinforced
priate safety and health practices and determine the applica-
Polymer-Matrix Composite Materials in Databases
bility of regulatory limitations prior to use. E1434Guide for Recording Mechanical Test Data of Fiber-
1.4 The values stated in either SI units or inch-pound units
Reinforced Composite Materials in Databases
are to be regarded separately as standard. Within the text the
E1471Guide for Identification of Fibers, Fillers, and Core
inch-pound units are shown in brackets. The values stated in Materials in Computerized Material Property Databases
each system are not exact equivalents; therefore, each system
3. Terminology
must be used independently of the other. Combining values
from the two systems may result in nonconformance with the
3.1 Definitions—Terminology D3878 defines terms relating
standard.
to high-modulus fibers and their composites. Terminology
D883definestermsrelatingtoplastics.TerminologyE6defines
2. Referenced Documents
terms relating to mechanical testing. Terminology E456 and
2.1 ASTM Standards: Practice E177 define terms relating to statistics. In the event of
a conflict between terms, Terminology D3878 shall have
D792Test Methods for Density and Specific Gravity (Rela-
tive Density) of Plastics by Displacement precedence over the other terminologies.
D883Terminology Relating to Plastics
3.2 Definitions of Terms Specific to This Standard:
NOTE 1—If the term represents a physical quantity, its analytical
1 dimensionsarestatedimmediatelyfollowingtheterm(orlettersymbol)in
This test method is under the jurisdiction of ASTM Committee D30 on
fundamental dimension form, using the following ASTM standard sym-
Composite Materials and is the direct responsibility of D30.06 on Interlaminar
bology for fundamental dimensions, shown within square brackets: [M]
Properties.
Current edition approved Oct. 1, 2013. Published October 2013. Originally for mass, [L] for length, [T] for time, [θ] for thermodynamic temperature,
ε1
approved in 1999. Last previous edition approved in 2006 as D6415–06A . DOI: and [nd] for nondimensional quantities. Use of these symbols is restricted
10.1520/D6415_D6415M-06AR13.
to analytical dimensions when used with square brackets, as the symbols
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
may have other definitions when used without the brackets.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
2 –2
3.2.1 applied moment, M [ML T ], n—the moment applied
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. to the curved test section of the specimen.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6415/D6415M − 06a (2013)
3.3.15 r, θ = cylindrical coordinates of any point in the
curved segment.
3.3.16 r ,r = inner and outer radii of curved segment.
i o
3.3.17 r = radial position of the maximum interlaminar
m
(radial) tensile stress.
3.3.18 S = standard deviation statistic of a sample popu-
n–1
lation for a given property.
3.3.19 t = average thickness of specimen.
3.3.20 w = width of the specimen.
3.3.21 x = test result for an individual specimen from the
sample population for a given property.
FIG. 1 Test Specimen Geometry (SI units)
3.3.22 x¯= mean or average (estimate of mean) of a sample
population for a given property.
3.3.23 ∆ = relative displacement between the top and
bottom halves of the four-point-bending fixture.
3.3.24 κ = parameter used in strength calculation.
3.3.25 ρ = parameter used in strength calculation.
3.3.26 φ = angle from horizontal of the specimen legs in
degrees.
3.3.27 φ= angle from horizontal of the specimen legs at the
i
start of the test in degrees (0.5 × angle between the legs).
3.3.28 σ = radial stress component in curved segment.
r
4. Summary of Test Method
4.1 A90° curved-beam test specimen is used to measure the
FIG. 2 Test Specimen Geometry (inch-pound)
curvedbeamstrengthofacontinuous-fiber-reinforcedcompos-
ite material (Fig. 1 and Fig. 2). The curved beam strength
1 –2
3.2.2 curved beam strength, CBS [ML T ], n—the moment
represents the moment per unit width which causes a delami-
per unit width, M/w, applied to the curved test section which
nation(s) to form. If the curved beam is unidirectional with all
causes a sharp decrease in applied load or delamination(s) to
fibersrunningcontinuouslyalongthelegsandaroundthebend
form.
and an appropriate failure mode is observed, an interlaminar
3u –1 –2
3.2.3 interlaminar tensile strength, F [ML T ], n—the
(through-the-thickness)tensilestrengthmayalsobecalculated.
strengthofthecompositematerialintheout-of-plane(through-
The curved beam is uniform thickness and consists of two
the-thickness) direction.
straight legs connected by a 90° bend with a 6.4-mm [0.25-in.]
3.3 Symbols:
inner radius. The curved beam is loaded in four-point bending
3.3.1 CBS = curved beam strength (see 3.2.2).
to apply a constant bending moment across the curved test
3.3.2 CV = coefficient of variation statistic of a sample
section.Anout-of-planetensilestressisproducedinthecurved
population for a given property (in percent).
region of the specimen to cause the failure.
3.3.3 d ,d = horizontal and vertical distances between two
x y
5. Significance and Use
adjacent top and bottom loading bars, respectively.
3.3.4 D = diameter of the cylindrical loading bars on the 5.1 Out-of-plane stress analyses are not easily performed.
four-point-bending fixture.
Failure criteria are varied and poorly validated. Interlaminar
3.3.5 E ,E = moduli in the radial and tangential directions, allowables are not readily available. However, stress analysts
r θ
respectively.
routinely encounter structural details in which they cannot
3u
3.3.6 F = interlaminar tensile strength (see 3.2.3).
ignore the out-of-plane loads. This test method is designed to
3.3.7 g = parameter used in strength calculation.
produce out-of-plane structural failure data for structural de-
3.3.8 l = distance between the centerlines of the bottom
sign and analysis, quality assurance, and research and devel-
b
loading bars on the four-point-bending fixture.
opment. For unidirectional specimens, this test method is
3.3.9 l = distance along the specimen’s leg between the
designed to produce interlaminar tensile strength data. Factors
centerlines of a top and bottom loading bar.
thatinfluencethecurvedbeamstrengthandshouldthereforebe
3.3.10 l = distance between the centerlines of the top
reported include the following: material, methods of material
t
loading bars on the four-point-bending fixture. preparation, methods of processing and specimen fabrication,
3.3.11 M = applied moment (see 3.2.1).
specimen preparation, specimen conditioning, environment of
3.3.12 P = total force applied to the four-point-bending testing,speedoftesting,timeattemperature,voidcontent,and
fixture.
volume percent reinforcement.
max
3.3.13 P = maximum force applied to the four-point-
6. Interferences
bending fixture before failure.
3.3.14 P = force applied to the specimen by a single 6.1 Failureinnon-unidirectionalspecimensmaybeinitiated
b
loading bar. from matrix cracks or free edge stresses. Consequently, the
D6415/D6415M − 06a (2013)
interlaminar strength calculated from non-unidirectional speci- indicate the force with an accuracy over the force range(s) of
mens may be in error. interest of within 61% of the indicated value.
7.1.4 Grips—Each head of the testing machine shall have a
6.2 The stress state of a curved beam in four-point bending
means to hold half of the four-point-bending fixture firmly in
iscomplex.Circumferentialtensilestressesareproducedalong
place.Aconvenientmeansofprovidinganattachmentpointfor
the inner surface, and circumferential compressive stresses are
each fixture half is through the use of a metal “T” in each grip.
produced on the outer surface. The radial tensile stress ranges
The lower part of the “T” is clamped in the grips, and the top
fromzeroattheinnerandoutersurfacestoapeakinthemiddle
part of the “T” provides a flat attachment surface for each
third of the thickness. Consequently, the failure should be
fixture half.
carefullyobservedtoensurethatadelamination(s)isproduced
across the width before the failure data are used. 7.2 Four-Point-Bending Fixture—Afour-point-bending test
apparatusasshowninFig.3shallbeusedtoloadthespecimen.
6.3 Sincestressesarenonuniformandthecriticalstressstate
Machine drawings for example fixtures are shown in the
occurs in a small region, the location of architectural charac-
appendix. Other designs that perform the necessary functions
teristics of the specimen (for example, fabric weave, and tow
are acceptable. The cylindrical loading bars shall have diam-
intersections) may affect the curved beam strength.
eters. D, of 6 to 10 mm [0.25 to 0.40 in.] and be mounted on
6.4 Nonlaminated, 3-D reinforced, or textile composites
roller bearings. The distance between the bar centers shall be
may fail by different mechanisms than laminates. The most
100 6 2 mm [4.00 6 0.05 in.] (l ) for the bottom fixture and
b
critical damage may be in the form of matrix cracking or fiber
75 6 2 mm [3.00 6 0.05 in.] (l) for the top fixture.
t
failure, or both, rather than delaminations.
7.3 Displacement Indicator—The relative axial displace-
6.5 Material and Specimen Preparation—Poormaterialfab-
mentbetweentheupperandlowerfixturesmaybeestimatedas
rication practices, lack of control of fiber alignment, and
the crosshead travel provided the deformation of the testing
damage induced by improper coupon machining are known
machine and support fixture is less than 2% of the crosshead
causes of high material data scatter in composites in general.
travel. If not, this displacement shall be obtained from a
Important aspects of specimen preparation that contribute to
properlycalibratedexternalgageortransducerlocatedbetween
data scatter include thickness variation, curve geometry, sur-
the two fixtures. The displacement indicator shall indicate the
face roughness, and failure to maintain the dimensions speci-
displacementwithanaccuracyof 61%ofthethicknessofthe
fied in section 8.2
specimen.
6.6 The curved beam and interlaminar strengths measured
7.4 Force Versus Displacement (P Versus ∆) Record—An
using this test method are extremely sensitive to reinforcement
X-Y plotter, or similar device, shall be used to make a
volume and void content. Consequently, the test results may
permanent record during the test of force versus displacement.
reflect manufacturing quality as much as material properties.
Alternatively, the data may be stored digitally and postpro-
Bothreinforcementvolumeandvoidcontentshallbereported.
cessed.
6.7 Specimens with low bending stiffness, or high values of
7.5 Micrometers—The micrometer(s) shall usea4to6mm
interlaminar strength, or both, may exhibit excessive bending
[0.16to0.25in.]ball-interfaceonirregularsurfacessuchasthe
of the specimen legs during flexural loading. This can create
bag-side of a laminate, and a flat anvil interface on machined
large errors in the calculated bending moment, resulting in
or very-smooth tooled surfaces. The accuracy of the instru-
unconservative strength calculations. A recommended limita-
ments shall be suitable for reading to within 1% of the sample
tion on crosshead displacement is provided in Section 12.
width and thickness. For typical specimen geometries, an
Although outside of the scope of this test method, a doubler
may be added to the legs to reduce the flexure.
7. Apparatus
7.1 Testing Machine—The testing machine shall be in con-
formance with Practices E4, and shall satisfy the following
requirements:
7.1.1 Testing Machine Configuration—The testing machine
shall have both an essentially stationary head and a movable
head.
7.1.2 Drive Mechanism—The testing machine drive mecha-
nism 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 in accordance with 11.3.
7.1.3 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 FIG. 3 Curved Beam in Four-Point Bending
D6415/D6415M − 06a (2013)
instrument with an accuracy of 625 µm [60.001 in.] is 8.3.1 A male tool is recommended for lay-up and cure to
desirable for both thickness and width measurements
...