ASTM C1468-19a

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.
Designation: C1468 − 19a
Standard Test Method for
Transthickness Tensile Strength of Continuous Fiber-
Reinforced Advanced Ceramics at Ambient Temperature
This standard is issued under the fixed designation C1468; 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* 1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
1.1 This test method covers the determination of transthick-
responsibility of the user of this standard to establish appro-
T
ness tensile strength S under monotonic uniaxial tensile
~ !
U
priate safety, health, and environmental practices and deter-
loading of continuous fiber-reinforced ceramics (CFCC) at
mine the applicability of regulatory limitations prior to use.
ambient temperature. This test method addresses, but is not
Additionalrecommendationsareprovidedin6.7andSection7.
restricted to, various suggested test specimen geometries, test
1.5 This international standard was developed in accor-
fixtures, data collection, and reporting procedures. In general,
round or square test specimens are tensile tested in the dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
direction normal to the thickness by bonding appropriate
hardware to the samples and performing the test. For a Development of International Standards, Guides and Recom-
Cartesiancoordinatesystem,the x-axisandthe y-axisareinthe mendations issued by the World Trade Organization Technical
plane of the test specimen. The transthickness direction is Barriers to Trade (TBT) Committee.
normal to the plane and is labeled the z-axis for this test
method. For CFCCs, the plane of the test specimen normally
2. Referenced Documents
containsthelargerofthethreedimensionsandisparalleltothe
2.1 ASTM Standards:
fiber layers for unidirectional, bidirectional, and woven com-
C1145 Terminology of Advanced Ceramics
posites. Note that transthickness tensile strength as used in this
C1239 Practice for Reporting Uniaxial Strength Data and
test method refers to the tensile strength obtained under
Estimating Weibull Distribution Parameters forAdvanced
monotonic uniaxial tensile loading, where “monotonic” refers
Ceramics
to a continuous nonstop test rate with no reversals from test
C1275 Test Method for Monotonic Tensile Behavior of
initiation to final fracture.
Continuous Fiber-Reinforced Advanced Ceramics with
1.2 This test method is intended primarily for use with all
Solid Rectangular Cross-Section Test Specimens at Am-
advanced ceramic matrix composites with continuous fiber
bient Temperature
reinforcement: unidirectional (1D), bidirectional (2D), woven,
C1468 Test Method for Transthickness Tensile Strength of
and tridirectional (3D). In addition, this test method also may
Continuous Fiber-Reinforced Advanced Ceramics at Am-
be used with glass (amorphous) matrix composites with 1D,
bient Temperature
2D, and 3D continuous fiber reinforcement. This test method
D3878 Terminology for Composite Materials
does not directly address discontinuous fiber-reinforced,
E4 Practices for Force Verification of Testing Machines
whisker-reinforced, or particulate-reinforced ceramics, al-
E6 Terminology Relating to Methods of Mechanical Testing
though the test methods detailed here may be equally appli-
E177 Practice for Use of the Terms Precision and Bias in
cable to these composites. It should be noted that 3D architec-
ASTM Test Methods
tures with a high volume fraction of fibers in the “z” direction
E337 Test Method for Measuring Humidity with a Psy-
may be difficult to test successfully.
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
1.3 Values are in accordance with the International System
peratures)
of Units (SI) and IEEE/ASTM SI 10.
E691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
This test method is under the jurisdiction of ASTM Committee C28 on
Advanced Ceramics and is the direct responsibility of Subcommittee C28.07 on
Ceramic Matrix Composites. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2019. Published July 2019. Originally approved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 2000. Last previous edition approved in 2019 as C1468 – 19. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
C1468-19A. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1468 − 19a
E1012 Practice for Verification of Testing Frame and Speci- 3.2.2 transthickness, n—the direction parallel to the
men Alignment Under Tensile and Compressive Axial thickness, that is, out-of-plane dimension, as identified in 1.1,
Force Application and also typically normal to the plies for 1D, 2D laminate, and
IEEE/ASTM SI 10 American National Standard for Metric woven cloth. For 3D laminates, this direction is typically taken
Practice to be normal to the thickness and associated with the “z”
direction.
3. Terminology
4. Significance and Use
3.1 Definitions:
3.1.1 The definitions of terms relating to tensile testing 4.1 This test method may be used for material development,
appearing in Terminology E6 apply to the terms used in this material comparison, quality assurance, characterization, and
test method. The definitions of terms relating to advanced design data generation.
ceramics appearing in Terminology C1145 apply to the terms
4.2 Continuous fiber-reinforced ceramic matrix composites
used in this test method. The definitions of terms relating to
generally are characterized by glass or fine grain-sized
fiber-reinforced composites appearing in Terminology D3878
(<50 µm) ceramic matrices and ceramic fiber reinforcements.
applytothetermsusedinthistestmethod.Pertinentdefinitions
CFCCs are candidate materials for high-temperature structural
as listed in Practice E1012 and Terminologies C1145, D3878,
applications requiring high degrees of corrosion and oxidation
and E6 are shown in the following with the appropriate source
resistance, wear and erosion resistance, and inherent damage
given in brackets. Terms used in conjunction with this test
tolerance, that is, toughness. In addition, continuous fiber-
method are defined as follows:
reinforced glass (amorphous) matrix composites are candidate
3.1.2 advanced ceramic, n—a highly engineered, high-
materials for similar but possibly less demanding applications.
performance, predominately nonmetallic, inorganic, ceramic
Although shear test methods are used to evaluate shear
material having specific functional attributes. [C1145]
interlaminar strength (τ , τ ) in advanced ceramics, there is
ZX ZY
–1
3.1.3 bending strain [LL ], n—the difference between the
significant difficulty in test specimen machining and testing.
strain at the surface and the axial strain. [E1012]
Improperly prepared notches can produce nonuniform stress
distribution in the shear test specimens and can lead to
3.1.4 breaking force [F], n—the force at which fracture
ambiguity of interpretation of strength results. In addition,
occurs, P , is the breaking force in units of N. [E6]
max
these shear test specimens also rarely produce a gage section
3.1.5 ceramic matrix composite (CMC), n—a material con-
that is in a state of pure shear. Uniaxially forced transthickness
sisting of two or more materials (insoluble in one another), in
tensile strength tests measure the tensile interlaminar strength
which the major, continuous component (matrix component) is
T
~S !, avoid the complications listed above, and provide infor-
U
a ceramic, while the secondary component(s) (reinforcing
mation on mechanical behavior and strength for a uniformly
component) may be ceramic, glass-ceramic, glass, metal, or
stressed material. The ultimate strength value measured is not
organic in nature. These components are combined on a
a direct measure of the matrix strength, but a combination of
macroscale to form a useful engineering material possessing
the strength of the matrix and the level of bonding between the
certain properties or behavior not possessed by the individual
fiber, fiber/matrix interphase, and the matrix.
constituents. [C1145]
4.3 CFCCs tested in a transthickness tensile test (TTT) may
3.1.6 continuous fiber-reinforced ceramic matrix composite
fail from a single dominant flaw or from a cumulative damage
(CFCC), n—a ceramic matrix composite in which the reinforc-
process; therefore, the volume of material subjected to a
ing phase consists of continuous filaments, fibers, yarn, or
uniform tensile stress for a single uniaxially forced TTT may
knitted or woven fabrics. [C1145]
be a significant factor in determining the ultimate strength of
3.1.7 gage length [L], n—the original length [L ] of that
GL
CFCCs.Theprobabilisticnatureofthestrengthdistributionsof
portion of the test specimen over which strain or change of
the brittle matrices of CFCCs requires a sufficient number of
length is determined. [E6]
test specimens at each testing condition for statistical analysis
–2
3.1.8 modulus of elasticity [FL ], n—the ratio of stress to
anddesign,withguidelinesfortestspecimensizeandsufficient
corresponding strain below the proportional limit. [E6]
numbers provided in this test method. Studies to determine the
exact influence of test specimen volume on strength distribu-
3.1.9 percent bending, n—the bending strain times 100
tions for CFCCs have not been completed. It should be noted
divided by the axial strain. [E1012]
–2 that strengths obtained using other recommended test speci-
3.1.10 tensile strength [FL ], n—the maximum tensile
mens with different volumes and areas may vary due to these
stress which a material is capable of sustaining. Tensile
volume differences.
strengthiscalculatedfromthemaximumforceduringatension
test carried to rupture and the original cross-sectional area of 4.4 The results of TTTs of test specimens fabricated to
standardized dimensions from a particular material, or selected
the test specimen. [E6]
portions of a part, or both, may not totally represent the
3.2 Definitions of Terms Specific to This Standard:
strength and deformation properties of the entire full-size end
3.2.1 fixturing, n—fixturing is referred to as the device(s)
product or its in-service behavior in different environments.
bonded to the test specimen. It is this device(s) that is actually
gripped or pinned to the load train. The fixturing transmits the 4.5 For quality control purposes, results derived from stan-
applied force to the test specimen. dardized TTT specimens may be considered indicative of the
C1468 − 19a
levels along the sides and interior of the test specimen were found to be
response of the material from which they were taken for given
uniform.
primary processing conditions and post-processing heat treat-
ments.
6. Apparatus
4.6 The strength of CFCCs is dependent on their inherent
6.1 Testing Machines—Machines used for TTT shall con-
resistance to fracture, the presence of flaws, damage accumu-
form to the requirements of Practices E4. The forces used in
lation processes, or a combination thereof.Analysis of fracture
determining tensile strength shall be accurate within 61%at
surfaces and fractography, though beyond the scope of this test
anyforcewithintheselectedforcerangeofthetestingmachine
method, is highly recommended.
as defined in Practices E4. A schematic showing pertinent
features of the TTT apparatus for two possible forcing con-
5. Interferences
figurations is shown in Figs. 1 and 2.
5.1 Test environment (vacuum, inert gas, ambient air, etc.)
6.1.1 Values for transthickness tensile strength can range a
including moisture content, for example, relative humidity,
great deal for different types of CFCC. Therefore, it is helpful
may have an influence on the measured strength. In particular,
to know an expected strength value in order to properly select
the behavior of materials susceptible to slow crack growth
a force range. Approximate transthickness tensile strength
fracture will be strongly influenced by test environment and
values (1) for several CFCCs are as follows: porous oxide/
testingrate.Testingtoevaluatethemaximumstrengthpotential
oxide composites range from 2 to 10 MPa; hot-pressed, fully
of a material should be conducted in inert environments or at
dense SiC/MAS-5 glass-ceramic composites range from 14 to
sufficiently rapid testing rates, or both, so as to minimize slow
27 MPa; Polymer Infiltrated and Pyrolyzed (PIP) SiC/SiNC
crack growth effects. Conversely, testing can be conducted in
range from 15 to 32 MPa; and hot-pressed SCS-6/Si N ranges
3 4
environments and testing modes and rates representative of
from 30 to 43 MPa.
service conditions to evaluate material performance under use
6.1.2 Foranytestingapparatus,theloadtrainwillneedtobe
conditions. When testing is conducted in uncontrolled ambient
aligned for angularity and concentricity. Alignment of the
air with the intent of evaluating maximum strength potential,
testing system will need to be measured and is detailed inA1.1
relative humidity and temperature must be monitored and
of Test Method C1275.
reported. Testing at humidity levels >65 % RH is not recom-
6.2 Gripping Devices:
mended and any deviations from this recommendation must be
6.2.1 General—Various types of gripping devices may be
reported.
used to transmit the force applied by the testing machine to the
5.2 Surface and edge preparation of test specimens can
test fixtures and into the test specimens. The brittle nature of
introducefabricationflawswhichmayhavepronouncedeffects
the matrices of CFCCs requires accurate alignment. Bending
on the measured transthickness strength (1). Machining dam-
moments can produce stresses leading to premature crack
age introduced during test specimen preparation can be either
initiation and fracture of the test specimen. Gripping devices
a random interfering factor in the determination of strength of
can be classified generally as those employing active and those
pristine material, that is, increased frequency of surface-
employing passive grip interfaces as discussed in the following
initiatedfracturescomparedtovolume-initiatedfractures,oran
inherent part of the strength characteristics. Universal or
standardized test methods of surface and edge preparation do
not exist. It should be understood that final machining steps
may, or may not, negate machining damage introduced during
the initial machining; thus, test specimen fabrication history
may play an important role in the measured strength distribu-
tions and should be reported. In addition, the nature of
fabrication used for certain composites, for example, chemical
vaporinfiltrationorhotpressing,mayrequirethetestingoftest
specimens in the as-processed condition.
5.3 Bending in uniaxial TTTs can cause or promote nonuni-
form stress distributions with maximum stresses occurring at
the test specimen edge, leading to nonrepresentative fractures.
Similarly, fracture from edge flaws may be accentuated or
suppressed by the presence of the nonuniform stresses caused
by bending.
NOTE 1—Finite element calculations were performed for the square
cross section test specimen for the forcing conditions and test specimen
thickness investigated in Reference (1). Stress levels along the four corner
edgeswerefoundtobelowerthantheinterior,exceptforthecornersatthe
bond lines where the stress was slightly higher than the interior. Stress
The boldface numbers in parentheses refers to the list of references at the end FIG. 1 Schematic Diagram of One Possible Apparatus for Con-
of this standard. ducting a Uniaxial Transthickness Tensile Test
C1468 − 19a
6.2.1.2 Passive Grip Interfaces—Passive grip interfaces
transmit the force applied by the test machine through a direct
mechanical link (4). Generally, these mechanical links transmit
the test forces to the test specimen via geometrical features of
the test fixturing. Passive grips may act through pin forcing via
pins at holes in the fixturing. Generally, close tolerances of
linear dimensions are required to promote uniform contact as
well as to provide for noneccentric forcing. In addition,
moderately close tolerances are required for center-line coin-
cidence and diameter [D] of the pins and holes.
6.3 Load Train Couplers:
6.3.1 General—Various types of devices (load train cou-
plers) may be used to attach the active or passive grip interface
assemblies to the testing machine (1, 5-7). The load train
couplers, in conjunction with the type of gripping device, play
major roles in the alignment of the load train, and thus,
subsequent bending imposed in the test specimen. Load train
couplers can be classified generally as fixed and non-fixed as
discussed in the following sections. Note that use of well-
FIG. 2 Schematic Diagram of a Second Possible Apparatus for
aligned fixed or self-aligning non-fixed couplers does not
Conducting a Uniaxial Transthickness Tensile Test
automatically guarantee low bending in the test specimen. The
type and operation of grip interfaces, as well as the as-
sections. Several additional gripping techniques are discussed
fabricated dimensions of the test specimen, can add signifi-
in Test Method C1275.
cantly to the final bending imposed in the test specimen.
6.2.1.1 Active Grip Interfaces—Active grip interfaces re-
Additional information pertaining to couplers can be found in
quire a continuous application of a mechanical, hydraulic, or
Test Method C1275.
pneumatic force to transmit the force applied by the test
6.3.1.1 Verifyalignmentofthetestingsystemasaminimum
machine to the test fixtures. Generally, these types of grip
at the beginning and end of a test series as detailed in A1.1 of
interfaces cause a force to be applied normal to the surface of
Test Method C1275, unless the conditions for verifying align-
the gripped section of the test fixturing. Transmission of the
ment additional times are met.Atest series is a discrete group
uniaxialforceappliedbythetestmachinethenisaccomplished
of tests on individual test specimens conducted within a
by friction between the test fixturing and the grip faces; thus,
discrete period of time on a particular material configuration,
important aspects of active grip interfaces are uniform contact
test specimen geometry, test condition, or other uniquely
between the gripped section of the test fixturing and the grip
definable qualifier. For example, a test series composed of
faces and constant coefficient of friction over the grip/fixture
MaterialAcomprisingtentestspecimensofGeometryBtested
interface. In addition, for active grips, uniform application of
at a fixed rate in force control to final fracture in ambient air.
gripping force and motion of the grips upon actuation are
An additional verification of alignment is recommended, al-
important factors to consider in ensuring proper gripping.
though not required, at the middle of the test series. Measure
(1) Face-forced grips, either by direct lateral pressure grip
alignment with a dummy test specimen and the alignment
faces (2) or by indirect wedge-type grip faces, act as the grip
verification procedures detailed in Test Method C1275.Allow-
interface (3). Generally, close tolerances are required for the
able bending values are discussed in 6.4. Alignment test
flatness and parallelism as well as for the wedge angle of the
specimens used for verification should be equipped with a
wedge grip faces. In addition, the thickness, flatness, and
recommended eight separate longitudinal strain gages to deter-
parallelism of the gripped section of the fixturing shall be
mine bending contributions from both concentric and angular
within similarly close tolerances to promote uniform contact at
misalignment of the grip heads. The length of the alignment
the fixture/grip interface. Tolerances will vary depending on
test specimen should be approximately the same length as the
the exact configuration.
test specimen and fixturing. Use a material (isotropic,
(2) Sufficient lateral pressure should be applied to prevent
homogeneous, continuous) with similar elastic modulus and
slippage between the grip face and the fixturing. Grip surfaces
elastic strain capability to the CFCC being tested.
that are scored or serrated with a pattern similar to that of a
single-cut file have been found satisfactory. A fine serration 6.3.2 Fixed Load Train Couplers—Fixed couplers may
appears to be the most satisfactory. The serrations should be incorporate devices which require either a one-time, pre-test
keptcleanandwelldefinedbutnotoverlysharp.Thelength[L] alignment adjustment of the load train which remains constant
and width [W] of the grip faces should be equal to or greater for all subsequent tests, or an in-situ, pre-test alignment of the
than the respective length and width of the fixturing to be load train which is conducted separately for each test specimen
gripped. and each test. Such devices (8) usually employ angularity and
(3) Grip inserts, called wedges, can be machined to accept concentricity adjusters to accommodate inherent load train
flatorroundfixturing.Thisallowsforawiderangeoffixturing misalignments. Fixed load trains have two translational de-
to be utilized. grees of freedom and three degrees of rotational freedom fixed.
C1468 − 19a
Regardless of which method is used, verify the alignment as a non-fixed load train is shown in Fig. 4, and this arrangement
discussed in 6.3.1.1. A schematic diagram of one possible corresponds to the load train identified in Fig. 2.
arrangement for a fixed load train is shown in Fig. 3, and this
NOTE 2—The use of non-fixed load train couplers allows for many test
arrangement corresponds to the load train identified in Fig. 1.
specimens to be prepared ahead of time using an alignment device. Once
6.3.2.1 Fixed load train couplers often are preferred for
the test specimens are bonded to the fixturing, they can all be tested in a
veryshortperiodoftime.Thisgreatlyincreasesthroughputandminimizes
monotonic testing CFCCs. During the fracture process, the
machine time.
fixed coupler tends to hold the test specimen in an aligned
position, and thus, provides a continuous uniform stress across 6.3.3.1 The forcing configuration shown in Fig. 4 uses
the remaining ligament of the gage section. For TTT, however, universal rod ends (sometimes called ball joint rod ends) at
this is not an issue, allowing for use of both methods. both ends of the fixtured test specimen. The universal rods
6.3.2.2 The use of fixed load train couplers typically will allow for a full range of angular motion and will allow for
require that the test specimens be bonded to the fixturing after some concentricity and angularity misalignment of the grips.A
the fixturing has been mounted in the test frame or grips. photograph showing assembly of the fixturing, test specimen,
CFCCs in general have low transthickness tensile strength, as and universal rod ends is shown in Fig. 5.
stated in 6.1.1, and this requirement will minimize the possi-
6.4 Allowable Bending—Analytical and empirical studies
bility of inducing bending when the fixturing is gripped. One
(11) have concluded that for negligible effects on the estimates
drawback to mounting the test specimen in the force frame is
of the strength distribution parameters (for example, Weibull
that it will reduce productivity. There will be a waiting period
modulus, mˆ, and characteristic strength, σ ) of monolithic
θ
astheadhesivecures.Caremustbetakentoensurethatthetest
advanced ceramics, allowable percent bending as defined in
specimen does not move on the fixturing during the cure cycle
Practice E1012 should not exceed five. Conclusions arrived at
of the adhesive.
in Ref (11) for the uniaxial tension strength along one of the
6.3.3 Non-Fixed Load Train Couplers—Non-fixed couplers
directions of reinforcement are also supposed to be valid for
may incorporate devices which promote self-alignment of the
thetransthicknesscase.Applyingtheseconclusionsforthistest
load train during the movement of the crosshead or actuator.
method (11) assumes that transthickness tensile strength frac-
Generally, such devices rely upon freely moving linkages to
tures are due to single fracture origins in the volume of the
eliminate applied moments as the load train components are
material, all test specimens experience the same level of
forced. Knife edges, universal joints, hydraulic couplers, or air
bending, and that Weibull modulus, mˆ, was constant.
bearings are examples (5, 9, 10) of such devices. Although
6.4.1 Studies of the effect of bending on the transthickness
non-fixed load train couplers are intended to be self-aligning,
tensile strength distributions of CFCCs do not exist. Until such
the operation of the couplers must be verified as discussed in
information is forthcoming for CFCCs, this test method adopts
6.3.1.1. A schematic diagram of one possible arrangement for
FIG. 4 Schematic Diagram of One Possible Arrangement for a
FIG. 3 Schematic Diagram of One Possible Arrangement for a Non-Fixed Load Train That Uses Couplers and Ball Joint Rod
Fixed Load Train End Adapters
C1468 − 19a
performing the TTT on just the adhesive. The tensile strength
of the adhesive then can be determined as described in 10.3.
6.7.2 Single-part adhesives that air cure at room tempera-
ture are the easiest to use, but generally exhibit low strength.
6.7.3 Two-part adhesives require a bulk resin, along with a
catalyst to activate curing. These adhesives demonstrate mod-
erate strength, and often require glass beads of a specific size
to produce a bond line of specific thickness for optimum
bonding.Often,thereisexcessadhesivepresentwhentryingto
ensure a complete bond line, and this can pose a problem, as
adhesive should not flow up or down the edges of the test
specimen; therefore, care should be taken in the amount of
adhesive used.
6.7.4 Single-partadhesivesthatcureatanelevatedtempera-
ture are very easy to handle and generally produce very
high-strength bonds. Several of these elevated temperature
FIG. 5 Photograph of a Transthickness Tensile Test Specimen
Bonded to Fixturing, With Fixturing Assembled with Universal
curing adhesives are produced in sheets that easily are cut to
Rod Ends (Ball Joint Rod Ends) for Improved Alignment
the desired shape using scissors or cutting blades.Atack agent
is often used to keep the film in place on the fixturing. Excess
film extending beyond the test specimen can easily be trimmed
the recommendations for tensile testing of monolithic ad-
off before the fixturing is placed in a furnace for cure. Use of
vanced ceramics and uniaxial tensile testing of CFCCs. The
thesetypesofadhesivesresultsinthesameamountofadhesive
recommended maximum allowable percent bending at the
being used during each test, thus minimizing the influence of
onset of the cumulative fracture process, for example, matrix
adhesives on transthickness strength.
cracking stress, for test specimens tested under this standard is
6.7.4.1 Adhesives that cure at an elevated temperature are
five at the anticipated fracture force.
usually sensitive to the maximum temperature; therefore,
6.5 Data Acquisition—At minimum, make an autographic
thermocouples should be attached to the fixturing (1) to ensure
recordofmaximumforce;however,itisdesirabletoalsomake
that the cure temperature is reached and maintained, and the
a record, where applicable, of applied force, crosshead
overall cure cycle is followed.
displacement, strain, and time. Use either digital data acquisi-
tion systems or analog chart recorders for this purpose, NOTE 4—Adhesives that cure at elevated temperature must reach the
cure temperature in order to be activated. Extra care should be used in
although a digital record is recommended for ease of later data
documenting that the temperature of the adhesive bond has been reached.
analysis. Recording devices shall be accurate to 1.0 % of full
It is not acceptable to simply record the temperature of the furnace and
scale. Data acquisition rates will depend on the forcing rates
assumethatthefixturingandadhesivehavereachedthesametemperature.
used to conduct the test. A data acquisition rate of at least
Improper curing of the adhesive (1) has been found to be the number one
20 Hz should be used, and the acquisition rate should be fast cause of bond line failures.
enough to capture the maximum force within 1 %.
6.7.5 Porous CFCCs may allow the adhesive to penetrate
6.6 Dimension-Measuring Devices—Micrometers and other into the interior of the CMC. Care must be taken to determine
iftheviscosityoftheadhesivewillallowittopenetrateintothe
devices used for measuring linear dimensions shall be accurate
and precise to at least one half the smallest unit to which the test specimen. For porous CFCC systems, extra material or a
spare test specimen should be bonded to blocks that are of the
individual dimension is required to be measured. For the
purposes of this test method, measure cross-sectional dimen- same material as the fixture, and then sectioned metallographi-
sions to within 0.02 mm, requiring measuring devices with cally to determine the depth of penetration of the adhesive into
accuracy of 0.01 mm. thetestspecimen.Theadhesiveshouldnotpenetratemorethan
one fiber ply or more than 10 % of the specimen thickness (6)
6.7 Adhesives—In conducting a TTT, an adhesive is re-
from each face.
quired to bond the test specimen to the fixturing, as it is not
normally possible to directly grip the test specimen. There are
6.8 Measurementofdisplacementonthickersamplescanbe
many types of adhesives available, and care should be taken to madeusingaverysmallgagelength[L ]extensometer,strain
GL
select an adhesive strong enough to conduct the test.
gages, video extensometers, or noncontacting laser extensom-
etry. No data exists to determine what effect the contacting
NOTE 3—Many adhesives contain hazardous chemicals. Manufacturers
measurement devices have on measured transthickness tensile
of adhesives routinely provide listings of the possible hazards associated
strength. Displacement measurements can be used to calculate
with particular adhesives, and commonly provide Material Safety Data
Sheets (MSDS) on their products. Read all safety handling requirements
a transthickness elastic modulus [E ] value.All displacement
ZZ
and follow the manufacturer’s recommended handling procedures. In
measurements are to be made directly on the test specimen.
general, always utilize protective face, eye, hand, and body gear. If the
adhesive produces gases, use only in vented hoods certified for those
7. Precautionary Statements
specific gases.
6.7.1 The strength of the adhesive can be evaluated by 7.1 Duringtheconductofthistestmethod,thepossibilityof
bonding the fixturing together without the test specimen and flying fragments of broken test material may be high. The
C1468 − 19a
brittle nature of advanced ceramics and the release of strain
energy contribute to the potential release of uncontrolled
fragments upon fracture. Means for containment and retention
of these fragments for later fractographic reconstruction and
analysis is highly recommended.
7.2 Exposed fibers at the edges of CFCC test specimens
present a hazard due to the sharpness and brittleness of the
NOTE 1—Faces of test specimen can be as processed or machined flat.
ceramic fiber. All persons required to handle these materials
All dimensions are in mm, and tolerances are: x.x 6 0.1, x.xx 6 0.03.
should be well informed of such conditions and the proper
handling techniques.
FIG. 6 Drawing of a Circular Cross Section Transthickness Ten-
sile Test Specimen 19.0 mm in Diameter
8. Test Specimen
8.1 Test Specimen Geometry:
8.1.1 General—The geometry of TTT specimens is depen- 8.1.1.2 Generally, circular cross section test specimens are
dent on the dimensions of the available material. For example, preferred. Test specimens using a diameter [D] of 19 mm have
if the strength of an as-fabricated component is required, then been shown to provide consistent results when compared to
the dimension of the resulting test specimen may reflect the other test specimen geometries having a similar cross-sectional
thickness and width of the component, up to limits of the area. Such test specimens generally incorporate more than two
testing machine and test fixturing available. If it is desired to unit cells of typical fiber weaves. A typical fiber weave for
evaluatepreviouslyconditionedtestspecimens,thenthesizeof CMCs is the eight harness satin weave (8HSW). An engineer-
the transthickness test specimen will be limited by the size of ing drawing of a circular cross section TTT specimen, 19 mm
the conditioned test specimen. One example of a previously in diameter, is shown in Fig. 6.
conditioned test specimen would be a tensile fatigue test 8.1.1.3 There may be instances when square or rectangular
specimen that was fatigued for a set number of cycles and the cross section test specimens may be desirable, especially when
test stopped before failure occurred.ATTT specimen could be testing sections cut out of other larger test specimens that have
machined out of the fatigued test specimen but would be been conditioned or tested using other test methods. For square
limited in size to the width of the fatigue test specimen. Size cross section test specimens, a width [W] and length [L]ofat
should not be determined without the consideration of the size least 16.8 mm have been shown to provide consistent results
of the fiber and the fiber preform architecture. when compared to other test specimen geometries having a
8.1.1.1 The following sections discuss the most common similar cross-sectional area (1). As the test specimen cross-
test specimen geometries. Test specimens must have a mini- sectional area is decreased, defects at the corners or edges may
mum cross-sectional dimension greater than the unit cell of the have more of an influence on the measured strength. For fully
fiber architecture, or a minimum of 10 mm. Any larger size is dense CFCC test specimens at least 16.8 mm square, the
acceptable if the required forces meet the machine limitations. strength appears to be controlled by the microstructure of the
Deviations from the recommended geometries may be neces- CFCC and not the geometry of the test specimen (1).An
sary depending upon the particular geometry of the available engineering drawing of a square cross section TTT specimen
material. 16.8 mm on a side is shown in Fig. 7.Adimension of 16.8 mm
NOTE 1—Faces of test specimen can be as processed or machined flat. All dimensions are in mm, and tolerances are x.x 6 0.1, x.xx 6 0.03.
FIG. 7 Drawing of a Square Cross Section Transthickness Tensile Test Specimen 16.8 mm Wide
C1468 − 19a
was selected as it is approximately the width of two unit cells both faces of the material and by using appropriate cutting and
of an 8HSW cloth woven produced with either silicon carbide grinding rates. These rates will have to be determined for each
or oxide fiber tows containing fibers with a diameter of 15 µm CFCC system.
or less. Both the circular and square cross section test speci-
8.2.5.2 The cutting and grinding should be performed in an
mens have been used and have been shown to be effective in
initial and final grinding operation using appropriate diamond
eliminating test specimen geometry effects for a fully dense
tooling. The initial rough grinding should use a material
CFCC if the cross-sectional area is maintained at approxi-
removal rate of 0.03 mm per pass and a 180 to 240 grit
mately 282 mm (1).
diamond grinding wheel for the entire initial rough grinding
process. Initial rough grinding should stop when 0.25 mm of
8.2 Test Specimen Preparation:
material remains to be removed. Final grinding should then be
8.2.1 Depending upon the intended application of the test
performed using a material removal rate of 0.015 mm per pass
results, use one of the following test specimen preparation
and a 320 to 400 grit diamond wheel. If a finer finish is
procedures. Regardless of the preparation procedure used,
requested, the 400 grit diamond wheel can be substituted with
sufficient details regarding the procedure must be reported to
a 600 grit diamond wheel.
allow replication.
8.2.5.3 TTT specimens using the circular cross section can
8.2.2 As-Fabricated—The TTT specimen should simulate
becoredrilledtoanoversizeddiameteranddiamondgroundto
the surface/edge conditions and processing route of an appli-
thefinaldimensionsusingthefinalgrindingprocedurelistedin
cation where no machining is used; for example, as-cast,
8.2.5.2.
sintered, hot-pressed, or injection-molded part. No additional
machining specifications are relevant. 8.2.5.4 Final grinding should be performed with the grind-
ing wheel rotating in a plane parallel to the plies in the x- and
NOTE 5—As-processed test specimens might possess rough surface
y-directions to avoid fraying the reinforcing ceramic fibers.
textures and non-parallel edges and may be prone to premature failure if
Machining should not be performed in the z-direction. Appro-
there are stress concentrations at the edges of the test specimen.
priate care should be taken to not damage the test specimen
8.2.3 Application-Matched Machining—The TTT specimen
during clamping of the material.
should have the same surface/edge preparation as that given to
8.2.5.5 The machined edges shall not be beveled.
the component. Unless the process is proprietary, the report
should be specific about the stages of material removal, wheel
8.3 Coated Material—CFCCs sometimes have a protective
grits, wheel bonding, amount of material removed per pass,
seal coat applied to the outer surface of the composite. In these
and type of coolant used.
instances, the coating should be removed prior to testing if
8.2.4 Customary Practices—Ininstanceswhereacustomary
determination of the transthickness tensile strength of the
machining procedure has been developed that is satisfactory
substrate CFCC is required. The procedures discussed in 8.2.2
for a class of materials (that is, it induces no unwanted
– 8.2.5 may be used to remove this exterior coating.
surface/subsurface damage or residual stresses), this procedure
8.3.1 Sometimes the seal coatings are an integral part of the
should be used.
CFCC, and the determination of the tensile adhesive strength
8.2.4.1 ItiscustomarytomachineonlytheedgesoftheTTT
between the seal coating and the substrate CFCC may be of
specimen. However, the faces can be machined to make them
interest. In this case, the seal coating should be retained.
parallel, to reduce the surface roughness, or to remove high
8.3.2 Sufficient details regarding the coating must be in-
spots.Suchmachiningwillfacilitatetheprocessofbondingthe
cluded in the report. The report should list if a seal coat was
testspecimentothefixturing.Itisimportanttonotethathigher
originally present, whether or not it was removed, and the
surface roughness may decrease bonding integrity. In addition,
procedure used to remove it if applicable.
machining the faces will generally damage fibers the surface
8.4 Handling Precaution—Exercise care in storing and
plies, and any machining of the faces should be reported.
handling finished test specimens to avoid the introduction of
8.2.5 Recommended Procedure—In instances where 8.2.2 –
random and severe flaws. In addition, give attention to pre-test
8.2.4 are not appropriate, 8.2.5 applies. Studies to evaluate the
storage of test specimens in controlled environments or desic-
machinability of CFCCs have not been completed.
cators to avoid unquantifiable environmental degradation of
NOTE 6—Several commercial machining companies were contacted to
test specimens prior to testing.
determine the optimum procedure for machining test specimens out of
CFCC material. This information has been condensed into 8.2.5. The 8.5 Number of Valid Tests—Aminimum of ten valid tests is
recommended procedure of 8.2.5 can be viewed as starting-point guide-
required for the purpose of estimating a mean. A greater
lines. A more stringent procedure may be necessary.
number of valid tests may be necessary if estimates regarding
8.2.5.1 Conduct grinding or cutting with an ample supply of the form of the strength distribution are required. The number
appropriate filtered coolant to keep the test material and of valid tests required by this test method has been established
grindingwheelconstantlyfloodedandparticlesflushed.Grind- with the intent of determining not only reasonable confidence
ing can be done in at least two stages, ranging from coarse to limits on strength distribution parameters, but also to discern
fine rate of material removal. All cutting can be done in one multiple fracture mechanisms. If material cost or test specimen
stage appropriate for the depth of cut. Care must be taken availability limits the number of tests to be conducted, a
during cutting and grinding to avoid “fraying” the edges of the minimum of three valid tests can be conducted to determine an
test specimen. Fraying can be avoided by supporting one or indication of material properties.
C1468 − 19a
8.6 Valid Tests—A valid individual test is one which meets 8.8 Bonding of Test Specimens to Fixturing:
all the following requirements: all the test requirements of this
8.8.1 It is extremely hard to grip a test specimen directly to
test method, and failure occurs within the test specimen (not at
conduct a TTT. Therefore, fixturing must be bonded to the test
the test specimen adhesive interface, or at any point or fraction
specimens. This fixturing is then gripped or connected to the
of the adhesive interface).
load train by pins and couplers. If fixed gripping is utilized,
then the test specimen normally is bonded to the fixturing
8.7 Test Specimen Dimensions—Conduct 100 % inspection/
directly in the force frame as discussed in 6.3.2.2. Engineering
measurements of all test specimens and test specimen dimen-
drawings for fixturing that accepts a 19.0-mm circular,
sions to ensure compliance with the drawing specifications.
16.8-mm square, and 10-mm square cross section test speci-
Generally, high-resolution optical methods or high-resolution
menareshowninFigs.8-10,respectively.Thedrawingsarefor
digital point contact methods are satisfactory as long as the
a pin and clevis arrangement, but can be easily modified to
equipment meets the specifications in 6.6.
acceptauniversalrodend.Itisrecommendedthatthefixturing
8.7.1 Determinethethicknessandwidthordiameterofeach
be made out of stainless steel to minimize oxidation during
test specimen to within 0.02 mm. Measurements should be
adhesive cure or adhesive removal.
made on at least three different cross-sectional planes at
8.8.1.1 Thoroughly clean the mounting surfaces of the
equally spaced locations around the test specimen. To avoid
fixture. Adhesive remaining on the fixturing can easily be
damage in the critical gage section area, make these measure-
removed by an intermediate temperature heat treatment to char
ments either optically or mechanically using a flat, anvil-type
the adhesive or a diamond honing stick. After the adhesive is
micrometer. In either case, the resolution of the instrument
removed, thoroughly clean the fixturing. In some cases, a very
shallbeasspecifiedin6.6.Exerciseextremecautiontoprevent
light sand blasting may be used to clean the mounting surface.
damage to the test specimen edges. Ball-tipped or sharp-anvil
Exercise care in using sandblasting, as it will slowly erode the
micrometers are not recommended because edge damage can
fixturing. The fixturing will need to be refaced if they get out
be induced. Record and report the measured dimensions and
of tolerance. Once all adhesive and residue are removed,
locations of the measurements for use in the calculation of the
thoroughly clean the bonding faces of the fixturing using
tensile stress. Use the average of the multiple
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