ASTM C1899-21

Standard Test Method for Flexural Strength of Continuous Fiber-Reinforced Advanced Ceramic Tubular Test Specimens at Ambient Temperature

SIGNIFICANCE AND USE
5.1 This test method may be used for material development, material comparison, quality assurance, characterization, and design data generation.  
5.2 Continuous fiber-reinforced ceramic composites (CFCCs) may be composed of continuous ceramic-fiber directional (1D, 2D, and 3D) reinforcements which are often contained in a fine-grain-sized (  
5.3 CFCC components have distinctive and synergistic combinations of material properties, interface coatings, porosity control, composite architecture (1D, 2D, and 3D), and geometric shape that are generally inseparable. Prediction of the mechanical performance of CFCC tubes (particularly with braid and 3D weave architectures) may not be possible by applying measured properties from flat CFCC plates to the design of tubes. This is because fabrication/processing methods may be unique to tubes and not replicable to flat plates, thereby producing compositionally similar but structurally and morphologically different CFCC materials. In particular, tubular components comprised of CFCC material form a unique synergistic combination of material, geometric shape, and reinforcement architecture that is generally inseparable. In other words, prediction of mechanical performance of CFCC tubes generally cannot be made by using properties measured from flat plates. Strength tests of transversely loaded CFCC tubes provide information on mechanical behavior and strength for a material subjected to a uniaxial, nonuniform stress.  
5.4 Unlike monolithic advanced ceramics that fracture catastrophically from a single dominant flaw, CMCs generally experience “graceful” fracture from a cumulative damage process. Therefore, while the volume of material subjected to a nonuniform, uniaxial flexural stress for transversely loaded tube test may be a significant factor for determining matrix cracking stress, this same volume may not be as significant a factor in determining the ultimate strength of a CMC. However, the probabilistic nature of the stre...
SCOPE
1.1 This test method covers the determination of flexural strength, including stress-strain response, under monotonic loading of continuous fiber-reinforced advanced ceramic tubes at ambient temperature. This test method addresses tubular test specimen geometries, test specimen/grip fabrication methods, testing modes (force, displacement, or strain-control), testing rates (force rate, stress rate, displacement rate, or strain rate), and data collection and reporting procedures.  
1.2 In this test method, an advanced ceramic composite tube/cylinder with a defined gage section and a known wall thickness is subjected to four-point flexure while supported in a four-point loading system utilizing two force-application points spaced an inner span distance that are centered between two support points located an outer span distance apart. The applied transverse force produces a constant moment in the gage section of the tube and results in uniaxial flexural stress-strain response of the composite tube that is recorded until failure of the tube. The flexural strength and the flexural fracture strength are determined from the resulting maximum force and the force at fracture, respectively. The flexural strains, the flexural proportional limit stress, and the flexural modulus of elasticity in the longitudinal direction are determined from the stress-strain data. Note that flexural strength as used in this test method refers to the maximum tensile stress produced in the longitudinal direction of the tube by the introduction of a monotonically applied transverse force, where ‘monotonic’ refers to a continuous, nonstop test rate without reversals from test initiation to final fracture. The flexural strength is sometimes used to estimate the tensile strength of the material.  
1.3 This test method is intended for advanced ceramic matrix composite tubes with continuous fiber reinforcement: unidirectional (1D, filament wound and tape l...

General Information

• Status: Published
• Publication Date: 30-Jun-2021
• ICS: 81.060.30 - Advanced ceramics
• Technical Committee: C28 - Advanced Ceramics
• Drafting Committee: C28.07 - Ceramic Matrix Composites

Relations

• Refers: ASTM D3878-19a - Standard Terminology for Composite Materials
• Effective Date: 15-Oct-2019
• Refers: ASTM C1145-19 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM C1683-10(2019) - Standard Practice for Size Scaling of Tensile Strengths Using Weibull Statistics for Advanced Ceramics
• Effective Date: 01-Jul-2019
• Refers: ASTM D3878-19 - Standard Terminology for Composite Materials
• Effective Date: 15-Apr-2019
• Refers: ASTM C1239-13(2018) - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Jul-2018
• Refers: ASTM D3878-18 - Standard Terminology for Composite Materials
• Effective Date: 01-Apr-2018
• Refers: ASTM D3878-16 - Standard Terminology for Composite Materials
• Effective Date: 01-Aug-2016
• Refers: ASTM D3878-15 - Standard Terminology for Composite Materials
• Effective Date: 01-Jul-2015
• Refers: ASTM C1683-10(2015) - Standard Practice for Size Scaling of Tensile Strengths Using Weibull Statistics for Advanced Ceramics
• Effective Date: 01-Jan-2015
• Refers: ASTM E4-14 - Standard Practices for Force Verification of Testing Machines
• Effective Date: 01-Jun-2014
• Refers: ASTM C1684-13 - Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature—Cylindrical Rod Strength
• Effective Date: 01-Aug-2013
• Refers: ASTM C1684-13e1 - Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature—Cylindrical Rod Strength
• Effective Date: 01-Aug-2013
• Refers: ASTM C1239-13 - Standard Practice for Reporting Uniaxial Strength Data and Estimating Weibull Distribution Parameters for Advanced Ceramics
• Effective Date: 01-Aug-2013
• Refers: ASTM D3878-07(2013) - Standard Terminology for Composite Materials
• Effective Date: 01-May-2013
• Refers: ASTM C1145-06(2013)e1 - Standard Terminology of Advanced Ceramics
• Effective Date: 01-Feb-2013

Overview

ASTM C1899-21 is the internationally recognized standard test method for determining the flexural strength of continuous fiber-reinforced advanced ceramic tubular specimens at ambient temperature. Developed by ASTM, this method provides a consistent approach for evaluating the mechanical properties of ceramic matrix composite (CMC) tubes, particularly those reinforced with continuous fibers in various orientations (1D, 2D, and 3D). The test is essential for applications in material development, comparison, quality control, and design data generation.

Due to the unique synergy between material composition, fiber architecture, and tubular geometry in continuous fiber-reinforced ceramic composites (CFCCs), their properties are highly dependent on the actual tube form. This standard highlights that mechanical performance of tubular CFCCs cannot be reliably inferred from flat specimens or plates, emphasizing the need for tube-specific testing.

Key Topics

• Test Scope: Covers determination of flexural strength and stress-strain response of advanced ceramic matrix composite tubes under monotonic loading at room temperature.
• Test Methodology: Utilizes a four-point flexure setup, with the tube supported at two points and loaded at two additional points, producing uniaxial flexural stress until failure.
• Specimen Flexural Response: Measures properties such as flexural strength, fracture strength, modulus of elasticity, and proportional limit.
• Composite Architecture: Applicable to a wide range of tube architectures and fiber reinforcement types, including 1D (filament wound), 2D (woven), and 3D (braided/weave).
• Unique Material Behavior: CMC tubes often exhibit "graceful" cumulative damage fracture, in contrast to catastrophic failure in monolithic ceramics, underlining the importance of appropriate testing.
• Data Collection: Specifies recommendations for test speeds, specimen dimensions and preparation, strain measurement techniques, and reporting procedures.
• Design and Statistical Analysis: Addresses the probabilistic nature of ceramic composite strength and the need for statistical treatment in test data.

Applications

• Material Development: CFCC tubes are increasingly used in high-performance applications thanks to their high strength, toughness, and damage tolerance. This standard enables reliable comparison and optimization of new materials.
• Design Data Generation: Data collected via ASTM C1899-21 support component design and structural integrity assessment, particularly in industries like aerospace, energy, and advanced manufacturing where tubular ceramics are employed.
• Quality Assurance: Consistent test methodology ensures manufacturers and users can verify that products meet specified strength requirements before deployment.
• Comparative Studies: Provides a reliable foundation for benchmarking performance between different ceramic matrix composites, processing methods, or fiber architectures.

Related Standards

• ASTM C1684 - Flexural Strength of Advanced Ceramics at Ambient Temperature - Cylindrical Rod Strength
• ASTM C1683 - Size Scaling of Tensile Strengths Using Weibull Statistics for Advanced Ceramics
• ASTM D3878 - Terminology for Composite Materials
• ASTM E4 - Practices for Force Verification of Testing Machines
• ASTM E83 - Practice for Verification and Classification of Extensometer Systems
• ASTM C1145 - Terminology of Advanced Ceramics

These related ASTM standards provide additional terminology, specimen preparation guidelines, and testing practices for advanced ceramic and composite materials, complementing the methods established under ASTM C1899-21.

---

Keywords: ASTM C1899-21, ceramic matrix composite tubes, flexural strength testing, continuous fiber-reinforced ceramic composites, mechanical properties, quality assurance, advanced ceramics, 4-point flexure test, CMC tubular specimens, material development.

Standards Content (Sample)

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: C1899 − 21
Standard Test Method for
Flexural Strength of Continuous Fiber-Reinforced Advanced
Ceramic Tubular Test Specimens at Ambient Temperature
This standard is issued under the fixed designation C1899; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope posites can be composed of a wide range of ceramic fibers
(oxide, graphite, carbide, nitride, and other compositions) in a
1.1 This test method covers the determination of flexural
wide range of crystalline and amorphous ceramic matrix
strength, including stress-strain response, under monotonic
compositions (oxide, carbide, nitride, carbon, graphite, and
loading of continuous fiber-reinforced advanced ceramic tubes
other compositions). This test method may also be applicable
atambienttemperature.Thistestmethodaddressestubulartest
to some types of functionally graded tubes such as ceramic
specimen geometries, test specimen/grip fabrication methods,
fiber-wound tubes comprised of monolithic advanced ceram-
testing modes (force, displacement, or strain-control), testing
ics. It is not the intent of this test method to dictate or
rates (force rate, stress rate, displacement rate, or strain rate),
normalize material fabrication including fiber layup or number
and data collection and reporting procedures.
of plies comprising the composite, but to instead provide an
1.2 In this test method, an advanced ceramic composite
appropriate and consistent methodology for discerning the
tube/cylinder with a defined gage section and a known wall
effects of different fabrication or fiber layup methods on
thickness is subjected to four-point flexure while supported in
flexural behavior of resulting tubular geometries.
a four-point loading system utilizing two force-application
1.4 Thistestmethoddoesnotdirectlyaddressdiscontinuous
points spaced an inner span distance that are centered between
fiber-reinforced, whisker-reinforced, or particulate-reinforced
two support points located an outer span distance apart. The
ceramics, although the test methods detailed here may be
applied transverse force produces a constant moment in the
equally applicable to these composites if it can be shown that
gage section of the tube and results in uniaxial flexural
these materials display the damage-tolerant behavior of con-
stress-strain response of the composite tube that is recorded
tinuous fiber-reinforced ceramics.
until failure of the tube. The flexural strength and the flexural
fracture strength are determined from the resulting maximum
1.5 Thetestmethodisapplicabletoarangeoftestspecimen
force and the force at fracture, respectively. The flexural
tubegeometriesbasedontheintendedapplicationthatincludes
strains, the flexural proportional limit stress, and the flexural
composite material property and tube radius. Therefore, there
modulus of elasticity in the longitudinal direction are deter-
isno“standard”testspecimengeometryforatypicaltestsetup.
minedfromthestress-straindata.Notethatflexuralstrengthas
Lengths of the composite tube, lengths of the inner span, and
used in this test method refers to the maximum tensile stress
lengthsoftheouterspanaredeterminedsoastoprovideagage
produced in the longitudinal direction of the tube by the
length with uniform bending moment. A wide range of com-
introduction of a monotonically applied transverse force,
binations of material properties, tube radii, wall thicknesses,
where ‘monotonic’ refers to a continuous, nonstop test rate
tube lengths, and lengths of inner and outer spans section are
without reversals from test initiation to final fracture. The
possible.
flexural strength is sometimes used to estimate the tensile
1.5.1 This test method is specific to ambient temperature
strength of the material.
testing.Elevatedtemperaturetestingrequireshigh-temperature
furnaces and heating devices with temperature control and
1.3 This test method is intended for advanced ceramic
measurement systems and temperature-capable testing meth-
matrix composite tubes with continuous fiber reinforcement:
ods that are not addressed in this test method.
unidirectional (1D, filament wound and tape lay-up), bidirec-
tional (2D, fabric/tape lay-up and weave), and tridirectional
1.6 This test method addresses tubular test specimen
(3D, braid and weave). These types of ceramic matrix com-
geometries, test specimen preparation methods, testing rates
(that is, induced applied moment rate), and data collection and
reporting procedures in the following sections:
This test method is under the jurisdiction of ASTM Committee C28 on
Scope Section 1
Advanced Ceramics and is the direct responsibility of Subcommittee C28.07 on
Referenced Documents Section 2
Ceramic Matrix Composites.
Terminology Section 3
Current edition approved July 1, 2021. Published August 2021. DOI: 10.1520/
Summary of Test Method Section 4
C1899-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1899 − 21
3.1.1 The definitions of terms relating to flexural testing
Significance and Use Section 5
Interferences Section 6
appearing in Terminology E6 apply to the terms used in this
Apparatus Section 7
test method. The definitions of terms relating to advanced
Hazards Section 8
ceramics appearing in Terminology C1145 apply to the terms
Test Specimens Section 9
Test Procedure Section 10
used in this test method. The definitions of terms relating to
Calculation of Results Section 11
fiber-reinforced composites appearing in Terminology D3878
Report Section 12
applytothetermsusedinthistestmethod.Pertinentdefinitions
Precision and Bias Section 13
Keywords Section 14
as listed in Practice E1012 and Terminologies C1145, D3878,
Appendixes
and E6 are shown in the following with the appropriate source
Overview of Flexural Test Configurations Appendix X1
Fixtures with Cradles Appendix X2 given in parentheses. Additional terms used in conjunction
with this test method are defined in the following:
1.7 Values expressed in this test method are in accordance
3.1.2 advanced ceramic, n—a highly engineered, high-
withtheInternationalSystemofUnits(SI)andIEEE/ASTMSI
performance, predominantly nonmetallic, inorganic, ceramic
10.
material having specific functional attributes. (C1145)
1.8 This standard does not purport to address all of the
3.1.3 breaking force [F], n—the force at which fracture
safety concerns, if any, associated with its use. It is the
occurs. (E6)
responsibility of the user of this standard to establish appro-
3.1.4 ceramic matrix composite (CMC), n—a material con-
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. sisting of two or more materials (insoluble in one another) in
whichthemajor,continuouscomponent(matrixcomponent)is
Specific hazard statements are given in Section 8.
1.9 This international standard was developed in accor- a ceramic, while the secondary component(s) (reinforcing
component) may be ceramic, glass-ceramic, glass, metal, or
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the organic in nature. These components are combined on a
Development of International Standards, Guides and Recom- macroscale to form a useful engineering material possessing
mendations issued by the World Trade Organization Technical certain properties or behavior not possessed by the individual
Barriers to Trade (TBT) Committee. constituents. (C1145)
3.1.5 continuous fiber-reinforced ceramic matrix composite
2. Referenced Documents
(CFCC), n—aceramicmatrixcompositeinwhichthereinforc-
2.1 ASTM Standards: ing phase consists of a continuous fiber, continuous yarn, or a
C1145Terminology of Advanced Ceramics
woven fabric. (C1145)
–2
C1239Practice for Reporting Uniaxial Strength Data and
3.1.6 flexural fracture strength [FL ], n—theflexuralstress
Estimating Weibull Distribution Parameters forAdvanced
at the moment induced when the material breaks.
Ceramics
3.1.6.1 Discussion—The flexural fracture strength defined
C1683Practice for Size Scaling of Tensile Strengths Using
heredoesnotaccountforthenonlinearstress-strainresponseof
Weibull Statistics for Advanced Ceramics
a material beyond the proportional limit and therefore, in its
C1684Test Method for Flexural Strength of Advanced
simplicity, may not represent the actual strength potential of
Ceramics at Ambient Temperature—Cylindrical Rod
that material.
Strength
–2
3.1.7 flexural strength [FL ], n—the maximum tensile
D3878Terminology for Composite Materials
component of flexural stress which a material is capable of
E4Practices for Force Verification of Testing Machines
sustaining.
E6Terminology Relating to Methods of MechanicalTesting
3.1.7.1 Discussion—Flexuralstrengthiscalculatedfromthe
E83Practice for Verification and Classification of Exten-
maximum bending moment induced during a flexural test
someter Systems
carried to rupture and the original cross-sectional dimensions
E337Test Method for Measuring Humidity with a Psy-
of the test specimen. The flexural strength defined here does
chrometer (the Measurement of Wet- and Dry-Bulb Tem-
not account for the nonlinear stress-strain response of a
peratures)
material beyond the proportional limit and therefore, in its
E1012Practice for Verification of Testing Frame and Speci-
simplicity, may not represent the actual strength potential of
men Alignment Under Tensile and Compressive Axial
that material.
Force Application
3.1.8 four-point- ⁄4-point flexure, n—configuration of flex-
IEEE/ASTM SI 10American National Standard for Metric
uralstrengthtestingwhereaspecimenissymmetricallyloaded
Practice
attwolocationsthataresituatedonequarteroftheoverallspan
away from the outer two support bearings. (C1145)
3. Terminology
3.1.9 gage length [L], n—the original length of that portion
3.1 Definitions:
of the specimen over which strain or change of length is
determined. (E6)
For referenced ASTM standards, visit the ASTM website, www.astm.org, or –2
3.1.10 matrix cracking stress [FL ], n—the applied tensile
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
–2
stress at which the matrix cracks into a series of roughly
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. parallel blocks normal to the tensile stress. (C1145)
C1899 − 21
3.1.10.1 Discussion—In some cases, the matrix cracking 4. Summary of Test Method
stress may be indicated on the stress-strain curve by deviation
4.1 In this test method, a composite tube/cylinder with
from linearity (proportional limit) or incremental drops in the
knownwallthicknessandsupportedoveranouterloadingspan
stress with increasing strain. In other cases, especially with
is loaded transversely over an inner loading span. The mono-
materials which do not possess a linear portion of the stress-
tonically applied transverse force results in a uniaxial, nonuni-
strain curve, the matrix cracking stress may be indicated as the
form flexural stress-strain response of the composite tube that
first stress at which a permanent offset strain is detected in the
is recorded until failure of the tube. The ultimate flexural
unloading stress-strain (elastic limit). (C1145)
strength and the fracture flexural strength are determined from
–2
the resulting maximum transverse force and the transverse
3.1.11 modulus of elasticity [FL ], n—the ratio of stress to
force at fracture, respectively. The flexural strains, the propor-
corresponding strain below the proportional limit. (E6)
tional limit flexural stress, and the modulus of elasticity in the
–3
3.1.12 modulus of resilience [FLL ], n—strain energy per
longitudinal direction are determined from the flexural stress-
unitvolumerequiredtoelasticallystressthematerialfromzero
strain data.
totheproportionallimitindicatingtheabilityofthematerialto
4.2 Flexuralstrengthasusedinthistestmethodreferstothe
absorb energy when deformed elastically and return it when
maximum tensile stress produced in the longitudinal direction
unloaded. (C1145)
of the tube by the introduction of a monotonically applied
–3
3.1.13 modulus of toughness [FLL ], n—strain energy per transverseforce.Monotonicreferstoacontinuous,nonstoptest
unit volume required to stress the material from zero to final rate without reversals from test initiation to final fracture.
fracture indicating the ability of the material to absorb energy
4.3 This test method is applicable to a range of test
beyond the elastic range (that is, damage tolerance of the
specimen tube geometries based on a nondimensional param-
material).
eter (β) that includes composite material properties, tube
3.1.13.1 Discussion—Themodulusoftoughnesscanalsobe
radius, and wall thickness. Therefore, there is no “standard”
referred to as the cumulative damage energy and as such is
test specimen geometry for a typical test setup. Lengths of the
regardedasanindicationoftheabilityofthematerialtosustain
composite tube and other test specimen parameters are deter-
damage rather than as a material property. Fracture mechanics
mined so as to provide an inner span length as a gage length
methods for the characterization of CMCs have not been that is subjected to a constant moment that results in a uniaxial
developed. The determination of the modulus of toughness as
but nonuniform flexural stress in the gage section. A range of
provided in this test method for the characterization of the combinations of material properties, tube radii, wall
cumulative damage process in CMCs may become obsolete thicknesses, tube lengths, inner gage lengths, and outer gage
whenfracturemechanicsmethodsforCMCsbecomeavailable. lengths are possible. It is not the intent of this test method to
(C1145) dictate or normalize material fabrication including fiber layup
or number of plies comprising the composite, but to instead
3.1.14 monotonic, adj—acontinuous,nonstoptestratewith-
provideanappropriateandconsistentmethodologyfordiscern-
out reversals from test initiation to final fracture.
ingtheeffectsofdifferentfabricationorfiberlayupmethodson
–2
3.1.15 proportional limit [FL ], n—the greatest stress that flexural behavior of resulting tubular geometries.
a material is capable of sustaining without any deviation from
5. Significance and Use
proportionality of stress to strain (Hooke’s law).
5.1 Thistestmethodmaybeusedformaterialdevelopment,
3.1.15.1 Discussion—Many experiments have shown that
material comparison, quality assurance, characterization, and
valuesobservedfortheproportionallimitvarygreatlywiththe
design data generation.
sensitivity and accuracy of the testing equipment, eccentricity
of loading, the scale to which the stress-strain diagram is
5.2 Continuous fiber-reinforced ceramic composites
plotted, and other factors. When determination of proportional
(CFCCs) may be composed of continuous ceramic-fiber direc-
limit is required, the procedure and sensitivity of the test
tional (1D, 2D, and 3D) reinforcements which are often
equipment should be specified. (E6)
contained in a fine-grain-sized (<50 µm) ceramic matrix with
controlled porosity. Usually these composites have an engi-
3.1.16 slow crack growth, n—subcritical crack growth (ex-
neered thin (0.1 to 10 µm) interface coating on the fibers to
tension) which may result from, but is not restricted to, such
produce crack deflection and fiber pull-out.
mechanisms as environmentally assisted stress corrosion or
diffusive crack growth. (C1145) 5.3 CFCC components have distinctive and synergistic
combinations of material properties, interface coatings, poros-
3.1.17 transverse loading, n—forces applied perpendicular
ity control, composite architecture (1D, 2D, and 3D), and
tothelongitudinalaxisofamember.Transverseloadingcauses
geometric shape that are generally inseparable. Prediction of
the member to bend and deflect from its original position, with
the mechanical performance of CFCC tubes (particularly with
internal tensile and compressive strains accompanying the
braid and 3D weave architectures) may not be possible by
change in curvature of the member. Also called flexural
applying measured properties from flat CFCC plates to the
loading.
design of tubes. This is because fabrication/processing meth-
3.1.18 unit cell size, n—thesmallestsectionoffabric-weave
ods may be unique to tubes and not replicable to flat plates,
architecture required to repeat the textile pattern. thereby producing compositionally similar but structurally and
C1899 − 21
morphologically different CFCC materials. In particular, tubu- ence of flaws, damage accumulation processes, or combina-
lar components comprised of CFCC material form a unique tions thereof. Analyses of fracture surfaces and fractography,
synergistic combination of material, geometric shape, and
though beyond the scope of this test method, are highly
reinforcement architecture that is generally inseparable. In recommended.
other words, prediction of mechanical performance of CFCC
tubes generally cannot be made by using properties measured
6. Interferences
from flat plates. Strength tests of transversely loaded CFCC
6.1 Inherent variability in constituents and their properties;
tubesprovideinformationonmechanicalbehaviorandstrength
variations in material fabrication practices, fiber alignment,
for a material subjected to a uniaxial, nonuniform stress.
delamination, and internal porosity; and damage induced by
5.4 Unlikemonolithicadvancedceramicsthatfracturecata-
improper specimen machining are all known causes of data
strophically from a single dominant flaw, CMCs generally
scatter in CMCs.
experience “graceful” fracture from a cumulative damage
6.2 Test environment (vacuum, inert gas, ambient air, etc.),
process.Therefore,whilethevolumeofmaterialsubjectedtoa
including moisture content (for example, relative humidity),
nonuniform, uniaxial flexural stress for transversely loaded
may have an influence on the measured flexural strength. In
tube test may be a significant factor for determining matrix
particular, the behavior of materials susceptible to slow crack
cracking stress, this same volume may not be as significant a
factor in determining the ultimate strength of a CMC. growthfracturewillbestronglyinfluencedbytestenvironment
and testing rate. Conduct testing to evaluate the maximum
However, the probabilistic nature of the strength distributions
of the brittle matrices of CMCs requires a statistically signifi- strength potential of a material in inert environments or at
cant number of test specimens for statistical analysis and sufficiently rapid testing rates, or both, so as to minimize slow
design. Studies to determine the exact influence of test speci- crack growth effects. Conversely, testing can be conducted in
men volume on strength distributions for CMCs have not been
environments and testing modes and rates representative of
completed. It should be noted that tensile flexural strengths
service conditions to evaluate material performance under use
obtained using different recommended test specimens with
conditions.When testing is conducted in uncontrolled ambient
different volumes of material in the gage sections may be
air with the intent of evaluating maximum strength potential,
different due to these volume effects. Practice C1683 provides
monitor and report relative humidity and temperature. Testing
guidance on the scaling of statistical parameters for strength to
at humidity levels >65% relative humidity (RH) is not
account for differences in effective volume, effective area, or
recommended. Report any deviations from this recommenda-
both.
tion.
5.5 Flexural strength tests provide information on the
6.3 Surface preparation of test specimens, although nor-
strength and deformation of materials under stresses induced
mally not considered a major concern in CMCs, can introduce
from transverse loading of tubes. Nonuniform but uniaxial
fabrication flaws that may have pronounced effects on flexural
stressstatesareinherentinthesetypesoftests,andsubsequent
stress mechanical properties and behavior (for example, shape
evaluation of any nonlinear stress-strain behavior must take
and level of the resulting stress-strain curve, tensile flexural
into account the asymmetric and anisotropic behavior of the
strengthandstrain,proportionallimitflexuralstressandstrain,
CMC under multiaxial stressing. This nonlinear behavior may
etc.). Machining damage introduced during test specimen
develop as the result of cumulative damage processes (for
preparation can be either a random interfering factor in the
example, matrix cracking, matrix/fiber debonding, fiber
determination of ultimate strength of pristine material (that is,
fracture, delamination, etc.) which may be influenced by
increased frequency of surface-initiated fractures compared to
testing mode, testing rate, processing effects, or environmental
volume-initiated fractures) or an inherent part of the strength
effects. Some of these effects may be consequences of stress
characteristics to be measured. Surface preparation can also
corrosion or subcritical (slow) crack growth that can be
lead to the introduction of residual stresses. Universal or
minimized by testing at sufficiently rapid rates as outlined in
standardizedtestmethodsofsurfacepreparationdonotexist.It
this test method.
should be understood that final machining steps may or may
5.6 The results of flexural strength tests of test specimens
not negate machining damage introduced during the initial
fabricated to standardized dimensions from a particular mate-
machining.Thus,testspecimenfabricationhistorymayplayan
rial or selected portions of a part, or both, may not totally
important role in the measured strength distributions and
represent the strength and deformation properties of the entire,
should be reported. In addition, the nature of fabrication used
full-size end product or its in-service behavior in different
forcertaincomposites(forexample,chemicalvaporinfiltration
environments.
orhotpressing)mayrequirethetestingoftestspecimensinthe
as-processed condition (that is, it may not be possible to
5.7 For quality control purposes, results derived from stan-
machine the test specimen faces).
dardized flexural strength test specimens may be considered
indicativeoftheresponseofthematerialfromwhichtheywere
6.4 Uniaxial flexural tests inherently produce nonuniform
taken for, given primary processing conditions and post-
stress distributions with maximum and minimum stresses
processing heat treatments.
occurring at the surface of the test specimen, leading to
fracturesoriginatingatsurfacesorneargeometricaltransitions.
5.8 The flexural behavior and flexural strength of a CMC
are dependent on its inherent resistance to fracture, the pres- In addition, when deformations or strains are measured at
C1899 − 21
surfaceswheremaximumorminimumstressesoccur,measure-
ment of strains will depend on the location of the strain-
measuringdeviceonthetestspecimen.Similarly,fracturefrom
surface flaws may be accentuated or suppressed by the pres-
ence of the nonuniform stresses caused by bending.
6.5 Fractures that initiate outside the inner load span (de-
fined as the gage section of the test specimen and subjected to
a constant moment) may be due to factors such as stress
concentrations or geometrical transitions, extraneous stresses
introduced by fixtures/load apparatuses, or strength-limiting
features in the microstructure of the specimen. Because such
non-gage section fractures will usually constitute invalid tests,
provide an explanation when differentiating between valid and
invalid tests.
6.6 Flexural testing of a tube can produce a tensile stress at
both the outer fiber of the outer diameter and seemingly on the
inner diameter as well. Therefore, there is a probability of
failureinitiationoccurringattheinnerdiameterofthetube.For
simplicity, calculations of stress in this test method implicitly
assumethatfailurewillinitiateattheouterfiber.However,itis
also possible that like failure could occur wherever there is a
sufficient axial tensile stress for such failure. This is a particu- FIG. 1 Schematic Diagram of One Possible Apparatus for Apply-
ing a Transverse Force to a Flexure Test Fixture for Conducting
larly important consideration if the outer diameter is machined
a Flexural Strength Test of a CMC Tube
during fabrication and the inner diameter is not.
(for illustration purposes only)
6.7 Dimensionsofas-fabricatedtubesmayproducegeomet-
ric dimensions and shapes (for example, noncircular cross
sections) that do not fit the assumptions of the stress calcula-
tions. Depending on the level of deviations from these
assumptions, these may need to be accounted for in the
subsequentinterpretationofthematerialbehaviorandresulting
strength calculations.
6.8 Nonlinear material behavior beyond the proportional
limit makes the definitions of flexural strength and flexural
fracture strength based on linear behavior overly simplistic.
Therefore, additional analyses to account for the nonlinear
behavior and its effect on the determination of the “true”
flexural strength and “true” flexural fracture strength may be
FIG. 2 Details of Terms Used to Calculate Applied Moment
necessary but are beyond the scope of this test method.
Examples of flexure test setups applied to CMC tubular test
7. Apparatus
specimens are contained in Appendix X1 and shown in Figs.
7.1 Testing Machines—Machines used for applying trans-
X1.1-X1.4.
verse forces to test fixtures for flexural strength testing shall
7.2.2 The only flexure test configuration used in this test
conformtotherequirementsofPracticesE4.Theforceusedto
method is four-point- ⁄4-point. The inner span (IS) is deter-
inducethetransverseforceshallbeaccuratetowithin 61%at
mined from analytical calculations based on test material
anyforcewithintheselectedforcerangeofthetestingmachine
propertiesandtubedimensionssuchasthetubeouterdiameter
asdefinedinPracticesE4.Anillustrationalschematicshowing
(OD). Once the IS is determined, the outer span (OS) is
pertinent features of the flexural strength testing apparatus is
determined as twice the inner span. Fig. 2 illustrates a
shown in Fig. 1.
four-point flexure test setup with nomenclature.
7.3 Test Spans:
7.2 Fixtures:
7.3.1 Based on previous studies (1, 2) inner span and outer
7.2.1 General—Flexural test fixtures are generally com-
span can be estimated from a material/geometry parameter
posed of two parts: (1) self-contained flexure test fixture with
such that:
two components: movable inner span assembly guided by a
fixed outer span assembly, and (2) attachments to the test
machine such as a threaded push rod attached to the movable 3
The boldface numbers in parentheses refer to a list of references at the end of
inner span and a flat platen on which the flexure test rests. this standard.
C1899 − 21
IS .9⁄ β IS-2×(cradle length) to minimize effects of the loading point
contacts within the inner span.
3 1 2 v
~ !
β 5Œ (1) 7.5.2 Strain Gages—Alternatively, strain can also be deter-
tube 2 2
~r ! t
i
mined directly from strain gages. Strain gages should be
OS 52 3IS
centered in the constant moment section of the flexure test
specimen bounded by the two loading points of the inner span.
where:
Maximum length of the longitudinal strain gage should be
v = Poisson’s ratio of test material,
tube
IS-2×(cradle length) to minimize effects of the loading point
r = inner radius of tubular test specimen, and
i
contacts within the inner span. In addition, minimum length of
t = wall thickness of tubular test specimen.
the longitudinal strain gage should be either three unit cells of
thefiberarchitectureor9to12mmandminimumwidthshould
be three unit cells of the fiber architecture or 6 mm. These
NOTE 1—Example 1 is for a commercial CMC (v = 0.15) tube with
outer diameter of 0.50 in. and tube wall thickness of 0.05 in. In this case, recommended strain gage dimensions may make the use of
strain gages on small-diameter tubular test specimens impos-
2 2
3 1 2 v 3 1 2 0.15
~ ! ~ !
β5Œ 5 513.08 ~1⁄in.! such sible because of strains due to initial curvature and averaging
tube 2 2 2
~r ! t 0.522~0.05!
i
ofstrainsasthestraingageinstallationcurvesuptheoutsideof
0.05
!SF GD
the test specimen toward the neutral axis.
9 9
that IS$ 5 50.69 in. and OS=2×IS=2× 0.69 in. = 1.38 in.
β 13.08
NOTE3—Notethatmeasuringstrainoncompositematerialsusingstrain
NOTE 2—Example 2 is for a commercial CMC (v = 0.15) tube with
gages is problematic that is further exacerbated by the curved surfaces of
outer diameter of 100 mm and tube wall thickness of 2 mm. In this case,
4 tubular test specimens and, therefore, may not be appropriate for certain
2 2
3 1 2 v 3 1 2 0.15 combinations of test materials and test specimen dimensions. Ideally, to
~ ! ~ !
β5Œ 5 50.133 ~1⁄mm! such that
tube 2 2 2 eliminate the effect of misaligned uniaxial strain gages, three-element
~r ! t 10022~2!
i
SF GD rosettestraingagesshouldbemountedonthetensilesurfaceofthetubular
!
testspecimentodeterminemaximumprincipalstrainthatshouldbeinthe
9 9
IS$ 5 567.38 mm and OS=2×IS=2× 67.38 mm = 134.77 longitudinaldirection.Unlessitcanbeshownthatstraingagereadingsare
β 0.133
not unduly influenced by localized strain events such as fiber crossovers,
mm.
straingagelengthsspecifiedin7.5.2shouldbeused.Notethatlargerstrain
7.3.2 An additional empirical condition (3) placed on the
gages may be required for fabric reinforcements to average the localized
strain effects of the fiber crossovers. However, larger strain gages adhered
inner and outer spans to avoid shear failures and emphasize
to the curved surfaces of the tubular test specimens may have an initial
flexuralstressesisIS≥2ODwhichisequaltoOS≥4OD.Use
strain due to tube curvature that may render the strain reading unusable.
the greater of the two IS and OS values calculated in 7.3.1.
Straingages,surfacepreparation,andbondingagentsshouldbechosenso
as to provide adequate performance on the subject materials. Suitable
7.4 Loading Points—“Cradles”areusedtoavoidpointloads
strain recording equipment should be employed. Note that many CMCs
at the contact point of the roller and curved surface of the
exhibit high degrees of porosity and surface roughness and therefore
tubular test specimen without crushing the thin wall of the
require surface preparation, including surface filling, before the strain
tube.These cradles take various forms as illustrated in Appen-
gages can be applied.
dix X1 and Appendix X2 (2, 4-6).
7.5.3 Whole-Field Strain Measurement—Digital image cor-
7.4.1 Acradlemaybelinedwithanelastomertoconformto
relation (DIC) is a whole-field, optical method that employs
the outer surface of the test specimen. As an example, rubber
trackingandimageregistrationtechniquesforaccurate2Dand
(2, 4) has been cut to shape using a water jet.
3D measurements of changes in images (7, 8). The resulting
7.5 Strain Measurement—When measured, strain on the
imageshowsthestraindistributionoverthesurfaceofthetube.
tensile surface of the tube in flexure should be determined by
NOTE 4—Several methods can be used to measure the whole-field
means of a suitable extensometer/deflectometer, strain gages,
displacement distribution using DIC. Typically, an image is recorded
or appropriate whole-field strain methods. If Poisson’s ratio is
before deformation at a particular brightness distribution and then a
to be determined, the tubular test specimen must be instru-
similar brightness distribution is searched for in the image after deforma-
mented to measure strain in both longitudinal and lateral tion. The displacement components of a pixel located at the center of the
subset are determined, and the displacement distributions are obtained by
transverse (that is, circumferential) directions.
repeating this procedure for corresponding pixels. To determine strain, a
7.5.1 Extensometry—Extensometer systems used for testing
local approximation is used in which a least-squares fit for five side-by-
ofCMCtubulartestspecimensshallsatisfyPracticeE83,Class
side data points and each point strain is determined using partial
B1 requirements and are recommended to be used in place of
differentiation. In this case, the length of the five data points is equivalent
to the “gage length” for the strain evaluation. The complete strain
strain gages for test specimens with gage lengths of ≥25 mm
distribution can be obtained by repeating this procedure for the full field.
and shall be used for high-performance tests beyond the range
DIC (7, 8) can employ a digital camera with minimum of 1940 × 1480
of strain gage applications. Calibrate extensometer systems
pixelscapabilityanda12-bitresolutionequippedwithatelecentriclensto
periodically in accordance with Practice E83. For “clip-on”
measure displacement and strain field on the surface of the tubular test
extensometers mechanically attached to the test specimen, specimen. A photograph is taken every second on an area of about 10 ×
12mm (or on the order of one unit cell of the fiber architecture). A
maketheattachmentsoastocausenodamagetothespecimen
high-contrastspecklepatterncanbeobtainedonthetestspecimensurface
surface. In addition, the “clip-on” extensometer should be
by applying a matte randomized painting in order to produce an efficient
centered in the constant moment section of the flexure test
image correlation. A ring-shaped source with a monochromatic light can
specimen bounded by the two loading points of the inner span.
provide a homogeneous and uniform illumination to improve the signal-
The gage length of the extensometer should not exceed to-noise ratio.
C1899 − 21
7.6 DataAcquisition—Ataminimum,obtainanautographic specimen geometry. Tubular test specimens in flexure experi-
record of transverse force and gage section transverse deflec- ence the highest tensile stresses at the outer diameter surface
tion or longitudinal strain versus time. Either analog chart with the lowest tensile stresses at the inner diameter surface.
recorders or digital data acquisition systems can be used for
9.1.1.1 The following subsections discuss the required flex-
thispurpose,althoughadigitalrecordisrecommendedforease
ural strength tubular test specimen geometries, although any
of later data analysis. Ideally, use an analog chart recorder or
geometry is acceptable if it meets requirements for fixture
plotter in conjunction with the digital data acquisition system
dimensionsandtestspecimendimensionsaswellasacceptable
to provide an immediate record of the test as a supplement to
fracture locations of this test method. Deviations from the
the digital record. Recording devices shall be accurate to
recommended geometries may be necessary depending upon
within 60.1 % for the entire testing system including readout
the particular CMC being evaluated. Stress analyses of untried
unit as specified in Practices E4, and shall have a minimum
test specimen geometries should be conducted to ensure that
data acquisition rate of 10 Hz, with a response of 50 Hz
stress concentrations that can lead to undesired fractures
deemed more than sufficient.
outside the gage section do not exist. Stress analyses can
7.6.1 Record strain or elongation of the gage section, or
indicate the magnitude of such stress concentrations while
both,similarlytotheforceorasindependentvariablesofforce.
revealingthesuccessofproducinganonuniformuniaxialstress
Crosshead displacement of the test machine may also be
state in the gage section of the test specimen. The CMC
recordedbutshouldnotbeusedtodefinedeflectionorstrainin
material designer/user, the CMC material producer, and the
the gage section.Adeflectometer (for example, mechanical or
testing house shall mutually agree to a test specimen geometry
optical) at the midpoint of the gage section can be used to
specification with defined specimen dimensions, tolerance
measure maximum transverse deflection.
requirements, and finishing conditions.
7.7 Dimension-Measuring Devices—Micrometers and other
9.1.2 Test Specimen Dimensions—Although the inner and
devicesusedformeasuringlineardimensionsshallbeaccurate
outer diameters as well as wall thickness of CMC tubes can
and precise to at least one half the smallest unit to which the
vary widely depending on the application, analytical and
individual dimension is required to be measured. For the
experimental studies have shown (
1-4) that one can maximize
purposes of this test method, measure cross-sectional dimen-
the chances of obtaining a successful test by using consistent
sions to within 0.02 mm, thereby requiring dimension-
ranges of overall tube length as follows.
measuring devices with accuracies of 0.01 mm.
L $OS16 3 unitcellsizeofthematerial (2)
~ !
t
8. Hazards
8.1 Duringtheconductofthistestmethod,thepossibilityof
NOTE 5—Example 3 uses the results of Note 1 for the example of a
flying fragments of broken test material is high. The brittle commercial CMC (v = 0.15) tube with outer diameter of 0.50 in. and tube
wallthicknessof0.05in.InnerandouterspansarecalculatedasIS≥0.69
nature of advanced ceramics and the release of strain energy
in. and OS ≥ 1.38 in., respectively, per 7.3.1. However, using 7.3.2,IS ≥
contribute to the potential release of uncontrolled fragments
2×OD=2× 0.50 = 1.00 in. and OS=2×IS= 2.00 in., both of which
upon fracture. Provide means for containment and retention of
are greater than those calculated in 7.3.1. For an example of a composite
these fragments for later fractographic reconstruction/analysis
test material comprised of a 5-harness satin weave (five warp yarns, five
weft yarns, 0.06 in. yarn spacing, and 0.06 in. yarn width), the unit cell
and to prevent respiration or injury. Polymer shields can be
length is 0.30 in. and unit cell length is 0.30 in. Using these unit cell
usedtoencirclethetestfixtureandtestspecimenandtocapture
dimensions, the overall tube length is L ≥OS+6× (unit cell size) = 2.00
t
specimen fragments.
in.+6× (0.30 in.) = 3.80 in.
8.2 Exposed fibers at the edges of CMC test specimens NOTE 6—Example 4 uses the results of Note 2 for the example of a
commercialCMC(v=0.15)tubewithouterdiameterof100mmandtube
present a hazard due to the sharpness and brittleness of the
wallthicknessof2mm.InnerandouterspansarecalculatedasIS≥67.38
ceramic fiber. Inform all those required to handle these
mm and OS ≥ 134.77 mm, respectively, per 7.3.1. However, using 7.3.2,
materials of such conditions and the proper handling tech-
IS ≥2×OD=2×100=200mmandOS=2×IS=400mm, both of
niques.
which are greater than those calculated in 7.3.1. For an example of a
composite test material comprised of a 5-harness satin weave (five warp
9. Test Specimens
yarns, five weft yarns, 1.48 mm yarn spacing, and 1.48 mm yarn width),
the unit cell length is 7.4 mm and unit cell length is 7.4 mm. Using these
9.1 Test Specimen Geometry:
unit cell dimensions, the overall tube length is L ≥OS+6× (unit cell
t
9.1.1 General—The geometry of tubular test specimens is
size) = 400 mm+6× (7.4 mm) = 444.4 mm.
dependent on the ultimate use of the flexural strength data. For
9.2 Test Specimen Preparation:
example,iftheflexuralstrengthofanas-fabricatedcomponent
9.2.1 Depending upon the intended application of the flex-
is required, the dimensions of the resulting test specimen may
ural strength data, use one of the following test specimen
reflectthewallthickness,tubediameter,andlengthrestrictions
preparation procedures. Regardless of the preparation proce-
of the component. If it is desired to evaluate the effects of
dure used, report sufficient details regarding the procedure to
interactions of various constituent materials for a particular
allow replication.
CMC manufactured via a particular processing route, then the
size of the test specimen and resulting gage section (that is, 9.2.2 As-Fabricated—The tubular test specimen should
innerspan,IS)willreflectthedesiredvolumetobesampled.In simulatethesurface/edgeconditionsandprocessingrouteofan
addition, calculated outer span, OS, plus the overall length of application where no machining is used; for example, as-cast,
the test specimen will influence the final design of the test sintered, or injection molded part. No additional machining
C1899 − 21
specifications are relevant. As-processed test specimens pos-
sessingroughsurfacetexturesandnonparalleledgesmaycause
excessive misalignment or be prone to non-gage section
fractures, or both.
9.2.3 Application-Matched Machining—The tubular test
specimen should have the same surface/edge preparation as
that given to the component. Unless the process is proprietary,
report specifics about the stages of material removal, wheel
grits, wheel bonding, amount of material removed per pass,
and type of coolant used.
9.2.4 Customary Practices—Ininstanceswhereacustomary
machining procedure has been developed that is completely
satisfactory for a class of materials (that is, it induces no
unwanted surface/subsurface damage or residual stresses), use
this procedure.
FIG. 3 Details of Terms Used to Determine Cross-Sectional
Dimensions of a Tubular Test Specimen
9.2.5 Standard Procedure—In instances where 9.2.2 – 9.2.4
are not appropriate, 9.2.5 shall apply. Studies to evaluate the
machinability of CMCs have not been completed. Therefore,
the standard procedure of 9.2.5 can be viewed as starting point
different cross-sectional planes, for a minimum of six (6)
guidelines and a more stringent procedure may be necessary.
measurements each of outer diameter and wall thickness. To
9.2.5.1 Conductallgrindingorcuttingwithamplesupplyof
avoid damage in the critical gage section area, it is recom-
appropriatefilteredcoolanttokeeptheworkpieceandgrinding
mended that these measurements be made either optically (for
wheel constantly flooded and particles flushed. Conduct grind-
example, an optical comparator) or mechanically using a
ing in at least two stages, ranging from coarse to fine rate of
self-limiting (friction or ratchet mechanism) flat, anvil-type
material removal. Conduct all cutting in one stage appropriate
micrometer.When measuring dimensions between the faces of
for the depth of cut.
woven materials using contacting metrology, employ a self-
9.2.5.2 Employ a stock removal rate on the order of
limiting (friction or ratchet mechanism) flat, anvil-type mi-
0.03mm per pass using diamond tools that have between 320
crometer having anvil cross-sectional dimensions of at least
and 600 grit. Remove equal stock where applicable.
5mm. In all cases, the resolution of the instrument shall be as
9.3 Test Specimen Handling and Storage—Exercise care in
specified in 7.7. Exercise caution to prevent damage to the test
handling, packaging, and storage of finished test specimens to
specimen gage section. Ball-tipped or sharp-anvil micrometers
avoid the introduction of surface flaws. In addition, test
may be preferred when measuring small-diameter test speci-
specimens may be stored in controlled environments or desic-
mens or materials with rough or uneven nonwoven surfaces.
cators to avoid environmental (for example, humidity) degra-
NoncontactingmetrologysuchasX-raycomputedtomography
dation of specimens prior to testing.
at different resolutions and magnifications, depending on the
dimensional tolerance, has been used to make dimensional
9.4 Number of Test Specimens—Test a minimum of five test
measurements on CMC tubes (9). Record and report the
specimens in a valid manner for the purposes of estimating a
measured dimensions and locations of the measurements for
mean. A greater number of test specimens tested validly may
use in the calculation of the flexural stress. Use the average of
be necessary if estimates regarding the form of the strength
the multiple measurements in the stress calculations.
distribution are required. If material cost or test specimen
10.1.1 Alternatively,toavoiddamagetothegagesection(or
availabilitylimitsthenumberofpossibletests,fewertestsmay
in cases where it is not possible to infer or determine gage
beconductedtodetermineanindicationofmaterialproperties.
sectionwallthickness),usetheproceduresdescribedin10.1to
9.5 ValidTest—Avalidindividualtestisonewhichmeetsall
make post-fracture measur
...