ASTM B771-87(2006)
(Test Method)Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides
Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides
SIGNIFICANCE AND USE
The property KIcSR determined by this test method is believed to characterize the resistance of a cemented carbide to fracture in a neutral environment in the presence of a sharp crack under severe tensile constraint, such that the state of stress near the crack front approaches tri-tensile plane strain, and the crack-tip plastic region is small compared with the crack size and specimen dimensions in the constraint direction. A KIcSR value is believed to represent a lower limiting value of fracture toughness. This value may be used to estimate the relation between failure stress and defect size when the conditions of high constraint described above would be expected. Background information concerning the basis for development of this test method in terms of linear elastic fracture mechanics may be found in Refs (1-4).3
This test method can serve the following purposes:
5.2.1 To establish, in quantitative terms significant to service performance, the effects of fabrication variables on the fracture toughness of new or existing materials, and
5.2.2 To establish the suitability of a material for a specific application for which the stress conditions are prescribed and for which maximum flaw sizes can be established with confidence.
SCOPE
1.1 This test method covers the determination of the fracture toughness of cemented carbides (K IcSR) by testing slotted short rod or short bar specimens.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: B771 – 87 (Reapproved 2006)
Standard Test Method for
Short Rod Fracture Toughness of Cemented Carbides
This standard is issued under the fixed designation B771; 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 4. Summary of Test Method
1.1 Thistestmethodcoversthedeterminationofthefracture 4.1 This test method involves the application of an opening
toughness of cemented carbides (K ) by testing slotted short load to the mouth of the short rod or short bar specimen which
IcSR
rod or short bar specimens. contains a chevron-shaped slot. Load versus displacement
1.2 The values stated in SI units are to be regarded as across the slot at the specimen mouth is recorded autographi-
standard. The values given in parentheses are for information cally.As the load is increased, a crack initiates at the point of
only. thechevronslotandslowlyadvanceslongitudinally,tendingto
1.3 This standard does not purport to address all of the split the specimen in half. The load goes through a smooth
safety concerns, if any, associated with its use. It is the maximum when the width of the crack front is about one third
responsibility of the user of this standard to establish appro- of the specimen diameter (short rod) or breadth (short bar).
priate safety and health practices and determine the applica- Thereafter, the load decreases with further crack growth. Two
bility of regulatory limitations prior to use. unloading-reloading cycles are performed during the test to
measure the effects of any macroscopic residual stresses in the
2. Referenced Documents
specimen. The fracture toughness is calculated from the
2.1 ASTM Standards:
maximumloadinthetestandaresidualstressparameterwhich
E399 Test Method for Linear-Elastic Plane-Strain Fracture is evaluated from the unloading-reloading cycles on the test
Toughness K of Metallic Materials
record.
Ic
3. Terminology Definitions 5. Significance and Use
−3/2
3.1 stress intensity factor, K,(dimensional units FL )—
5.1 The property K determined by this test method is
l
IcSR
the magnitude of the ideal-crack-tip stress field for mode 1 in believedtocharacterizetheresistanceofacementedcarbideto
a linear-elastic body.
fracture in a neutral environment in the presence of a sharp
crack under severe tensile constraint, such that the state of
NOTE 1—Values of K for mode l are given by:
stress near the crack front approaches tri-tensile plane strain,
K 5limit [s 2pr#
=
l y
and the crack-tip plastic region is small compared with the
r→0 (1)
cracksizeandspecimendimensionsintheconstraintdirection.
AK value is believed to represent a lower limiting value of
IcSR
where:
fracture toughness. This value may be used to estimate the
r = distance directly forward from the crack tip to a
relation between failure stress and defect size when the
location where the significant stress s is calculated,
y
conditions of high constraint described above would be ex-
and
pected. Background information concerning the basis for
s = principal stress normal to the crack plane.
y
development of this test method in terms of linear elastic
3.2 Abbreviations:fracture toughness of cemented carbide,
−3/2
fracture mechanics may be found in Refs (1-4).
K ,(dimensional units FL )—the material-toughness
IcSR
5.2 This test method can serve the following purposes:
property measured in terms of the stress-intensity factor K by
l
5.2.1 To establish, in quantitative terms significant to ser-
the operational procedure specified in this test method.
vice performance, the effects of fabrication variables on the
fracture toughness of new or existing materials, and
This test method is under the jurisdiction of ASTM Committee B09 on Metal
5.2.2 To establish the suitability of a material for a specific
Powders and Metal Powder Products and is the direct responsibility of Subcom-
application for which the stress conditions are prescribed and
mittee B09.06 on Cemented Carbides.
Current edition approved April 1, 2006. Published April 2006. Originally for which maximum flaw sizes can be established with
approved in 1987. Last previous edition approved in 2001 as B771–87 (2001).
confidence.
DOI: 10.1520/B0771-87R06.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
the ASTM website. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B771 – 87 (2006)
Standard Dimensions
Standard Dimensions
Short Rod
Short Bar
(mm) (in.)
(mm) (in.)
B = 12.700 6 0.025 0.500 6 0.001
B = 12.700 6 0.025 0.500 6 0.001
W = 19.050 6 0.075 0.750 6 0.003
H = 11.050 6 0.025 0.435 6 0.001
t = 0.381 6 0.025 0.015 6 0.001
W = 19.050 6 0.075 0.750 6 0.003
For Curved Slot Option
t = 0.3816 0.025 0.015 6 0.001
a = 6.3506 0.075 0.250 6 0.003
o For Curved Slot Option
u = 58.0° 6 0.5°
a = 6.3506 0.075 0.250 6 0.003
o
R = 62.23 6 1.27 02.45 6 0.05
u = 58.0° 6 0.5°
For Straight Slot Option
R = 62.23 6 1.27 2.45 6 0.05
a = 6.7446 0.075 0.266 6 0.003
o
For Straight Slot Option
u = 55.2° 6 0.5°
a = 6.744 6 0.075 0.266 6 0.003
o
R=``
u = 55.2° 6 0.5°
R=``
FIG. 1 Short Rod Specimen
FIG. 2 Short Bar Specimen
6. Specimen Configuration, Dimensions, and Preparation
6.1 Both the round short rod specimen and the rectangular
shaped short bar specimen are equally acceptable and have
been found to have the same calibration (5). The short rod
dimensions are given in Fig. 1; the short bar in Fig. 2.
6.2 Grip Slot—Depending on the apparatus used to test the
specimen, a grip slot may be required in the specimen front
face,asshowninFig.3.Thesurfacesinthegripslotshallhave
a smooth ground finish so that the contact with each grip will
be along an essentially continuous line along the entire grip
slot, rather than at a few isolated points or along a short
segment within the grip slot.
6.3 Crack-Guiding Slots—These may be ground using a
diamondabrasivewheelofapproximately124 63mm(4.9 6
0.1 in.) diameter, with a thickness of 0.36 6 0.01 mm (0.0140
6 0.0005 in.). The resulting slots in the specimen are slightly
thicker than the diamond wheel (0.38 6 0.02 mm, or 0.015 6
NOTE 1—The dashed lines show the front face profile of Figs. 1 and 2
0.001 in.). A diamond concentration number of 50, and a grit
without grip slot.
size of 150 are suggested. Dimensions are given in Fig. 1 and
FIG. 3 Short Rod and Short Bar Grip Slot in Specimen Front Face
Fig. 2 for two slotting options: (1) Specimens with curved slot
bottoms made by plunge feeding the specimen onto a diamond
cutting wheel of a given radius, and (2) Specimens with
7. Apparatus
straight slot bottoms made by moving the specimen by a
cutting wheel. The values of a and u for the two slot 7.1 Theprocedureinvolvestestingofchevron-slottedspeci-
o
configurations are chosen to cause the specimen calibration to mens and recording the load versus specimen mouth opening
remain constant. displacement during the test.
B771 – 87 (2006)
FIG. 4 Grip Design
7.2 Grips and Fixtures for Tensile Test Machine Loading—
Grip slots are required in the specimen face for this test
method,asshowninFig.3.Fig.4showsthegripdesign.Grips
shall have a hardness of 45 HRC or greater, and shall be
capable of providing loads to at least 1560 N (350 lbf). The
gripsareattachedtothearmsoftensiletestmachinebythepin
and clevis arrangement shown in Fig. 5. The grip lips are
inserted into the grip slot in the specimen, and the specimen is
loaded as the test machine arms apply a tensile load to the
grips.Atransducerformeasuringthespecimenmouthopening
FIG. 5 Tensile Test Machine Test Configuration
displacement during the test, and means for automatically
recording the load-displacement test record, such as an X-Y
recorder, are also required when using the tensile test machine
controlled displacement loading is necessary in order to allow
apparatus.Asuggesteddesignforthespecimenmouthopening
the crack to arrest well before passing beyond the critical
displacement gage appears in Fig. 6. The gage shall have a
location. The large pop-in load is then ignored, and the
−6
displacement resolution of 0.25 µm (10 310 in.) or better.
subsequent load maximum as the crack passes through the
However, it is not necessary to calibrate the displacement axis
critical location is used to determine K . Stiff machine
IcSR
ofthetestrecordsinceonlydisplacementratiosareusedinthe
loading is also required in order to maintain crack growth
data analysis.
stability to well beyond the peak load in the test, where the
7.3 DistributedLoadTestMachine —Analternativespecial
second unloading-reloading cycle is initiated.
purpose machine that has been found suitable for the test
requires no grip slot in the front face of the specimen. A thin
8. Procedure
stainlesssteelinflatablebladderisinsertedintothechevronslot
8.1 Number of Tests—A minimum of 3 replicate tests shall
in the mouth of the specimen. Subsequent inflation of the
be made.
bladder causes it to press against the inner surfaces of the slot,
8.2 Specimen Measurement:
thusproducingthedesiredloading.Themachineprovidesload
8.2.1 Measure and record all specimen dimensions. If the
and displacement outputs, which must be recorded externally
dimensions are within the tolerances shown in Fig. 1 and Fig.
on a device such as an X-Y recorder.
2, no correction to the data need be made for out-of-tolerance
7.4 Testing Machine Characteristics—It has been observed
dimensions. If one or more of the parameters a , W, u or t are
o
that some grades of carbides show a “pop-in” type of behavior
out of tolerance by up to 3 times the tolerances shown in Fig.
in which the load required to initiate the crack at the point of
1andFig.2,validtestsmaystillbemadebytheapplicationof
the chevron slot is larger than the load required to advance the
the appropriate factors to account for the deviation from
crack just after initiation, such that the crack suddenly and
standard dimensions (see 9.3). If the slot centering is outside
audibly jumps ahead at the time of its initiation. Occasionally,
the indicated tolerance, the crack is less likely to follow the
the load at crack initiation can exceed the load maximum
chevron slots. However, the test may still be considered
whichoccursasthecrackpassesthroughthecriticallocationin
successful if the crack follows the slots sufficiently well, as
the specimen. When this occurs, a very stiff machine with
discussed in 9.2.
8.2.2 The slot thickness measurement is critical on speci-
mens to be tested on a Fractometer. It should be measured to
Fractometer, a trademark of Terra Tek Systems, 360 Wakara Way, Salt Lake
City, UT 84108, has been found satisfactory for this purpose. within 0.013 mm (0.0005 in.) at the outside corners of the slot
B771 – 87 (2006)
FIG. 6 Suggested Design for a Specimen Mouth Opening Gage
using a feeler gage. If a feeler gage blade enters the slot to a as possible into the grip slot, resulting in contact lines (load
depth of 1 mm or more, the slot is said to be at least as thick lines) at 0.63 mm (0.025 in.) from the specimen front face.
as the blade. Because the saw cuts forming the chevron slot Correct any deviations from the desired specimen alignment.
overlap somewhat in the mouth of the specimen, and because 8.3.1.3 Install the specimen mouth opening displacement
the cuts may not meet perfectly, the slot width near the center gageonthespecimen.Thegagemustsensethemouthopening
of the mouth may be larger than the width at the outside no farther than 1 mm (0.040 in.) from the front face of the
corners. If the slot width near the center exceeds the slot width specimen.IfthegagedesignofFig.6isused,thecontactforce
at the corners by more than 0.10 mm (0.004 in.), a test of that between the gage arms and the specimen can be adjusted with
specimen by a Fractometer is invalid. a rubber elastic band so the gage will support itself, as
8.3 Specimen Testing Procedure: indicated in Fig. 5. However, the contact force must not be
8.3.1 Load Transducer Calibration: more than 2 N (0.5 lb), as it increases the measured load to
8.3.1.1 Calibrate the output of the load cell in the test fracture the specimen.
machine to assure that the load cell output, as recorded on the 8.3.1.4 Adjust the displacement (x-axis) sensitivity of the
load versus displacement recorder, is accurately translatable load-displacement recorder to produce a convenient-size data
into the actual force applied to the specimen. In those cases in trace. A70° angle between the x-axis and the initial elastic
which a distributed load test machine is used (see 7.3), the loadingtraceofthetestissuggested.Aquantitativecalibration
calibration shall be performed according to the instructions in of the displacement axis is not necessary.
Annex A1. 8.3.1.5 With the load-displacement recorder operating, test
8.3.1.2 Installthespecimenonthetestmachine.Ifusingthe the specimen by causing the specimen mouth to open at a rate
tensile test machine (see 7.2), operate the test machine in the of 0.0025 to 0.0125 mm/s (0.0001 to 0.0005 in./s). The
“displacementcontrol”mode.Bringthegripssufficientlyclose specimen is unloaded by reversing the motion of the grips
together such that they simultaneously fit into the grip slot in twice during the test. The first unloading is begun when the
thespecimenface.Thenincreasethespacingbetweenthegrips slope of the unloading line on the load-displacement record
very carefully until an opening load of 10 to 30 N (2 to 7 lb) will be approximately 70% of the initial elastic loading slope.
is applied to the specimen. Check the alignment of the (For estimating the point at which the unloadings should be
specimen with respect to the grips, and the alignment of the initiated, it can be assumed that the unloading paths will be
grips with respect to each other. The grips shall be centered in linearandwillpointtowardtheoriginoftheload-displacement
the specimen grip slot to within 0.25 mm (0.010 in.). The record.) The second unloading is begun when the unloading
vertical offset between the grips shall not exceed 0.13 mm slope will be approximately 35% of the initial elastic loading
(0.005 in.). Using a magnifying glass, observe the grips in the slope. Each unloading shall be continued until the load on t
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:B 771–87(Reapproved2001) Designation: B771 – 87 (Reapproved 2006)
Standard Test Method for
Short Rod Fracture Toughness of Cemented Carbides
This standard is issued under the fixed designation B771; 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
1.1 Thistestmethodcoversthedeterminationofthefracturetoughnessofcementedcarbides(K )bytestingslottedshortrod
IcSR
or short bar specimens.
1.2Values stated in SI units are to be regarded as the standard. Inch-pound units are provided for information only.
1.2
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
E399Test Method for Plane-Strain Fracture Toughness of Metallic Materials ASTM Standards:
E616Terminology Relating to Fracture Testing 399 Test Method for Linear-Elastic Plane-Strain Fracture Toughness K of
Ic
Metallic Materials
3. Terminology Definitions
−3/2
3.1 stress intensity factor, K,(dimensional units FL )—the magnitude of the ideal-crack-tip stress field for mode 1 in a
l
linear-elastic body.
NOTE 1—Values of K for mode l are given by:
(1) Kl 5limit [sy2pr#r→0
where:
r = distance directly forward from the crack tip to a location where the significant stress s is calculated, and
y
s = principal stress normal to the crack plane.
y
−3/2
3.2 Abbreviations:fracture toughness of cemented carbide, K ,(dimensional units FL )—the material-toughness property
IcSR
measured in terms of the stress-intensity factor K by the operational procedure specified in this test method.
l
4. Summary of Test Method
4.1 This test method involves the application of an opening load to the mouth of the short rod or short bar specimen which
contains a chevron-shaped slot. Load versus displacement across the slot at the specimen mouth is recorded autographically. As
the load is increased, a crack initiates at the point of the chevron slot and slowly advances longitudinally, tending to split the
specimen in half.The load goes through a smooth maximum when the width of the crack front is about one third of the specimen
diameter (short rod) or breadth (short bar). Thereafter, the load decreases with further crack growth. Two unloading-reloading
cycles are performed during the test to measure the effects of any macroscopic residual stresses in the specimen. The fracture
toughness is calculated from the maximum load in the test and a residual stress parameter which is evaluated from the
unloading-reloading cycles on the test record.
This test method is under the jurisdiction of ASTM Committee B09 on Metal Powders and Metal Powder Products and is the direct responsibility of Subcommittee
B09.06 on Cemented Carbides.
Current edition approved Aug. 28, 1987. Published October 1987.
Current edition approved April 1, 2006. Published April 2006. Originally approved in 1987. Last previous edition approved in 2001 as B771–87 (2001). DOI:
10.1520/B0771-87R06.
Annual Book of ASTM Standards, Vol 03.01.
ForreferencedASTMstandards,visittheASTMwebsite,www.astm.org,orcontactASTMCustomerServiceatservice@astm.org.ForAnnualBookofASTMStandards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B771 – 87 (2006)
5. Significance and Use
5.1 ThepropertyK determinedbythistestmethodisbelievedtocharacterizetheresistanceofacementedcarbidetofracture
IcSR
inaneutralenvironmentinthepresenceofasharpcrackunderseveretensileconstraint,suchthatthestateofstressnearthecrack
front approaches tri-tensile plane strain, and the crack-tip plastic region is small compared with the crack size and specimen
dimensions in the constraint direction. A K value is believed to represent a lower limiting value of fracture toughness. This
IcSR
value may be used to estimate the relation between failure stress and defect size when the conditions of high constraint described
above would be expected. Background information concerning the basis for development of this test method in terms of linear
elastic fracture mechanics may be found in Refs (1-4).
5.2 This test method can serve the following purposes:
5.2.1 To establish, in quantitative terms significant to service performance, the effects of fabrication variables on the fracture
toughness of new or existing materials, and
5.2.2 To establish the suitability of a material for a specific application for which the stress conditions are prescribed and for
which maximum flaw sizes can be established with confidence.
6. Specimen Configuration, Dimensions, and Preparation
6.1 Both the round short rod specimen and the rectangular shaped short bar specimen are equally acceptable and have been
found to have the same calibration (5). The short rod dimensions are given in Fig. 1; the short bar in Fig. 2.
6.2 Grip Slot—Depending on the apparatus used to test the specimen, a grip slot may be required in the specimen front face,
as shown in Fig. 3.The surfaces in the grip slot shall have a smooth ground finish so that the contact with each grip will be along
an essentially continuous line along the entire grip slot, rather than at a few isolated points or along a short segment within the
grip slot.
6.3 Crack-GuidingSlots—Thesemaybegroundusingadiamondabrasivewheelofapproximately124 63mm(4.9 60.1in.)
diameter, with a thickness of 0.36 6 0.01 mm (0.0140 6 0.0005 in.). The resulting slots in the specimen are slightly thicker than
the diamond wheel (0.38 6 0.02 mm, or 0.015 6 0.001 in.). A diamond concentration number of 50, and a grit size of 150 are
The boldface numbers in parentheses refer to the list of references at the end of this standard.
Standard Dimensions
Short Rod
(mm) (in.)
B = 12.700 6 0.025 0.500 6 0.001
W = 19.050 6 0.075 0.750 6 0.003
t = 0.381 6 0.025 0.015 6 0.001
For Curved Slot Option
a = 6.3506 0.075 0.250 6 0.003
o
u = 58.0° 6 0.5°
R = 62.23 6 1.27 02.45 6 0.05
For Straight Slot Option
a = 6.7446 0.075 0.266 6 0.003
o
u = 55.2° 6 0.5°
R=``
FIG. 1 Short Rod Specimen
B771 – 87 (2006)
Standard Dimensions
Short Bar
(mm) (in.)
B = 12.700 6 0.025 0.500 6 0.001
H = 11.050 6 0.025 0.435 6 0.001
W = 19.050 6 0.075 0.750 6 0.003
t = 0.3816 0.025 0.015 6 0.001
For Curved Slot Option
a = 6.3506 0.075 0.250 6 0.003
o
u = 58.0° 6 0.5°
R = 62.23 6 1.27 2.45 6 0.05
For Straight Slot Option
a = 6.744 6 0.075 0.266 6 0.003
o
u = 55.2° 6 0.5°
R=``
FIG. 2 Short Bar Specimen
NOTE 1—The dashed lines show the front face profile of Figs. 1 and 2
without grip slot.
FIG. 3 Short Rod and Short Bar Grip Slot in Specimen Front Face
suggested. Dimensions are given in Fig. 1 and Fig. 2 for two slotting options: (1) Specimens with curved slot bottoms made by
plunge feeding the specimen onto a diamond cutting wheel of a given radius, and (2) Specimens with straight slot bottoms made
bymovingthespecimenbyacuttingwheel.Thevaluesofa and uforthetwoslotconfigurationsarechosentocausethespecimen
o
calibration to remain constant.
B771 – 87 (2006)
7. Apparatus
7.1 The procedure involves testing of chevron-slotted specimens and recording the load versus specimen mouth opening
displacement during the test.
7.2 Grips and Fixtures for Tensile Test Machine Loading—Grip slots are required in the specimen face for this test method, as
showninFig.3.Fig.4showsthegripdesign.Gripsshallhaveahardnessof45HRCorgreater,andshallbecapableofproviding
loads to at least 1560 N (350 lbf). The grips are attached to the arms of tensile test machine by the pin and clevis arrangement
shown in Fig. 5. The grip lips are inserted into the grip slot in the specimen, and the specimen is loaded as the test machine arms
apply a tensile load to the grips.Atransducer for measuring the specimen mouth opening displacement during the test, and means
forautomaticallyrecordingtheload-displacementtestrecord,suchasanX-Yrecorder,arealsorequiredwhenusingthetensiletest
machine apparatus.Asuggested design for the specimen mouth opening displacement gage appears in Fig. 6.The gage shall have
−6
a displacement resolution of 0.25 µm (10 310 in.) or better. However, it is not necessary to calibrate the displacement axis of
the test record since only displacement ratios are used in the data analysis.
7.3 Distributed Load Test Machine —An alternative special purpose machine that has been found suitable for the test requires
no grip slot in the front face of the specimen.Athin stainless steel inflatable bladder is inserted into the chevron slot in the mouth
of the specimen. Subsequent inflation of the bladder causes it to press against the inner surfaces of the slot, thus producing the
desired loading. The machine provides load and displacement outputs, which must be recorded externally on a device such as an
X-Y recorder.
7.4 Testing Machine Characteristics—It has been observed that some grades of carbides show a “pop-in” type of behavior in
which the load required to initiate the crack at the point of the chevron slot is larger than the load required to advance the crack
justafterinitiation,suchthatthecracksuddenlyandaudiblyjumpsaheadatthetimeofitsinitiation.Occasionally,theloadatcrack
initiation can exceed the load maximum which occurs as the crack passes through the critical location in the specimen.When this
occurs, a very stiff machine with controlled displacement loading is necessary in order to allow the crack to arrest well before
passing beyond the critical location. The large pop-in load is then ignored, and the subsequent load maximum as the crack passes
through the critical location is used to determine K . Stiff machine loading is also required in order to maintain crack growth
IcSR
stability to well beyond the peak load in the test, where the second unloading-reloading cycle is initiated.
8. Procedure
8.1 Number of Tests—A minimum of 3 replicate tests shall be made.
8.2 Specimen Measurement:
8.2.1 Measure and record all specimen dimensions. If the dimensions are within the tolerances shown in Fig. 1 and Fig. 2, no
correction to the data need be made for out-of-tolerance dimensions. If one or more of the parameters a , W, u or t are out of
o
tolerance by up to 3 times the tolerances shown in Fig. 1 and Fig. 2, valid tests may still be made by the application of the
appropriate factors to account for the deviation from standard dimensions (see 9.3). If the slot centering is outside the indicated
tolerance, the crack is less likely to follow the chevron slots. However, the test may still be considered successful if the crack
follows the slots sufficiently well, as discussed in 9.2.
8.2.2 The slot thickness measurement is critical on specimens to be tested on a Fractometer. It should be measured to within
0.013 mm (0.0005 in.) at the outside corners of the slot using a feeler gage. If a feeler gage blade enters the slot to a depth of 1
Fractometer, a trademark of Terra Tek Systems, 360 Wakara Way, Salt Lake City, UT 84108, has been found satisfactory for this purpose.
FIG. 4 Grip Design
B771 – 87 (2006)
FIG. 5 Tensile Test Machine Test Configuration
FIG. 6 Suggested Design for a Specimen Mouth Opening Gage
mm or more, the slot is said to be at least as thick as the blade. Because the saw cuts forming the chevron slot overlap somewhat
in the mouth of the specimen, and because the cuts may not meet perfectly, the slot width near the center of the mouth may be
B771 – 87 (2006)
larger than the width at the outside corners. If the slot width near the center exceeds the slot width at the corners by more than
0.10 mm (0.004 in.), a test of that specimen by a Fractometer is invalid.
8.3 Specimen Testing Procedure:
8.3.1 Load Transducer Calibration:
8.3.1.1 Calibratetheoutputoftheloadcellinthetestmachinetoassurethattheloadcelloutput,asrecordedontheloadversus
displacementrecorder,isaccuratelytranslatableintotheactualforceappliedtothespecimen.Inthosecasesinwhichadistributed
load test machine is used (see 7.3), the calibration shall be performed according to the instructions in Annex A1.
8.3.1.2 Install the specimen on the test machine. If using the tensile test machine (see 7.2), operate the test machine in the
“displacement control” mode. Bring the grips sufficiently close together such that they simultaneously fit into the grip slot in the
specimenface.Thenincreasethespacingbetweenthegripsverycarefullyuntilanopeningloadof10to30N(2to7lb)isapplied
tothespecimen.Checkthealignmentofthespecimenwithrespecttothegrips,andthealignmentofthegripswithrespecttoeach
other. The grips shall be centered in the specimen grip slot to within 0.25 mm (0.010 in.). The vertical offset between the grips
shall not exceed 0.13 mm (0.005 in.). Using a magnifying glass, observe the grips in the grip slot from each side of the specimen
to assure that the specimen is properly installed. The grips should extend as far as possible into the grip slot, resulting in contact
lines(loadlines)at0.63mm(0.025in.)fromthespecimenfrontface.Correctanydeviationsfromthedesiredspecimenalignment.
8.3.1.3 Install the specimen mouth opening displacement gage on the specimen. The gage must sense the mouth opening no
farther than 1 mm (0.040 in.) from the front face of the specimen. If the gage design of Fig. 6 is used, the contact force between
the gage arms and the specimen can be adjusted with a rubber elastic band so the gage will support itself, as indicated in Fig. 5.
However, the contact force must not be more than 2 N (0.5 lb), as it increases the measured load to fracture the specimen.
8.3.1.4 Adjust the displacement (x-axis) sensitivity of the load-displacement recorder to produce a convenient-size data trace.
A70° angle between the x-axis and the initial elastic loading trace of the test is suggested. A quantitative calibration of the
displacement axis is not necessary.
8.3.1.5 With the load-displacement r
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