ASTM B771-87(2001)
(Test Method)Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides
Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides
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 Values stated in SI units are to be regarded as the standard. Inch-pound units are provided for information only.
1.3 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems 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: B 771 – 87 (Reapproved 2001)
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 (e) 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 Values stated in SI units are to be regarded as the across the slot at the specimen mouth is recorded autographi-
standard. Inch-pound units are provided for information only. cally. As the load is increased, a crack initiates at the point of
1.3 This standard does not purport to address all of the thechevronslotandslowlyadvanceslongitudinally,tendingto
safety concerns, if any, associated with its use. It is the split the specimen in half. The load goes through a smooth
responsibility of the user of this standard to establish appro- maximum when the width of the crack front is about one third
priate safety and health practices and determine the applica- of the specimen diameter (short rod) or breadth (short bar).
bility of regulatory limitations prior to use. Thereafter, the load decreases with further crack growth. Two
unloading-reloading cycles are performed during the test to
2. Referenced Documents
measure the effects of any macroscopic residual stresses in the
2.1 ASTM Standards: specimen. The fracture toughness is calculated from the
E399 Test Method for Plane-Strain Fracture Toughness of
maximumloadinthetestandaresidualstressparameterwhich
Metallic Materials is evaluated from the unloading-reloading cycles on the test
E616 Terminology Relating to Fracture Testing
record.
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
Powders and Metal Powder Products and is the direct responsibility of Subcom-
mittee B09.06 on Cemented Carbides.
Current edition approved Aug. 28, 1987. Published October 1987. Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
Annual Book of ASTM Standards, Vol 03.01. this standard.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B 771 – 87 (2001)
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
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,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
NOTE 1—The dashed lines show the front face profile of Figs. 1 and 2
0.1 in.) diameter, with a thickness of 0.36 6 0.01 mm (0.0140
without grip slot.
6 0.0005 in.). The resulting slots in the specimen are slightly
FIG. 3 Short Rod and Short Bar Grip Slot in Specimen Front Face
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 suggested. Dimensions are given in Fig. 1 and
cutting wheel of a given radius, and (2) Specimens with
Fig. 2 for two slotting options: (1) Specimens with curved slot
straight slot bottoms made by moving the specimen by a
bottoms made by plunge feeding the specimen onto a diamond
B 771 – 87 (2001)
FIG. 4 Grip Design
cutting wheel. The values of a and u for the two slot
o
configurations are chosen to cause the specimen calibration to
remain constant.
7. Apparatus
7.1 Theprocedureinvolvestestingofchevron-slottedspeci-
mens 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
FIG. 5 Tensile Test Machine Test Configuration
method,asshowninFig.3.Fig.4showsthegripdesign.Grips
shall have a hardness of 45 HRC or greater, and shall be
7.4 Testing Machine Characteristics—It has been observed
capable of providing loads to at least 1560 N (350 lbf). The
that some grades of carbides show a “pop-in” type of behavior
gripsareattachedtothearmsoftensiletestmachinebythepin
in which the load required to initiate the crack at the point of
and clevis arrangement shown in Fig. 5. The grip lips are
the chevron slot is larger than the load required to advance the
inserted into the grip slot in the specimen, and the specimen is
crack just after initiation, such that the crack suddenly and
loaded as the test machine arms apply a tensile load to the
audibly jumps ahead at the time of its initiation. Occasionally,
grips.Atransducerformeasuringthespecimenmouthopening
the load at crack initiation can exceed the load maximum
displacement during the test, and means for automatically
whichoccursasthecrackpassesthroughthecriticallocationin
recording the load-displacement test record, such as an X-Y
the specimen. When this occurs, a very stiff machine with
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
−6
location. The large pop-in load is then ignored, and the
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
ofthetestrecordsinceonlydisplacementratiosareusedinthe
IcSR
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
in the mouth of the specimen. Subsequent inflation of the 8.1 Number of Tests—A minimum of 3 replicate tests shall
bladder causes it to press against the inner surfaces of the slot,
be made.
thusproducingthedesiredloading.Themachineprovidesload 8.2 Specimen Measurement:
and displacement outputs, which must be recorded externally
8.2.1 Measure and record all specimen dimensions. If the
on a device such as an X-Y recorder. 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
o
out of tolerance by up to 3 times the tolerances shown in Fig.
Fractometer, a trademark of Terra Tek Systems, 360 Wakara Way, Salt Lake
City, UT 84108, has been found satisfactory for this purpose. 1andFig.2,validtestsmaystillbemadebytheapplicationof
B 771 – 87 (2001)
FIG. 6 Suggested Design for a Specimen Mouth Opening Gage
the appropriate factors to account for the deviation from very carefully until an opening load of 10 to 30 N (2 to 7 lb)
standard dimensions (see 9.3). If the slot centering is outside is applied to the specimen. Check the alignment of the
the indicated tolerance, the crack is less likely to follow the specimen with respect to the grips, and the alignment of the
chevron slots. However, the test may still be considered grips with respect to each other. The grips shall be centered in
successful if the crack follows the slots sufficiently well, as the specimen grip slot to within 0.25 mm (0.010 in.). The
discussed in 9.2. vertical offset between the grips shall not exceed 0.13 mm
8.2.2 The slot thickness measurement is critical on speci- (0.005 in.). Using a magnifying glass, observe the grips in the
mens to be tested on a Fractometer. It should be measured to grip slot from each side of the specimen to assure that the
within 0.013 mm (0.0005 in.) at the outside corners of the slot specimen is properly installed. The grips should extend as far
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. A 70° angle between the x-axis and the initial elastic
which a distributed load test machine is used (see 7.3), the loading trace of the test is suggested.Aquantitative calibration
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
B 771 – 87 (2001)
“overhang”ofthefracturesurfaceovertheslotbottomoneach
side of the chevron at a distance of 10.8 mm (0.425 in.) from
the mouth of the specimen (Fig. 7). If the sum, Db,ofthe
overhangsoneachsideofthechevronexceeds0.25mm(0.010
in.), the test is invalid.
NOTE 2—Imperfect crack follow often results from poor centering of
the chevron slot in the specimen. However, it can also result from strong
residual stresses in the test specimen.
9.3 Out-of-Tolerance Dimension Corrections—If the speci-
men dimensi
...
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