ASTM B771-87(1997)
(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 1997)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
Standard Test Method for
Short Rod Fracture Toughness of Cemented Carbides
This standard is issued under the fixed designation B 771; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope across the slot at the specimen mouth is recorded autographi-
cally. As the load is increased, a crack initiates at the point of
1.1 This test method covers the determination of the fracture
the chevron slot and slowly advances longitudinally, tending to
toughness of cemented carbides (K ) by testing slotted short
IcSR
split the specimen in half. The load goes through a smooth
rod or short bar specimens.
maximum when the width of the crack front is about one third
1.2 Values stated in SI units are to be regarded as the
of the specimen diameter (short rod) or breadth (short bar).
standard. Inch-pound units are provided for information only.
Thereafter, the load decreases with further crack growth. Two
1.3 This standard does not purport to address all of the
unloading-reloading cycles are performed during the test to
safety concerns, if any, associated with its use. It is the
measure the effects of any macroscopic residual stresses in the
responsibility of the user of this standard to establish appro-
specimen. The fracture toughness is calculated from the
priate safety and health practices and determine the applica-
maximum load in the test and a residual stress parameter which
bility of regulatory limitations prior to use.
is evaluated from the unloading-reloading cycles on the test
2. Referenced Documents
record.
2.1 ASTM Standards:
5. Significance and Use
E 399 Test Method for Plane-Strain Fracture Toughness of
5.1 The property K determined by this test method is
IcSR
Metallic Materials
2 believed to characterize the resistance of a cemented carbide to
E 616 Terminology Relating to Fracture Testing
fracture in a neutral environment in the presence of a sharp
3. Terminology Definitions crack under severe tensile constraint, such that the state of
−3/2
stress near the crack front approaches tri-tensile plane strain,
3.1 stress intensity factor, K,(dimensional units FL )—
l
and the crack-tip plastic region is small compared with the
the magnitude of the ideal-crack-tip stress field for mode 1 in
crack size and specimen dimensions in the constraint direction.
a linear-elastic body.
A K value is believed to represent a lower limiting value of
IcSR
NOTE 1—Values of K for mode l are given by:
fracture toughness. This value may be used to estimate the
K 5 limit @s 2pr# relation between failure stress and defect size when the
=
l y
r → 0 (1)
conditions of high constraint described above would be ex-
pected. Background information concerning the basis for
where:
development of this test method in terms of linear elastic
r 5 distance directly forward from the crack tip to a
fracture mechanics may be found in Refs (1-4).
location where the significant stress s is calculated,
y
5.2 This test method can serve the following purposes:
and
5.2.1 To establish, in quantitative terms significant to ser-
s 5 principal stress normal to the crack plane.
y
vice performance, the effects of fabrication variables on the
3.2 fracture toughness of cemented carbide, K ,(dimen-
IcSR
−3/2 fracture toughness of new or existing materials, and
sional units FL )—the material-toughness property mea-
5.2.2 To establish the suitability of a material for a specific
sured in terms of the stress-intensity factor K by the opera-
l
application for which the stress conditions are prescribed and
tional procedure specified in this test method.
for which maximum flaw sizes can be established with
confidence.
4. Summary of Test Method
4.1 This test method involves the application of an opening
6. Specimen Configuration, Dimensions, and Preparation
load to the mouth of the short rod or short bar specimen which
6.1 Both the round short rod specimen and the rectangular
contains a chevron-shaped slot. Load versus displacement
shaped short bar specimen are equally acceptable and have
been found to have the same calibration (5). The short rod
This test method is under the jurisdiction of ASTM Committee B-9 on Metal
dimensions are given in Fig. 1; the short bar in Fig. 2.
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. The boldface numbers in parentheses refer to the list of references at the end of
Annual Book of ASTM Standards, Vol 03.01. this standard.
B 771
Standard Dimensions
Standard Dimensions
Short Rod
Short Bar
(mm) (in.)
(mm) (in.)
B 5 12.700 6 0.025 0.500 6 0.001
B 5 12.700 6 0.025 0.500 6 0.001
W 5 19.050 6 0.075 0.750 6 0.003
H 5 11.050 6 0.025 0.435 6 0.001
t5 0.381 6 0.025 0.015 6 0.001
W 5 19.050 6 0.075 0.750 6 0.003
For Curved Slot Option
t5 0.3816 0.025 0.015 6 0.001
a 5 6.3506 0.075 0.250 6 0.003
o
For Curved Slot Option
u5 58.0° 6 0.5°
a 5 6.3506 0.075 0.250 6 0.003
o
R 5 62.23 6 1.27 02.45 6 0.05
u5 58.0° 6 0.5°
For Straight Slot Option
R 5 62.23 6 1.27 2.45 6 0.05
a 5 6.7446 0.075 0.266 6 0.003
o
For Straight Slot Option
u5 55.2° 6 0.5°
a 5 6.744 6 0.075 0.266 6 0.003
o
R 5‘ ‘
u5 55.2° 6 0.5°
R 5‘ ‘
FIG. 1 Short Rod Specimen
FIG. 2 Short Bar Specimen
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-Guiding Slots—These may be ground using a
diamond abrasive wheel of approximately 124 6 3 mm (4.96
0.1 in.) diameter, with a thickness of 0.36 6 0.01 mm
(0.01406 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 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 by moving the specimen by a
cutting wheel. The values of a and u for the two slot
o
NOTE 1—The dashed lines show the front face profile of Figs. 1 and 2
configurations are chosen to cause the specimen calibration to
without grip slot.
remain constant.
FIG. 3 Short Rod and Short Bar Grip Slot in Specimen Front Face
7. Apparatus
7.1 The procedure involves testing of chevron-slotted speci- method, as shown in Fig. 3. Fig. 4 shows the grip design. Grips
mens and recording the load versus specimen mouth opening shall have a hardness of 45 HRC or greater, and shall be
displacement during the test. capable of providing loads to at least 1560 N (350 lbf). The
7.2 Grips and Fixtures for Tensile Test Machine Loading— grips are attached to the arms of tensile test machine by the pin
Grip slots are required in the specimen face for this test and clevis arrangement shown in Fig. 5. The grip lips are
B 771
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. A thin
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 just after initiation, such that the crack suddenly and
FIG. 4 Grip Design
audibly jumps ahead at the time of its initiation. Occasionally,
the load at crack 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
IcSR
loading is also required in order to maintain crack growth
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
o
out of 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 speci-
FIG. 5 Tensile Test Machine Test Configuration
mens 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
inserted into the grip slot in the specimen, and the specimen is
depth of 1 mm or more, the slot is said to be at least as thick
loaded as the test machine arms apply a tensile load to the
as the blade. Because the saw cuts forming the chevron slot
grips. A transducer for measuring the specimen mouth opening
overlap somewhat in the mouth of the specimen, and because
displacement during the test, and means for automatically
the cuts may not meet perfectly, the slot width near the center
recording the load-displacement test record, such as an X-Y
of the mouth may be larger than the width at the outside
recorder, are also required when using the tensile test machine
corners. If the slot width near the center exceeds the slot width
apparatus. A suggested design for the specimen mouth opening
displacement gage appears in Fig. 6. The gage shall have a
−6
displacement resolution of 0.25 μm (10 3 10 in.) or better.
Fractometer, a trademark of Terra Tek Systems, 360 Wakara Way, Salt Lake
However, it is not necessary to calibrate the displacement axis City, UT 84108, has been found satisfactory for this purpose.
B 771
FIG. 6 Suggested Design for a Specimen Mouth Opening Gage
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. A quantitative 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 Install the specimen on the test machine. If using the 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
“displacement control” mode. Bring the grips sufficiently close 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
the specimen face. Then increase the spacing between the grips 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 linear and will point toward the origin of the load-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 the
grip slot from each side of the specimen to assure that the specimen has decreased to less than 10 % of the load at the
specimen is properly installed. The grips should extend as far initiation of the unloading. The specimen shall be immediately
as possible into the grip slot, resulting in contact lines (load reloaded and the test continued after each unloading. The test
lines) at 0.63 mm (0.025 in.) from the specimen front face. record generated by t
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