1、Designation: B771 11Standard Test Method forShort Rod Fracture Toughness of Cemented Carbides1This standard is issued under the fixed designation B771; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A nu
2、mber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the determination of the fracturetoughness of cemented carbides (KIcSR) by testing slotted shortrod or short bar
3、specimens.1.2 The values stated in SI units are to be regarded asstandard. The values given in parentheses are for informationonly.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establi
4、sh appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E399 Test Method for Linear-Elastic Plane-Strain FractureToughness KIcof Metallic Materials3. Terminology Definitions3.1 stress intensity fac
5、tor, Kl,(dimensional units FL3/2)the magnitude of the ideal-crack-tip stress field for mode 1 ina linear-elastic body.NOTE 1Values of K for mode l are given by:Kl5 limit sy=2pr#r0 (1)where:r = distance directly forward from the crack tip to alocation where the significant stress syis calculated,ands
6、y= principal stress normal to the crack plane.3.2 Abbreviations:fracture toughness of cemented carbide,KIcSR,(dimensional units FL3/2)the material-toughnessproperty measured in terms of the stress-intensity factor Klbythe operational procedure specified in this test method.4. Summary of Test Method4
7、.1 This test method involves the application of an openingload to the mouth of the short rod or short bar specimen whichcontains a chevron-shaped slot. Load versus displacementacross the slot at the specimen mouth is recorded autographi-cally. As the load is increased, a crack initiates at the point
8、 ofthe chevron slot and slowly advances longitudinally, tending tosplit the specimen in half. The load goes through a smoothmaximum when the width of the crack front is about one thirdof the specimen diameter (short rod) or breadth (short bar).Thereafter, the load decreases with further crack growth
9、. Twounloading-reloading cycles are performed during the test tomeasure the effects of any macroscopic residual stresses in thespecimen. The fracture toughness is calculated from themaximum load in the test and a residual stress parameter whichis evaluated from the unloading-reloading cycles on the
10、testrecord.5. Significance and Use5.1 The property KIcSRdetermined by this test method isbelieved to characterize the resistance of a cemented carbide tofracture in a neutral environment in the presence of a sharpcrack under severe tensile constraint, such that the state ofstress near the crack fron
11、t approaches tri-tensile plane strain,and the crack-tip plastic region is small compared with thecrack size and specimen dimensions in the constraint direction.A KIcSRvalue is believed to represent a lower limiting value offracture toughness. This value may be used to estimate therelation between fa
12、ilure stress and defect size when theconditions of high constraint described above would be ex-pected. Background information concerning the basis fordevelopment of this test method in terms of linear elasticfracture mechanics may be found in Refs (1-4).35.2 This test method can serve the following
13、purposes:5.2.1 To establish, in quantitative terms significant to ser-vice performance, the effects of fabrication variables on thefracture toughness of new or existing materials, and5.2.2 To establish the suitability of a material for a specificapplication for which the stress conditions are prescr
14、ibed andfor which maximum flaw sizes can be established withconfidence.1This test method is under the jurisdiction of ASTM Committee B09 on MetalPowders and Metal Powder Products and is the direct responsibility of Subcom-mittee B09.06 on Cemented Carbides.Current edition approved Oct. 1, 2011. Publ
15、ished October 2011. Originallyapproved in 1987. Last previous edition approved in 2006 as B771 87 (2006).DOI: 10.1520/B0771-11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information
16、, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references at the end ofthis standard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.6. Specimen Configurati
17、on, Dimensions, and Preparation6.1 Both the round short rod specimen and the rectangularshaped short bar specimen are equally acceptable and havebeen found to have the same calibration (5). The short roddimensions are given in Fig. 1; the short bar in Fig. 2.6.2 Grip SlotDepending on the apparatus u
18、sed to test thespecimen, a grip slot may be required in the specimen frontface, as shown in Fig. 3. The surfaces in the grip slot shall havea smooth ground finish so that the contact with each grip willbe along an essentially continuous line along the entire gripslot, rather than at a few isolated p
19、oints or along a shortsegment within the grip slot.6.3 Crack-Guiding SlotsThese may be ground using adiamond abrasive wheel of approximately 124 6 3 mm (4.9 60.1 in.) diameter, with a thickness of 0.36 6 0.01 mm (0.01406 0.0005 in.). The resulting slots in the specimen are slightlythicker than the d
20、iamond wheel (0.38 6 0.02 mm, or 0.015 60.001 in.). A diamond concentration number of 50, and a gritsize of 150 are suggested. Dimensions are given in Fig. 1 andFig. 2 for two slotting options: (1) Specimens with curved slotbottoms made by plunge feeding the specimen onto a diamondcutting wheel of a
21、 given radius, and (2) Specimens withstraight slot bottoms made by moving the specimen by acutting wheel. The values of aoand u for the two slotconfigurations are chosen to cause the specimen calibration toremain constant.7. Apparatus7.1 The procedure involves testing of chevron-slotted speci-mens a
22、nd recording the load versus specimen mouth openingdisplacement during the test.Standard DimensionsShort Rod(mm) (in.)B = 12.700 6 0.025 0.500 6 0.001W = 19.050 6 0.075 0.750 6 0.003t = 0.381 6 0.025 0.015 6 0.001For Curved Slot Optionao= 6.3506 0.075 0.250 6 0.003u = 58.0 6 0.5R = 62.23 6 1.27 02.4
23、5 6 0.05For Straight Slot Optionao= 6.7446 0.075 0.266 6 0.003u = 55.2 6 0.5R=FIG. 1 Short Rod SpecimenStandard DimensionsShort Bar(mm) (in.)B = 12.700 6 0.025 0.500 6 0.001H = 11.050 6 0.025 0.435 6 0.001W = 19.050 6 0.075 0.750 6 0.003t = 0.3816 0.025 0.015 6 0.001For Curved Slot Optionao= 6.3506
24、0.075 0.250 6 0.003u = 58.0 6 0.5R = 62.23 6 1.27 2.45 6 0.05For Straight Slot Optionao= 6.744 6 0.075 0.266 6 0.003u = 55.2 6 0.5R=FIG. 2 Short Bar SpecimenNOTE 1The dashed lines show the front face profile of Figs. 1 and 2without grip slot.FIG. 3 Short Rod and Short Bar Grip Slot in Specimen Front
25、 FaceB771 1127.2 Grips and Fixtures for Tensile Test Machine LoadingGrip slots are required in the specimen face for this testmethod, as shown in Fig. 3. Fig. 4 shows the grip design. Gripsshall have a hardness of 45 HRC or greater, and shall becapable of providing loads to at least 1560 N (350 lbf)
26、. Thegrips are attached to the arms of tensile test machine by the pinand clevis arrangement shown in Fig. 5. The grip lips areinserted into the grip slot in the specimen, and the specimen isloaded as the test machine arms apply a tensile load to thegrips.Atransducer for measuring the specimen mouth
27、 openingdisplacement during the test, and means for automaticallyrecording the load-displacement test record, such as an X-Yrecorder, are also required when using the tensile test machineapparatus.Asuggested design for the specimen mouth openingdisplacement gage appears in Fig. 6. The gage shall hav
28、e adisplacement resolution of 0.25 m (10 3 106in.) or better.However, it is not necessary to calibrate the displacement axisof the test record since only displacement ratios are used in thedata analysis.7.3 Distributed Load Test Machine4An alternative specialpurpose machine that has been found suita
29、ble for the testrequires no grip slot in the front face of the specimen. A thinstainless steel inflatable bladder is inserted into the chevron slotin the mouth of the specimen. Subsequent inflation of thebladder causes it to press against the inner surfaces of the slot,thus producing the desired loa
30、ding. The machine provides loadand displacement outputs, which must be recorded externallyon a device such as an X-Y recorder.7.4 Testing Machine CharacteristicsIt has been observedthat some grades of carbides show a “pop-in” type of behaviorin which the load required to initiate the crack at the po
31、int ofthe chevron slot is larger than the load required to advance thecrack just after initiation, such that the crack suddenly andaudibly jumps ahead at the time of its initiation. Occasionally,the load at crack initiation can exceed the load maximumwhich occurs as the crack passes through the crit
32、ical location inthe specimen. When this occurs, a very stiff machine withcontrolled displacement loading is necessary in order to allowthe crack to arrest well before passing beyond the criticallocation. The large pop-in load is then ignored, and thesubsequent load maximum as the crack passes throug
33、h thecritical location is used to determine KIcSR. Stiff machineloading is also required in order to maintain crack growthstability to well beyond the peak load in the test, where thesecond unloading-reloading cycle is initiated.8. Procedure8.1 Number of TestsA minimum of 3 replicate tests shallbe m
34、ade.8.2 Specimen Measurement:8.2.1 Measure and record all specimen dimensions. If thedimensions are within the tolerances shown in Fig. 1 and Fig.2, no correction to the data need be made for out-of-tolerancedimensions. If one or more of the parameters ao, W, u or t areout of tolerance by up to 3 ti
35、mes the tolerances shown in Fig.1 and Fig. 2, valid tests may still be made by the application ofthe appropriate factors to account for the deviation fromstandard dimensions (see 9.3). If the slot centering is outsidethe indicated tolerance, the crack is less likely to follow thechevron slots. Howev
36、er, the test may still be consideredsuccessful if the crack follows the slots sufficiently well, asdiscussed 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 towithin 0.013 mm (0.0005 in.) at the outside corners of the slot4Fr
37、aQ WC of Dijon Investment Inc, 1948 Michigan Ave, Salt Lake City, UT84108, has been found satisfactory for this purpose.FIG. 4 Grip DesignFIG. 5 Tensile Test Machine Test ConfigurationB771 113using a feeler gage. If a feeler gage blade enters the slot to adepth of 1 mm or more, the slot is said to b
38、e at least as thickas the blade. Because the saw cuts forming the chevron slotoverlap somewhat in the mouth of the specimen, and becausethe cuts may not meet perfectly, the slot width near the centerof the mouth may be larger than the width at the outsidecorners. If the slot width near the center ex
39、ceeds the slot widthat the corners by more than 0.10 mm (0.004 in.), a test of thatspecimen by a Fractometer is invalid.8.3 Specimen Testing Procedure:8.3.1 Load Transducer Calibration:8.3.1.1 Calibrate the output of the load cell in the testmachine to assure that the load cell output, as recorded o
40、n theload versus displacement recorder, is accurately translatableinto the actual force applied to the specimen. In those cases inwhich a distributed load test machine is used (see 7.3), thecalibration shall be performed according to the instructions inAnnex A1.8.3.1.2 Install the specimen on the te
41、st machine. If using thetensile test machine (see 7.2), operate the test machine in the“displacement control” mode. Bring the grips sufficiently closetogether such that they simultaneously fit into the grip slot inthe specimen face. Then increase the spacing between the gripsvery carefully until an
42、opening load of 10 to 30 N (2 to 7 lb)is applied to the specimen. Check the alignment of thespecimen with respect to the grips, and the alignment of thegrips with respect to each other. The grips shall be centered inthe specimen grip slot to within 0.25 mm (0.010 in.). Thevertical offset between the
43、 grips shall not exceed 0.13 mm(0.005 in.). Using a magnifying glass, observe the grips in thegrip slot from each side of the specimen to assure that thespecimen is properly installed. The grips should extend as faras possible into the grip slot, resulting in contact lines (loadlines) at 0.63 mm (0.
44、025 in.) from the specimen front face.Correct any deviations from the desired specimen alignment.8.3.1.3 Install the specimen mouth opening displacementgage on the specimen. The gage must sense the mouth openingno farther than 1 mm (0.040 in.) from the front face of thespecimen. If the gage design o
45、f Fig. 6 is used, the contact forcebetween the gage arms and the specimen can be adjusted witha rubber elastic band so the gage will support itself, asindicated in Fig. 5. However, the contact force must not bemore than 2 N (0.5 lb), as it increases the measured load tofracture the specimen.8.3.1.4
46、Adjust the displacement (x-axis) sensitivity of theload-displacement recorder to produce a convenient-size datatrace. A70 angle between the x-axis and the initial elasticloading trace of the test is suggested.Aquantitative calibrationof the displacement axis is not necessary.8.3.1.5 With the load-di
47、splacement recorder operating, testthe specimen by causing the specimen mouth to open at a rateof 0.0025 to 0.0125 mm/s (0.0001 to 0.0005 in./s). Thespecimen is unloaded by reversing the motion of the gripstwice during the test. The first unloading is begun when theslope of the unloading line on the
48、 load-displacement recordwill be approximately 70 % of the initial elastic loading slope.(For estimating the point at which the unloadings should beinitiated, it can be assumed that the unloading paths will belinear and will point toward the origin of the load-displacementrecord.) The second unloadi
49、ng is begun when the unloadingslope will be approximately 35 % of the initial elastic loadingslope. Each unloading shall be continued until the load on thespecimen has decreased to less than 10 % of the load at theinitiation of the unloading. The specimen shall be immediatelyFIG. 6 Suggested Design for a Specimen Mouth Opening GageB771 114reloaded and the test continued after each unloading. The testrecord generated by the above procedure should be similar tothat of Fig. 8.8.3.2 Crack-Pop-InIf a sudden load drop occurs simulta-neously with an audibl
