1、Designation: E 1949 03 (Reapproved 2009)Standard Test Method forAmbient Temperature Fatigue Life of Metallic BondedResistance Strain Gages1This standard is issued under the fixed designation E 1949; the number immediately following the designation indicates the year oforiginal adoption or, in the ca
2、se of revision, the year of last revision. A number 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 a uniform procedure for thedetermination of strain gage fatigue li
3、fe at ambient tempera-ture. A suggested testing equipment design is included.1.2 This test method does not apply to force transducers orextensometers that use bonded resistance strain gages assensing elements.1.3 Strain gages are part of a complex system that includesstructure, adhesive, gage, leadw
4、ires, instrumentation, and (of-ten) environmental protection. As a result, many things affectthe performance of strain gages, including user technique. Afurther complication is that strain gages, once installed, nor-mally cannot be reinstalled in another location. Therefore, it isnot possible to cal
5、ibrate individual strain gages; performancecharacteristics are normally presented on a statistical basis.1.4 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 establish appro-priate safety and
6、 health practices and determine the applica-bility of regulatory limitations prior to its use.2. Referenced Documents2.1 ASTM Standards:2E 1237 Guide for Installing Bonded Resistance StrainGages3. Terminology3.1 strain gage fatigue life, nthe number of fully reversedstrain cycles corresponding to th
7、e onset of degraded gageperformance, whether due to excessive zero shift or otherdetectable failure mode (see Section 9.6).4. Significance and Use4.1 Strain gages are the most widely used devices formeasuring strains and for evaluating stresses in structures. Inmany applications there are often cycl
8、ic loads which can causestrain gage failure. Performance parameters of strain gages areaffected by both the materials from which they are made andtheir geometric design.4.2 The determination of most strain gage parameters re-quires mechanical testing that is destructive. Since gages testedfor fatigu
9、e life cannot be used again, it is necessary to treat datastatistically. In general, longer and wider gages with lowerresistances will have greater fatigue life. Optional additions togages (integral leads are an example) will often reduce fatiguelife.4.3 To be used, strain gages must be bonded to a
10、structure.Good results, particularly in a fatigue environment, dependheavily on the materials used to clean the bonding surface, tobond the gage, and to provide a protective coating. Skill of theinstaller is another major factor in success. Finally, instrumen-tation systems must be carefully selecte
11、d and calibrated toensure that they do not unduly degrade the performance of thegages.4.4 This test method encompasses only fully reversed straincycles.4.5 Fatigue failure of a strain gage may not involve visiblecracking or fracture of the gage, but merely sufficient zero shiftto compromise the accu
12、racy of the gage output for static straincomponents.5. Interferences5.1 In order to ensure that strain gage test data are within adefined accuracy, the gages must be properly bonded andprotected with acceptable materials. Aids in the strain gageinstallation and verification thereof can be found in G
13、uideE 1237. It is important to note that good performance in cyclicapplications requires the best installations possible.6. Hazards6.1 WarningIn the specimen surface cleaning, gage bond-ing, and protection steps of strain gage installations, hazardouschemicals are often employed. Users of these test
14、 methods areresponsible for contacting manufactures of such chemicals forapplicable Material Safety Data Sheets, and to adhere to therequired precautions.1This test method is under the jurisdiction of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E28.01 onC
15、alibration of Mechanical Testing Machines and Apparatus.Current edition approved April 1, 2009. Published September 2009. Originallyapproved in 1998. Last previous edition approved in 2003 as E 1949 03.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Serv
16、ice at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.7. Apparatus7.1 Test Measurement Requirement
17、s:7.1.1 For fatigue life determination the uncertainty of therelative resistance change measurement shall not exceed 65V/V or 60.1 % of the actual value, whichever is greater.7.1.2 Several methods are available for measuring thechange of gage resistance with sufficient resolution and accu-racy. In g
18、eneral, any method that is convenient may be usedafter it has been shown that the particular combination ofinstruments or components used produces a system with therequired accuracy.7.1.3 Many types of instruments are available for obtainingstrain data directly from a resistance strain gage. Thesein
19、struments use various types of excitation and read-outsystems. Such indicators may be used only after their resolu-tion, accuracy, and stability have been verified by connecting aresistor that can be varied in accurately known increments inplace of the gage and calibrating the strain indicator over
20、theentire range for which it will be used. The calibrating resistorsteps shall be accurate to 0.1 % of the resistance change or 2ppm of the total resistance, whichever is greater. Effects fromthe following influences on measurement accuracy must bequantified and found within limits that preserve the
21、 requiredoverall system accuracy: thermal emfs within the bridge circuitand within the gage leadwire, reactive changes within thebridge and lead circuits, initial bridge unbalance, and batteryconditions or power line fluctuations.7.2 Mechanical Equipment Requirements:7.2.1 A suggested cantilever tes
22、t beam is shown in Fig. 1.The beam must have a fatigue life exceeding that of the straingages to be tested. One material which meets this requirementis 3Ms3Aerospace FP 525, which is a unidirectional glass-reinforced epoxy composite material, with all fibers alignedwith the long axis of the beam. Su
23、rface spalling of metallic testbeams and crazing of plastic specimens are examples of beamfailures that will produce faulty, misleadingly low, strain gagefatigue life.7.2.1.1 Beam specimens must be cut such that the glassfibers are aligned with the long dimension of the specimen. Acantilever specime
24、n is recommended for this testing because itprovides a range of strain levels in a single test. (A conse-quence is that the specimens strain level near the clamp is veryhigh. Normal structural materials will not survive such highlevels and may fail in ways that imply strain gage failure whensuch is
25、not the case.) A test beam should be used for one testonly.7.2.2 A suggested fatigue testing machine is illustrated inFig. 2. For a specimen with overall dimensions as shown inFig. 1 and a thickness of 9.5 mm (0.375 in.), the crank shoulddeflect the beam approximately 15 mm (0.6 in.) to produce asui
26、table strain range from 6500 m/m to 63500 m/m. Aloading rate of 1800 cycles/min has proven efficient, but not sofast as to cause higher mode bending. While not absolutelyessential, there are several features that provide for a safer andmore accurate machine, as follows:7.2.2.1 A thick plastic shield
27、 to prevent injury in case ofspecimen or machine failure.7.2.2.2 A shut off device consisting of micro switchespositioned above and below the specimen (near the crank) andwired in the motor power circuit to shut off power in case ofspecimen rupture; and7.2.2.3 An electric counter geared to the drive
28、 system, orsome other counting device appropriately connected to themachine, so the machine can be programmed to shut off or takedata at preselected intervals.8. Conditioning8.1 Ambient (Room Temperature) ConditionsThe nominaltemperature and relative humidity shall be 23C (73F) and50 %, respectively
29、. In no case shall the temperature be less than18C (64F) or greater than 25C (77F) and the relativehumidity less than 35 % or more than 60 %.9. Procedure9.1 Strain levels for the test should be selected based on theexpected fatigue life for the test gages. Typical values might be62000 m/m, 62400 m/m
30、, and 62800 m/m. (It may benecessary to select at least one substantially lower strain levelif it is desirable to indicate a no-failure strain level; see 9.6.2)Normally six or more strain gages are tested at each strainlevel.9.2 Strain Gage Attachment Requirements:9.2.1 The attachment conditions sha
31、ll correspond exactly tothe instructions published by the gage manufacturer anddiscussed in Guide E 1237. Most fatigue failures occur in thetab and transition areas. Use care in attaching leadwires.9.2.2 In many applications strain gage damage will occur inthe lead attachment/tab areas first. Conseq
32、uently sensor sur-vival will be enhanced by placing the solder tabs in the lowestpossible strain field. When conducting fatigue tests, orient thetabs toward the low-strain end of the test beam.9.3 The rectangular beam of Fig. 1 is convenient in provid-ing a nearly linear strain variation from one en
33、d to the other. Ifit is important to test at precisely known strain levels, the beamshould first be surveyed with linear strain gages to determinelocations of the desired strain levels. Survey gages are placedat regular intervals along the length of the beam; installed withthe major measurement axis
34、 of the gage aligned with the longaxis of the beam. The beam is deflected an amount equal to themaximum test deflection and the strain levels recorded. Ifnecessary, linear interpolation can be used to locate strain3The sole source of supply of this material known to the committee at this timeis 3M,
35、Product Information Ctr., at Bldg. 515-3N-06, St. Paul, MN 55144-1000. Ifyou aware of alternative suppliers, please provide this information to ASTMHeadquarters. Your comments will receive careful consideration at a meeting of theresponsible technical committee,1which you may attend.FIG. 1 Cantileve
36、r Test BeamE 1949 03 (2009)2levels in between two survey gage locations. Test gages areinstalled with the major measurement axis of the gage alignedwith the long axis of the beam at the predetermined locations.The center of the gage grid should coincide with the line ofdesired strain, as shown in Fi
37、g. 3. (Do not scribe the beam. Thiswill produce a strain concentration within the gage grid area.)In some cases, an exact cyclic strain level is not important andtest gages are installed where experience indicates the approxi-mate desired strain is located. To achieve the most precise andconsistent
38、test results (by staying within the well defined strainarea of the beam), test gages should be installed at least 50 mm(2 in.) from either the beam restraining clamp or the loadingarea. For best survival rate, route instrumentation leads 90degrees from the long axis of the beam and anchor them firml
39、yto the gage tab and beam with a suitable coating.9.4 Each gages zero reading and alternating strain rangemust be recorded using an instrumentation system with suffi-cient resolution and accuracy. Since the fatigue failure of agage is typically defined as a zero reading shift of 100 in./in.,the meas
40、urement system must be able to accurately resolve aminimum of 10 in./in.This data can be collected eitherstatically or dynamically. To obtain zeros statically, it isnecessary to disconnect the crank arm from the specimen toremove all load from the part. The alternating strain range isthen obtained b
41、y re-connecting the crank arm and rotating thedrive to get maximum and minimum static strain levels.Dynamic data must be collected using an instrument with ascanning speed of at least 10 times the loading rate to preventaliasing and possible erroneous data. The alternating strainrange and zero are d
42、etermined by examining 10 to 20 loadingcycles. The alternating strain range is determined by calculat-ing the difference between the mean maximum and meanminimum strains over the period. The zero is found bycalculating the average of the mean maximum and meanminimum readings. Recorded data should be
43、 examined care-fully to ensure that no “spikes” occur in the data which wouldlead to false peaks (see 9.6.1) and, therefore, false calculatedzeros.FIG. 2 Strain Gage Fatigue RigFIG. 3 Gage Layout on Fatigue Test Beam (No Gages in Cross-Hatched Areas)E 1949 03 (2009)39.4.1 Regardless of data collecti
44、on method (static or dy-namic), all initial gage zeros should be within 50 m/m of themidpoint of the alternating strain range.9.5 Data should be taken at the following number of cycles:100, 500, 1000, 10 000, 500 000, 1 000 000, 2 000 000, 5 000000, and 10 000 000 cycles, or less if gages have faile
45、d or donot need qualification at such high cyclic lives. For staticmeasurements, cycle the beam to the set number of cycles andstop. Repeat the static portion of 9.4. Continue this procedurefor all other set number of cycles. Conveniently, dynamicdata-taking does not require stopping the machine, an
46、d the testmay be run continuously with periodic checks from theoperator. Ten to twenty cycles of data should be taken at eachset number of approximate cycles. Continue this for allapplicable number of cycles.9.6 Failure Criteria:9.6.1 Strain gages are subject to fatigue failure in three ways(other t
47、han complete rupture): (A) zero shift, (B) change ingage factor, and (C) super sensitivity. Change in gage factor israre and, if encountered, is probably an indication of faultystrain gages under test. Super Sensitivity can be seen bymonitoring gage output on an oscilloscope during dynamiccycling. I
48、t is caused by the onset of grid cracking and thesymptom is the occurrence of spikes at the top of the tensioncycle (on the wave form). Again, super sensitivity is arelatively rare occurrence until long after gages have failed thezero shift test.9.6.2 The standard level of zero shift defining strain
49、 gagefailure is 100 m/m. At this level strain gages rarely exhibitappreciable gage factor change or super sensitivity. Laborato-ries using this test method may elect to assign a higher valuefor zero shift failure but must clearly indicate doing so. Atypical case when a higher standard level of zero shift failuremight be selected is that of isoelastic strain gages, usedprimarily for dynamic testing. For such gages a standard levelof zero shift failure of 300 m/m is often chosen. However, asthe testing laboratory raises its standard level of zero shiftfailur
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