ASTM E1949-2003(2014) Standard Test Method for Ambient Temperature Fatigue Life of Metallic Bonded Resistance Strain Gages.pdf

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1、Designation: E1949 03 (Reapproved 2014)Standard Test Method forAmbient Temperature Fatigue Life of Metallic BondedResistance Strain Gages1This standard is issued under the fixed designation E1949; the number immediately following the designation indicates the year oforiginal adoption or, in the case

2、 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 life

3、 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, lead wi

4、res, instrumentation, and(often) environmental protection. As a result, many thingsaffect the performance of strain gages, including user tech-nique. A further complication is that strain gages, onceinstalled, normally cannot be reinstalled in another location.Therefore, it is not possible to calibr

5、ate individual strain gages;performance characteristics are normally presented on a statis-tical 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 h

6、ealth practices and determine the applica-bility of regulatory limitations prior to its use.2. Referenced Documents2.1 ASTM Standards:2E1237 Guide for Installing Bonded Resistance Strain Gages3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 strain gage fatigue life, nthe number

7、 of fully re-versed strain cycles corresponding to the onset of degradedgage performance, whether due to excessive zero shift or otherdetectable failure mode (see 9.6).4. Significance and Use4.1 Strain gages are the most widely used devices formeasuring strains and for evaluating stresses in structu

8、res. Inmany applications there are often cyclic 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

9、 is destructive. Since gages testedfor fatigue 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

10、To be used, strain gages must be bonded to a 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, instr

11、umen-tation systems must be carefully selected 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 merel

12、y sufficient zero shiftto compromise the accuracy 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 gageinstallati

13、on and verification thereof can be found in GuideE1237. It is important to note that good performance in cyclicapplications requires the best installations possible.6. Hazards6.1 WarningIn the specimen surface cleaning, gagebonding, and protection steps of strain gage installations,hazardous chemica

14、ls are often employed. Users of these test1This test method is under the jurisdiction of ASTM Committee E28 onMechanical Testing and is the direct responsibility of Subcommittee E28.01 onCalibration of Mechanical Testing Machines and Apparatus.Current edition approved April 15, 2014. Published Augus

15、t 2014. Originallyapproved in 1998. Last previous edition approved in 2009 as E1949 03(2009).DOI: 10.1520/E1949-03R14.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer t

16、o the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1methods are responsible for contacting manufactures of suchchemicals for applicable Material Safety Data Sheets, and toadhere to

17、the required precautions.7. Apparatus7.1 Test Measurement Requirements:7.1.1 For fatigue life determination the uncertainty of therelative resistance change measurement shall not exceed 65/ or 60.1 % of the actual value, whichever is greater.7.1.2 Several methods are available for measuring thechang

18、e of gage resistance with sufficient resolution and accu-racy. In general, 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 o

19、btainingstrain data directly from a resistance strain gage. Theseinstruments use various types of excitation and read-outsystems. Such indicators may be used only after theirresolution, accuracy, and stability have been verified by con-necting a resistor that can be varied in accurately knownincreme

20、nts in place of the gage and calibrating the strainindicator over the entire range for which it will be used. Thecalibrating resistor steps shall be accurate to 0.1 % of theresistance change or 2 ppm of the total resistance, whichever isgreater. Effects from the following influences on measurementac

21、curacy must be quantified and found within limits thatpreserve the required overall system accuracy: thermal emfswithin the bridge circuit and within the gage leadwire, reactivechanges within the bridge and lead circuits, initial bridgeunbalance, and battery conditions or power line fluctuations.7.2

22、 Mechanical Equipment Requirements:7.2.1 A suggested cantilever test 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 3Ms3, which is a unidirectional glass-reinforced epoxycomposite material, with al

23、l fibers aligned with the long axis ofthe beam. Surface spalling of metallic test beams and crazingof plastic specimens are examples of beam failures that willproduce faulty, misleadingly low, strain gage fatigue life.7.2.1.1 Beam specimens must be cut such that the glassfibers are aligned with the

24、long dimension of the specimen. Acantilever specimen 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

25、in ways that imply strain gage failure whensuch is 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

26、 beam approximately 15 mm (0.6 in.) to produce asuitable 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

27、 machine, as follows:7.2.2.1 A thick plastic shield 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;

28、 and7.2.2.3 An electric counter geared to the drive 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 relativ

29、e humidity shall be 23C (73F) and50 %, respectively. 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

30、 gages. Typical values might be62000 m/m, 62400 m/m, 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 Attachme

31、nt Requirements:9.2.1 The attachment conditions shall correspond exactly tothe instructions published by the gage manufacturer anddiscussed in Guide E1237. Most fatigue failures occur in thetab and transition areas. Use care in attaching leadwires.9.2.2 In many applications strain gage damage will o

32、ccur inthe lead attachment/tab areas first. Consequently 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 provi

33、d-ing a nearly linear strain variation from one end to the other. Ifit is important to test at precisely known strain levels, the beamshould first be surveyed with linear strain gages to determine3The sole source of supply of this material known to the committee at this timeis 3M, Product Informatio

34、n 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 Cantilever Test BeamE1949 0

35、3 (2014)2locations of the desired strain levels. Survey gages are placedat regular intervals along the length of the beam; installed withthe major measurement axis of the gage aligned with the longaxis of the beam. The beam is deflected an amount equal to themaximum test deflection and the strain le

36、vels recorded. Ifnecessary, linear interpolation can be used to locate strainlevels 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 coinci

37、de with the line ofdesired strain, as shown in Fig. 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 lo

38、cated. To achieve the most precise andconsistent 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 fro

39、m the long axis of the beam and anchor them firmlyto 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 define

40、d as a zero reading shift of 100 in./in.,the measurement 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 pa

41、rt. The alternating strain range isthen obtained by reconnecting 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

42、 data. The alternating strainrange and zero are determined by examining 10 to 20 loadingcycles. The alternating strain range is determined by calculat-ing the difference between the mean maximum and meanFIG. 2 Strain Gage Fatigue RigFIG. 3 Gage Layout on Fatigue Test Beam (No Gages in Cross-Hatched

43、Areas)E1949 03 (2014)3minimum strains over the period. The zero is found bycalculating the average of the mean maximum and meanminimum readings. Recorded data should be examined care-fully to ensure that no “spikes” occur in the data which wouldlead to false peaks (see 9.6.1) and, therefore, false c

44、alculatedzeros.9.4.1 Regardless of data collection method (static ordynamic), all initial gage zeros should be within 50 m/m ofthe midpoint 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, 2 000 000,5 000 000, and

45、10 000 000 cycles, or less if gages have failedor do not 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-ta

46、king does not require stopping the machine, and 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 su

47、bject to fatigue failure in three ways(other than 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 outp

48、ut on an oscilloscope during dynamiccycling. It 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 Th

49、e standard level of zero shift defining strain 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