Stress-Strain Measurement and Strain Gauge

1 stress - Strain Measurement and Strain GaugeStrain Measurement and Strain GaugesStrain xx, yyand xyasStrain Measurement and Strain Gauges uxxx vyyy XY = vuxy , of Good StrainoCheaper, , , ( ,temperature,humidity,vibrationetc.). of Strain GaugesStraingaugesofalltypesareessential lydevicesthatsensethechangeinlength, Strain Gauges Thesemechanicaldevicesaregenerallyknowna sextensometersandareusedtomeasurestrainu nderstaticorgraduallyvaryingloadingcondi tions. Anextensometerisusuallyprovidedwithtwokn ifeedgeswhichareclampedfirmlyincontactwi ththetestcomponentataspecificdistanceorg augelengthapart. Whenthetestcomponentisstrained,thetwokni feedgesundergoasmallrelativedisplacement . and disadvantage of Mechanical Strain GaugesAdvantage oSelf contained magnification auxillaryequipment to high inertia of the Gauge , it is unsuitable for dynamic measurements and varying response (due to high inertia)oNon-availability of adequate surface area on the test specimen and clearance above it to accommodate the Gauge together with its mounting Strain Gauges Opticalgaugesacombinationofmechanicaland opticalleversareusedtoamplifytherelative displacementbetweentheknifeedges.

2 Anydeformationofthestructuretowhichthisg augeisfixed, : Martens optical Gauge Optical Strain Gauges AschematicdiagramoftheTuckermanopticalga ugeandtheautocollimatorusedwithitisgiven inFigure. Theautocollimatorcarriesboththesourceofa parallelbeamoflighttomeasurethedeflectio nofthereflectedray. , optical Gauge Electrical Resistance Strain GaugeoStrainGaugeisapassivetransducertha tconvertsamechanicalelongationordisplace mentproducedduetoaforceintoitscorrespond ingchangeinresistance(R) , ,ifthemetalissubjectedtocompressivestres s,thelengthwilldecrease, of Strain , of Strain GaugeWorking of Strain of Strain GaugeThesensitivityofastraingaugeisdescr ibedintermsofacharacteristiccalledthegau gefactor,definedasunitchangeinresistance perunitchangeinlength. K=thegaugefactor R=theinitialresistanceinohms(withoutstra in) =thechangeininitialresistanceinohms L=theinitiallengthinmeters(withoutstrain ) = L o = the resistivity of the material (Ohms*m)oL = the length of the conductor (m)oA = the cross-sectional area of the conductor (m2)R= L/AWhat s the Wheatstone Bridge?

3 Wheatstonebridgeisanelectriccircuitsuita blefordetectionofminuteresistancechanges ,thereforeusedtomeasureresistancechanges ofastraingage Thebridgeisconfiguredbycombiningfourresi storsasshowninFigure. InitiallyR1=R2=R3=R4,inthisconditionnoou tputvoltageisthere,e=0 WhenoneoftheResistancesisreplacedbystrai nGaugeattachedtotheobjectwhosestrainisto bemeasuredandloadisapplied,thenthereissm allchangeintheresistanceofgauge,hencesom eoutputvoltageistherewhichcanberelatedto strainas Fromthis,straincanbeeasilydeterminedusin gtherelationHalf Bridge ConfigurationToincreasethesensitivityofm easurement,twostraingaugesareconnectedin thebridge,thistypeofconfigurationiscalle dasHalfbridgeasshowninfigureandtheoutput voltageandstraincanberelatedasWhengauges areconnectedtoadjacentarmsandWhengaugesa reconnectedtooppositearmsFull Bridge ConfigurationTofurtherenhancethesensitiv ity, , + R1,R2+ R2,R3+ R3andR4+ R4,respectively,thebridgeoutputvoltage,e .

4 Of a Strain GaugeoNatureofthestraintobedetectedoGaug eLengthoNumberofGaugesinGaugePatternoArr angementofGaugesinGaugePatternoGridResis tanceoTemperaturesensitivityoCarrierMate rialoGaugeWidthoAvailabilityolowcostElec trical Resistance Type Strain GaugeBasedonmounting: Bondedstraingauge Un-bondedstraingaugeBasedonconstruction: Foilstraingauge SemiconductorstraingaugeBonded Strain GaugeAbondedstrain-gageelement,consistin gofametallicwire,etchedfoil,vacuum-depos itedfilm, Strain Strain Strain GaugeFormeasurementsofsmallstrain,semico nductorstraingauges,socalledpiezoresisto rs, , BeamStrain GaugeStrain GaugeBending BeamStrain GaugeIn TensionStrain Gaugein compressionAdvantages & stressoVibration measurementoTorque measurementoBending and deflection measurementoCompression and tension measurementoStrain measurementBonding Procedure ,rustandplating,ifany,withagrinderorsand blastbeforepolishing DecideBondingPositionUsingapenciloramark ing-offpin, ,takecarenottodeeplyscratchthestrain-gag ebondingsurfaceBonding Procedure RemovegreasefrombondingsurfaceandcleanUs inganindustrialtissuepaper(SILBON paper)

5 Dippedinacetone, ,curingisadverselyaccelerated,therebylow eringtheadhesivestrengthBonding Procedure BondstraingagetomeasuringsiteAfterapplyi ngadropoftheadhesive,putthestraingageont hemeasuringsitewhileliningupthecentermar kswiththemarkingofflines ,donotliftittoadjusttheposition CompletebondingworkAfterpressingthestrai ngagewithathumbforoneminuteorso, ,goodmeasurementresultsareavailableafter 60minutesofcompletecuringoftheadhesiveMe asuring Strain with a Strain -IndicatoroFirst install a Strain gaugeoConnect the wires from the Strain Gauge to the Strain loading conditionsoRead Strain from Strain indicator Note that the indicator always displays 4 digits and reads in microstrain! Thus, 0017 means 17 micro-inches / inch of Strain .