Strain Gauge Selection

8/2/2019 Strain Gauge Selection

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1.0 Introduction

The initial step in preparing or any strain gage installationis the selection o the appropriate gage or the task. Itmight at rst appear that gage selection is a simple exercise,o no great consequence to the stress analyst; but quitethe opposite is true. Careul, rational selection o gage

characteristics and parameters can be very important in:optimizing the gage perormance or specied environmentaland operating conditions, obtaining accurate and reliablestrain measurements, contributing to the ease o installation,and minimizing the totalcost o the gage installation.

The installation and operating characteristics o a straingage are aected by the ollowing parameters, which areselectable in varying degrees:

strain-sensitivealloy

backingmaterial(carrier)

gridresistance

gagepattern

self-temperaturecompensationnumber

gagelength

options

Basically, the gage selection process consists o determiningthe particular available combination o parameters whichis most compatible with the environmental and otheroperating conditions, and at the same time best satisiesthe installation and operating constraints. Theseconstraints are generally expressed in the orm orequirements such as:

accuracy testduration

stability cyclicendurance

temperature easeofinstallation

elongation environment

The cost o the strain gage itsel is not ordinarily a primeconsideration in gage selection, since the signicant economicmeasure is the total cost o the complete installation, owhich the gage cost is usually but a small raction. In manycases, the selection o a gage series or optional eaturewhich increases the gage cost serves to decrease the totalinstallation cost.

It must be appreciated that the process o gage selectiongenerally involves compromises. This is because parameter

choices which tend to satisy one o the constraints orrequirements may work against satisying others. Forexample,inthecaseofasmall-radiusfillet,wherethespace available or gage installation is very limited, andthe strain gradient extremely high, one o the shortestavailable gages might be the obvious choice. At thesame time, however, gages shorter than about 0.125 in

[3 mm] are generally characterized by lower maximumelongation, reduced atigue lie, less stable behavior, andgreater installation diculty. Another situation which oteninfuences gage selection, and leads to compromise, is thestockofgagesathandforday-to-daystrainmeasurements.

While compromises are almost always necessary, thestress analyst should be ully aware o the eects o suchcompromises on meeting the requirements o the gageinstallation. This understanding is necessary to make the bestoverall compromise or any particular set o circumstances,and to judge the eects o that compromise on the accuracyand validity o the test data.

The strain gage selection criteria considered here relate

primarily to stress analysis applications. The selectioncriteria or strain gages used on transducer spring elements,while similar in many respects to the considerationspresented here, may vary signicantly rom application toapplicationandshouldbetreatedaccordingly.TheMicro-Measurements Transducer Applications Department can

assist in this selection.

2.0 Gage Selection Parameters2.1 Strain-Sensing Alloys

The principal component which determines the operatingcharacteristicsof a strain gage isthestrain-sensitivealloy

used in the oil grid. However, the alloy is not in every casean independently selectable parameter. This is because eachMicro-Measurementsstraingageseries(identiedbythersttwo,orthree,lettersinthealphanumericgagedesignation)is designed as a complete system. That system is comprisedo a particular oil and backing combination, and usuallyincorporatesadditionalgageconstructionfeatures(suchasencapsulation,integralleadwires,orsolderdots)specicto

the series in question.

Micro-Measurementssuppliesavarietyofstraingagealloysasfollows(withtheirrespectiveletterdesignations):
mailto:micro-measurements%40vishaypg.com?subject=


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A: Constantaninself-temperature-compensatedform.

P: Annealed constantan.

D: Isoelastic.

K:Nickel-chromium al loy, a modified Karma inself-temperature-compensatedform.

2.1.1 Constantan Alloy

O all modern strain gage alloys, constantan is the oldest,and still the most widely used. This situation refects theact that constantan has the best overall combination oproperties needed or many strain gage applications. Thisalloy has, or example, an adequately high strain sensitivity,or gage factor, which is relatively insensitive to strain level

and temperature. Its resistivity is high enough to achievesuitable resistance values in even very small grids, and itstemperature coeicient o resistance is not excessive. Inaddition, constantan is characterized by good atigue lieand relatively high elongation capability. It must be noted,however, that constantan tends to exhibit a continuous drit attemperatures above +150F [+65C]; and this characteristicshould be taken into account when zero stability o thestrain gage is critical over a period o hours or days.

Veryimportantly,constantancanbeprocessedforself- temperature-compensation(see boxat right)tomatcha

wide range o test material expansion coecients. A alloy issuppliedinself-temperature-compensation(S-T-C)numbers

00, 03, 05, 06, 09, 13, 15, 18, 30, 40 and 50, or use on testmaterials with corresponding thermal expansion coecients(expressedinppm/F).

Forthemeasurementofverylargestrains,5%(50000 )orabove,annealedconstantan(Palloy)isthegridmaterial

normally selected. Constantan in this orm is very ductile;and, in gage lengths o 0.125 in [3 mm] and longer, can bestrained to >20%. It should be borne in mind, however,that under high cyclic strains the P alloy will exhibit somepermanent resistance change with each cycle, and causea corresponding zero shit in the strain gage. Because othis characteristic, and the tendency or premature gridailure with repeated straining, P alloy is not ordinarilyrecommended or cyclic strain applications. P alloy isavailablewithS-T-Cnumbersof08and40foruseonmetals

and plastics, respectively.

2.1.2 Isoelastic Alloy

When purely dynamic strain measurements are to bemade that is, when it is not necessary to maintain astablereferencezeroisoelastic(Dalloy)offerscertain

advantages. Principal among these are superior atigue lie,comparedtoAalloy,andahighgagefactor(approximately3.2)whichimprovesthe signal-to-noise ratioin dynamictesting.

Dalloy isnotsubjectto self-temperature-compensation.

Moreover,asshowninthegraph(seebox),itsthermaloutputissohigh(about80/F[145/C])thatthis

alloy is not normally usable or static strain measurements.There are times, however, when D alloy nds application inspecial-purposetransducerswhereahighoutputisneeded,andwhereafull-bridgearrangementcanbeusedtoachievereasonable temperature compensation within the circuit.

Self-Temperature-Compensation

An important property shared by constantan andmodied Karma strain gage alloys is their responsivenessto specialprocessing forself-temperature-compensation.Self-temperature-compensatedstrain gagesaredesignedto produce minimum thermal output (temperature-induced apparent strain) over the temperature rangerom about 50 to +400F [45 to +200C]. Whenselectingeitherconstantan(A-alloy)ormodifiedKarma(K-alloy)strain gages, the self-temperature-compensation(S-T-C)numbermustbespecied.TheS-T-Cnumberistheapproximatethermalexpansioncoefficientinppm/Fofthe structural material on which the strain gage will displayminimum thermal output.The accompanying graph illustrates typical thermal outputcharacteristics or A and K alloys. The thermal output ouncompensated isoelastic alloy is included in the same graphforcomparison purposes. In normalpractice, theS-T-CnumberforanA-orK-alloygageisselectedtomostcloselymatch the thermal expansion coecient o the test material.However, the thermal output curves or these alloys can berotatedabouttheroom-temperaturereferencepointtofavora particular temperature range. This is done by intentionallymismatchingtheS-T-Cnumberandtheexpansioncoefcientintheappropriatedirection.WhentheselectedS-T-Cnumberis lower than the expansion coecient, the curve is rotatedcounterclockwise. An opposite mismatch produces clockwiserotation o the thermal output curve. Under conditions oS-T-Cmismatch,thethermaloutputcurvesforAandKalloysdo not apply, o course, and it will generally be necessary tocalibrate the installation or thermal output as a unction otemperature.For additional inormation on strain gage temperature eects,seeTechNoteTN-504.
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Other properties o D alloy should also be noted when

considering the selection o this grid material. It is, orinstance, magnetoresistive; and its response to strain issomewhat nonlinear, becoming signicantly so at strainsbeyond 5000.

2.1.3 Karma Alloy

Modiied Karma, or K alloy, with its wide areas oapplication, represents an important member in the amilyo strain gage alloys. This alloy is characterized by goodatigue lie and excellent stability; and is the preerred choiceor accurate static strain measurements over long periodsof time (monthsoryears)atroom temperature, orlesser

periods at elevated temperature. It is recommended orextended static strain measurements over the temperaturerange rom 452 to +500F [269 to +260C]. For shortperiods,encapsulatedK-alloystraingagescanbeexposed

to temperatures as high as +750F [+400C]. An inertatmosphere will improve stability and extend the useul gagelie at high temperatures.

Among its other advantages, K alloy oers a muchlatter thermal output curve than A alloy, and thuspermits more accurate correction or thermal outputerrors at temperature extremes. Like constantan, Kalloycan be self-temperature-compensatedfor use on

materials with dierent thermal expansion coeicients.The available S-T-C numbers inK alloy are limited,

however, to the ollowing: 00, 03, 05, 06, 09, 13, and 15.K alloy isthe normalselection whena temperature-

compensated gage is required that has environmentalcapabilities and perormance characteristics not attainableinA-alloygages.

Due to the diculty o soldering directly to K alloy, theduplex copper eature, which was ormerly oered as anoption,isnowstandardonallMicro-Measurementsopen-aced strain gages produced with K alloy. The duplex copperfeature isa precisely formedcoppersolderingpad(DP)ordot(DD),dependingontheavailabletabarea.AllK-alloy

gages which do not have leads or solder dots are speciedwithDPorDDaspartofthedesignation(inplaceof,orinadditionto,theoptionspecier).ThespecicstyleofcoppertreatmentwillbeadvisedwhentheCustomerServiceDepartmentiscontacted.Open-facedK-alloygagesmayalso

be ordered with solder dots.

2.2 Backing Materials

Conventionalfoilstraingageconstructioninvolvesaphoto-etched metal oil pattern mounted on a plastic backing orcarrier. The backing serves several important unctions:

providesameansforhandlingthefoilpatternduring

installation presents areadilybondablesurface foradheringthe

gage to the test specimen

provides electrical insulationbetweenthemetalfoil

and the test object

Backingmaterials supplied on Micro-Measurementsstraingagesareoftwobasictypes:polyimideandglass-fiber-reinforcedepoxy-phenolic.Asin thecaseof thestrain-sensitivealloy, thebacking isnot completely an

independently speciable parameter. Certain backing andalloy combinations, along with special construction eatures,are designed as systems, and given gage series designations.As a result, when arriving at the optimum gage type or

a particular application, the process does not permit thearbitrary combination o an alloy and a backing material,but requires the specication o an available gage series.Micro-Measurementsgageseriesandtheirpropertiesare

described in the ollowing Section 2.3. Each series has itsown characteristics and preerred areas o application; andselection recommendations are given in the tables that ollow.The individual backing materials are discussed here, as thealloys were in the previous section, to aid in understandingthe properties o the series in which the alloys and backingmaterials occur.

TheMicro-MeasurementspolyimideEbackingisatoughand extremely fexible carrier, and can be contoured readily

to t small radii. In addition, the high peel strength o thefoilonthepolyimidebackingmakespolyimide-backedgagesless sensitive to mechanical damage during installation.With its ease o handling and its suitability or use over thetemperature range rom 320 to +350F [195 to +175C],polyimideis anidealbackingmaterialforgeneral-purposestatic and dynamic stress analysis. This backing is capableo large elongations, and can be used to measure plasticstrains in excess o 20%. Polyimide backing is a eature oMicro-MeasurementsEA-,CEA-,EP-,EK-,S2K-,N2A-,andED-Seriesstraingages.

For outstanding perormance over the widest range otemperatures, the glass-fiber-reinforcedepoxy-phenolic

backing material is the most suitable choice. This backingcan be used or static and dynamic strain measurementfrom452to+550F[269to+290C].Inshort-term

applications, the upper temperature limit can be extendedto as high as +750F [+400C]. The maximum elongationo this carrier material is limited, however, to about 1 to2%.Reinforcedepoxy-phenolicbackingisemployedonthefollowinggageseries:WA,WK,SA,SK,WD,andSD.


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STaNdaRd STRaIN GaGe SeRIeS SeleCTIoN ChaRT

GageSeries

Description and Primary Application Temperature Range Strain Gage

Fatigue Life

StrainLevelin

Numberof Cycles

EA Constantan oil in combination with a tough, lexible,polyimide backing. Wide range o options available.Primarily intended or general-purpose static anddynamic stress analysis. Not recommended or highestaccuracy transducers.

Normal:100 to +350F [75 to +175C]

Special or Short-Term:320 to +400F [195 to +205C]

3% or gagelengths under

1/8 in [3.2 mm];5% or 1/8 in

and over

180015001200

105

106

108

CEA Universal general-purpose strain gages. Constantangrid completely encapsulated in polyimide, with large,rugged copper-coated tabs. Primarily used or general-purpose static and dynamic stress ana lysis. C-Featuregages are specially highlighted throughout the gagelistings o our Precision Strain Gages Data Book.

Normal:100 to +350F [75 to +175C]

Stacked rosettes limited to+150F [+65C]

3% or gagelengths under 1/8

in [3.2 mm];5% or 1/8 in

and over

15001500

105

106*

*Fatigue lie improvedusing low-modulussolder.

N2A Open-aced constantan oil gages with a thin, laminated,

polyimide-ilm backing. Primarily recommendedor use in precision transducers, the N2A Series ischaracterized by low and repeatable creep perormance.

Also recommended or stress analysis applicationsemploying large gage patterns, where the especially latmatrix eases gage installation.

Normal Static Transducer Service:100 to +200F [75 to +95C]

3% 17001500

106

107

WA Fully encapsulated constantan gages with high-endurance leadwires. Useul over wider temperatureranges and in more extreme environments than EASeries. Option W available on some patterns, butrestricts atigue lie to some extent.

Nml:100 t +400F [75 t +205c]

spl sht-Tm:320 t +500F [195 t +260c]

2%200018001500

105

106

107

SA Fully encapsulated constantan gages with solder dots.Same matrix as WA Series. Same uses as WA Seriesbut derated somewhat in maximum temperature andoperating environment because o solder dots.

Normal:100 to +400F [75 to +205C]


2%18001500

106

107

EP Specially annealed constantan oil with tough, high-elongation polyimide backing. Used primarily ormeasurements o large post-yield strains. Available

with Options E, L, and LE (may restrict elongationcapability).

100 to +400F [75 to +205C]

10% or gagelengths under

1/8 in [3.2 mm];

20% or 1/8 inand over

1000 104

EP gages show zeroshit under high-cyclicstrains.

ED Isoelastic oil in combination with tough, lexiblepolyimide backing. High gage actor and extendedatigue lie excellent or dynamic measurements. Notnormally used in static measurements due to very highthermal-output characteristics.

Dynamic:320 to +400F [195 to +205C]

2%Nonlinear atstrain levelsover 0.5%

25002200

106

107

WDFully encapsulated isoelastic gages with high-endur-ance leadwires. Used in wide-range dynamic strainmeasurement applications in severe environments.

Dynamic:320 to +500F [195 to +260C]

1.5%Nonlinear atstrain levelsover 0.5%

300025002200

105

107

108

SD Equivalent to WD Series, but with solder dots insteado leadwires.

Dynamic:320 to +400F [195 to +205C]

1.5%See above note

25002200

106

107

EK K-alloy oil in combination with a tough, lexible polyimidebacking. Primarily used where a combination o highergrid resistances, stability at elevated temperature, andgreatest backing lexibility are required.

Normal:320 to +350F [195 to +175C]


1.5% 1800 107

WK Fully encapsulated K-alloy gages with high-enduranceleadwires. Widest temperature range and most extremeenvironmental capability o any general-purpose gagewhen sel-temperature-compensation is required.Option W available on some patterns, but restricts bothatigue lie and maximum operating temperature.

452 to +550F [269 to +290C]Special or Short-Term:

452 to +750F [269 to +400C]1.5%

22002000

106

107

SK Fully encapsulated K-alloy gages with solder dots.Same uses as WK Series, but derated in maximumtemperature and operating environment because osolder dots.

Normal:452 to +450F [269 to +230C]


1.5%22002000

106

107

S2K K-alloy oil laminated to 0.001 in [0.025 mm] thick,high-perormance polyimide backing, with a laminatedpolyimide overlay ully encapsulating the grid andsolder tabs. Provided with large solder pads or ease oleadwire attachment.

Normal:100 to +250F [75 to +120C]


1.5%18001500

106

107

The perormance data given here arenominal, and apply primarily to gages o 0.125-in [3-mm] gage length or larger.


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2.3 Gage Series

AsnotedinSections2.1and2.2,thestrain-sensingalloyand

backing material are not subject to completely independentselection and arbitrary combination. Instead, a selectionmust be made rom among the available gage systems, orseries, where each series generally incorporates special designor construction eatures, as well as a specic combination oalloy and backing material. For convenience in identiying theappropriate gage series to meet specied test requirements,the inormation on gage series perormance and selection ispresented in the two tables above, in condensed orm.

Thersttablegivesbriefdescriptionsofallgeneral-purposeMicro-Measurements gage series including in each

case the alloy and backing combination and the principalconstruction eatures. This table denes the perormance oeach series in terms o operating temperature range, strainrange, and cyclic endurance as a unction o strain level.It must be noted, however, that the perormance data arenominal, and apply primarily to gages o 0.125 in [3 mm] orlonger gage length.

The second table gives the recommended gage seriesor speciic test proiles, or sets o test requirements,categorized by the ollowing criteria:

50 to +150F [45 to +65C]

50 to +400F [45 to +205C]

452 to +450F [269 to +230C]


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typeofstrainmeasurement(static,dynamic,etc.)

operatingtemperatureofgageinstallation

testduration

accuracyrequired

cyclic endurance required

This table provides the basic means or preliminary selectiono the gage series or most conventional applications. Italso includes recommendations or adhesives, since theadhesive in a strain gage installation becomes part o thegage system, and correspondingly aects the perormanceo the gage. This selection table, supplemented by theinormation in the irst table, is used in conjunctionwithourPrecisionStrainGagesDataBook, to arrive

at the complete gage selection. The procedure oraccomplishing thisis described inSection3.0 of this

Tech Note.

When a test prole is encountered that is beyond the rangesspecied in the above table, it can usually be assumed thatthe test requirements approach or exceed the perormancelimitations o available gages. Under these conditions, theinteractions between gage perormance characteristicsbecome too complex or presentation in a simple table.In such cases, the user should consult our ApplicationsEngineering Department or assistance in arriving at thebest compromise.

As indicated in the previoustable, theCEASeries isusuallythepreferredchoiceforroutinestrain-measurementsituations, not requiring extremes in perormance orenvironmentalcapabilities(and not requiring theverysmallestingagelengths,orspecializedgridcongurations).CEA-Series strain gages are polyimide-encapsulatedA-alloygages, featuringlarge, rugged, copper-coated tabsforeaseinsolderingleadwiresdirectlytothegage(below).These thin, fexible gages can be contoured to almost any

radius. In overall handling characteristics, or example,

convenience,resistance todamageinhandling,etc.,CEA-Seriesgagesareoutstanding.

2.4 Gage Length

Thegagelengthofastraingageistheactiveorstrain-

sensitive length o the grid, as shown below. The endloops and solder tabs are considered insensitive to strainbecauseoftheirrelativelylargecross-sectionalareaandlowelectrical resistance. To satisy the widely varying needs oexperimental stress analysis and transducer applications,Micro-Measurementsoffersgagelengthsrangingfrom

0.008 in [0.2 mm] to 4 in [100 mm].

Gagelengthisoftenaveryimportantfactorindeterminingthe gage perormance under a given set o circumstances.For example, strain measurements are usually made atthe most critical points on a machine part or structure that is, at the most highly stressed points. And, verycommonly, the highly stressed points are associated withstress concentrations, where the strain gradient is quite steep

and the area o maximum strain is restricted to a very smallregion. The strain gage tends to integrate, or average, thestrainovertheareacoveredbythegrid.Sincetheaverage

o any nonuniorm strain distribution is always less than themaximum, a strain gage which is noticeably larger than themaximum strain region will indicate a strain magnitude thatis too low. The sketch above illustrates a representative strain

GAGE LENGTH

BACKING

ENCAPSULATION

COPPER-COATED TABS

PEAKSTRAIN

INDICATEDSTRAIN

STRAIN

X


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distribution in the vicinity o a stress concentration, and

demonstrates the error in strain indicated by a gage which istoo long with respect to the zone o peak strain.

As a rule o thumb, when practicable, the gage length shouldbe no greater than 0.1 times the radius o a hole, llet, ornotch, or the corresponding dimension o any other stressraiser at which the strain measurement is to be made. Withstress-raisercongurationshavingthesignicantdimensionless than, say, 0.5 in [13 mm], this rule o thumb can lead tovery small gage lengths. Because the use o a small straingage may introduce a number o other problems, it is otennecessary to compromise.

Straingagesoflessthanabout0.125in[3mm]gagelength

tend to exhibit degraded perormance particularly in

terms o the maximum allowable elongation, the stabilityunder static strain, and endurance when subjected toalternating cyclic strain. When any o these considerationsoutweigh the inaccuracy due to strain averaging, a largergage may be required.

When they can be employed, larger gages oer severaladvantages worth noting. They are usually easier to handle(ingagelengthsupto,say,0.5inor13mm)innearly

every aspect o the installation and wiring procedure thanminiature gages. Furthermore, large gages provide improvedheat dissipation because they introduce, or the samenominal gage resistance, lower wattage per unit o grid area.This consideration can be very important when the gage

is installed on a plastic or other substrate with poor heattranser properties. Inadequate heat dissipation causes hightemperatures in the grid, backing, adhesive, and test specimensurace, and may noticeably aect gage perormance andaccuracy(seeTechNoteTN-502,Optimizing Strain GageExcitation Levels).

Stillanotherapplicationof largestraingages inthis

case, oten very large gages is in strain measurementon nonhomogeneous materials. Consider concrete, orexample,whichisamixtureofaggregate(usuallystone)andcement. When measuring strains in a concrete structure itis ordinarily desirable to use a strain gage o sucient gagelength to span several pieces o aggregate in order to measure

the representative strain in the structure. In other words, itis usually the average strain that is sought in such instances,not the severe local fuctuations in strain occurring at theinteraces between the aggregate particles and the cement.In general, when measuring strains on structures made ocomposite materials o any kind, the gage length shouldnormally be large with respect to the dimensions o theinhomogeneities in the material.

As a generally applicable guide, when the oregoingconsiderations do not dictate otherwise, gage lengths in therange rom 0.125 to 0.25 in [3 to 6 mm] are preerable. Thelargest selection o gage patterns and stock gages is available

in this range o lengths. Furthermore, larger or smaller sizes

generally cost more, and larger gages do not noticeablyimprove atigue lie, stability, or elongation, while shortergages are usually inerior in these characteristics.

2.5 Gage Pattern

The gage pattern reers cumulatively to the shape o the grid,thenumberandorientationofthegridsinamultiple-grid

gage, the solder tab conguration, and various constructioneatures which are standard or a particular pattern.All details o the grid and solder tab congurations areillustratedintheGagePatterncolumnsofourstraingage

data book. The wide variety o patterns in the list is designedto satisy the ull range o normal gage installation and

strain measurement requirements.

Withsingle-gridgages, patternsuitabilityforaparticularapplication depends primarily on the ollowing:

Solder tabs These should, o course, be compatiblein size and orientation with the space available at thegage installation site. It is also important that thetab arrangement be such as to not excessively tax theprociency o the installer in making proper leadwireconnections.

Grid width When severe strain gradientsperpendicular to the gage axis exist in the test specimensurace, a narrow grid will minimize the averagingerror. Wider grids, when available and suitable to theinstallation site, will improve the heat dissipation andenhance gage stability particularly when the gageis to be installed on a material or specimen with poorheat transer properties.

Gage resistance In certain instances, the onlydierence between two gage patterns available inthe same series is the grid resistance typically 120ohmsvs.350ohms.Whenthechoiceexists,thehigher-

resistance gage is preerable in that it reduces the heatgenerationratebyafactorofthree(forthesameappliedvoltageacrossthegage).Highergageresistancealso

has the advantage o decreasing leadwire eects suchas circuit desensitization due to leadwire resistance,and unwanted signal variations caused by leadwireresistance changes with temperature luctuations.Similarly,whenthegagecircuitincludesswitches,sliprings, or other sources o random resistance change, thesignal-to-noiseratioisimprovedwithhigherresistancegages operating at the same power level.

Inexperimental stressanalysis, a single-grid gagewould

normally be used only when the stress state at the point omeasurement is known to be uniaxial and the directionso the principal axes are known with reasonable accuracy(5).


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These requirements severely limit the meaningul applicability

ofsingle-gridstraingagesinstressanalysis;andfailuretoconsider biaxiality o the stress state can lead to large errorsin the stress magnitude inerred rom measurements madewithasingle-gridgage.

For a biaxial stress state a commoncase necessitating strain measurementatwo-orthree-elementrosetteisrequired in order to determine theprincipal stresses. When the directionso the principal axes are known inadvance,a two-element90-degree (ortee)rosettecanbeemployedwiththegage axes aligned to coincide with the principal axes. The

directions o the principal axes can sometimes be determinedwith sucient accuracy rom one o several considerations.For example, the shape o the test object and the mode oloading may be such that the directions o the principalaxes are obvious rom the symmetry o the situation, as ina cylindrical pressure vessel. The principal axes can also bedenedbyPhotoStress testing.

In the most general case o suracestresses, when the directions o theprincipal axes are not known romotherconsiderations,athree-element

rosette must be used to obtain theprincipal stress magnitudes. The

rosette can be installed with anyorientation, but is usually mountedso that one o the grids is aligned with some signicant

axis of the test object. Three-

element rosettes are available in both45-degreerectangularand60-degreedelta conigurations. The usualchoice is the rectangular rosette sincethedata-reductiontaskissomewhat

simpler or this conguration.

When a rosette is to be employed, careul consideration shouldalways be given to the dierence in characteristics betweensingle-planeandstackedrosettes.Foranygivengagelength,thesingle-plane rosetteis superior tothe stackedrosettein terms o heat transer to the testspecimen, generally providing betterstability and accuracy or static strain

measurements. Furthermore, whenthere is a signicant strain gradientperpendiculartothetestsurface(asinbending),thesingle-planerosette

will produce more accurate straindata because all grids are as close as possible to the testsurface.Stillanotherconsiderationisthat stackedrosettesare generally less conormable to contoured suraces thansingle-planerosettes.

On the other hand, when there are large strain gradients intheplaneofthetestsurface,asisoftenthecase,thesingle-

plane rosette can produce errors in strain indication becausethe grids sample the strain at dierent points. For these

applications the stacked rosette is ordinarily preerable.The stacked rosette is also advantageous when the space ormounting the rosette is limited.

90-degree rosette

45-degree rosette

Stacked rosette

Micro-Measurementsoffersaselectionofoptionalfeatures

or its strain gages and special sensors. The addition ooptions to the basic gage construction usually increases thecost, but this is generally oset by the benets. Examplesare:

Signicantreductionofinstallationtimeandcosts Reductionoftheskilllevelnecessarytomakedepend-

able installations

Increasedreliabilityofapplications

Simpliedinstallationofsensorsindifcultlocationsoncomponents or in the eld

Increased protection, both in handl ing duringinstallation and shielding rom the test environment

Achievementofspecialperformancecharacteristics

Availability o each option varies with gage series andpattern.Standardoptionsarenotedforeachsensorinour

strain gage data book.

Shownbelowisasummaryoftheoptionalfeaturesoffered.

optin Brif dscriptin

W Integral Terminals and Encapsulation

E Encapsulation with Exposed Tabs

SE Solder Dots and Encapsulation

L Preattached Leads

LE Preattached Leads and Encapsulation

STaNdaRd CaTaloG oPTIoNS

2.6 Optional Features

60-degree rosette


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Option W SeriesAvailability:EA, EP, WA, ED, WD, EK, WK

General Description: This option provides encapsulation, and thin, printed circuit terminals at the tab endo the gage. Beryllium copper jumpers connect the terminals to the gage tabs. The terminals are 0.0014 in[0.036 mm] thick copper on polyimide backing nominally 0.0015 in [0.038 mm] thick. Option W gages arerugged and well protected, and permit the direct attachment o larger leadwires than would be possible withopen-facedgages.ThisoptionisprimarilyusedonEA-Seriesgagesforgeneral-purposeapplications.Solder:+430F [+220C]tin-silveralloysolderjointsonE-backedgages,+570F[+30 0C]lead-tin-silveralloysolderjointsonW-backedgages.Temperature Limit: +400F [+200C]forE-backedgages,+500F[+260C] orW-backedgages.Grid Protection: Entire grid and part o terminals are encapsulated with polyimide. FatigueLife:Somelossinfatiguelifeunlessstrainlevelsattheterminallocationarebelow1000. Size: Option Wextends rom the soldering tab end o the gages and thereby increases gage size. With some patterns width is slightly greater.Strain Range:Withsomegageseries,notablyE-backedgages,strainrangewillbereduced.ThiseffectisgreatestwithEP

gages, and Option W should be avoided with them i possible. Flexibility: Option W adds encapsulation, making gagesslightly thicker and stier. Conormance to curved suraces will be somewhat reduced. In the terminal area itsel, stiness

is markedly increased. Resistance Tolerance:OnE-backedgages,resistancetoleranceisnormallydoubled.

Option E SeriesAvailability: EA, ED, EK, EP

General Description: Option E consists o a protective encapsulation o polyimide lm approximately 0.001 in[0.025mm]thick.Thisprovidesruggednessandexcellentgridprotection,withlittlesacriceinexibility.Sol-dering is greatly simplied since the solder is prevented rom tinning any more o the gage tab than is deliber -

ately exposed or lead attachment. Option E protects the grid rom ngerprints and other contaminating agentsduringinstallationand,therefore,contributessignicantlyto long-termgagestability.Heavierleadsmaybeattacheddirectlytothegagetabsforsimplestaticloadtests.Supplementaryprotectivecoatingsshouldstillbe

applied ater lead attachment in most cases. Temperature Limit: No degradation. Grid Protection: Entire gridand part o tabs are encapsulated. Fatigue Life: When gages are properly wired with small jumpers, maximum

endurance is easily obtained. Size:Gagesizeisnotaffected. Strain Range:Strainrangeofgageswillbereducedbecausethe additional reinorcement o the polyimide encapsulation can cause bond ailure beore the gage reaches its ull straincapability. Flexibility:OptionEgagesarealmostasconformableoncurvedsurfacesasopen-facedgages,sincenointernalleads or solder are present at the time o installation. Resistance Tolerance: Resistance tolerance is normally doubled whenOption E is selected.

Option SE SeriesAvailability: EA, ED, EK, EP

General Description:OptionSEisthecombinationofsolderdotsonthegagetabswitha0.001-in[0.025-mm]polyimide encapsulation layer that covers the entire gage. The encapsulation is removed over the solder dotsproviding access or lead attachment. These gages are very fexible, and well protected rom handling damageduringinstallation.OptionSEisprimarilyintendedforsmallgagesthatmustbeinstalledinrestrictedareas,

since leadwires can be routed to the exposed solder dots rom any direction. The option does not increaseoverallgagedimensions,sothematrixmaybeeld-trimmedveryclosetotheactualpatternsize.OptionSEissometimesusefulonminiaturetransducersofmedium-orlow-accuracyclass,orinstressanalysisworkonminiature parts. Solder: +570F [+300C]tin-silveralloy.Topreventlossoflong-termstability,gageswithOptionSEmustbesolderedwithnoncorrosive(rosin)ux,andalluxresidueshouldbecarefullyremovedwithRosinSol-vent ater wiring. Protective coatings should then be used. Temperature Limit: No degradation. Grid Protection: Entire gageis encapsulated. Fatigue Life: When gages are properly wired with small jumpers, maximum endurance is easily obtained.Size:Gagesizeisnotaffected. Strain Range:Strainrangeofgageswillbereducedbecausetheadditionalreinforcement

o the polyimide encapsulation can cause bond ailure beore the gage reaches its ull strain capability. Flexibility: OptionSEgagesarealmostasconformableoncurvedsurfacesasopen-facedgages. Resistance Tolerance: Resistance tolerance isnormallydoubledwhenOptionSEisselected.


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Option LE SeriesAvailability: EA, ED, EK, EP

General Description: This option provides the same conormable sot copper lead ribbons as used in Option L,butwiththeadditionofa0.001-in[0.025-mm]thickencapsulationlayerofpolyimidelm.Theencapsulationlayer provides excellent protection or the gage during handling and installation. It also contributes greatly toenvironmentalprotection,thoughsupplementarycoatingsarestillrecommendedforelduse.GageswithOp-tionLEwillnormallyshowbetterlong-termstabilitythanopen-facedgageswhicharewaterproofedonlyafter

installation. A good part o the reason or this is that the encapsulation layer prevents contamination o the gridsurace rom ngerprints or other agents during handling and installation. The presence o such contaminantswill cause some loss in gage stability, even though the gage is subsequently coated with protective compounds.Leads: Nominal ribbon size or most gages is 0.012 wide x 0.004 in thick [0.30 x 0.10 mm] copper ribbons. Leadsare approximately 0.8 in [20 mm] long. Solder: +430F [+220C] tin-silveralloy.Temperature Limit: +400F[+200C]. Grid Protection: Entire gage is encapsulated. A short extension o the backing is let uncovered at theleadwire end to prevent contact between the leadwires and the specimen surace. Fatigue Life: Fatigue lie willnormally be degraded by Option LE. This occurs primarily because the copper ribbon has limited cyclic endurance. Option LEisnotoftenrecommendedforveryhighendurancegagessuchastheEDSeries.Size: Matrix size is unchanged. Strain Range:StrainrangewillusuallybereducedbytheadditionofOptionLE.Flexibility:GageswithOptionLEarenotasconformableas standard gages. Resistance Tolerance: Resistance tolerance is normally doubled by the addition o Option LE.

Leadwire Orientation for Options L and LE

These illustrations show the standard orientation o leadwires relative to the gage pattern geometry or Options L and LE.The general rule is that the leads are parallel to the longest dimension o the pattern. The illustrations also apply to leadwireorientationforWA-,WK-andWD-Seriesgages,whenthepatternshownisavailableinoneoftheseseries.

(3 or 4 tabs)

Option L SeriesAvailability: EA, ED, EK, EP

General Description: OptionListheadditionof softcopperleadribbonstoopen-facedpolyimide-backedgages. The use o this type o ribbon results in a thinner and more conormable gage than would be the casewith round wires o equivalent cross section. At the same time, the ribbon is so designed that it orms almostas readily in any desired direction. Leads: Nominal ribbon size or most gages is 0.012 wide x 0.004 in thick[0.30 x 0.10 mm]. Leads are approximately 0.8 in [20 mm] long. Solder: +430F [+220C] tin-silveralloy.Temperature Limit: +400F [+200C]. Fatigue Life: Fatigue lie will normally be degraded by Option L. Thisoccurs primarily because the copper ribbon has limited cyclic endurance. When it is possible to careullydress the leads so that they are not bonded in a high strain eld, the perormance limitation will not apply.OptionLisnotoftenrecommendedforveryhighendurancegagessuchastheEDSeries.Size: Matrix size isunchanged. Strain Range:StrainrangewillusuallybereducedbytheadditionofOptionL. Flexibility:Gageswith Option L are not as conormable as standard gages. Resistance Tolerance: Not aected.


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As in other aspects o strain gage selection, the choice ooptions ordinarily involves a variety o compromises. Forinstance, an option which maximizes a particular gage

perormance parameter such as atigue lie may at the sametime require greater skill in installing the gage. Becauseo the many interactions between installation attributesand perormance parameters associated with the options,the relative merits o all standard options are summarizedqualitatively in the chart on page 60 as an aid to optionselection. For comparison purposes, the correspondingcharacteristicsoftheCEASeriesaregivenintheright-mostcolumn o the table.

Since,instrainmeasurementforstressanalysis,thestandardoptionsaremostfrequentlyappliedtoEA-Seriesstraingages, the inormation supplied in this section is directedprimarily toward such option applications.

WhencontemplatingtheapplicationofanEA-Seriesgage

with an option, the rst consideration should usually bewhether there isan equivalentCEA-Series gage thatwill

satisy the test requirements. Comparing, or example, anEA-SeriesgageequippedwithOptionWandasimilarCEA-Seriespattern,itwillbefoundthatthelatterischaracterizedby lower cost, greater fexibility and conormability, andsuperior atigue lie. The only possible advantages or theselection o Option W are the wider variety o availablepatterns and the occasional need or large solderingterminals.

It should also be noted that many standard strain gage types,without options, are normally available rom stock; whilegages with options are commonly manuactured to order,and may thus involve a minimum order requirement.

Option P SeriesAvailability: EA, N2A

General Description: OptionPistheadditionofpreattachedleadwirecablestomanypatternsofEA-andN2A-Seriesstraingages.Encapsulationsealssmalljumperleadwiresatgageend,andcableinsulationprotectssolder

joints at cable end. Option P virtually eliminates need or soldering during gage installation. Leads:Apairof1-in[25-mm]M-LINE134-AWP(solidcopper,polyurethaneenamel)singleconductorjumperleadwires.Cable: 10ft[3.1m]ofcolor-coded,at,three-conductor26-gauge[0.404mmdia.],stranded,tinnedcopperwithvinylin-sulation(similartoM-LINE326-DFV).Solder: +430F[+220C]tin-silveralloysolderjoints,jumpertogage.

Cable conductors and jumpers joined with +430F [+220C] solder beneath cable insulation. Exposed leadwireson unattached end o cable are pretinned or ease o hookup. Temperature Limit:60 to +180F [50 to +80C];limited by vinyl insulation on cable. Grid Encapsulation: Entire grid and tabs are encapsulated with Option E.Fatigue Life: Fatigue lie will normally be degraded by Option P, primarily because the copper jumper wireshave limited cyclic endurance. Pattern Availability: MostEA-andN2A-Seriessingle-gridpatternsthatare0.062

in [1.5 mm] or greater gage length, with parallel solder tabs on one end o the grid, and suitable or encapsulation.

(ConsultourApplicationsEngineeringDepartmentforavailabilityofOptionPonothergageseries/patterns,andfornonstandardcablelengths.)Size: Matrix size is unchanged. Strain Range: Strainrangewillusuallybereducedby the addition o Option P. Flexibility:E-backedgageswithOptionParenotasconformableasstandardgages.Resistance Considerations:Eachconductorofthecablehasanominalresistanceof0.04ohm/ft[0.13ohm/m].Gageresistanceis measured at gage tabs. Gage Factor:Gagefactorisdeterminedforgageswithoutpreattachedcable.

Option P2 SeriesAvailability: CEA

General Description: OptionP2istheadditionofpreattachedleadwirecablestoCEA-Seriesstraingages.Op-tion P2 virtually eliminates need or soldering during gage installation. Cable: 10ft[3.1m]ofcolor-coded,at,three-conductor30-gauge[0.255mm],stranded,tinnedcopperwithvinylinsulation(similartoM-LINE330-DFV).Solder: +361F[+180C]tin-leadalloysolderjoints.Exposedleadwiresonunattachedendof

cable are pretinned or ease o hookup. Temperature Limit:60 to +180F [50 to +80C]; limited by vinyl

insulation on cable. Grid Encapsulation: Entiregridisencapsulated.(Soldertabsarenotencapsulated.)FatigueLife: Fatigue lie will normally be unchanged by Option P2. Pattern Availability:MostCEA-Seriessingle-andmultiple-gridpatterns.Size: Matrix size is unchanged. Strain Range: StandardforCEA-Seriesgages.Flexibil-ity: No appreciable increase in stiness. Resistance Considerations: Each conductor o the cable has a nominalresistanceof0.11ohm/ft[0.36ohm/m].Gageresistanceismeasuredatgagetabs. Gage Factor: Gagefactorisdetermined or gages without preattached cable.

2.7 Characteristics of Standard Catalog Options on EA-Series Gages


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anopen-faced EA-Seriesgagewithout optionsarearbitrarily assigned a value o 5. Numbers greater than 5

indicate a particular parameter is improved by addition o

the option, while smaller numbers indicate a reduction inperormance.

Insttin attribut Stnr optins Cea or Prfrmnc Prmtr

W e Se l le Sris

Overall Ease o Gage Installations 8 7 6 5 6 10

Ease o Leadwire Attachment 10 8 7 7 8 10

Protection o Grid rom Environmental Attack 8 8 8 5 8 8

Cyclic Strain Indurance 2 7 8 3 4 4

Elongation Capability 2 3 3 4 3 3

Resistance Tolerance 3 3 3 5 3 3Reinorcement Eects 2 3 3 5 3 3

3.0 Gage Selection Procedure

The perormance o a strain gage in any given application isaected by every element in the design and manuacture othegage.Micro-Measurementsoffersagreatvarietyofgage

types or meeting the widest range o strain measurementneeds. Despite the large number o variables involved, theprocess o gage selection can be reduced to only a ew basicsteps. From the ollowing diagram that explains the gage

designation code, it is evident that there are but ve parametersto select, not counting options. These are: the gage series, theS-T-C number, thegagelength and pattern, and the

resistance.

O the preceding parameters, the gage length and pattern arenormally the rst and second selections to be made, basedon the space available or gage mounting and the natureo the stress eld in terms o biaxiality and expected straingradient. A good starting point or initial consideration ogage length is 0.125 in [3 mm]. This size oers the widestvariety o choices rom which to select remaining gage

parameters such as pattern, series and resistance. Thegage and its solder tabs are large enough or relatively easyhandling and installation. At the same time, gages o thislength provide perormance capabilities comparable to thoseo larger gages.

The principal reason or selecting a longer gage wouldcommonlybeoneofthefollowing:(a)greatergridareaforbetterheatdissipation;(b)improvedstrainaveragingon inhomogeneousmaterialssuch as fiber-reinforcedcomposites;or(c)slightlyeasierhandlingandinstallation(forgagelengthsupto0.50in[13mm]).Ontheotherhand,

a shorter gage length may be necessary when the object isto measure localized peak strains in the vicinity o a stressconcentration, such as a hole or shoulder. The same is true,o course, when the space available or gage mounting is verylimited.

In selecting the gage pattern, the irst consideration iswhetherasingle-gridgageorrosetteisrequired(seeSection2.5). Single-gridgagesareavailablewithdifferentaspect(length-to-width)ratiosandvarioussoldertabarrangementsforadaptabilitytodifferinginstallationrequirements.Two-element90-degreerosettes,when applicable, canalso beselected rom a number o dierent grid and solder tabcongurations.Withthree-elementrosettes(rectangularordelta),theprimarychoiceinpatternselection,oncethegage

length has been determined, is between planar and stackedconstruction,asdescribedinSection2.5.

The ormat o our strain gage data book is designed tosimplifyselectionofthegagelengthandpattern.Similarpatterns available in each gage length are grouped together,and listed in order o size. The strain gages in the main

ea-06-250BF-350 oPTIoN le

GaGe leNGTh oPTIoN (IF aNY)

GaGe PaTTeRN ReSISTaNCe

GaGe SeRIeS S-T-C NUMBeR

Strin Gg Sctin Stps

Stp 4

Stp 5

Stp 6

Stp 1

Stp 2

Stp 3


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listing(largepictures)arethemostwidelyusedforstress

analysis applications. This section should always be reviewedrst to locate an appropriate gage.

With an initial selection o the gage size and patterncompleted, the next step is to select the gage series, thusdetermining the oil and backing combination, and anyother eatures common to the series. This is accomplished byreerring to the earlier chart, which gives the recommendedgage series or speciic test proiles, or sets o testrequirements. I the gage series is to have a standard optionapplied, the option should be tentatively specied at thistime, since the availability o the desired option on theselected gage pattern in that series requires veriicationduring the procedure outlined in the ollowing paragraph.

Afterselectingthegageseries(andoption,ifany),referenceismadeagaintoourPrecisionStrainGagesDataBooktorecord the gage designation o the desired gage size andpattern in the recommended series. I this combination is notlisted as available in the catalog, a similar gage pattern in thesame size group, or a slightly dierent size in an equivalentpattern, can usually be selected or meeting the installationand test requirements. In extreme cases, it may be necessaryto select an alternate series and repeat this process. Quiterequently, and especially or routine strain measurement,more than one gage size and pattern combination will besuitable or the specied test conditions. In these cases, it iswiseto selectagagefromthemainlistings(largepictures)

to eliminate the likelihood o extended delivery time or aminimum order requirement.

As noted under the gage pattern discussion on page 55, thereareoftenadvantagesfromselectingthe350-ohmresistance

i this resistance is compatible with the instrumentationto be used. This decision may be infuenced, however, bycost considerations, particularly in the case o very smallgages. Somereductioninfatiguelife canalsobeexpectedforthe high-resistancesmall gages.Finally, inrecordingthecompletegagedesignation,theS-T-Cnumbershouldbe inserted rom the list o available numbers or each alloygiven in this Tech Note.

This completes the gage selection procedure. In each step o

theprocedure,theStrainGageSelectionChecklistprovidedinSection4.0shouldbereferredtoasanaidinaccountingor the test conditions and requirements which could aectthe selection.

4.0 Strain Gage Selection Checklist

This checklist is provided as a convenient, rapid means orhelping make certain that no critical requirement o the testproile which could aect gage selection is overlooked.

It should be borne in mind in using the checklist thatthe considerations listed apply to relatively routine andconventional stress analysis situations, and do not embraceexotic applications involving nuclear radiation, intensemagnetic elds, extreme centriugal orces, and the like.

CoNSIdeRaTIoNS FoR

PaRaMeTeR SeleCTIoN

Selection Step: 1 strain gradients Parameter: Gg lngt area o maximum strain

accuracy required

static strain stability

maximum elongation

cyclic endurance

heat dissipation

space or installation

ease o installation

Selection Step: 2 strain gradients (in-plane

Parameter: Gg Pttrn and normal to surace)

biaxiality o stress heat dissipation

space or installation

ease o installation

gage resistance availability

Selection Step: 3 type o strain measurement

Parameter: Gg Sris application (static, dynamic,

post-yield, etc.)

operating temperature

test duration

cyclic endurance

accuracy required

ease o installation

Selection Step: 4 type o measurement (static,

Parameter: optins dynamic, post-yield, etc.)

installation environment

laboratory or eld

stability requirements

soldering sensitivity o

substrate (plastic, bone, etc.)

space available or installation

installation time constraints

Selection Step: 5 heat dissipation

Parameter: Gg Rsistnc leadwire desensitization

signal-to-noise ratio

Selection Step: 6 test specimen material Parameter: S-T-C Numbr operating temperature range

accuracy required


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5.0 Gage Selection Examples

Inthissection,threeexamplesaregivenofthegage-selection procedure in representative stress analysissituations. An attempt has been made to provide theprincipal reasons or the particular choices which aremade. It should be noted, however, that an experiencedstress analyst does not ordinarily proceed in the samestep-by-stepfashionillustratedin these examples.Instead,

simultaneously keeping in mind the test conditions andenvironment, the gage installation constraints, and the testrequirements, the analyst reviews our strain gage data bookand quickly segregates the more likely candidates romamongtheavailablegage-patternandseriescombinationsinthe appropriate sizes. The selection criteria are then rened

inaccordancewiththeparticularstrain-measurementtaskto converge on the gage or gages to be specied or the testprogram. Whether ormally or otherwise, the knowledgeablepractitioner does so in the light o parameter selectionconsiderations such as those itemized in the precedingchecklist.

A. Design Study of a Pressure Vessel

Strainmeasurementsaretobemadeonascaled-downplastic model o a pressure vessel. The model will be testedstatically at, or near, room temperature; and, althoughthe tests may be conducted over a period o several

months, individual tests will take only a ew hours to run.

Gage Selection:

1. Gage Length Very short gage lengths should beavoided in order to minimize heat dissipation problemscaused by the low thermal conductivity o the plastic.

The model is quite large, and apparently ree o severestraingradients;therefore,a0.25-in[6.3-mm]gagelength is specied, because the widest selection o gagepatterns is available in this length.

2. Gage Pattern In some areas o the model, thedirections o the principal axes are obvious romconsiderationsofsymmetry,andsingle-gridgagescan

be employed. O the patterns available in the selectedgage length, the 250BF pattern is a good compromisebecause o its high grid resistance which will helpminimize heat dissipation problems.

In other areas o the model, the directions o theprincipalaxesarenotknown, anda three-elementrosette will be required. For this purpose, a planar

rosette should be selected, since a stacked rosette wouldcontribute signiicantly to reinorcement and heatdissipationproblems.Becauseofitshigh-resistance

grid, the 250RD pattern is a good choice.

3. Gage SeriesThepolyimide(E)backingispreferredbecause its low elastic modulus will minimizereinorcement o the plastic model. Because the normalchoice o grid alloy or static strain measurement atroomtemperatureistheAalloy,theEASeriesshould

be selected or this application.

4. Options Excessive heat application to the test modelduring leadwire attachment could damage the material.OptionL(preattachedleads)isthereforeselectedso

that the instrument cable can be attached directly tothe leads without the application o a soldering iron tothe gage proper. Option L is preerable over Options LEand P because the encapsulation in the latter optionswould add reinorcement.

5. Resistance In this case, the resistance was determinedinStep2whenthehigherresistancealternativewas

selected rom among the gage patterns; i.e., in selectingthe250BFoverthe250BG,andthe250RDoverthe250RA. The selected gage resistance is thus 350 ohms.

6. S-T-C NumberIdeally,thegagesshould beself-temperature-compensatedtomatchthemodelmate-rial, but this is not always easible, since plastics particularly reinorced plastics vary widelyin thermal expansion coeicient. For unreinorcedplastic,S-T-C30,40or50shouldusuallybeselected.

I a mismatch between the model material and theS-T-Cnumberisnecessary,S-T-C13shouldbeselected(becauseofstockstatus),andthetestperformedatconstant temperature.

Gage Designations:

From the above steps, the strain gages to be used are:

EA-30-250BF-350/OptionL(single-grid)EA-30-250RD-350/OptionL(rosette)


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B. Dynamic Stress Analysis Study

of a Spur Gear in an Hydraulic PumpStrainmeasurementsaretobemadeattherootofthegear

tooth while the pump is operating. The llet radius at thetoothrootis0.125in(orabout3mm)andtesttemperatures

are expected to range rom 0 to +180F [20 to +80C].

Gage Selection:

1. Gage Length A gage length which is small withrespect to the llet radius should be specied or thisapplication. A length o 0.015 in [0.38 mm] is preerable,but reerence to our strain gage data book indicatesthat such a choice severely limits the available gagepatterns and grid alloys. Anticipating problems whichwouldotherwisebeencounteredinSteps2and3,agage length o 0.031 in [0.8 mm] is selected.

2. Gage Pattern Because the gear is a spur gear, thedirectionsoftheprincipalaxesareknown,andsingle-

grid gages can be employed. A gage pattern with bothsolder tabs at the same end should be selected so thatleadwire connections can be located in the clearancearea along the root circle between adjacent teeth. Inthe light o these considerations, the 031CF pattern ischosen or the task.

3. Gage Series Low strain levels are expected in thisapplication; and, urthermore, the strain signalsmust be transmitted through slip rings or through atelemetry system to get rom the rotating component

tothestationaryinstrumentation.Isoelastic(Dalloy)ispreferredforitshighergagefactor(nominally3.2,incontrastto2.1forAandKalloys).Becausethegage must be very lexible to conorm to the smallllet radius, the E backing is the most suitable choice.The maximum test temperature is not a considerationin this case, since it is well within the recommendedtemperature range or any o the standard backings.The combination o the E backing and the D alloydenes the ED gage series.

4. Options For protection o the gage grid in the testenvironment, Option E, encapsulation, should bespecied. Because o the limited clearance between the

outside diameter o one gear and the root circle o themating gear, a particularly thin gage installation mustbe made; and very small leadwires will be attached tothe gage tabs at 90 to the grid direction, and run overthe sides o the gear or connection to larger wires.This requirement necessitates attachment o the smallleadwires ater gage bonding, and prevents the use opreattached leads.

5. ResistanceIntheED-Seriesversionofthe031CF

gage pattern, our strain gage data book lists theresistance as 350 ohms. The higher resistance shouldusually be selected whenever the choice exists, andwill be advantageous in this instance in improving thesignal-to-noiseratiowhenslipringsareused.

6. S-T-C NumberDalloyisnotsubjecttoself-temperature-compensation, nor is compensationneeded or these tests since only dynamic strain is tobemeasured.IntheED-Seriesdesignationthetwo-digitS-T-CnumberisreplacedbythelettersDYfor

dynamic.

Gage Designation:

Combining the results o the above selection procedure, thegage to be employed is:

ED-DY-031CF-350/OptionE

C. Flight-Test Stress Analysisof a Titanium Aircraft Wing Tip Section

With, and Without, a Missile Module Attached

The operating temperature range or strain measurementsis rom 65 to +450F [55 to +230C], and will be adominant actor in the gage selection.

Gage Selection:

1. Gage Length Preliminary design studies using thePhotoStressphotoelasticcoatingtechniqueindicatethat a gage length o 0.062 in [1.6 mm] represents thebest compromise in view o the strain gradients, areaso peak strain, and space or gage installation.


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2. Gage Pattern With inormation about the stressstate and directions o principal axes gained rom thephotoelastic coating studies, there are some areas othewingtipwheresingle-gridgagesandtwo-elementtee rosettes can be employed. In other locations,where principal strain directions vary with the natureoftheightmaneuver,45-degreerectangularrosettes

are required.

The strain gradients are suciently steep that stackedrosettes should be selected. From our strain gage databook, the oregoing requirements suggest the selectiono 060WT and 060WR gage patterns or the stackedrosettes,andthe062APpatternforthesingle-grid

gage. In making this selection, attention was given tothe act that all three patterns are available in the WKSeries,whichiscompatiblewiththespeciedoperatingtemperature range.

3. Gage Series The maximum operating temperature,along with the requirement or static as well as dynamicstrain measurement, clearly dictates use o K alloy orthegridmaterial.EithertheSKorWKSeriescouldbe

selected, but the WK gages are preerred because theyhave integral leadwires.

4. Options For ease o gage installation, Option W,with integral soldering terminals, is advantageous.This option is not applicable to stacked rosettes,however,andisthereforespeciedforonlythesingle-grid gages.

5. ResistanceWhenavailable,asinthiscase,350-ohm gages should be specied because o the benetsassociated with the higher gage resistance.

6. S-T-C Number The titanium alloy used in the wingtipsectionisthe6Al-4Vtype,withathermalexpansion

coeicient o 4.9106 per F [8.8106 per C].KalloyofS-T-Cnumber05istheappropriatechoice.

Gage Designations:

WK-05-062AP-350/OptionW WK-05-060WT-350 WK-05-060WR-350

Documents

Strain Gauge Selection