lecture_14_ strain gage installation

8/11/2019 Lecture_14_ Strain Gage Installation

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M L/NE ACCESSORIES

Student Instruction Bulletin

Strain G age Installations withM Bond 200 and AE 10 Adhesive Systems

1.0 INTRODUCTION accurate an d reliable strain gage measurem ents. it is stronnlvrecomm ended t hat a certified-adhesive suchas -~ond f0i)Because the strain gage is an extremely sensitive device ca- methyl-2-cyanoacrylate or M-Bond epoxy adhesivepable of registering the smallest effects of an imperfect bond, be selected or most general labora torv installations.considerable attention to detail must be takefi to assurestable, creep-free installations. However, the techniquesinvolved are very simple, and readily mastered. 2.1 M Bond 200

This man ual gives explicit step-by-step instructions fo r mak- Micro-Measurements certified M-Bond 200 is a n excellenting consistently successful strain gage installations with general-purpose laboratory adhesive because of its fastM-Bond 200 and M-Bond AE-10 Adhesives. These direc- room-tem perature cure and ease of application. It is com pat-tions should be followed precisely. More detailed informa- ible with all Micro-Measurements strain gages and all com-tion may be found in the Measurements Group VideoTechTn mon str uctu ral materials. M-Bon d 200 Adhesive can be usedLibrary and in the following publications: for high-elon gation tests (+60 OOOpe), for fatig ue studies,

and f i r one-cycle proof tests within a no rm 2 operatingInstru ction Bulletin B-129,Surface Preparation or temperature range of -25 to +150 F (-32 to +65 C .Strain Gage Bonding.

Instruction Bulletin B-127, Strain Gage Installa-tions with M-Bo nd 200 Adhesive.

Instruction Bulletin B-137, Strain Gage Installa-tiom wit M BondAE:10/15andM sondGA 2A~Systems.

All operations described in this manual can be performedwith the use of the S tude nt Strain Gage Application Kit. Theprocedures outlined here are ideally suited to the classroomor teaching laboratory. For most teachingllearning activi-ties involving strain gage technology, the specially priced,first-quality Student Gages manufactured by Micro-Measurements Division of the M easurements Gro up may beused with ex cellent results.

The catalyst supplied with M-Bond 200 is specially formu-lated to control the reactivity rate. For best results, thecatalyst should be used sparingly. Since M-Bond 200 bon dsare weakened by exposure to high humidity, adequate pro-tective coatings are essential. Because this adhesive willbecome harder and more brittle with time, M-Bond 200 isnot generally recommended for permanent installations overone or two years in duration.

HANDLING PRECAUTIONS

M-Bond 200 is a cyanoacrylate compound.Immediatebonding of eye, skin, or mouth m ay result upon contact.Causes irritation. The user is cautioned to (1)avoid con-tact with skin; (2) avoidprolo nged or repeated breathingof vapors; and (3) use with adequate ventilation. Foradditional health and safety information, consult themateria lsafety d ata sheet which is available upon request.

2 .0 ST RA IN GAGE ADHESIVES The shelf life of M-Bond 200 is six months when storedunder normal laboratory conditions. Life of unopened

Because consistently successful installation of strain gages material can be extended by refrigeration [+40F(+5 C ] .requires the use of an adhe sive certified for str ain gage use, Due to possible condensation problems, care should beMicro-MeasurementsM LINEadhesivesundergoextensive taken to allow the unopened bottle to return to roomlaboratory testing to ensure reliability and consistency of temperature before opening. Refrigeration after open ing isthose properties required in strain gage bonding. To assure not recommended.


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Micro-Measurements certified M-Bond AE-10 is a 100

solids epoxy system for use with strain gages. It offers theadvantages of high elongatio n (10 ) and wider operatingtem pera ture ra nge [-320 to +200F (-195' to +9JoC)].Because it is highly resistant to m oisture and m ost chemic als,M-Bond AE-10 is recommended for perman ent 'installationsover one year in duration.

M-Bond AE-10 Adhesive is supplied in kit form with pre-weighed resin and sufficient curing agent for six separatemixes of adhesive. Allow the materials to attain roomtemp erature before opening the containers. Each of the indi-vidual units of resin can be separately activated by filling oneof the calibrated d ropp ers with curing agentexactly to thenumber 10 and dispensing the contents int o the center of the

jar of resin. Immediately cap the bottle of curing agent toavoid moisture absorption Mix the resin and curing agentfor five minutes, using one of the plastic stirring rods. Th epot life or working time after m ixing is 15 to 20 m inutes at+75OF (+24O C). Th e po t life can be som ew hat extend ed byoccasionally stirring the mixture, by cooling the jar, or byspreading the adhesive on a chemically clean aluminumplate. Discard the dropp er and stirring rod after use.

HANDLING PRECAUTIONS

While M-Bond AE-10 is considered relatively safe tohandle, contact with skin and inhalation of its vaporsshould be avoided Immediately washing with ordinarysoap and water is effective in cleansing should skin con-tact occur. F or eye contact, rinse thoroug hly with copiousamo unts of water and consult aphysicia n. For additio nalhealth and safety inform ation, consult the material safetyda ta sheet which is available upon request.

3 0 SURFAC E PREPARATION

Strain gages can be bonded satisfactorily to almost any solidmaterial if the material surface is properly prepared. Whilethere are m any surface preparation techniques available, thespecific procedures an d techniques described here are a care-fully developed and thoroughly proven system. They areideal for bo th M -Bond 200 and M -Bond AE-10 Strain GageAdhesives.

The purp ose of surface preparation is to develop a chemi-

cally clean surface having a roughness appropriate to thegage installation requirements, a surface alkalinity of thecorrect pH, and visible gage layout lines for locating andorienting the strain gage. The Micro-Measurem ents systemof surface preparation will accomplish these objectives foraluminum alloys and steels in five basic operations:

Solvent degreasing

Surface abrading

Application of gage layout lines

Surface conditioning

Neutralizing

To ensure maximum cleanliness and best results, the follow-ing should be avoided in all steps:

Touching the surface with the fingers

Wiping back and fo rth or reusing swabs or sponges

Dragging contaminants into the cleaned area fromthe uncleaned boundary of that area

Allowing a cleaning solution t o evaporate on thesurface

Allowing partially prepared surface t o sit betweensteps in the prepara tion process or a prepared su r-

face to sit before bondingConsult Instruction Bulletin B-129 for other test materialsand f or special-precautions and considerations for surfacepreparation.

3 1 Solvent egreasing

The shelf life of unmixed com ponen ts is one year at ro omtemp erature. D uring storage, crystals may form in the resin.These crystals do not affect adhesive performance, but +should be reliquefied prior t o m ixing by warming the resinjar to +120F +JOO for approximately one-half hour. Deg reasing is perform ed to remov e oils, greases, organicBecause excess heat will shorten pot life, allow the resin to con tam inan ts, and solubleche mical residues. Degreasingreturn to room temperature before adding the curing agent. shouldalways be the first operation.


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4.0 STRAIN GAGE BONDINGThe electrical resistance strain gage is capable of makingaccurate and sensitive indications of strains on the su rface ofthe test part. Its performance is absolutely depend ent on thebond between itself and the test part. The procedures out-lined below will help ensure satisfactory b onds wh en usingM-Bond 200 or AE-10 Adhesives. While the steps mayappear unduly elaborate, these techniques have been usedrepeatedly in strain gage installations which have yieldedconsistent and accurate results. The steps shown assume th at

a terminal strip will be used. When CEA-Series gages areused, no strip is required.

The stra in gage is now prepared for positioning o n the testspecimen. Position the gageltape assembly so the trianglealignment marks on the gage are over the layout lines on thespecimen. Holding the tape at a shallow angle, wipe theassembly onto the specimen surface. If the assembly is mis-aligned, lift the tape again a t a shallow angle until the assem-bly is free of the specim en. Repo sition a nd w ipe the assemblyagain with a shallow angle.

4.1 Handling and Preparation

Wcro-Measurements strain gages are specially treated foroptimum bond formation with all approp riate gage adhe-sives. No f urth er cleaning is necessary if c ontamin ation ofthe prepared bonding surface is avoided during handling.Should contamination occur, clean with a cotton swab

moistened with a low residue solvent suchas M LINE Neu-

tralizer 5A orGC 6 Isopropyl Alcohol. Allow the gage to dryfor several minu tes before bond ing.) Gages should never betouched with the hands.

Remove the strain gage from its acetate envelope by graspingthe edge of the gage backing with tweezers, and place on achemically clean glass plate or empty gage box ) with thebonding side of the gage down. Place the app ropriate termi-nals if any) next to the strai n gage solder tabs, leaving aspace of approximately I / 16 in 1.5 mm ) between the gagebacking and terminal.

Using a 4-to-6-in 100-to-150-mm) length of M - L I N EPCT-2A cellophane tape, anchor one end of the tape to th eglass plate behind the gage and terminal. Wipe the tapefirmly down over the gage and terminals. Pick the g age andterminals up by carefully lifting the tap e at a shallow angle30 to 45 degrees) until the tape comes free with the gage and

terminal attached. The shallow angle is importan t to avoidover-stressing the gage and causing permanent resistancechanges.) Caution: Some tapes may contaminate the bonding surface or react with the bonding adhesive. Use onlytapes certified for strain gage installations.

In prep aration for applying the adhesive, lift the end of thet+pe opposite the solder tabs at a shallow angle until the gageand term inal are free of the specimen. Tack the loose end ofthe tapelund er and press t o the surface so the gage lies flat

2 with the bonding side exposed.

The app ropr iate adhesive may now be applied. The proce-dures for M-Bond 200 and M-Bond AE-10 are described inthe two sections which follow.


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4.2 Bonding with M Bond 2 Holding the tape slightly taut and beginning from the tabend of the gage, slowly andfirmly make a single wipingstroke over the gageltape assembly with a clean gauzesponge to bring the gage back down over the alignmentmarks o n the specimen. Release the tape.

M-Bond 200 Catalyst should be applied sparingly in a thinuniform coat. Wipe the brush against the lip of the bottleapprox imately ten times to remove most of the catalyst. Setthe brush down on the gage and swab the gage backing bysliding -no t brushing in the painting style he brush over Immediately upon com pletion of the above step,discard thethe entire gage surface. Move the brush to an adjacent tape gauze and apply firm thumb pressure to the gage and termi-area p rior t o lifting from the surface. Allow the catalyst to nal area. This pressure should be held for a t least one minute.dry at least one minute under normal ambient laboratory Wait two minutes before the next step tape removal).conditions.

The next three steps must be completed in sequence withinthree to five seconds. R ead these steps before proceeding.

Lift the tucked-under tape. Holding the gageltape assem blyin a fixed position, apply one or two drops of M-Bond 200Adhesive at the junction of the tape and specim en surface,about 1 2 in 13 m m outside the actual gage installationarea.

Immediately rotate the tape to approximately a 30-degreeangle so that the gage is bridged over the installation area.

Th e gage and terminals should now be bonded to the speci-men. To remove the tape, pull it back directly over itself,peeling it slowly and steadily off the surface.


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4.3 Bonding with M Bond AE 10 Place a silicone rubber pad an d a back-up plate over the gageinstallation. Apply force by dead weight or spring clamp

This section follows 4.1 w hen using M-Bond AE-IO Adhe- until pressure of to 20 psi 35 to 35 ~ N I M ?s attained.sive.) M ix the Resin AE with Curing A gent Type 10 per the Take ca re to ensure the pressure is equal over the e ntire gageinstructions in Instruction Bulletin B-137 supplied with the dace.adhesive.

Coa t the specimen and back of the gage and terminal withthe prepared M-Bond AE-10 Adhesive; The mixing rod maybe used to a pply a thin layer of adhesive over bot h surfaces.Be careful not to pick up any unm ixed components of the

adhesive.T o ensure this, wipe the mixing rod clean and thenpick up a very small amou nt of adhesive from the centralarea of th e adhesive ar. After applying the adhesive, proceedimmediately to the next step.

Lift the tucked-over end of the tape and bridge over thespecimen installation area at approximately a 30-degreeangle. Beginning from the ta b end of the gage and using aclean gauze sponge, slowly and firmly make a single wipingstroke over the gag elta pe assembly to bring the gage backdown ov er the alignment marks on the specimen.


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5 0 SOLDERING TECHNIQUES Holding the t ip of a finger on the tip of the tinned wire forsafety, cut each wire with diagonal wire cu tters leaving 118 in3 mm) of exposed, tinned wire.

Tack the lead-in wires to the specimen with drafting tap e sothe tinned end of th e wire is spring-loaded in contact with thesolder bead. Complete the solder connection s before byapplying solder and iron tip for one second and removingsimultaneously.

Apply rosin solvent liberally to the solde r joints. Draftingtape may be removed by loosening the mastic with rosinsolvent. Remove all solvent with a gauze sponge, using adabbing action. Repeat.

If the strain gage is without encapsulation or preattachedlead ribbons, mask the gage grid area with drafting tape,leaving only the ta bs exposed.

After the soldering iron has reached operating tempe rature,clean the tip with a gauze sponge an d tin it with fresh solder.Tin the gage tabs and terminal tabs (if used). Melt a smallamount of solder on the tip of the soldering iron, lay therosin-core solder wire across the gage tab or cop per terminal.

Firmly apply the iron tip for one second, then simultane- -ously lift both solder and tip. bright, s&y, even moun d of tsolder should have been deposited on the tab. If not, repe atthe process. If spikes are formed rather than sm ooth beads, itis a sign of inadequate flux, dwelling too long with the iron,and/or an improper iron temperature. Feeding the coredsolder into the ta b are a during heat application will increase Tap e or otherwise secure the lead-in wires to the specimen tothe am ount of flux available. prevent the wires from being accidentally pulled from the

tabs. A stress relief loop should be placed between the tapeand th e solder connections.

Apply a protective coating over the entire gage and terminal

area. For most laboratory uses, M-Coat A will provideadequate long-term protection. The coating should be con-tinuous up to a nd over a t least the first 118 in 3 mm) ofleadwire insulation.

Fo r a three-conductor lead-in wire, separate the individualleads for 314 in 20 mm). Str ip away 112 in 13 mm) ofinsulation by using the soldering tip to melt the insulation o nboth sides of each end of the wire 112 in 13 mm) from theends and quickly pulling off the insulation.Warning: D o notuse a knife or other blade to cut the insulation When themain leadwire is stranded and terminal ?trips are used, it isoften convenient to cut all strands bu t one t o fit the size of thecopper pad. The long strand ca n then be used as the jumper The properly installed s train gage will have a resistance towire. Soldering is made considerably easier by this m ethod. ground of at least 10 000 to 20 000 megohms. CheckingThis is unnecessary when the leadwires are bonded directly leakage resistance with the Model 1300 Gage Installationto the solder tabs on C EA-Series strain gages. Tester is highly recommended.


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6.0 TWO AND THREE WIRE CIRCUITS

All commercial static strain indicators employ some form ofthe W heatstone bridge circuit to detect the resistance changein the gage with strain.

When a single active gage is connected t o the W heatstonebridge with only two wires, as shown in the accompanyingschema tic, both the wires will be in series with the gage in thesame arm of the bridge circuit. One of the effects of thisarrangem ent is that tem perature-induced resistance changes

in the leadwires are manifested as thermal output by thestrain indicator.

A s kr ro r n

0 2

h m m y R s b a

0

r

Tw o-wir e circuit for single active gage quarter bridge)

The errors due t o leadwire resistance changes in single-gageinstallations with two-wire circuits can be minimized byminimizing the total leadwire resistance; that is, by usingshort leadwires of the largest practicable cross-section.

TT " ""P

Two -wire circuit for two gages ha y bridge)

When two matche d gages are connectedas adjacent arm s ofthe bridge circuit (with the same length leadwires, main-tained at the same temperature), the temperature effectscancel since they are the same in each arm , and like resis-tance c hanges, in adjacent arm s of the bridge circuit a reself-nullifying.

Leadw ire effects can be virtually eliminated in single activegage insta llation s by use of the three-wire circuit. In thiscase a third le ad, representing the centerpoint conn ection ofthe bridge circuit, is brought o ut to one of the gage terminals.Resistance changes in the bridge centerpoint lead do notaffect bridge balance.

For this method of leadwire compensation to be effective,the two leadwires in the adjac ent bridge arms should be thesame length, and should be maintained a t the same tempera-ture. The three-wire circuit is the standa rd metho d of con-

nection for a single active temperature-compensated strain-gage in a quarter-bridge arrangement.

uppbpMY

P

Three-w ire circuit for single active gage quarter bridge)

Contact resistance at mechanical connections within theWhe atstone bridge circuit can lead to errors in the measure-ment of strain. Connections should be snugly made. Follow-ing bridge balance, a wiggle test shou ld be made on wiresleading to mechanical connections. No change in balanceshould occur if good connections have been made.

If necessary, contact surfac es may be cleaned of oils with alow residue solent such as isopropyl alcoh ol. If long periodsof disuse have caused contact surfaces to tarnish, clean the mby scraping lightly with a knife blade.


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LAB - train Gage Mounting

Surface preparation

Step1 - Solvent Degreasing- Degrease with CSM-1 Degreaser- Avoid back contamination

Step2 - Surface Abrading- Place M-Prep Conditioner on the surface- Wet lapping with silicon carbide paper

- Wipe out with clean gauze

Step3 - Layout Line- Line should be burnished not be scored or scribed.- After marking use Conditioner Aand cotton tipped

applicator for cleaning

Step4 - Neutralizing-

In order to provide optimum chemical condition for thestrain gage adhesives- Use M-prep Neutralizer 5A and cotton tipped applicator- Wipe out with a single slow stroke of a clean gauze

* Strain gage bonding

Step5 - Strain gage preparation- Place strain gage and terminal on the clean glass plate-

Using cellophane tape anchor one end of the tape to theglass plate

- Wipe the tape down


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-Pick the strain gage and terminal by carefully lifting thetape at a low angle (30 to 45degrees)

-Position the strain gageltape assembly so the trianglemarks on the gage are over the layout line.

-Wipe the tape with holding shallow angle

- In preparation for applying the adhesive, lift the end of thetape at a low angle

- Tack the loose end of the tape

Step6 - Strain gage bonding- M-Bond 200 catalyst should be applied very thinly- Wipe the brush against the lip of the bottle ten times to

remove the catalyst

lecture_14_ strain gage installation

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