Whether you are doing quality controltesting or materials research for develop-ing new materials, driving test machinevariability out of the material testingequation is critical for your success. Withas many as 30 parameters potentiallyaffecting measurement uncertainty, it isimportant to identify and minimizesources of possible error. Verifying theproper load frame alignment on a regu-lar basis is one of the more significanttesting parameters to assess.

Frame Alignment Bending strains within the specimenhave long been identified as a source oferror and scatter in LCF,HCF,Tensile, andCreep testing. Axial material propertiesmeasured in load frames can vary due tomisalignment. Misalignment imposes abending moment on the clamped speci-men,causing non-uniform strain. One sidewill have higher strain than intended forthe applied axial load. The higher straincan cause specimens to exhibit loweraxial strength than uniformly appliedstrain from a pure axial force.

ASTM E8(M) 04 describes standard testmethods for tension testing of metallicmaterials. Note 5 within subsection5.2.1 (gripping devices), is instructiveof the impact misalignment has oninducing undesirable bending stressesand strains into gripped specimens.

NOTE 5:For a standard 12.5-mm diameterspecimen, the stress increase is 1.5 % for each0.025 mm of eccentricity [with regard tocenterline gripping]. This error increases toabout 2.5 % / 0.025 mm for a 9-mm diameterspecimen and to about 3.2 % / 0.025 mm for a6-mm diameter specimen.

Alignment is especially important infatigue testing of brittle materials wheremisalignment can cause abnormal crackinitiation and growth.The crack initiationsite should be relatively random, unlessthe material has texture. Repeated failureson one side of the specimen probablyindicate the machine is out of alignment.An upper limit of 5% bending strain inLCF testing has proven to be readilyachievable through the control of loadframe alignment.

Statistical batch testing (via round robins)has shown that load frame misalignmentsignificantly contributes to error.Thereseems to be,however, little awareness ofthis source of error and how to resolveit. Misalignment leads to data scatter byinducing specimen bending stresses andstrains. Increased data scatter creates theneed to test larger batches of specimensto arrive at statistically similar results. Amore aligned load frame creates less datascatter.

Typically, two conditions will exist whentesting materials or components

Under-testing Performing tests onmaterials or components below the pre-scribed test criteria can result in false con-fidence in materials or components.

Over-testing Performing tests on mate-rials or components beyond the pre-scribed test criteria can result in overdesign and waste of material and time.

Balancing under-testing (risky) againstthe over-testing condition (operationallycostly) should be a goal objective. Areturn on investment is achieved withaligned load frames.Fewer samples needto be tested to achieve statistically signifi-cant results.

The inevitability of load frame misalignment If you assume that your load frames aredelivering correct and reliable databecause they are accurately calibratedfor load and displacement you may bemaking a serious error. Even properly

Driving Variability from the Material Testing Equation

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Load train misalignment introduces data variability seen most often as data scatter

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calibrated load frames can deliver incor-rect and unreliable data if they areunaligned or misaligned. Load framescan drift out of alignment due to anumber of factors. The initiation of anew test program, cross head position,a fixturing change, or a collet or wedgechange can all lead to load frame mis-alignment and ultimately jeopardize thereliability of your test data. Even pre-cisely machined MTS load frames assem-bled under our long-standing laser align-ment techniques need to be periodicallychecked for alignment. Tolerance stackups occur throughout the load train.Grips, spiral washers, load cells, actuatorsand crossheads cannot be machined per-fectly enough to avoid some impact onconcentric and angular misalignment.

Ever-Tighter Alignment Standards Increasingly, industry is recognizing theimportance of maintaining properlyaligned load frames and insisting ontighter alignment requirements. Manycompanies are now auditing their sup-pliers for compliance with the existingalignment standards. Major revisionshave been introduced or are underrevision for:

ASTM E1012 (GE) S-400-E ISO TC 164 SC5 WG11 VAMAS Report No. 42 ISSN 1016-2186

February 2003

VAMAS Report No. 42 ISSN 1016-2186defines the Alignment Cellas: a carefullymachined test specimen instrumented withstrain gauges for use in the measurementof alignment of the testing machine. Analignment cell is meant for use in succes-sive alignment verifications and, therefore,must only be subjected to elastic deforma-tions. The following is also cited from VAMASReport No. 42:

Determining the sources of bendingThe contribution due to the testmachines misalignment to the totalbending measured on the alignmentcell surface can be evaluated by:

Ensure your Alignment Solution retainsits true strain measuring integrityMTS supports efforts to define standardpractices for the periodic validation andcalibration of bending strain-measuringinstruments used to verify load framealignment.We 100% validate the measur-ing integrity of every MTS 709 alignmentspecimen.We can also periodically assessthe as-foundcondition of 709 alignmentspecimens to determine if a specimen inuse has been compromised (bent) follow-ing a series of frame alignment events.Our A2LA Accredited MTS Metrologydepartment has the expertise to bothperiodically validate 709-specimenintegrity and calibrate the 709 measur-ing and data acquisition electronics.

Axis Definition

Front ofMachine

Angular

Concentric

A 270

C 270

A 90

C 90

Subjecting the alignment cell to anaxial load in one orientation

Recording the strain gauge readings,and by

Repeating this after rotating the align-ment cell 180 about its vertical axis.

By rotating the alignment cell, its bend-ing contribution rotates relative to themachine while the machines bendingcomponent remains stationary. Themachines contribution corresponds to of the difference between the local bend-ing strains. Averaging the bending strainsfor any single gauge at two diametricallyopposite positions results in the bendingcomponent at the location of that gaugedue to inherent imperfections in the strain-gauged specimen or alignment cell.

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1Text is referenced from Dr. F.A.Kandils, "A

Procedure for the Measurement of MachineAlignment in Axial Testing" VAMAS Report No.42, ISSN 1016-2186, National PhysicalLaboratory, February 2003.

Precision is found in a MTS Alignment SolutionMTS has created a turnkey alignmentsolution to address the concerns in thelab. It is designed with the knowledge-able user in mind as a tool of precisionand with ease of usefeatures to accu-rately assess and confirm the presentand as-adjusted state of your loadframe(s) alignment. The system is com-prised of:

1. MTS Model 709 PC-based Alignment System

2. MTS Strain Gaged Specimen3. MTS Model 609 Alignment Fixture4. MTS Alignment Software

Smooth Shank Specimen

Laterally Stiff Frame

Parameters to Consider in Aligning Load Frames

Guided Column

Threaded Shank Specimen

Laterally Soft Frame

ElectromechanicalFrame

ServohydraulicFrame

Clevic and PinConnections

609 AlignmentFixture

Hydraulic Grips

MTS 709 AlignmentPackage

Preloaded Joints

Smooth ProfiledWedge/Collets

Self-CenteringJoints

Unguided BallScrews

No AlignmentFixture

MechanicalGrips

Serated Wedges/Collets

Easier MoreDifficult

MTS Alignment Solutions

Which alignment solution is right for your laboratory?

While MTS offers a range of alignmentsolutions to choose from, the initialchallenge may most often be one ofassessing your current configurations offorce-applying load frames to see howthey position on the ease of alignmentscale. It is important to note the differ-ences in verifying the as-found alignmentcondition as compared to adjusting align-ment to an improved state.

Conduct alignments yourself PurchaseMTS alignment hardware, fixtures andsoftware

Equip your load frames with MTSalignment fixtures and applications.Speed the alignment process

Contract with MTS to conduct thealignments on-site.MTS North AmericaService is accredited by A2LA.

Send your MTS 709 Alignment systemand MTS alignment specimens to ourA2LA accredited metrology lab for vali-dation and calibration

SummaryIn Materials testing, several machineattributes should be addressed for opti-mum results. Precision alignment ofboth the basic frame and the grippedspecimen is critical to controlling / mini-mizing bending strain in the specimen.The MTS alignment solution is designedto help ensure that your load frames areproperly aligned and consistently deliver-ing accurate and repeatable data. Integralto this solution is the ability to periodicallyvalidate the integrity of your alignmentmeasuring devices.

Contact MTS to Learn MoreTo see a demonstration of the conven-ient and economical new alignmentsolution from MTS, contact your localMTS sales engineer or browse tohttp://www.mts.com/align.

For More Information Regarding MTS Alignment ServicesContact your local MTS Service-Salesrepresentative, or contact MTS at 800-328-2255. (fax) 952-937-4515, orinfo@mts.com.

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EqualStrain

EqualStrain Positive

Strain

NegativeStrain

MTS is a registered trademark of MTS SystemsCorporation. RTM No. 211177 2006 MTS Systems Corporation.

100-161-434a Variability Printed in U.S.A. 8/06

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