Introduction to Tensile Testing

CHAPTER 1

Introduction to Tensile Testing

Fig. 1 Typical tensile specimen, showing a reduced gage section and enlarged shoulders. To avoid end effects from the shoulders,the length of the transition region should be at least as great as the diameter, and the total length of the reduced section should

be at least four times the diameter.

TENSILE TESTS are performed for severalreasons. The results of tensile tests are used inselecting materials for engineering applications.Tensile properties frequently are included in ma-terial specifications to ensure quality. Tensileproperties often are measured during develop-ment of new materials and processes, so that dif-ferent materials and processes can be compared.Finally, tensile properties often are used to pre-dict the behavior of a material under forms ofloading other than uniaxial tension.

The strength of a material often is the primaryconcern. The strength of interest may be mea-sured in terms of either the stress necessary tocause appreciable plastic deformation or themaximum stress that the material can withstand.These measures of strength are used, with ap-propriate caution (in the form of safety factors),in engineering design. Also of interest is the ma-terial’s ductility, which is a measure of howmuch it can be deformed before it fractures.Rarely is ductility incorporated directly in de-sign; rather, it is included in material specifica-tions to ensure quality and toughness. Low duc-tility in a tensile test often is accompanied bylow resistance to fracture under other forms ofloading. Elastic properties also may be of inter-est, but special techniques must be used to mea-sure these properties during tensile testing, andmore accurate measurements can be made byultrasonic techniques.

This chapter provides a brief overview ofsome of the more important topics associatedwith tensile testing. These include:

● Tensile specimens and test machines● Stress-strain curves, including discussions of

elastic versus plastic deformation, yieldpoints, and ductility

● True stress and strain● Test methodology and data analysis

It should be noted that subsequent chapters con-tain more detailed information on these topics.Most notably, the following chapters should bereferred to:

● Chapter 2, “Mechanical Behavior of Mate-rials Under Tensile Loads”

● Chapter 3, “Uniaxial Tensile Testing”● Chapter 4, “Tensile Testing Equipment and

Strain Sensors”

Tensile Specimens andTesting Machines

Tensile Specimens. Consider the typical ten-sile specimen shown in Fig. 1. It has enlargedends or shoulders for gripping. The importantpart of the specimen is the gage section. Thecross-sectional area of the gage section is re-duced relative to that of the remainder of thespecimen so that deformation and failure will be

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2 / Tensile Testing, Second Edition

Fig. 2 Systems for gripping tensile specimens. For round specimens, these include threaded grips (a), serrated wedges (b), and, forbutt end specimens, split collars constrained by a solid collar (c). Sheet specimens may be gripped with pins (d) or serrated

wedges (e).

localized in this region. The gage length is theregion over which measurements are made andis centered within the reduced section. The dis-tances between the ends of the gage section andthe shoulders should be great enough so that thelarger ends do not constrain deformation withinthe gage section, and the gage length should begreat relative to its diameter. Otherwise, thestress state will be more complex than simpletension. Detailed descriptions of standard spec-imen shapes are given in Chapter 3 and in sub-sequent chapters on tensile testing of specificmaterials.

There are various ways of gripping the spec-imen, some of which are illustrated in Fig. 2.The end may be screwed into a threaded grip, orit may be pinned; butt ends may be used, or thegrip section may be held between wedges. Thereare still other methods (see, for example, Fig. 24in Chapter 3). The most important concern inthe selection of a gripping method is to ensurethat the specimen can be held at the maximumload without slippage or failure in the grip sec-tion. Bending should be minimized.

Testing Machines. The most common testingmachines are universal testers, which test ma-

terials in tension, compression, or bending.Their primary function is to create the stress-strain curve described in the following sectionin this chapter.

Testing machines are either electromechanicalor hydraulic. The principal difference is themethod by which the load is applied.

Electromechanical machines are based on avariable-speed electric motor; a gear reductionsystem; and one, two, or four screws that movethe crosshead up or down. This motion loads thespecimen in tension or compression. Crossheadspeeds can be changed by changing the speed ofthe motor. A microprocessor-based closed-loopservo system can be implemented to accuratelycontrol the speed of the crosshead.

Hydraulic testing machines (Fig. 3) are basedon either a single or dual-acting piston thatmoves the crosshead up or down. However,most static hydraulic testing machines have asingle acting piston or ram. In a manually op-erated machine, the operator adjusts the orificeof a pressure-compensated needle valve to con-trol the rate of loading. In a closed-loop hydrau-lic servo system, the needle valve is replaced byan electrically operated servo valve for precisecontrol.



Introduction to Tensile Testing / 3

Fig. 3 Components of a hydraulic universal testing machine

In general, electromechanical machines arecapable of a wider range of test speeds andlonger crosshead displacements, whereas hy-draulic machines are more cost-effective forgenerating higher forces.

Stress-Strain Curves

A tensile test involves mounting the specimenin a machine, such as those described in the pre-vious section, and subjecting it to tension. Thetensile force is recorded as a function of the in-crease in gage length. Figure 4(a) shows a typ-ical curve for a ductile material. Such plots oftensile force versus tensile elongation would beof little value if they were not normalized withrespect to specimen dimensions.

Engineering stress, or nominal stress, s, is de-fined as

s � F/A (Eq 1)0

where F is the tensile force and A0 is the initialcross-sectional area of the gage section.

Engineering strain, or nominal strain, e, is de-fined as

e � DL/L (Eq 2)0

where L0 is the initial gage length and DL is thechange in gage length (L � L0).

When force-elongation data are converted toengineering stress and strain, a stress-straincurve (Fig. 4b) that is identical in shape to theforce-elongation curve can be plotted. The ad-vantage of dealing with stress versus strainrather than load versus elongation is that thestress-strain curve is virtually independent ofspecimen dimensions.

Elastic versus Plastic Deformation. Whena solid material is subjected to small stresses, thebonds between the atoms are stretched. Whenthe stress is removed, the bonds relax and thematerial returns to its original shape. This re-




Fig. 4 (a) Load-elongation curve from a tensile test and (b) corresponding engineering stress-strain curve. Specimen diameter, 12.5mm; gage length, 50 mm.

versible deformation is called elastic deforma-tion. (The deformation of a rubber band is en-tirely elastic). At higher stresses, planes of atomsslide over one another. This deformation, whichis not recovered when the stress is removed, istermed plastic deformation. Note that the term“plastic deformation” does not mean that the de-formed material is a plastic (a polymeric mate-rial). Bending of a wire (such as paper-clip wire)with the fingers (Fig. 5) illustrates the difference.If the wire is bent a little bit, it will snap backwhen released (top). With larger bends, it willunbend elastically to some extent on release, butthere will be a permanent bend because of theplastic deformation (bottom).

For most materials, the initial portion of thecurve is linear. The slope of this linear region iscalled the elastic modulus or Young’s modulus:

E � s/e (Eq 3)

In the elastic range, the ratio, t, of the mag-nitude of the lateral contraction strain to the ax-ial strain is called Poisson’s ratio:

t � �e /e (in an x-direction tensile test)y x (Eq 4)

Because elastic strains are usually very small,reasonably accurate measurement of Young’smodulus and Poisson’s ratio in a tensile test re-quires that strain be measured with a very sen-sitive extensometer. (Strain gages should beused for lateral strains.) Accurate results can alsobe obtained by velocity-of-sound measurements(unless the modulus is very low or the dampingis high, as with polymers).

When the stress rises high enough, the stress-strain behavior will cease to be linear and thestrain will not disappear completely on unload-ing. The strain that remains is called plasticstrain. The first plastic strain usually corre-sponds to the first deviation from linearity. (Forsome materials, the elastic deformation may benonlinear, and so there is not always this corre-spondence). Once plastic deformation has be-gun, there will be both elastic and plastic con-tributions to the total strain, eT. This can beexpressed as eT � ee � ep, where ep is the plas-

Fig. 5 Elastic and plastic deformation of a wire with the fin-gers. With small forces (top), all of the bending is elastic

and disappears when the force is released. With greater forces(below), some of the bending is recoverable (elastic), but most ofthe bending is not recovered (is plastic) when the force is re-moved.




Fig. 6 The low-strain region of the stress-strain curve for aductile material

tic contribution and ee is the elastic contribution(and still related to the stress by Eq 3).

It is tempting to define an elastic limit as thestress at which plastic deformation first occursand a proportional limit as the stress at whichthe stress-strain curve first deviates from linear-ity. However, neither definition is very useful,because measurement of the stress at which plas-tic deformation first occurs or the first deviationfrom linearity is observed depends on how ac-curately strain can be measured. The smaller theplastic strains that can be sensed and the smallerthe deviations from linearity can be detected, thesmaller the elastic and proportional limits.

To avoid this problem, the onset of the plas-ticity is usually described by an offset yieldstrength, which can be measured with greaterreproducibility. It can be found by constructinga straight line parallel to the initial linear portionof the stress-strain curve, but offset by e �0.002 or 0.2%. The yield strength is the stress atwhich this line intersects the stress-strain curve(Fig. 6). The rationale is that if the material hadbeen loaded to this stress and then unloaded, theunloading path would have been along this off-set line and would have resulted in a plasticstrain of e � 0.2%. Other offset strains are

sometimes used. The advantage of defining yieldstrength in this way is that such a parameter iseasily reproduced and does not depend heavilyon the sensitivity of measurement.

Sometimes, for convenience, yielding in met-als is defined by the stress required to achieve aspecified total strain (e.g., eT � 0.005 or 0.5%elongation) instead of a specified offset strain.In any case, the criterion should be made clearto the user of the data.

Yield Points. For some materials (e.g., low-carbon steels and many linear polymers), thestress-strain curves have initial maxima fol-lowed by lower stresses, as shown in Fig. 7(a)and (b). After the initial maximum, all the de-formation at any instant is occurring within arelatively small region of the specimen. Contin-ued elongation of the specimen occurs by prop-agation of the deforming region (Luders band inthe case of steels) along the gage section ratherthan by increased strain within the deformingregion. Only after the entire gage section hasbeen traversed by the band does the stress riseagain. In the case of linear polymers, a yieldstrength is often defined as the initial maximumstress. For steels, the subsequent lower yieldstrength is used to describe yielding. This is be-cause measurements of the initial maximum orupper yield strength are extremely sensitive tohow axially the load is applied during the tensiletest. Some laboratories cite the minimum,whereas others cite a mean stress during this dis-continuous yielding.

The tensile strength (ultimate strength) is de-fined as the highest value of engineering stress*(Fig. 8). Up to the maximum load, the defor-mation should be uniform along the gage sec-tion. With ductile materials, the tensile strengthcorresponds to the point at which the deforma-tion starts to localize, forming a neck (Fig. 8a).Less ductile materials fracture before they neck(Fig. 8b). In this case, the fracture strength is thetensile strength. Indeed, very brittle materials(e.g., glass at room temperature) do not yieldbefore fracture (Fig. 8c). Such materials havetensile strengths but not yield strengths.

Ductility. There are two common measuresused to describe the ductility of a material. One

*Sometimes the upper yield strength of low-carbon steel ishigher than the subsequent maximum. In such cases, someprefer to define the tensile strength as the subsequent max-imum instead of the initial maximum, which is higher. Insuch cases, the definition of tensile strength should be madeclear to the user.




Fig. 8 Stress-strain curves showing that the tensile strength is the maximum engineering stress regardless of whether the specimennecks (a) or fractures before necking (b and c).

Fig. 7 Inhomogeneous yielding of a low-carbon steel (a) and a linear polymer (b). After the initial stress maxima, the deformationoccurs within a narrow band, which propagates along the entire length of the gage section before the stress rises again.

is the percent elongation, which is defined sim-ply as

%El � [(L � L )/L ] � 100 (Eq 5)f 0 0

where L0 is the initial gage length and Lf is thelength of the gage section at fracture. Measure-ments may be made on the broken pieces or un-der load. For most materials, the amount of elas-tic elongation is so small that the two areequivalent. When this is not so (as with brittlemetals or rubber), the results should statewhether or not the elongation includes an elasticcontribution. The other common measure ofductility is percent reduction of area, which isdefined as

%RA � [(A � A )/A ] � 100 (Eq 6)0 f 0

where A0 and Af are the initial cross-sectionalarea and the cross-sectional area at fracture, re-spectively. If failure occurs without necking, onecan be calculated from the other:

%El � %RA/(100 � %RA) (Eq 7)

After a neck has developed, the two are nolonger related. Percent elongation, as a measureof ductility, has the disadvantage that it is reallycomposed of two parts: the uniform elongationthat occurs before necking, and the localizedelongation that occurs during necking. The sec-ond part is sensitive to the specimen shape.When a gage section that is very long (relativeto its diameter), the necking elongation con-verted to percent is very small. In contrast, witha gage section that is short (relative to its di-




ameter), the necking elongation can account formost of the total elongation.

For round bars, this problem has been reme-died by standardizing the ratio of gage length todiameter to 4:1. Within a series of bars, all withthe same gage-length-to-diameter ratio, thenecking elongation will be the same fraction ofthe total elongation. However, there is no simpleway to make meaningful comparisons of percentelongation from such standardized bars with thatmeasured on sheet tensile specimens or wire.With sheet tensile specimens, a portion of theelongation occurs during diffuse necking, andthis could be standardized by maintaining thesame ratio of width to gage length. However, aportion of the elongation also occurs duringwhat is called localized necking, and this de-pends on the sheet thickness. For tensile testingof wire, it is impractical to have a reduced sec-tion, and so the ratio of gage length to diameteris necessarily very large. Necking elongationcontributes very little to the total elongation.

Percent reduction of area, as a measure ofductility, has the disadvantage that with veryductile materials it is often difficult to measurethe final cross-sectional area at fracture. This isparticularly true of sheet specimens.

True Stress and Strain

If the results of tensile testing are to be usedto predict how a metal will behave under otherforms of loading, it is desirable to plot the datain terms of true stress and true strain. True stress,r, is defined as

r � F/A (Eq 8)

where A is the cross-sectional area at the timethat the applied force is F. Up to the point atwhich necking starts, true strain, e, is defined as

e � ln(L/L ) (Eq 9)0

This definition arises from taking an incrementof true strain, de, as the incremental change inlength, dL, divided by the length, L, at the time,de � dL/L, and integrating. As long as the de-formation is uniform along the gage section, thetrue stress and strain can be calculated from theengineering quantities. With constant volumeand uniform deformation, LA � L0A0:

A /A � L/L (Eq 10)0 0

Thus, according to Eq 2, A0/A � 1 � e. Equa-tion 8 can be rewritten as

r � (F/A )(A /A)0 0

and, with substitution for A0/A and F/A0, as

r � s(1 � e) (Eq 11)

Substitution of L/L0 � 1 � e into the expressionfor true strain (Eq 9) gives

e � ln(1 � e) (Eq 12)

At very low strains, the differences betweentrue and engineering stress and strain are verysmall. It does not really matter whether Young’smodulus is defined in terms of engineering ortrue stress strain.

It must be emphasized that these expressionsare valid only as long as the deformation is uni-form. Once necking starts, Eq 8 for true stressis still valid, but the cross-sectional area at thebase of the neck must be measured directlyrather than being inferred from the length mea-surements. Because the true stress, thus calcu-lated, is the true stress at the base of the neck,the corresponding true strain should also be atthe base of the neck. Equation 9 could still beused if the L and L0 values were known for anextremely short gage section centered on themiddle of the neck (one so short that variationsof area along it would be negligible). Of course,there will be no such gage section, but if therewere, Eq 10 would be valid. Thus the true straincan be calculated as

e � ln(A /A) (Eq 13)0

Figure 9 shows a comparison of engineering andtrue stress-strain curves for the same material.

Other FactorsInfluencing the Stress-Strain Curve

There are a number of factors not previouslydiscussed in this chapter that have an effect onthe shape of the stress-strain curve. These in-clude strain rate, temperature, and anisotropy.For information on these subjects, the readershould refer to Chapters 2 and 3 listed in theintroduction to this chapter as well as Chapter12, “Hot Tensile Testing” and Chapter 15, “HighStrain Rate Tensile Testing.”




Fig. 9 Comparison of engineering and true stress-straincurves. Prior to necking, a point on the r-e curve can

be constructed from a point on the s-e curve using Eq 11 and 12.Subsequently, the cross section must be measured to find truestress and strain.

Test Methodology and Data Analysis

This section reviews some of the more im-portant considerations involved in tensile test-ing. These include:

● Sample selection● Sample preparation● Test set-up● Test procedure● Data recording and analysis● Reporting

Sample Selection. When a material is tested,the objective usually is to determine whether ornot the material is suitable for its intended use.

The sample to be tested must fairly representthe body of material in question. In other words,it must be from the same source and have un-dergone the same processing steps.

It is often difficult to match exactly the testsamples to the structure made from the material.A common practice for testing of large castings,forgings, and composite layups is to add extramaterial to the part for use as “built-in” test sam-ples. This material is cut from the completed partafter processing and is made into test specimensthat have been subjected to the same processingsteps as the bulk of the part.

In practice, these specimens may not exactlymatch the bulk of the part in certain importantdetails, such as the grain patterns in critical areasof a forging. One or more complete parts maybe sacrificed to obtain test samples from themost critical areas for comparison with the“built-in” samples. Thus, it may be determined

how closely the “built-in” samples represent thematerial in question.

There is a special case in which the object ofthe test is to evaluate not the material, but thetest itself. Here, the test specimens must be asnearly identical as possible so the differences inthe test results represent, as far as possible, onlythe variability in the testing process.

Sample Preparation. It should be remem-bered that a “sample” is a quantity of materialthat represents a larger lot. The sample usuallyis made into multiple “specimens” for testing.Test samples must be prepared properly toachieve accurate results. The following rules aresuggested for general guidance.

First, as each sample is obtained, it should beidentified as to material description, source, lo-cation and orientation with respect to the bodyof material, processing status at the time of sam-pling, and the data and time of day that the sam-ple was obtained.

Second, test specimens must be made care-fully, with attention to several details. The spec-imen axis must be properly aligned with the ma-terial rolling direction, forging grain pattern, orcomposite layup. Cold working of the test sec-tion must be minimized. The dimensions of thespecimen must be held within the allowable tol-erances established by the test procedure. Theattachment areas at each end of the specimenmust be aligned with the axis of the bar (see Fig.10). Each specimen must be identified as be-longing to the original sample. If total elonga-tion is to be measured after the specimen breaks,the gage length must be marked on the reducedsection of the bar prior to testing.

The test set-up requires that equipment beproperly matched to the test at hand. There are

Fig. 10 Improper (left) and proper (right) alignment of speci-men attachment areas with axis of specimen




three requirements of the testing machine: forcecapacity sufficient to break the specimens to betested; control of test speed (or strain rate or loadrate), as required by the test specification; andprecision and accuracy sufficient to obtain andrecord properly the load and extension infor-mation generated by the test. This precision andaccuracy should be ensured by current calibra-tion certification.

For grips, of which many types are in com-mon use in tensile testing, only two rules apply:the grips must properly fit the specimens (or viceversa), and they must have sufficient force ca-pacity so that they are not damaged during test-ing.

As described earlier in the section “TensileSpecimens and Testing Machines,” there areseveral techniques for installing the specimen inthe grips. With wedge grips, placement of thespecimen in the grips is critical to proper align-ment (see Fig. 11). Ideally, the grip faces shouldbe of the same width as the tab ends of the testbar; otherwise, lateral alignment is dependentonly on the skill of the technician. The wedgegrip inserts should be contained within the gripbody or crosshead, and the specimen tabs shouldbe fully engaged by the grips (see Fig. 12).

Other types of grips have perhaps fewer trapsfor the inexperienced technician, but an obviousone is that, with threaded grips, a length of

threads on the specimen equal to at least onediameter should be engaged in the threadedgrips.

There are several potential problems that mustbe watched for during the test set-up, includingspecimen misalignment and worn grips. Thephysical alignment of the two points of attach-ment of the specimen is important, because anyoff-center loading will exert bending loads onthe specimen. This is critical in testing of brittlematerials, and may cause problems even for duc-tile materials. Alignment will be affected by thetesting-machine loadframe, any grips and fix-tures used, and the specimen itself. Misalign-ment may also induce load-measurement errorsdue to the passage of bending forces through theload-measuring apparatus. Such errors may bereduced by the use of spherical seats or “U-joints” in the set-up.

Worn grips may contribute to off-center load-ing. Uneven tooth marks across the width of thespecimen tab are an indication of trouble inwedge grips. Split-collar grips may also causeoff-center loading. Uneven wear of grips andmismatching of split-shell insert pairs are poten-tial problem areas.

Strain measurements are required for manytests. They are commonly made with extensom-eters, but strain gages are frequently used—especially on small specimens or where Pois-

Fig. 11 Improper (left, center) and proper (right) alignment of specimen in wedge grips




son’s ratio is to be measured. If strainmeasurements are required, appropriate strain-measuring instruments must be properly in-stalled. The technician should pay particular at-tention to setting of the extensometer gagelength (mechanical zero). The zero of the strainreadout should repeat consistently if the me-chanical zero is set properly. In other words,once the extensometer has been installed and ze-roed, subsequent installations should requireminimal readjustment of the zero.

Test Procedure. The following general rulesfor test procedure may be applied to almostevery tensile test.

Load and strain ranges should be selected sothat the test will fit the range. The maximumvalues to be recorded should be as close to thetop of the selected scale as convenient withoutrunning the risk of going past full scale. Rangesmay be selected using past experience for a par-ticular test, or specification data for the material(if available). Note that many computer-basedtesting systems have automatic range selectionand will capture data even if the range initiallyselected is too small.

The identity of each specimen should be ver-ified, and pertinent identification should be ac-curately recorded for the test records and report.

The dimensions needed to calculate the cross-sectional area of the reduced section should bemeasured and recorded. These measurementsshould be repeated for every specimen; it shouldnot be assumed that sample preparation is per-fectly consistent.

The load-indicator zero and the plot-load-axiszero, if applicable, should be set before the spec-imen is placed in the grips. Zeroes should neverbe reset after the specimen is in place.

The specimen is placed in the grips and issecured by closing the grips. If preload is to beremoved before the test is started, it should bephysically unloaded by moving the loadingmechanism. The zero adjustment should neverbe used for this purpose. Note that, in somecases, preload may be desirable and may be de-liberately introduced. For materials for whichthe initial portion of the curve is linear, the strainzero may be corrected for preload by extendingthe initial straight portion of the stress-straincurve to zero load and measuring strain fromthat point. The strain valve at the zero-load in-tercept is commonly called the “foot correction”and is subtracted from readings taken from strainscale (see Fig. 10 in Chapter 3, “Uniaxial TensileTesting”).

When the extensometer, if applicable, is in-stalled, the technician should be sure to set themechanical zero correctly. The strain-readoutzero should be set after the extensometer is inplace on the specimen.

The test procedure should be in conformancewith the published test specification and should

Fig. 12 Proper and improper engagement of a specimen inwedge grips




be repeated consistently for every test. It is im-portant that the test specification be followed forspeed of testing. Some materials are sensitive totest speed, and different speeds will give differ-ent results. Also, many testing machine load-and strain-measuring instruments are not capa-ble of responding fast enough for accuraterecording of test results if an excessive test speedis used.

The technician should monitor the test closelyand be alert for problems. One common sign oftrouble is a load-versus-strain plot in which theinitial portion of the curve is not straight. Thismay indicate off-center loading of the specimen,improper installation of the extensometer, or thespecimen was not straight to begin with.

Another potential trouble sign is a sharp dropin indicated load during the test. Such a dropmay be characteristic of the material, but it alsocan indicate problems such as slippage betweenthe specimen and the grips or stick-slip move-ment of the wedge grip inserts in the grip body.Slippage may be caused by worn inserts withdull teeth, particularly for hard, smooth speci-mens.

The stick-slip action in wedge grips is morecommon in testing of resilient materials, but italso can occur in testing of metals. Specimenscut from the wall of a pipe or tube may havecurved tab ends that flatten with increasingforce, allowing the inserts to move relative tothe grip body. Short tab ends on round speci-mens also may be crushed by the wedge grips,with the same result. If the sliding faces are notlubricated, they may move in unpredictablesteps accompanied by drops in the load reading.Dry-film molybdenum disulfide lubricants areeffective in solving stick-slip problems in wedgegrips, particularly when testing is done at ele-vated temperature.

When wedge grips are used, the specimenmust be installed so that the clamping force iscontained within the grip body. Placing the spec-imen too near the open end of the grip bodyresults in excessive stress on the grip body andinserts and is a common cause of grip failure.WARNING: Grip failures are dangerous andmay cause injury to personnel and damage toequipment.

Data generally may be grouped into “rawdata,” meaning the observed readings of themeasuring instruments, and “calculated data,”meaning the test results obtained after the firststep of analysis.

In the most simple tensile test, the raw datacomprise a single measurement of peak force

and the dimensional measurements taken to de-termine the cross-sectional area of the test spec-imen. The first analysis step is to calculate the“tensile strength,” defined as the force per unitarea required to fracture the specimen. Morecomplicated tests will require more information,which typically takes the form of a graph offorce versus extension. Computer-based testingmachines can display the graph without paper,and can save the measurements associated withthe graph by electronic means.

A permanent record of the raw test data isimportant, because it allows additional analysesto be performed later, if desired, and because itallows errors in analysis to be found and cor-rected by reference to the original data.

Data Recording. Test records may be neededby many departments within an organization, in-cluding metallurgy, engineering, commercial,and legal departments.

Engineering and metallurgy departments typ-ically are most interested in material properties,but may use raw data for error checking or ad-ditional analyses. The metallurgy departmentwants to know how variations in raw materialsor processing change the properties of the prod-uct being produced and tested, and the engi-neering department wants to know the propertiesof the material for design purposes.

Shipping, receiving, and accounting depart-ments need to know whether or not the materialmeets the specifications for shipping, accep-tance, and payment. The sales department needsinformation for advertising and for advising pro-spective customers.

If a product incorporating the tested materiallater fails—particularly if persons are injured—the legal department may need test data as evi-dence in legal proceedings. In this case, a recordof the raw data will be important for support ofthe original analysis and test report.

Analysis of test data is done at several levels.First, the technician observes the test in pro-gress, and may see that a grip is slipping or thatthe specimen fractures outside the gage section.These observations may be sufficient to deter-mine that a test is invalid.

Immediately after the test, a first-level anal-ysis is performed according to the calculationrequirements of the test procedure. ASTM testspecifications typically show the necessaryequations with an explanation and perhaps anexample. This analysis may be as simple asdividing peak force by cross-sectional area, orit may require more complex calculations. The




outputs of this first level of analysis are themechanical properties of the material beingtested.

Upon completion of the group of tests per-formed on the sample, a statistical analysis maybe made. The statistical analysis produces av-erage (mean or median) values for representa-tion of the sample in the subsequent databaseand also provides information about the unifor-mity of the material and the repeatability of thetest.

The results of tests on each sample of materialmay be stored in a database for future use. Thedatabase allows a wide range of analyses to beperformed using statistical methods to correlatethe mechanical-properties data with other infor-mation about the material. For example, it mayallow determination of whether or not there is asignificant difference between the materialtested and similar material obtained from a dif-ferent supplier or through a different productionpath.

Reporting. The test report usually containsthe results of tests performed on one samplecomposed of several specimens.

When ASTM specifications are used for test-ing, the requirements for reporting are definedby the specification. The needs of a particularuser probably will determine the form for iden-tification of the material, but the reported resultswill most likely be as given in the ASTM testspecification.

The information contained in the test reportgenerally should include identification of thetesting equipment, the material tested, and thetest procedure; the raw and calculated data foreach specimen; and a brief statistical summaryfor the sample.

Each piece of test equipment used for the testshould be identified, including serial numbers,capacity or range used, and date of certificationor date due for certification.

Identification of the material tested should in-clude the type of material (alloy, part number,etc.); the specific batch, lot, order, heat, or coilfrom which the sample was taken; the point inthe processing sequence (condition, temper, etc.)at which the sample was taken; and any test orpretest conditions (test temperature, aging, etc.).

Identification of the test procedure usuallywill be reported by reference to a standard testprocedure such as those published by ASTM orperhaps to a proprietary specification originatingwithin the testing organization.

The raw data for each specimen are recorded,or a reference to the raw data is included so that

the data can be obtained from a file if and whenthey are needed. Frequently, only a portion ofthe raw data—dimensions, for example—is re-corded, and information on the force-versus-ex-tension graph is referenced.

A tabulation of the properties calculated foreach specimen is recorded. The calculations atthis stage are the first level of data analysis. Thecalculations required usually are defined in thetest procedure or specification.

A brief statistical summary for the sample isa feature that is becoming more common withthe proliferation of computerized testing sys-tems, because the computations required can bedone automatically without added operatorworkload. The statistical summary may includethe average (mean) value, median value, stan-dard deviation, highest value, lowest value,range, etc. The average or median value wouldbe used to represent this sample at the next levelof analysis, which is the material database.

Examination of this initial statistical infor-mation can tell a great deal about the test as wellas the material. A low standard deviation orrange indicates that the material in the samplehas uniform properties (each of several speci-mens has nearly the same values for the mea-sured properties) and that the test is producingconsistent results. Conversely, a high standarddeviation or range indicates that a problem ofinconsistent material or testing exists and needsto be investigated.

A continuing record of the average propertiesand the associated standard deviation and rangeinformation is the basis for statistical processcontrol, which systematically interprets this in-formation so as to provide the maximum infor-mation about both the material and the test pro-cess.

ACKNOWLEDGMENTS

This chapter was adapted from:

● W.F. Hosford, Overview of Tensile Testing,Tensile Testing, P. Han, Ed., ASM Interna-tional, 1992, p 1–24

● P.M. Mumford, Test Methodology and DataAnalysis, Tensile Testing, P. Han, Ed., ASMInternational, 1992, p 49–60

● R. Gedney, Guide To Testing Metals UnderTension, Advanced Materials & Processes,February, 2002, p 29–31



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Introduction to Tensile Testing