tension test report

TENSION TESTEM 327: MECHANICS OF MATERIALS LABORATORY

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EXPERIMENT: TENSION TESTVIDEO TITLE: TENSION TEST

OBJECTIVES:

(1) Obtain a general understanding of howdifferent materials behave under uniaxial tensileloading.(2) Determine and compare material propertiesof various materials.

INTRODUCTION:

This experiment will consist of two parts. Thefirst will serve to introduce the MTS testingequipment and testing procedures. For the firstexperiment, a 0.25-inch nominal diameter hot-rolled steel sample will be tested to failure. Load-versus-strain and load-versus-stroke diagramswill be produced during the test and thesediagrams will subsequently be used to determinematerial properties. The second part to thisexperiment, which will be performed during thefirst laboratory rotation, will consist of similartests on aluminum and stainless steel specimens.

The student will learn how to properly conduct atension test and obtain the relevant materialproperties from the results. Further, the studentwill discover how different materials behaveunder similar loading conditions as well as howmaterial properties differ.

BACKGROUND:

Stress is a measure of the intensity of an internalforce. Stress is defined as the force per unit area:

Stress= = Load/Area = P/A [psi]

When a specimen is loaded so that the resultantforce passes through the centroid of the specimencross-section, the loading is categorized as axialand can be either tensile or compressive. Tests

utilizing axial loading are generally performed todetermine material properties.

When materials for engineering projects areprocured, the engineer often must specifymaterial property requirements to themanufacturer. After the material is received it isgenerally good practice, if not mandatory, toperform acceptance tests to verify the materialproperties before the materials are used.Therefore, it is important to understand whichmaterial properties are relevant and how thoseproperties are obtained.

Results from simple tension tests, similar to thetest described in this experiment, can provideinformation from which several materialproperties can be determined. The experiments tobe completed for Tension I and Tension II willillustrate the usefulness of the simple tension testand demonstrate the mechanical behavior ofdifferent materials. Later tests in this course willintroduce other relevant properties.

Figure 1 shows a typical tensile specimen bothbefore and after testing. Notice that the cross-section decreased significantly (necked) at thefailure location, indicating ductile material.Brittle materials display significantly less neckingand thus the cross-sectional area does notdecrease appreciably prior to failure.

Strain is a measure of the deformation that hasoccurred in a material. In the case where themagnitude of deformation is the same over theentire length of a body, strain may be defined as:

o

of

LLL

= [in/in]

Where: Lo is the initial length

Lf is the final length

For cases where the deformation differsthroughout the body, the lengths Lo and Lf must


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be reduced to a sub-region of the body where thedeformation is constant.

FIGURE 1

didf

.

.

.

.

.

.

.

.

.

.

A typical stress-strain diagram from a tension testfor hot-rolled steel is shown in Figure 2. Thisdiagram provides a great deal of usefulinformation about the material. The particularproperties are designated on the figure and areindividually discussed in the following list.

DUCTILITY: Characteristic of a material wherethe material can undergo large plasticdeformations before fracture, especially intension.

ENGINEERING FRACTURE STRENGTH:Engineering stress at the point of final fracture.Units: lb/in2 (psi)

ENGINEERING STRESS, : The load dividedby the initial cross-sectional area. Note that stressbased on the initial cross-section decreasesbeyond the ultimate strength.

Units: psiELASTIC LIMIT, E: Maximum stress for whichstress will be directly proportional to strain. Theend to the straight-line portion of the stress-straincurve. Equal to proportional limit.Units: psi

ELASTIC MODULUS, E: The ratio of stress tostrain for the initial straight-line portion of thestress-strain curve. Determined by:

Ab

ABE

=

Units: psi

MODULUS OF RESILIENCE, UR: Themaximum energy the material will absorb withoutinelastic deformation. Equal to the area under theelastic portion of the stress-strain curve.Determined by:

EU plR 2

2=

Where pl is the proportional limit,defined later in this section.

Units: (in-lb)/in3

MODULUS OF TOUGHNESS, UT: Energy perunit volume required to rupture the material.Equal to the area under the entire stress-straincurve. For materials with a stress-strain similar tothat shown in Figure 2, a trapezoidalapproximation can be used:

( )( )( )( )%1001%21 elongationU ultplT +=Units: (in-lb)/in3

PERCENT ELONGATION: A measure of thedeformation at the point of final fracture.Determined by:


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Percent elongation ( )%100o

of

LLL

=

Where: Lo is the initial lengthLf is the final length

Generally the percent elongation is obtained afterthe test by fitting the two halves of the specimentogether and measuring the change in lengthbetween two existing punch marks. The percentelongation will vary depending on the gage length(distance between punch marks) used. Thereforethe gage length should be reported along with thepercent elongation.Units: in/in,

PERCENT REDUCTION OF AREA: Ameasurement of the fracture ductility. Defined as:

%RA = Ao-Af x 100%

Ao Where: Ao is the initial cross-sectional area

Af is the final cross-sectional areaat the location of fracture.

Values for %RA range from near zero for brittlematerials to high values (approaching 100%) forductile materials which can neck severely atfailure.

[in/in]

[psi]

0.002 0.004

E

FractureStrength

Stress-Strain CurveTypical Sample

Ultimate Strength

Yield Strength(2% offset)

ProportionalLimit

FIGURE 2


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PROPORTIONAL LIMIT, pl : Engineeringstress at the point where the straight-line portionof the stress-strain curves ends. It is the limitingvalue for which the stresses and strains areproportional to one another. Some materials donot have a well-defined proportional limit and inmany cases the value may vary with the judgmentof the engineer and the precision of the scale ofthe graph. Equal to the elastic limit.Units: psi

TANGENT MODULUS, Et: Slope of the stress-strain curve at a particular stress level.Units: psi

ULTIMATE STRENGTH, ult: Highestengineering stress reached at any time during thetest. Also known as the tensile (or compressive)strength.Units: psi

TRUE STRESS, : Load divided by the actualcross-sectional area of the specimen at theparticular load level.Units: psi

TRUE FRACTURE STRESS, f: Load atfracture divided by the final cross-sectional area.Note that the true stress increases until ruptureoccurs due to the decrease in the cross-sectionalarea (referred to as necking).Units: psi

YIELD STRENGTH, y: Engineering stress thatcauses a specified amount of permanent strain.The specified permanent strain is referred to asthe offset or permanent set. The most commonlyused offset is 0.002 in/in or 0.2%. The magnitudeof the offset should be reported with the value ofthe yield strength. The method followed is todraw a line parallel to the initial slope of thestress-strain curve, but offset by a specified

amount of strain. The point at which this lineintersects the stress-strain curve is the yield pointat the specified offset. Yield strength is aparticularly useful measurement for materialswith no definite proportional limit.

Some materials exhibit a decrease in stressin the yield region, or a yield drop, as shown inFigure 3. In such cases it is common to report anupper yield strength which is the highest stressreached prior to the drop; and a lower yieldstrength which is the lowest stress reached beforethe stress again begins to increase. Mild steel isthe most common material to exhibit a yield drop.The upper yield stress is dependent on suchfactors as the alignment of the specimen in thegrips of the machine and the rate at which thespecimen is loaded. Poor alignment or very slowloading rates may result in no yield drop andinitial yielding may begin around the value thelower yield point would have in a standard test.Units: psi

FIGURE 3

, Strain, in/in

, S

tress

, psi 0.2

LY

Yield DropUY

PL

MATERIAL TO BE TESTED:Tension testing will be performed on a total ofthree materials:

Hot-Rolled Steel (SAE 1020)Stress-proof SteelAluminum (6061-T6)

All three materials will be provided in the form of0.25-inch nominal diameter rods cut to 12-inchlengths.


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EQUIPMENT TO BE USED:MTS Testing Machine (20,000-lb Capacity)Extensometer (0.2 in/in Capacity)

SAFETY CONSIDERATIONS:Never operate machine when someone's hands arebetween the grips. Make sure all lab participantsare clear of equipment before beginning orresuming testing.

PROCEDURE:

SPECIMEN PREPARATIONS:The diameter of each specimen must be measuredand recorded. Punch marks must be made at 2inch intervals along each sample. These should bemeasured and recorded after making the punchmarks.

DATA REQUIREMENTS:The student will need to produce a stress-versus-strain curve for all three specimens on one graph.

MTS SET-UP1.) Follow Start- up Procedures

Station Manager tensionMPT tension.000

2.) Turn hydraulics on.3.) Make sure the 'MANUAL OFFSET' = 0 for

Stroke.4.) Adjust 'SET POINT'' to 0.05.) 'AUTO OFFSET' Load6.) Set-up Scope to plot a/b.

Load 500 lbf 2000Stroke 0.1 in 0.5Time 15 min

TESTING PROCEDURE:

1.) Create specimen file tens*.2.) Install Specimen in lower grip. Leave upper

grip open.3.) Install Extensometer onto specimen.4.) Pull Pin out of Extensometer.5.) Close upper grip.6.) Measure the distance between the grips

(gage length).7.) 'AUTO OFFSET' Strain.8.) Start the scope.9.) Lock MPT and select specimen.10.) Press 'RUN'. Let the test proceed through

elastic range until yielding is clearly presenton the scope.

11.) Press 'Pause' (1st operator button) to haltloading once yielding begins. (Do not allowmore that 0.005in/in strain)

12.) Replace pin in Extensometer and removeExtensometer from specimen.

13.) Press 'Continue' (2nd operator button) andlet test run to failure.

14.) Once failure occurs, press 'STOP'.15.) Remove specimen pieces from grips.16.) Unlock MPT and adjust SET POINT to 0.0.17.) Measure distance between punch marks and

final diameter at location of failure.18.) Repeat procedure for additional specimens19.) Turn hydraulics off.16.) Copy data files to diskette.

c:\em327data\tens*\specimen.dat17.) Delete specimen tens*.

POST TESTING PROCEDURE:1.) Record final punch mark distances and

diameter at the failure section on the datasheet.

2.) Indicate where the failure occurred, on thedata sheet.


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REPORT:

The report outline found in Appendix A shouldbe used for all formal reports handed in forEM327.

REPORT REQUIREMENTS:For each of the three materials tested;1. Determine and tabulate the following

properties:a. Proportional Limitb. Yield Strengthc. Ultimate Strengthd. Modulus of Elasticity

e. Percent elongation for each 2" gage length between punch marks (including

the segment containing the failure) and forthe largest combined gage length (6"or8")inside the grip markings.f. Percent reduction in areag. Modulus of Resilienceh. Modulus of Toughnessi. True Fracture Strength

2. Compare b,c, and d to reference values foundin Appendix B. by calculating the percentageerror.

3. Discuss possible reasons for thediscrepancies in (2).

4. Provide stress versus strain plot,appropriately labeled, for all three specimenstested. (Refer to Appendix A for example).

5. Discuss the consistency of the percentelongation measurements using different gagesections. Comment on the possible reasonsfor discrepancies obtained for a givenspecimen.

6. Briefly summarize, in words, the similaritiesand differences in material properties for thethree materials tested. When observed,present relationships between variousmaterial properties for the three materialstested (example: increasing Modulus of

Toughness for the three materials wasaccompanied by increasing percent reductionin area and decreasing Modulus ofResilience).

QUESTIONS:1. Chances are that the specimens failed

somewhere other than directly in the middle.What determines where a specimen fails?

2. For the steel specimen compare the stress inthe bar at rupture, as computed from the areaat the break, with the ultimate strength. Whyisn't the actual area of the fractured cross-section a suitable basis for defining strength?

3. Why is it often difficult to evaluate the elasticlimit?

4. What is the effect of poor alignment of thespecimen? Why does a specimen that isproperly aligned provide a more accurateestimate of the tensile strength compared tothe estimate provided from results from a testwhere the specimen was not accuratelyaligned?

5. Why would a stress-strain diagram bepreferable to a load-elongation diagram forpresenting the results of a tension test?

6. Why is it important to know the gage lengthwhen using percent elongation information?

7. Explain why the percent elongation in a 2inch gage section may exceed that of an 8inch gage section.

8. Can any conclusions be drawn regarding thepossible effect of the punch marks on thestrength of the bar at the punched sections?

9. Can the elongation of a specimen bedetermined accurately by measuring themovement of the test machine cross head?Why?


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EXAMPLE DATA

The data obtained from the MTS machine is similar to the following data set.

Data Acquisition Linear Region Time: 55.541504Axial Load Axial Strain Axial Strokekip in/in in0.62910855 1.846371e-004 2.8323776e-0031.1272926 3.8605928e-004 5.1649134e-0031.6147344 5.8748142e-004 7.6640593e-0032.1035187 7.8890362e-004 1.0329815e-0022.5761893 9.903257e-004 1.3162179e-002

Data Acquisition Non-Liniear Region Time: 112.58374Axial Stroke Axial Loadin kip0.10663022 8.50202940.15677974 8.64302540.20692927 8.65578170.25707877 8.63094040.3072283 8.58461280.3573778 8.47651580.40752733 7.55870250.45767686 -5.3712581e-003

Data Acquisition Ultimate Load Time: 112.5918Sec

Axial Load Axial Strokekip in8.6584673 0.46100906-1.8738974 5.6147482e-002

GRAPHS

In order to make the stress versus strain graphs for each specimen, the student must use both the linearand non-linear data.

The load must be converted to the normal stress, , using the following equation.

initialAreaLoad

=

For the linear region, the strain can be taken directly from the data.

For the non-linear region, the stroke must be converted to strain, , using the following equation.

initialLengthStroke

=

Put all three specimens on the same graph in order to compare the different materials. Be sure to includea legend and label the axis.

tension test report

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