A1_TensileTest_2003

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EXPERIMENT A1

Mechanical Testing of Materials

Summary:

Welcome to the ME-242 laboratory! In this laboratory you will perform hands-onexperiments with the Instron tensile test machine and conduct analyses that will allow you to determine

the materials properties of several test articles. hese materials properties will include

yield stren"th

tensile stren"th

elon"ation

#efore testin"$ you will learn to calibrate the load cell and extensometer and select appropriateoperatin" conditions. %fter testin" you will need the "raphs from your chart recorders alon" with thevarious measurements of sample "eometry to calculate sample properties. &our samples will includesome or all of the followin"

a hot wor'ed steel

a cold wor'ed steel

an aluminum alloy and several plastics with various properties

Instructions:&our 'ey to success in this lab is to come prepared!

#efore arrivin" at the lab$ read throu"h this lab module so that you will understand what the lab

procedure is and how the lab e(uipment is used.

Each "roup should answer all the (uestions on the preliminary question sheetto be turned in at

the eginningof the lab.

Each "roup will write one report. )eneral "uidelines for writin" this report may be found in the

section on wee'ly laboratory reports contained in the lab manual.

Timing: his lab ta'es the ma*ority of one afternoon +approximately 4-, hours.

Whats an Instron?he term Instron is fre(uently bandied about test labs and in industry. %nInstron is a universal test machine. #ut be careful! Instron is a brand nameand there are many brands of universal test machines includin" M andinius-/lsen$ amon" others. he labs in the 0epartment of MechanicalEn"ineerin" feature both Instron and M test machines.

Instron

was established in 143 in #oston$ Massachusetts by aroldindman and )eor"e #urr. Mr. indman was wor'in" on a pro*ect todetermine the properties of new materials to be used in parachutes. incetest machines available at that time did not have the necessaryperformance criteria to ade(uately evaluate these new materials$ Mr.indman teamed up with Mr. #urr to desi"n a material testin" machinebased on strain "au"e load cells and servo control systems.

he resultin" prototype was so successful that Mr. indman and Mr. #urrformed Instron En"ineerin" 5orporation. he name was derived from the6ins7 in the word instruments and the 6tron7 in the word electronics.

Example of a 8niversalest Machine

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!ac"groun#:

he bac'"round for this lab can be found in most introductory materials science texts such asMaterials Science an# Engineering$ by 5allister.

En"ineerin" stress is the force per unit +ori"inal area.

En"ineerin" strain is the elon"ation per unit +ori"inal len"th. hey are represented by thefollowin" symbols

En"ineerin" tress$ S= F

Ao

and En"ineerin" train$ e=l

lo

Where Ao 9 ori"inal cross sectional area of specimen

lo 9 ori"inal len"th of the "au"e section

F 9 applied force

l 9 chan"e in len"th

:or a linear elastic material$ these parameters are related by oo'e;s law$

S = Eewhere E is &oun";s modulus. It is implicit here that only axial stresses and strains are of interest./therwise$ oo'e;s


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= ln l

o+ llo

= ln 1 +e( ) e .

:or strains of about 1>$ the ?error? is of order of 2

or 1@ -4. 5onse(uently$ there is no si"nificant

difference in the en"ineerin" and true strains when all measurements are of small strains. he true stress

and strain are also related by the modulus E$ = E since the modulus is established at a small

strain level where Ai is approximately e(ual to Aoand li is approximately e(ual to l .

:or lar"e strains in plastic deformation$ the volume of specimens is approximately conserved. #ecause of

this$ the instantaneous area Ai can be calculated from the true strain. %ssumin" volume is conserved$

Aolume 9 Aolo = Aili . /r$ rearran"in" and ta'in" the natural lo"arithm$ we obtain

= lnA

o

Ai

= ln

li

lo

.

hus$ Ai = Aoexp() . =ote that a tensile true strain followed by an e(ual compressive true strainreproduces that initial len"th of the specimen. his is not true for en"ineerin" strain.

Tensile Test Specimens0urin" a tension test$ it is desirable to apply forces to the specimenlar"e enou"h to brea' it. ence$ some test en"ineers spend theircareers brea'in" thin"s for a livin"!

In order to collect useful data in a tension test the "rip re"ion of thetest specimen must have a lar"e enou"h area to transmit the forceswithout si"nificant deformation or slippin". 5onse(uently$ mostspecimens have a reduced "au"e len"th and enlar"ed "rip re"ions.

While most material properties are supposed to be specimen

"eometry and "rip independent$ there are some wea'dependencies. hus$ the %merican ociety for estin" Materials+%M has specified standard specimen "eometries.

%M has also prescribed test methods so that data reported fordesi"n purposes is obtained in a standardiBed way. he specimen"eometry is usually reported as part of the test results.More info on the %M may be found atwww.astm.or"

Example of a tensile testspecimen

Ceturnin" to our discussion of the properties$ the data we will record is the load elon"ation curve. incemany materials are rate sensitive$ the rate of elon"ation is controlled durin" the tensile test by movin" oneof the "rips at a fixed displacement rate relative to the other. 8sual testin" rates correspond to

en"ineerin" strain rates of about 1310 = se where the ( ) represents differentiation with respect totime. :or example$ if the specimen had one inch "au"e len"th$ the displacement of the machine is 1@

-D

inches per sec. and the load is recorded on a strip chart travelin" at constant speed$ say 11@ inch per

second$ then it is clear that the 10-3

s-1strain rate will produce 1@

-Dinch displacement in 11@ inch of chart

or 1> strain in one inch of chart. 5hart len"th and strain are then parametric variables$ both dependenton time. his is the simplest way of measurin" the load-elon"ation curve and is the most common.owever$ the elon"ation determined in this way also included the elon"ation of the "rips$ the ends ofspecimen$ the load measurin" transducer +load cell and the deflection of all the test frame. ypically$ atthe yield stren"th of a steel$ the other elon"ation outside the "au"e len"th is about , times lar"er than theelon"ation insidethe "au"e len"th.

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Gauge Length

Grip Area
http://www.astm.org/http://www.astm.org/http://www.astm.org/


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5onse(uently$ we cannot measure the elastic modulus from the slope of the load vs. elon"ation curvedetermined in this way. o circumvent this problem and ma'e direct measurements$ an extensometer isinstalled on the specimen that measures displacement within the "au"e len"th. his transducer isdesi"ned to produce a linear volta"e output with respect to displacement. ince the initial "au"e len"th isfixed$ the output is then proportional to the en"ineerin" strain. If the load si"nal +volta"e which isproportional to the applied force and the extensometer si"nals are plotted usin" an F-& plotter$ the initialslope is then the elastic modulus.

:or stability$ the load must increase all the time. The tensile deformation is unstable and strain is nolonger uniform when the load reaches a maximum. 0eformation stability is achieved when the specimenhardens durin" deformation. he result is uniform elon"ation. If the hardenin" rate is too low$ a runawaysituation called nec'in" develops. o avoid nec' formation$ the hardenin" rate must be faster than thedecrease in cross sectional area

d

dA

A.

=ow if the volume remains constant AlV= or dV = 0 = A dl + l dA ,he strain can be written in terms of the chan"e in area as

d =dl

l=

dA

A.

ubstitutin"$ we obtain the re(uirement for stability

d

d .

Whend

d= $ then dF = 0 and the sample is unstable. This can be shown as follows. By definition

= F

Aor F= A .

Differentiate this equation to obtain

dF = A d + dA .

When the load is maximum$ dF = 0 and

A d + dA = 0 ord

d= .

his is the critical value for the wor' hardenin" rate. %s a result the specimen may nec' down and be"inlocal deformation. his occurs at the pea' load. o determine the true stress strain behavior beyond thepea' load re(uires 'nowled"e of the non-uniform "eometry of the nec' in both the calculation of strainand the stress distribution. In certain materials$ the true stress at fracture can be several times the

en"ineerin" stress.

Most data you will be exposed to are en"ineerin" stress and strain unless otherwise specified. If there isa yield point$ namely$ a sharp transition between elastic and plastic deformation$ yield stress is defined asthe stress at the yield point. If there is a yield drop$ the maximum stress is the upper yield point and theminimum stress is the lower yield point. If the curve is smooth$ yield stress is defined at a specific amountof plastic strain. 8sually @.2> permanent strain is used to define the yield stress. hen the yield stress is

so identified as (0.2%). he proportional limit is the stress where the flow curve first deviates from

linearity. his is intrinsically difficult to measure because it is related to the sensitivity of your instruments.ry to estimate the proportional limit when you analyBe your data. he ultimate tensile stren"th is thelar"est en"ineerin" stress achieved durin" the test to failure.

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he elon"ation to failure is the permanent en"ineerin" strain at fracture determined at Bero load. It doesnot include elastic strain but does include both the uniform strain and the localiBed$ nec'in"$ strain. heelon"ation to failure is usually stated as percent strain over a "iven "au"e len"th. he reduction in areais also a measure of ductility. he true strain at fracture is determined by measurin" the areas of thefractured specimen at the fracture site. Cecall usin" the constant volume approximation that

= A

o

Ai

.

he area under the en"ineerin" stress-strain curve is a measure of the ener"y needed to fracture thespecimen. It has units of ener"y per unit volume of the "au"e len"th and it is sometimes referred to as ameasure of a material;s ?tou"hness.? owever$ the term fracture tou"hness more commonly refers to theener"y re(uired to propa"ate a crac' per unit area increase of crac' siBe.

Advanced Test Applications

he Instron you will be usin" today can apply aloadGeither tensile or compressiveGin one axis.

In industry$ test en"ineers mi"ht want to applymultiple loads across a variety of axes in order todetermine ultimate performance of a product ordevice.

/ne example of this is the structural tests that wereperformed on the 5handra pace elescope7soptical meterin" bench. he 5handra elescope isone of =%%7s )reat /bservatories and it7s opticalmeterin" bench was desi"ned$ fabricated$ andtested by the Eastman Hoda' 5ompany inCochester$ =&.

In order to perform a structural test a special D-story test frame was constructed in which doBensof actuators were mounted. he actuators wereattached to the 5handra optical bench and variousloads were applied that simulated conditions thatthe structure mi"ht experience durin" it7s launchinto orbit. In function these actuators are similar tothe Instron7s in that they apply a controlled load in aspecific direction.

:or more information httpchandra.harvard.edu

his 23 ft-lon" by 1@@in. diameter optical benchwas desi"ned and built by Hoda' for =%%;s5handra F-ray observatory. Wei"hin" 3, lbs.$ the

honeycomb structure is the lar"est compositemeterin" device ever built for use in space.

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http://chandra.harvard.edu/http://chandra.harvard.edu/


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Apparatus

In this experiment we will use an Instron machine desi"ned to do tensile tests of specimens. hemachine has a ,@@@ lb. capacity. It consists of a lar"e heavy duty test frame with a fixed beam at the top$a movin" beam +referred to as the crosshead and a "earbox and very lar"e motor located in its base.he specimen is mounted between two "rips$ one attached to the fixed top beam and the other attachedto the movin" crosshead. he fixed beam at the top contains a load cell +which wor's on the principle ofstrain "au"es. It measures the applied force on the tensile specimen. he movement of the crossheadrelative to the fixed beam "enerates the strain within the specimen and conse(uently the correspondin"

load. he "earbox below selects hi"h and low speed ran"es for movement of the crosshead.

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SpecimenGrips

Fixed Beam

Start / Stopand

Speed controlConsole

Load Cellbridge gain

Load CellShuntCircuit

Calibration

Speed

Selector

Up / Down /Stop

Buttons

Figures 2 & 3:Control Consoles for Instron Tensile Tester

crosshead

Load CellBridge control

andChart Recorder

Console

Figures 1Instron Tensile Tester


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=ext to the test frame is the associated electronics consoles. hey contain the main startstop controlsfor testin" and the ad*ustments for the sensitivity of the strain "au"e load cell +a strain "au"e brid"e aswell as a chart recorder to read the output of the load cell brid"e. he electronics consoles also containsthe "ear speed selection box for the "earbox +allows us to select the various strain rates and the mainonoff switches for the instrument$ one to turn the instrument on directly and the other to turn the amplifierfor the "earbox motor onoff +called the %mplidyne switch.

In order to enhance the accuracy of our measurements of &oun";s Modulus we will add an extensometerdirectly to the sample to measure the actual elon"ation between two "iven points on the sample to recordthe load vs. elon"ation curve for the elastic re"ion of the sample only.

:inally$ a data ac(uisition system utiliBin" a J5$ a =ational Instruments data ac(uisition card$ and


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E$perimental proce#ure:

1% Equipment &aliration:5alibration coefficients for the Instron load cell and extensometer must be "enerated in order toconvert the volta"e data ac(uired durin" a tensile test to real data.

ince the extensometer +an


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4. Install the calibrated extensometer on the specimen. #e sure that it is centered and strai"ht andthat it is fully closed. CeBero the extensometer with the Bero control on the


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.% /oule chec" the follo-ing:a% 0our test specimen is properly loa#e# in the grips of the tensile tester% The lo-er grip pin is in placec% The correct crosshea# spee# is selecte#%#% The material specs are input into the a2IE3 2I%e% The Strain units utton is #epresse# +in the a2IE3 2I,%f% 0ou are sa)ing #ata%

1@. tart the test by pressin" the down button on the Instron control console.

11. /bserve the specimen. 0o not "et too close because fracture of the specimen liberates all thestored elastic ener"y in the specimen. 0o you see bands propa"atin" alon" the steel specimenP

hese are < ders bands indicatin" the multiplication and motion of dislocations. hey will not bevisible unless the specimen is hi"hly polished.

12. #e sure to record both load vs. time and load vs. strain for the initial portion of the test. Remo)ethe e$tensometer -hen the a2IE3 2I #isplays the 4REM52E EXTENS5METER6 on theoa# 7 Strain 8raph and continue the test recordin" the load vs. time curve until fracture./bserve the nec' formation. =ote that it occurs ri"ht after the maximum load.

1D. 0o this for all of you specimens. +&ou will not use the extensometer on the @., in diameter plastic

specimen. 8se the conditions "iven in the chart in the appendix for each of these samples.

Figure 6:As the specimen approaches ultimate stress the reduction inarea becomes clearly visible. This is referred to as necking.

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Reduced area observed


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/iscussion:

Ceport the followin" data for each of the samples if it exists

&oun";s modulusJroportional limit8pper and lower yield [email protected]> yield stren"th

8ltimate stren"th> Elon"ation at fracture> Ceduction in area

5ompare your results of &oun"7s modulus$ yield stress$ and ultimate stren"th with published values.Explain any discrepancies.

In your discussion please address the followin"

1. 0etermine the error that would result if you calculated &oun";s modulus from the loaddisplacement curve without the extensometer clipped on the specimen. Explain the cause of this error.5onsider an in-series sprin" representin" the machine stiffness +that would also include the "rips and thepart of the specimen outside of the "a"e len"th. 0etermine and compare the values of the machinesprin" constant calculated from the data for each specimen. Why are these values differentP

2. Jlot a true-stress versus true-strain curve for the cold rolled steel specimen +int use aconstant volume approximation and compare to the en"ineerin" stress versus en"ineerin" strain plot.Jlot only for the re"ion where the calculation is valuable. What is the limit of the calculation and whyP

D. he stress-strain curves for plastics are very different for those of metals +e.". aluminum andsteel. Explain in terms of the differences in atomic or molecular deformation mechanisms.

4. he cold wor'ed steel specimen does not show a yield point$ the hot wor'ed steel does.WhyP %fter plastically deformin" the sample$ would either of these samples show a yield point uponreloadin"P WhyP

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%JJE=0IF

Cecommended chart speed$ crosshead speed and full scale

Materials 5olor 5hart speed 5rossheadpeed

:ull cale

+% 1@1N cold-wor'ed white-blue @., inmin @.@2 inmin ,$@@@ lb

+# %M %-D3 hot-wor'ed blue-blue @., inmin @.@, inmin ,$@@@ lb

+5 2@24-D,1 Ced @., inmin @.@, inmin ,$@@@ lb

+0 =ylon-1@1 #lue 1.@ inmin @.2 inmin ,$@@@ lb

+E Jolyethylene-i-density )reen @., inmin 1.@ inmin 2$@@@ lb

+: JA5 )ray @., inmin @.1 inmin 2$@@@ lb

&aution5han"in" the chart speed re(uires replacement of "ears. he chart motor has considerable inertia andre(uires several seconds to stop. 0o not touch the "ears while they are movin".

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EXPERIMENT A1 ME&9ANI&A TESTIN8

PREIMINAR0 ;ESTI5NS

)roup number +namesQQQQQQQQQQQQQQQQQQQQQQQQQQQQQ 0ateQQQQQQQQQQQQQQ

1. What are potential safety concerns for this experimentP

2. ow does a1@D@ steel differ from a1@4@ steelP #e specific. %ddress what the numbers mean aswell as how the properties differ.

D. ow does a cold-wor'ed steel differ from a hot-wor'ed steelP #e specific.

4. :or a 1@2@ steel sample with a len"th of 2.2,? and a diameter of .2D,? calculate the maximumload you would expect to have to apply to fracture the sample. #ased on this value$ what load cell ran"ewould you choose and whyP %lso$ estimate the maximum elon"ation a 2 inch sample would experience

before plastic deformation +estimate this value assumin" yield occurs at @.2> strain. #ased on this value$what crosshead rate would you choose for your experiment and whyP %t this crosshead rate$ how lon"would you predict it would ta'e to fracture the specimenP

,. %ssume that the load cell bein" used is set to a 2@@@ lb. full scale and has an accuracy of 2> fullscale.

+a What will be the accuracy in readin" a 1@@@ lb. load +in terms of a > of theactual loadP

+b What will be the accuracy in readin" a 2@@ lb. load +in terms of a > of the

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actual loadP

3. Why do we put an extensometer on the sample rather than *ust use the extension of the frame ofthe InstronP Is use of the extensometer important in measurin" the elastic modulusP Is the use of anextensometer valuable for measurin" the ultimate stren"thP

. 5onsider the Instron machine +with stiffness 'm and the sample +with stiffness 's as sprin"s inseries with total stiffness 't. What is the relationship between these three stiffnessesP 0urin" the test$ youmust 'eep trac' of the scales on each of your charts and label them appropriately. If your computer "ivesa plot of force versus crosshead position and another plot "ives the force versus sample elon"ation fromthe extensometer clipped on the specimen$ what stiffness would be "iven by the slopes of each of theseplotsP

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A1_TensileTest_2003

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