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Johannes Schneider

Lab Partners: Joshua Gafford, Robert Kalwarowsky

02/17/2008

2.002 Lab 1 Report

bubby@mit.edu

Uniaxial Tension TestUniaxial Tension TestUniaxial Tension TestUniaxial Tension Testinginginging

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1. AbstractAbstractAbstractAbstract

Prior to entering engineering studies at university, only few engineering students have had

exposure to material analysis. In order for engineering students to be successful in their field it is

significant for them to acquire a good understanding for material analysis in the course of their studies.

The uniaxial tension test is one of the most fundamental tests in mechanical engineering specifically

devised for determining mechanical material properties that are related to forces acting on bodies in a

uniform direction; properties such as the Young’s Modulus, the Yield stress, the Tensile Strain to Failure,

and the Ultimate Tensile Strength. In the course of this laboratory experiment, five material specimens

were tested and analyzed on their stress-strain relationships: Aluminum, two 4140 Steels, one rubbery

polymer, and Polycarbonate. This laboratory experiment is meant to convey a basic understanding for

material testing and to trigger further interest in the study of material engineering.

2. Experimental MethodsExperimental MethodsExperimental MethodsExperimental Methods

2.0 Manual MeasurementsManual MeasurementsManual MeasurementsManual Measurements

Before the start of each test it is significant to have measured and recorded the respective initial

diameters of each specimen’s narrow mid-section as well as that specimen’s initial gauge length. At the

end of each test, after material failure has occurred, it is important to also have measured and recorded

the final diameter of that same specimen’s necking region.

2.1 6160616061606160----T6 T6 T6 T6 Aluminum Axial Tensile TestAluminum Axial Tensile TestAluminum Axial Tensile TestAluminum Axial Tensile Test

In the aluminum axial tensile test, a specimen of 6160-T6 Aluminum was fastened to the testing

grips of an Advanced Materials Testing machine, short Admat. The shape of the test sample resembled

that of an hour glass. Its top and bottom sections were considerably wider in diameter than that of the

narrow mid-section, which the extensiometer attached to. During the tensile test, the bottom section of

the specimen was statically attached to the testing machine’s frame whereas the top section was tightly

gripped and pulled on by the machine’s crosshead. The test was designed to simulate varying stages of

static loading, thus the machine’s crosshead moved at a very slow, constant upward motion. As the

machine reported information on loading force and overall crosshead deflection, an electrical

extensiometer accurately measured and reported the sample’s overall elongation over the its gauge

length. This data was recorded in time intervals of .5 seconds during the course of the test until

controlled failure occured in the narrow mid-section of the specimen.

2.2 Rubbery Polymer Rubbery Polymer Rubbery Polymer Rubbery Polymer Axial Tensile TestAxial Tensile TestAxial Tensile TestAxial Tensile Test

The rubbery polymer tensile test was done using a lesser forceful, more accurate Zwick/Roell

Z2.5 testing machine which was built for maximum loading conditions of less than 2.5 kN. The shape of

the rubbery specimen used in this test again resembled that of an hour-glass, designed to constrain the

area of greatest stress as well as to constrain the point of most probable failure to the gauge of the

specimen. For this test the sample was not driven to the point of failure but rather set on continuous

loading cycles. Markings on the surface of the specimen illustrated the degrees of deformation and

elongation of the sample throughout the tested cycles. For even better visualization the sample was put

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between two pieces of Polaroid foil revealing color-patterns on the surface of the material that

qualitatively related to levels of strain that were presented on the surface of the material. The data for

axial loading and total elongation was recorded and saved for further formal analysis.

2.3 Polycarbonate Polycarbonate Polycarbonate Polycarbonate Axial Tensile TestAxial Tensile TestAxial Tensile TestAxial Tensile Test

The polycarbonate tensile test was conducted using an Instron Servo mechanical testing machine.

Analogous to the setup of the aluminum test, the polycarbonate sample was fastened to the testing

machine. An electronic extensiometer recorded overall elongations throughout the gauge length.

Likewise, crosshead displacement data and loading forces were documented for constant time intervals of

.5 seconds. The Sample failed in the relative center of the long neck that had formed.

2.4 HeatHeatHeatHeat----treated 4140 Chromale Steel treated 4140 Chromale Steel treated 4140 Chromale Steel treated 4140 Chromale Steel AAAAxial Tensile Testxial Tensile Testxial Tensile Testxial Tensile Test

The final two tests of this laboratory exercise resembled in setup and execution the ones that

were used in the cases of the aluminum and polycarbonate specimen. This time though, two specimen of

the same basic material – 4140 Chromale Steel – were set in direct comparison to another when tested.

One of the two steel specimens had been annealed prior to testing, the other one had been kept in

original “as-received” condition. Both samples had been manufactured to meet industry specifications for

bolting of high pressure vessels. The narrow mid-section was to hold the extensiometer as well as to mark

the preferred location for elongations, necking, and ultimate material failure. As in previous testing, the

recorded data consisted of machine loading and elongation of the extensiometer gauge.

3. ResultsResultsResultsResults

3.1. IntroductionIntroductionIntroductionIntroduction

Engineering requires much more than merely having great ideas. It is just as significant to know

about manufacturing techniques that will enable you to turn your idea into reality and ultimately, on the

most fundamental level, it is significant to be familiar with materials behaviors as to being able to decide

on the materials that are best suited for a certain goal. By conducting uniaxial tensile tests, engineers can

study material relationships between stresses and strains as well as related material properties that

ultimately help uniquely defining materials for future reference. Many of these unique material

properties were originally determined by studying and comparing various stress-strain-curve.

3.2. CalculationsCalculationsCalculationsCalculations and and and and AnalysisAnalysisAnalysisAnalysis

All five uniaxial tension tests that were conducted as parts of this laboratory exercise recorded

raw data in Load versus elongation pairs. These can be converted into engineering stresses and strains

using:

������������ � ���� [1]

and

������������ � ������ , [2]

As well as into true stresses and strains using:

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��� � � �������������1 � ������������� [3]

and

��� � � ln ������� � � ln �1 � �������������. [4]

For the conversion of loading data into stress data, and elongation data into strain data, the values

for initial gauge length L0 and initial cross-sectional Area of the narrow mid-section A0 that were taken

before the start of each test need to be used. Thereafter the raw data that was received in each of the

tensile tests was converted into stress and strain data-pairs and plotted in engineering stress versus

engineering strain graphs, as well as in True Stress versus True Strain graphs. Since stress and strain

account for the geometry of each specimen, this data is more useful for purposes of general material

analysis than raw force vs. elongation data that is taken straight from the testing machines. Thus stress-

strain graphs are extremely powerful tools for material analysis. Simply by studying each material’s

characteristic stress-strain graph is it possible to tell what happened to that material during the loading

process, both on levels of the macrostructure as well as on that of microstructure. Most engineering

calculations are done using engineering stress and engineering strain data, true stress and true strain are a

more accurate representation of the momentary stresses and strains within a material. This is only

significant when dealing with large deformations, such as for rubbery materials or when studying plastic

deformations.

Figure A.1 is a representation of engineering stresses versus engineering strains that occurred

within the 6160-T6 Aluminum specimen ranging from the initially unloaded condition to the ultimate

material failure. As can be seen in this graph, aluminum, as all other materials that were tested, at first

behaves linearly like a spring with spring constant equal to the slope of that line. This constant is called

the Young’s modulus:

� �������� [5]

The Young’s Modulus is one of many unique material properties that are attained by studying

material deformations under uniaxial loading conditions. The Young’s Modulus describes material

behaviors under uniaxial loading conditions during the elastic regime. The point at which the material

exits the elastic regime and enters the plastic regime is called the yield strength of a material. This is the

point at which the stress-strain-curve stops showing a linear behavioral pattern. The yield strength is

commonly approximated by the intersection between the 0.2% offset yield strength line and the data

points taken in the course of the tension test. The 0.2% offset yield strength line intersects the strain axis

at 0.002 and uses the material’s Young’s modulus for its slope:

�.� %��� � �� � 0.2%� [6]

�� � ���������������� � �.� %���� � ��� � 0.2%� [7]

When entering the plastic regime of deformation, aluminum begins to stress harden. That means

in other words, that the stress-strain-curve continues to climb, yet with a continually diminishing slope

until reaching the point of ultimate tensile strength, as can be seen in Figure A.1:

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���� � ���� [8]

After reaching the point of maximum tensile stress, the stress-strain curve of aluminum starts

declining until material failure. The stress-strain-curves of both 4041 chromale steel specimen, as can be

seen in Figures D.1 and E.1, reveal similar basis functional behaviors to that found in the case of the

aluminum specimen. Major differences rather lie in the exact values of each material’s Young’s modulo

[5], yield strengths [7], and ultimate tensile strengths [8]. The rubbery specimen, as shown in Figure B.1,

maintained its linear behavior throughout complete loading cycle. In other words, the rubbery material

never left it elastic regime. As also presented in the case of the metal specimen and as illustrated in Figure

C.1, the polycarbonate specimen also shows a phase of plastic deformation that follows the initial linear

elastic regime. Yet unlike it was the case for the metals, the PC plot drops considerably ultimately after

the yield strength [7] is reached, and increases for the final stretch of material deformation until eventual

fracture.

4.4.4.4. DiscussionDiscussionDiscussionDiscussion

The behaviors that were just described using experimental data were also visually evident on the

surface of each specimen during different testing phases. Material that was deforming elastically, merely

stretched without revealing any signs of stress. As soon as a material entered the plastic phase, strain

marks appeared on that sample’s surface. After passing the state of ultimate tensile stress, “necking”

occurred in the narrow mid-section of the specimen. In other words, the cross-sectional area of a very

small section of the overall sample began shrinking disproportionately quickly until the point of failure.

In the special case of the polycarbonate sample, necking occurred just after reaching the plastic regime,

yet instead of causing immediate material failure, instead the neck propagated outwards until, in an ideal

case, the entire narrow mid-section was made into a large neck. Only then, failure would occur.

Such material behaviors are even better explained in the specific context of each material’s

microstructure. For metals, each atom is bound within an overall crystalline structure. In the elastic

regime of metals, atomic and molecular bonds are merely stretched, yet not broken. Once the stresses

acting upon the atomic bonds exceed a certain level, atomic and molecular bonds begin breaking. This

transition is marked by a material’s yield stress. Any bond, once broken, will not grow back upon

relieving material loading. The rubbery specimen has a very different molecular structure to that of the

metals. It is made of very long, tangled molecules that can be imagined as spaghettis on a plate. When

pulled on, these spaghetti-like molecules slowly untangle and align in parallel with the direction that the

loading is applied to the material. The molecular bonds in rubbery materials only break just before

failure, this explains their extensive elastic regime. Polycarbonate is yet another case. On a molecular

level, PC consists of an unordered mess of very large, interconnected molecules. These gigantic molecules

provide PC with a greater structural stability than that of rubbers, yet much less than that of metals. In

the case of uniaxial loading, molecular bonds of polycarbonate first stretch until the material’s yield

strength is reached, at which point the weaker connections-bonds in-between the larger molecule-

strands break. Such behavior results in a drop of PC’s stress-strain-graph (as presented in Figure

C.1)immediately after the yield strength [7] is reached. After most of these weak bonds are broken, the

molecular structure resembles that of rubber for at this point most of polycarbonate’s molecules have

basically attained a linear structure. This explains why, after initial necking has occurred, this neck tends

to propagate outwards, rather than causing almost immediate facture as is the case for metals.

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The major difference between the two steels consisted in the fact that the Annealed Steel was

heat-treated, which was meant to strengthen the steel’s microstructure. Yet when comparing the values

for yield stress [7] and ultimate tensile stress [8] between the two steels (see figure 1), it is obvious that

the “as-received” steel has much greater strength than the annealed steel. Also the ductility [9], which is

a measure for the overall strength of a material, suggests a greater material stiffness for the “as-received”

steel than for the annealed steel.

� � ���� [9]

This is an unexpected result, yet can be contributed to small errors that might have occurred

during the annealing process. It also seems surprising that the ductility of both steel specimen lies above

that of aluminum (figure 1). This phenomenon might simply be related to the fact that both steels are

chrome alloys and thus softer and probably also more ductile than regular steel.

The most significant findings of this laboratory exercise are summarized in a table of properties

below (see figure 1). The results presented in this table approximately agree with those found in

literature.

Material Samples E σy σuts q εeng f σeng for εeng=1

[Units] [MPa] [MPa] [MPa] [ ] [ ] [MPa]

Annealed 4140 Chromale Steel 223,724 473 766 0.618 0.226 -

As-Revieved 4140 Chromale Steel 205,770 917 988 0.599 0.205 -

6061-T6 Aluminum 70,569 319 350 0.628 0.116 -

Polycarbonate 2,173 59.2 59.2 - 1.730 46.44

EPDM Rubber 4.1 - - - - 3.00

Figure 1:Figure 1:Figure 1:Figure 1: Table of PropertiesTable of PropertiesTable of PropertiesTable of Properties