Lab 2 - Mechanical Properties Sophomore Year

8/2/2019 Lab 2 - Mechanical Properties Sophomore Year

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Mechanical Properties

Amy Lautenbach, Katie Burzynski, Brian Hower, Austin Schader, Thomas Agasid

Dept. of Materials Engineering, California Polytechnic SLO, San Luis Obispo, CA 93407

Abstract:

This experiment used Rockwell hardness tests and Instron tensile tests with ASTM standards to better

compare mechanical properties of steel, brass, and aluminum samples. Also, including the collective

data from the twelve aluminum pieces, twelve brass, and twelve steel (one of each given to each group

of twelve). The trends observed from the samples within group 11 were that Youngs Modulus depends

on the amount of available slip planes which allow the material to more easily elastically deform.

Strength, on the other hand, varied depending on the materials electron configuration. Brasss valence

electrons are held more loosely, and can deform more elastically without fracture than aluminum and

thus have a higher tensile strength. The trends observed through all of the data in all twelve groups

were that hardness and tensile strength had a linear correlation. This can be explained because

hardness is the measurement of resistance to the plastic deformation used during a tensile test.

Introduction:

The purpose of this experiment was to compare

the properties among different materials in

order to investigate the connection between

mechanical properties and the structure of

materials. The samples provided for

investigation were: aluminum, steel, and brass.These materials were put through hardness

tests and tensile tests using the specified by the

American Society for Testing and Materials

(ASTM) standards. The hardness test will help

to determine whether the steel sample is a

1010 alloy, with hardness readings in the 90s, or

4130, readings in the 100s. Performing tensile

tests on the materials will generate data to

create a stress-strain curve. All of the data

including data collected by other groups with

comparable samples will be used to analyze and

compare elastic moduli, strengths, ductility, and

hardness.

Experimental Procedure:

Hardness Tests:

The samples were hardness tested using the

Rockwell hardness tester available at Cal Poly

using a scale of Rockwell F. ASTM standards

require that this test be conducted at ambient

temperature within the limits of 50 to 95

Fahrenheit. The test piece is to be supported

rigidly. The Rockwell hardness test method

then measures the permanent depth of an

indentation left by an indenter which first uses

a minor load to find a reference position. Next,

a major load is used to reach the total required

test force. During testing the apparatus must

be protected from vibrations. After this major

load is released and the final position of theindenter is measured and converted to a

Rockwell hardness number. This was repeated

five times, with the center of each indentation

to an edge of the test piece at least two and a

half times the diameter of the indentation

apart, on each sample to ensure there was no


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error in measurement. The resulting average

hardness numbers were entered into an excel

spreadsheet where they were compared to

results from other teams. The hardness values

were also compared to the Elastic Moduli and

to strength to observe correlations between

these values.

Tensile Tests:

The samples were tensile tested individually

using the Instron tensile tester using the ASTM

standard for tensile testing. The standard

requires that the cross-sectional area of the test

specimen be determined as well as the

dimensions of the cross section at the center of

the reduced section. The samples were then

placed into the Instron tester and force was

applied in axial tension until the samples

underwent fracture. The Instron Tensile Tester

then generated a Stress vs. Strain curve of each

material measuring the change in length versus

force applied. Each strain curve was then

plotted in reference to one and other on the

same graph. Trends were then noted between

stiffness and hardness along with yield strengthand maximum strength in addition to ductility.

Periodic trends were then used to further

analyze and organize the results.

Results and Discussion:

I. Comparison of 3 Material SamplesUsing the data that was calculated from the

tensile test and referencing Cambridge

Engineering Selector (CES), the unknown steelwas able to be determined. By comparing the

yield strength values of the 1010 steel and 4130

steel in CES to that of the steel from the tensile

test, the yield strength value of the sample was

within the range of the 1010 steel and outside

of the range of the 4130 steel. In addition, the

hardness values taken from the sample were

similar to data of other groups who performed

tests on 1010 steel. From this, the sample steel

was determined to be 1010 steel.

Table 1Tensile Test Readings

Metal E y uts % el

Al 44.24 136.26 148.73 11.02

Cu+Zn 89.79 321.58 398.95 31.86

Fe+C 112.84 270.35 376.09 26.05

Among the three samples tested, aluminum had

the lowest Youngs modulus, yield and ultimate

tensile strengths, while also being the least

ductile as shown in Table 1. The crystal

structures for each of the metals have a direct

influence on the mechanical properties of the

samples.

The initial prediction that the aluminum sample

would be the least stiff was correct. Due to

aluminums face-centered cubic (FCC) crystal

structure, which has 12 slip planes, this allows

for more dislocations to slide on the available

slip planes. With an increased number of slipplanes, aluminum is more easily elastically

deformed, thus has a low Youngs modulus. In

comparison, the brass and the steel samples are

alloys. NOTE: Find out the crystal structures of

steel and brass to mention here. In a

generalization, alloys have additional strain on

their lattice due to the impurities, zinc in copper

for brass, and carbon in iron for steel. These

impurities make it difficult for dislocations to

move through the lattices. This additionalstrain results in a higher Youngs modulus for

brass and steel in comparison to aluminum.

The initial prediction that the aluminum would

have the least strength was correct. However,

the initial prediction that steel would have a

higher yield and ultimate tensile strength than


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brass was incorrect. The results in Table 1 show

that brass has a yield strength and ultimate

tensile strength greater than the 1010 steel. As

mentioned above, the crystal structures of the

samples have a direct influence on the

mechanical properties of the samples. For

similar reasons that the crystal structure affects

the Youngs modulus, the crystal structure also

influences a materials yield and ultimate tensile

strengths.

The yield strength of a material is defined as the

point at which a predetermined amount of

permanent deformation occurs. At this

particular point the material transitions from

elastic to plastic deformation. As discussedearlier, aluminums crystal structure causes the

material to have a low elastic modulus. Due to

aluminums low elastic modulus, in comparison

to brass and steel, the sample did not require as

much stress to be applied to it in order to reach

its yield strength. That is why the yield strength

of aluminum is lower than the brass and steel

samples.

Furthermore, the ultimate tensile strength isalso influenced by a materials crystal structure.

The ultimate tensile strength is defined as the

point at which a material can be stretched

before necking occurs and is the greatest

amount of axial stress a material can withstand.

The results in Table 1 show aluminum with the

lowest ultimate tensile strength, while brass

and steel both had higher ultimate tensile

strengths. The results make sense that

aluminum has the lowest ultimate tensilestrength of the tested samples after having the

lowest Youngs modulus, and the lowest yield

strength. The reason the brass and steel

samples have significantly greater ultimate

tensile strengths is due to the fact that they are

both alloys. The different sized atoms in the

crystal structures of brass and steel impede the

movement of dislocations. By impeding the

movement of the dislocations, a greater

amount of stress is required for the dislocations

to move, thus resulting in a higher ultimate

tensile strength.

Aluminum was found to be the least ductile

material. Brass and steel are both more ductile

than aluminum. Aluminum rests in the group 3B

column in row 3. Brass is composed of copper

and zinc which are both in row 4 which resides

lower down on the periodic table. Copper and

zinc have more electrons and protons than

aluminum. Because copper and zinc have so

many more electrons near the core, their outervalence electrons are located farther from their

nucleus than the valence electrons in

aluminum. The attraction between protons and

electrons decreases as the distance between

them increases. This causes the valence

electrons of copper or zinc to be held looser

than the valence electrons of aluminum. The

shielding effect becomes more prevalent due to

the extra D orbital electrons in the copper and

zinc, which weakens the bond strength

between the atoms in steel or brass compared

to aluminum. The electrons are held less tightly

by their nuclei. This allows the atoms in steel

and brass to move past each other easier than

atoms in the aluminum crystal structure.

Aluminum has a FCC crystal structure which

because of the multiple slip planes is usually a

more ductile structure. However, due to how

high up on the periodic table and how few free

electrons are present the bond strength

between aluminum atoms is higher. The

intermolecular forces between aluminum atoms

are stronger than those in Steel and Brass. The

higher the forces attracting the aluminum

atoms to one another, the harder it is to pull

the atoms apart. Steel has weaker bonds


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between its motifs due to the shielding effect

on the Fe atoms. Because of the weaker

intermolecular forces between Steel molecules,

the Steels slip planes easily slide past one

another, causing steel to be more ductile than

aluminum.

The Rockwell Hardness Readings are an

measurement of the materials hardness or

resistance to plastic deformation. Again, the

solid solution materials, brass and steel are

harder than aluminum, as seen in Table 2.

Table 2

Rockwell Hardness Readings: F-scale

Sample 1 2 3 4 5

Aluminum 16.3 21.7 25.2 25.4 21.0

Brass 93.7 94.3 94.4 94.5 94.4

Steel 96.2 96.2 95.9 95.7 95.8

II. Comparison of Class DataOur statistical analysis of Hardness vs. tensile

strength showed an approximate linearproportionality between the two characteristic.

When fitted with a best fit line our R^2 value

was 92.1% meaning 92.1% of the data could be

explained by this linear trend, seen in Figure I.

This relationship can be explained because

hardness is a measurement resistance to plastic

deformation and during a test a material is

stressed until plastic deformation occurs.

Similarly yield strength is also related to

hardness as can easily be shown using theCambridge Engineering Selector software to

plot hardness vs tensile strength.

Figure I

The collective data amongst the class of therelation between hardness values and ultimate

tensile strength.

In Figure II, the hardness vs elastic modulus

data shows a slight linear like trend, and during

a CES plot of the data there also appears to be a

linear like trend, however, the two properties

are not mathematically linked in any way.

Hardness does not depend on stiffness and the

test for hardness is independent of stiffness.

Figure II

The collective data of the relation between

hardness and elastic modulus.

600500400300200100

120

100

80

60

40

20

UTS (MPa)

H

avg(HRF

)

Al

Brass

S-1010

S-4130

Metal

Scatterplot of H avg (HRF) vs UTS (MPa)

180160140120100806040200

110

100

90

80

70

60

50

40

30

20

E (GPa)

HRF

Al

Brass

S-1010

S-4130

Metal

Scatterplot of HRF vs E (GPa)


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Elastic modulus and yield strength are also not

mathematically linked although our data also

appears to show a trend, as seen in Figure III. As

demonstrated by materials such as elastomers

elastic modulus has little to do with yield

strength. When plotted in CES the two

properties appear to have some sort of linear

correlation especially for metals, however,

there is little evidence to mathematically link

the two as being dependent on one and other.

Figure III


elastic modulus and yield strength.

When analyzing yield strength vs ductility our

data showed no correlation between the two

properties. When using CES to plot the two

properties, Figure IV displays random scattering

of points. Yield strength has no correlation to

ductility. This can be demonstrated by

comparing the graphs of yield strength vs elastic

modulus of metals and of elastomers; the lack

of consistency across materials clearly displays

the lack of relationship.

Figure IV


yield strength and ductility.

400350300250200150100

175

150

125

100

75

50

Yield Stress

E(GPa)_

1

Al

Brass

S-1010

S-4130

Metal

Scatterplot of E (GPa)_1 vs Yield Stress

353025201510

400

350

300

250

200

150

100

%elong

YieldStress

Al

Brass

S-1010

S-4130

Metal

Scatterplot of Yield Stress vs %elong

Lab 2 - Mechanical Properties Sophomore Year

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Transcript of Lab 2 - Mechanical Properties Sophomore Year