Materials Characterization Lab Report

8/10/2019 Materials Characterization Lab Report

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TENSILE TEST

Objective(s):

To ascertain and compare the following mechanical properties between three materials (i.e.

aluminum, mild steel and brass): strength, ductility, and stiffness.

Strength: to calculate yield strength and tensile / ultimate strength (UTS) via the stress-

strain curve.

Ductility: to determine % elongation and % reduction of area.

Stiffness: to determine the modulus of elasticity ().

Theory:

The tensile test also known as tension test, is carried out by pulling on something to ascertain

how the material reacts to forces being applied in tension. As the material is pulled, one can

determine the materials tensile strength and by how much it will elongate. During testing,

tensile strength is the stress at which a material breaks or permanently deforms (i.e. load is

applied gradually and is increased until the material fails). This test is therefore a destructive test.

The type of equipment that will be used in the laboratory to conduct this test, is a hydraulic

machine which is the tensile test machine. An extensometer will also be used to measure the

change in length in each specimen when the load is applied. From this test the following will be

determined: yield strength, ultimate strength, ductility and stiffness.


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Equipment and materials:

Three samples: Mild Steele, Aluminum and Brass

Extensometer

Gauge Mark Punch

Tensile Test Machine (Tinius Olsen:400000 pound capacity)
http://www.google.tt/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=VkqTDpc0pGD3EM&tbnid=yqZDhDcGBcy66M:&ved=0CAUQjRw&url=http://www.substech.com/dokuwiki/doku.php?id=tensile_test_and_stress-strain_diagram&ei=8_h5UsHBLs3NkQfAiYDYCw&bvm=bv.55980276,d.eW0&psig=AFQjCNGpr2KjBhwMuXNXqZrGRbiUrIp4uQ&ust=1383811684421851http://www.google.tt/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=tU-q3PO8woAvrM&tbnid=avLUNbqKhGeBKM:&ved=0CAUQjRw&url=http://www.lboro.ac.uk/microsites/mechman/research/mechanics-adv-materials/equipment/machines/hounsfield.html&ei=k_h5Uuz9Msr6kQean4DoCw&bvm=bv.55980276,d.eW0&psig=AFQjCNFK6vk8cWTZ1triOgY8wc-PduRFaQ&ust=1383811581697007http://www.google.tt/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=VkqTDpc0pGD3EM&tbnid=yqZDhDcGBcy66M:&ved=0CAUQjRw&url=http://www.substech.com/dokuwiki/doku.php?id=tensile_test_and_stress-strain_diagram&ei=8_h5UsHBLs3NkQfAiYDYCw&bvm=bv.55980276,d.eW0&psig=AFQjCNGpr2KjBhwMuXNXqZrGRbiUrIp4uQ&ust=1383811684421851http://www.google.tt/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=tU-q3PO8woAvrM&tbnid=avLUNbqKhGeBKM:&ved=0CAUQjRw&url=http://www.lboro.ac.uk/microsites/mechman/research/mechanics-adv-materials/equipment/machines/hounsfield.html&ei=k_h5Uuz9Msr6kQean4DoCw&bvm=bv.55980276,d.eW0&psig=AFQjCNFK6vk8cWTZ1triOgY8wc-PduRFaQ&ust=1383811581697007


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Procedure:

The vernier caliper was used to measure the gauge length of each sample between punch

marks and measurements were recorded. The thickness and width of the samples were also

measured and recorded. The sample was then placed in the testing machine ensuring that the

axis of the specimen was in alignment with the direction of the pull. The values of gauge

length, thickness and width were then entered into the computer software. The tensile

machine was then zeroed and the test was started using the computer software. The sample

was pulled until it fractured. A force vs. elongation graph was then obtained for each

specimen and this was used for calculations. The broken sample was removed and placed

together and the new distance was measured between the gauge marks. The thickness and

width around the fracture point were also measured.


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Results and calculations:

Table 1 showing measured and calculated readings

Mild Steele (mm) Aluminum (mm) Brass (mm)

Gauge length 90.24 91.16 91.2

Thickness 2.92 3.12 3.19

Width 7.89 7.33 8.16

Final Length 110.98 97.66 105.11

Final Thickness 2.24 2.92 2.59

Final width 6.24 6.499 7.15

Initial Cross Sectional Area

(mm2)

2.92*7.89

= 23.0388

3.13*7.33

= 22.9429

3.19*8.16

=26.0304

Final Cross Sectional Area (mm ) 2.24*6.24

=13.98

2.92*6.499

=18.98

2.59*7.15

=18.52


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Table 2 showing comparison of results

Mild Steel Aluminum Brass

Ultimate Tensile Stress (MPa) 342.22 229.56 352.16

Yield Strength (MPa) 253.18 205.87 290.27

% Elongation (%) 23.0 7.13 15.25

% Reduction in Area (%) 39.2 17.27 28.9

Modulus of Elasticity () 3238.13mm 1353mm 3873.24mm

Sample calculations:

Tensile Strength/Ultimate tensile strength (UTS):

Maximum load/Original Area = Maximum Load/ (width * thickness of specimen)

Mild Steel: 8000/ (2.92*7.89) = 347.22

Yield Strength:

Yield Load/Original area or Yield Load (0.2% offset)/Original area

Mild Steel: 5833/23.04 = 253.18

% Elongation:

Increase in length * 100

Initial Length

Mild Steel: (20.74/90.24)* 100


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% Reduction of Area = (Decrease in Area/ Initial Area) * 100:

Mild steel = (9.0588/23.0388)*100

Modulus of Elasticity ()

Stress/Strain = y/x

Mild steel: 5833.40-166.68/1.75-0 = 3.24N/mm

Discussion:

Table 2 shows measurements of mechanical properties obtained through tensile testing which

includes yield strength, tensile strength, ductility and modulus of elasticity. According to table 2,

brass showed the highest value for both ultimate tensile strength and yield strength. These values

were 352.16 and 253.18 MPa respectively, followed by mild steel and aluminum. From the

results generated, it was observed that brass was the strongest and aluminum was the weakest of

the three specimens being tested (i.e. order of strength was: brass > mild steel> aluminum).

According to Calister (2007), the stress level at which plastic deformation begins is the point of

yielding. For metals that experience this elastic-plastic transition, the point of yielding was

determined by the initial departure from the linearity of the stress-strain curve. This was the

proportional limit and it was established by the construction of a straight line parallel to the

elastic portion of the stress-strain curve; on the strain axis, 0.02% was offsetted by the

constructed line. The stress corresponding to the intersection of this line and the stress-strain

curve as it bends over to the plastic region was the yield strength and this was determined from


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both aluminum and brass (4708.35 N and 7555.60 N) strength test graphs respectively. The

magnitude of the yield strength for a metal was a measure of its resistance to plastic deformation.

Theoretically, yield strength ranges from 35MPa for aluminum to over 1400 MPa for high-

strength steels. This range generally corresponded to the results obtained from the experiment

which was 253.18, 205.87 and 290.27 MPa for mild steel, aluminum and brass respectively.

Tensile strength however, was the stress at maximum on the stress-strain curve. This

corresponded to the maximum stress that could be sustained by a structure in tension; all

deformation up to this point was confined at the neck. This phenomenon is called necking and

fracture occurs at that point. Tensile stress vary from approximately 50 MPa for aluminum to as

high as 3000 MPa for high-strength steels. This range also corresponded relatively to the results

obtained from the experiment conducted which was 342.22, 229.56 and 352.16 MPa for mild

steel aluminum and brass respectively. The deviation between experimental and theoretical

results could be attributed to the difference in sample size taken. Since the theoretical results

obtained was for a sample of 55 mm in length, whereas the gauge length used in this experiment

was 91.20 mm, 91.16 mm, and 90.24 mm for brass, aluminum and mild steel respectively.

Consequently, results obtained from this experiment would be higher than the theoretical results,

because dimensions from the samples given were different from the theoretical sample.

Another mechanical property tested was ductility, for the three samples given. Ductility, was a

measure of the degree of plastic deformation that has been sustained at fracture (Calister, 2007).

A material that showed negligible or no plastic deformation upon fracture is termed brittle. In

this experiment ductility was expressed quantitatively as percent elongation and percent

reduction in area. Table 2 showed mild steel was relatively most ductile, whereas brass was

relatively the least amongst specimens given. This was expected, because materials that were


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increasing in strength exhibit decreasing ductility. Furthermore, ductility could be attributed to

the presence of metallic bonds. In metallic boning, valence shell electrons are delocalized and

shared between many atoms; these delocalized electrons allow metal atoms to slide past one

another without being subjected to strong repulsive forces from one another. This is made

possible by the presence of slip systems. As a result, the more slip systems a metal has the more

ductile is the material. From table 2 it was inferred that mild steel has the most slip systems

relative to aluminum and brass which accounted for its high ductility. Also, the degree of

impurities present was another factor that affected ductility for the samples given. The more

concentration of impurities a metal has the more brittle and harder the metal becomes and

therefore it was inferred that mild steel had least amount of impurities in comparison to

aluminum and brass.

Lastly, mild steel showed the highest percentage in reduction in area relative to aluminum and

brass. This demonstrated why mild steel was the most ductile in comparison to aluminum and

brass, since materials with high ductility also showed high values of reduction in area and

percentage elongation.

Modulus of resilience or Youngs modulus was the strain energy per unit volume required to

stress a material from unloaded state up to the point of yielding. Resilient materials were

identified by having high yield strengths and low moduli of elasticity (i.e. aluminum and mild

steel). Thus, the higher the modulus of resilience value of the material the stiffer the material and

the less deflection would occur when load was applied (for example brass).


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Conclusion

Tensile test was conducted to deduce and specify the physical and mechanical properties of

materials given. Tensile test data was used to construct a stress-strain curve for each test

specimen. The results represent the action of the material in service (Horath 2001). Two main

points that arise from the stress-strain curve were the yield point and ultimate tensile stress of the

material. These values provided information on the load-bearing capabilities of each sample

material. Percentage elongation and reduction in area gave an indication of the deformation of

material under experimental conditions. The amount of deformation a material exhibits before

failure, was used to determine whether the specimen was brittle or ductile. Young modulus

showed the resilience or elastic stiffness of the material. It was determined by finding the

gradient within the elastic limit of the stress-strain curve.


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References:

Callister.William.D. Jr.,2007.Material Science and Engineering.7th

edition.John Wiley &

Sons:York PA.

Horath.Larry.,2001.Fundamentals of Materials Science for Technologist.2nd

Edition.Prince Hall

Inc: New Jersey.


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CHARPY TEST

Objective(s):

1. To determine whether the material samples given experienced ductile to brittle transition

at 280

C using the Charpy energy impact test.

Theory:

The Charpy test is a technique that is designed to measure the impact energy which is also

known as notch toughness. The principle of operation is that a pendulum is allowed to fall from a

fixed height and in doing so, develops kinetic energy. Part of this energy is absorbed in

fracturing the specimen so that the pendulum rises to a reduce height after it has broken the

specimen (as shown in figure 1 below). The energy absorbed by specimen is indicated on the

machine.


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Figure 1 showing impact testing apparatus along with specimen used (Calister, 2007)


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Experiment and Materials:

Three sample materials i.e. mild steel, aluminum and brass

Charpy Impact tester

Procedure:

The impact testing scale was tested for possible error and the error was calibrated and recorded

as 0.5 oat. The hammer was then lifted and set into the start position. A v-notched square

material sample was placed in the line of impact of hammer of the machine. The hammer was

released and struck the sample and the impact on the sample was recorded. This procedure was

repeated for brass, aluminum and mild steel samples.

Results:

Table 1 showing original results obtained

Sample Energy absorbed % cleavage

Aluminum 10.0 J 9 mm

Brass 8 J 9 mm

Mild steel 19 J 9 mm


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Table 2 showing results after 0.5 J deducted due to friction

Sample Energy absorbed

Aluminum 9.5

Brass 7.5

Steel 19

Table 3 showing class results (group 2)

Sample Energy absorbed %cleavage

Aluminum 10-.05 J = 9.5 J 9

Brass 7.5-.05 J = 7.0 J 9

Steel 39.5 -0.5 J = 39 J 9

Discussion:

The Charpy impact test determines the amount of energy sustained by a material upon impact.

The energy absorbed by the material was recorded as material toughness in Joules. Toughness

mainly depended on ductility and strength of a material. Amongst the samples being tested, it

was found that mild steel was relatively tougher than aluminum and brass. This was an indication

how well a material could withstand shock loading due to ductility and strength of that material.

One of the main objective of the Charpy test was to determine the ductiletobrittle transition

of the material at a given temperature. According to Calister (2007), the ductile to brittle


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transition was related to the temperature dependence of the measured impact energy absorption.

At higher temperatures Charpy v-notch energy would be relatively large in correlation with a

ductile mode of fracture. When temperatures decreased, the impact energy drops abruptly over a

narrow temperature range, below which the energy has a constant but small value; that is, the

mode of fracture is brittle. From results obtained (including class results), impact energy for mild

steel was relatively the highest at 280C, this showed a ductile mode of fracture. In addition,

appearance of the failure surface of a material was also an indication of the nature of the fracture.

For mild steel, the fracture appeared fibrous or dull and this was indicative of ductile fracture. In

contrast to brass and aluminum, the fractured surface appeared shinny in texture and this was

indicative of a brittle fracture.

Conclusion:

The Charpy test was an energy impact test which determined the behavior of materials under

sudden sharp blows. In this test the, a pendulum of known weight was suspended at a known

angle above the sample. The pendulum was then released and swings through an arc through the

specimen. The angle through which the pendulum breaks the specimen gave the energy absorbed

by the specimen tester. For this experiment it was found that mild steel absorbed more energy

upon impact than aluminum and brass and therefore its fracture was ductile in nature relative to

aluminum and brass.

References:



Sons:York PA.


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Hardness Test

Objective:

To determine the hardness of aluminum, brass and mild steel using Brinnel and Rockwell

testing methods.

To estimate the ultimate tensile stress for each sample.

To determine each materials ability withstand compressive loading and abrasion.

To determine if the samples meet the requirements of a nationally recognized standard

for hardness and/or ultimate tensile strength.

Theory:

Hardness is usually referred to as a materials property that indicates resistance to surface

penetration. In this experiment the Brinnel and Rockwell test methods were utilized to

determine the samples mechanical property, namely hardness. The Brinnel test is a static

hardness test that involves pressing a hardened steel ball into a test sample. Usually, a 10-cm

case-hardened steel or tungsten carbide ball under a 3000-kg load for hard metals, a 1500-kg

load for metal of intermediate hardness and a 500-kg or lower for soft materials. The Rockwell

test is similar to the Brinnel test in that the hardness number found is the degree of indentation of

the test piece caused by an indenter under a given static load. However, the Rockwell test differ

from the Brinnel test in principle in that it is conducted with a choice of three different loads and

three different indenters. It differs from the Brinnel test in that the indenters and the loads are

smaller, and the indentation made by the load is smaller and shallower. The Rockwell test is


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applicable to testing of materials beyond the limitations of the Brinnel test, and it is faster due to

random direct readings (Horath, 2001).

Experiment and Materials:

Samples of steel, aluminum alloys and brass.

Rockwell Hardness tester.

Brinnel Hardness tester.

File.

Samples were flat and free of burrs, dirt or scale which may crush during testing (giving low

readings) or may damage the indentor. A suitable load and indentor for each tester was selected.

The center of all test locations must be at least 2 diameters from the edge of the specimen or

the edge of other test locations.

Brinnel Hardness Tester

Each sample was raised on the table until it touches the 10 mm ball indentor. The hydraulic valve

was then closed and pressure was applied until weights raise approximately inch. Full test load

was then applied for ten to fifteen seconds. The hydraulic release valve was then opened and

sample was removed. The hand microscope was used to measure the indentation diameter twice,

each at 90 degrees to each other (0.05 mm accuracy). The results were then averaged and

converted to a Brinnel hardness number using the chart or by calculation.


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Rockwell Hardness Tester

The sample was raised on the table until large needle rotates three times and was nearly vertical.

The small needle was on the dot then the mirror load was applied. The outside ring was rotated

on the dial so that the large needle was set on zero. The loading lever was released clockwise

turn and the major load was then applied. When the large needle stops moving (approximately

six seconds), the loading lever was returned to its original position. The hardness number from

scale was then read (Black scale for Brale indentor and Red for steel ball indentor).

File

Each sample was stroked with a file and resulting marks were compared.

Results:

Table 1 showing result from Brinnel test

Properties Mild steel Aluminum Brass

d 4.5mm 2.4mm 3.4mm

D 10 mm 10 mm 10 mm

Pre-load kgf 10 10 10

Major load kgf 3000 500 1000


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Table 1a showing results from Brinnel test (for group 1)


d 4.55 mm 2.4 mm 3.5 mm

D 10 mm 10 mm 10 mm



Table 1b showing results from Brinnel test (for group 2)


d 3.5 mm 2.3 mm 4.4mm

D 10 mm 10 mm 10 mm



Table 2 showing results from Rockwell test

Materials 1stReading 2

nReading 3

rReading Avg Reading

Mild steel B68 B74 B71 B71

Aluminum B56 B58 B57 B57

Brass B67 B67 B68 B67


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Table 2.a showing results from Rockwell test (for group 2)


nReading 3



Aluminum B54 B58 B57 B56


Table 2.b showing results from Rockwell test (for group 1)


nReading 3



Aluminum B61 B62 B62.5 B61.8


Table 3 showing calculated results from Brinnel test

Sample material HB h

Aluminum 109.8 kgmm-

0.15 mm

Brass 106 kgmm-

0.300 mm

Mild steel 179 kgmm-

0.53 mm

Key:

HB = Brinnel Hardness, h = depth of indentation in milliliters


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Sample calculations

HB = 2F/ [.D (D(D2 - d

2)1/2

)] , h = F/(.D * HD)

Where

F is the major load, = 3.14, D = diameter of ball in milliliters, d = mean diameter of indentation

in milliliters and h = depth of indentation in milliliters.

Aluminum:

HB = 2(500)/ [(3.14*10(10-(102- 2.4

2)

1/2)] = 109.8 km/mm

2

h = 500/ (3.14*10*5.23) = 0.15 mm

Discussion:

The mechanical property hardness, usually measures the material ability to resist localized

plastic deformation (i.e. scratch or dent). Early hardness tests were based on natural minerals

with a scale constructed solely on the ability of one mineral to scratch another that was softer

(Calister, 2007). Two methods used for measuring hardness of a material were the Brinnel and

Rockwell test. The Brinnel hardness numbers usually falls within a range of 90 to 630, with

higher numbers indicating greater hardness. Brinnel hardness number was the pressure per unit

area, in kilograms per square milliliter of the indentation that remained after the load was

removed. The deeper the penetration, the larger the diameter of the indentation and the lower the

hardness number. The Results from table 3 indicated, that mild steel had the highest Brinnel

number which was 179.0 kg/mm2which showed greater hardness relative to aluminum and brass

which was 109.8 and 106 kg/mm2respectively. This general trend also corresponded to the class

results for the other groups. However, there were limitations associated with the Brinnel test.


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Firstly, since the Brinnel test was a destructive test, indentation to the material was permanent

and this rendered the sample unfit for further use. Thus, it was not well adapted for very hard

materials as well as very thin pieces, where indentation may be much greater than the thickness

of the material. This phenomenon could attributed to the deviation in results generated for mild

steel and brass between all three groups as shown in tables 1, 1a and 1b respectively.

The Rockwell test was another test employed in measuring hardness of sample materials given

for the experiment. With this system, a hardness number was determined by the difference in

depth penetration resulting from the application of an initial minor load followed by a major

load. The use of the minor load was to enhance test accuracy. The minor and major load was

10kg, and 100kg respectively. This method uses many scales and each scale was represented by

a letter of the alphabet; this corresponded with the indenter and load used. The dial on the

machine has two sets of figures one red and one black which differ by 30 hardness numbers. This

dial was designated to standardize the B and C scales. The red scale was used for readings

obtained through the use of ball indenters.

For the purpose of this experiment, the B-scale was used, because the B-scale usually test

materials of medium hardness between the ranges of 0 to 100. In a similar vein to the Brinnel

test, the Rockwell test results from tables 2 and 2.a, showed that mild steel was found to have

greater hardness than aluminum and brass.

Limitations to the Rockwell test were as follows: for each scale, hardness may range up to 130;

however, as hardness values rose above 100 or drop below 20 on any scale, they become

inaccurate and because the scales have some overlap, in such situation it may be necessary to

choose a another suitable scale (Calister, 2007). Secondly, inaccuracies could result, if sample

material was too thin, and indentation was too close to upon one another. Thus indentation


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diameter measurements would have been inaccurate and as a result group 1 results for mild steel

and brass differs from groups two and three.

Conclusion:

In this experiment two methods were used to measure hardness of sample materials, these were

the Brinnel and Rockwell test. From both test it was determined that mild steel was much harder

in comparison to brass and aluminum. Though both test methods yielded relatively the same

result, the scale by which they use were different in measuring indentation resistance.

Reference:



Sons:York PA.

Horath.Larry.,2001.Fundamentals of Materials Science for Technologist.2nd

Edition.Prince Hall

Inc: New Jersey.

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Materials Characterization Lab Report