Tensile Strength and Deformation Characteristics of Engineering Materials

8/10/2019 Tensile Strength and Deformation Characteristics of Engineering Materials

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Tensile Strength and Deformation

Characteristics of Engineering

Materials

Chris Powell

Word Count: 2,741

Figure 1: Vice configuration used in the tensile testing of the plastic samples (Powell,2012)


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Contents

ABSTRACT ........................................................................................................................................................ 1

INTRODUCTION ............................................................................................................................................... 1

METHOD .......................................................................................................................................................... 3

SAFETY................................................................................................................................................................. 4

EXPERIMENTAL ERRORS........................................................................................................................................... 4

RESULTS ........................................................................................................................................................... 5

DISCUSSION ..................................................................................................................................................... 5

METALS................................................................................................................................................................ 6

PLASTICS............................................................................................................................................................... 7

CONCLUSIONS ................................................................................................................................................. 7

REFERENCES ..................................................................................................................................................... 1

List of Figures

FIGURE 1:VICE CONFIGURATION USED IN THE TENSILE TESTING OF THE PLASTIC SAMPLES (POWELL,2012) 1

FIGURE 2:TABLE SHOWING AN EXAMPLE OF A TENSILE PROFILE SHOWING SOME OF ITS KEY FEATURES (INSTRON,N.D.) 2

FIGURE 3:NECKING OF POLYETHYLENE DURING TESTING (POWELL,2012) 2

FIGURE 4:PHOTOGRAPH OF THE MONSANTO TESOMETER(POWELL,2012) 3

FIGURE 5:PLASTIC SPECIMENS (POWELL,2012) 3

FIGURE 6:METAL SPECIMENS (POWELL,2012) 3

FIGURE 7:EMERGENCY STOP BUTTON ALONGSIDE THE SWITCH USED TO CONTROL THE CROSSHEAD (POWELL,2012) 3

FIGURE 8:PHOTOGRAPH SHOWING THE TORN ENDS AFTER FAILURE OF POLYPROPYLENE (POWELL,2012) 4

FIGURE 9:PHOTOGRAPH SHOWING POLYPROPYLENE AFTER FAILURE (POWELL,2012) 4

FIGURE 10:PHOTOGRAPH SHOWING POLYSTYRENE AFTER FAILURE (POWELL,2012) 5
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Abstract

The behaviour of certain materials vary greatly under tensile loads and it is important to

understand and predict these characteristics, particularly as the materials being tested are

regularly applied in engineering circumstances. The experiment being undertaken involved

placing the sample between two grips (Figure 1.), one of which moved vertically upwards

thereby applying a tensile load to the samples which eventually caused them to fail. Whilst

this was happening a computer logged the extension and load values which produced a graph

showing how the sample responded to the tensile forces.

Introduction

The experiments undertaken on the three polymer and three metal samples are commonly

known as tensile tests, and are widely renowned for being probably the most fundamentaltype of mechanical test that can be carriedout on a material (Instron,n.d.). Once the sample

is subjected to a tensile force it begins to deform producing an extension, this extension is

then logged against the load required to create it which produces a tensile profile

(Instron,n.d.) the area under which shows how much energy the material absorbs before

failing (Bingham,n.d.). These tensile profiles are important to engineers as it is important to

be able to produce structures which will not permanently deform under their standard loads

and also to be able to construct machinery which is capable of forming and manipulating

different materials into desired shapes (Ashby,Jones,2012).

All materials exhibit some form of elastic properties but this may happen for such a short

extension or load range it is difficult to measure or unnoticeable. On the load/extension

graphs this elastic region is identified as the linear portion that originates from the

intersection of the axis (where both load and extension are equal to zero). The area under this

straight section of the graph is the stored elastic energy, which means that once the load is

removed this energy is released and the material springs back to its original shape

(Ashby,Jones,2012). The straight line relationship between the extension produced and the

load applied to it means they are proportional and therefore obey Hookes Law. As Hookes

Law is being obeyed the ratio of stress to strain is constant (Instron,n.d.) and therefore the

slope of the line is equal to a constant know as the Modulus of Elasticity or Youngs

Modulus. This is derived from the relationship where stress is equal to

and the strain is the ratio of the extension over the original length. For

both stress and strain to be equal to one another a constant must be introduced which then

gives , where E is the YoungsModulus and is a gauge of the materials stiffness

(Instron,n.d.). Stiffness is defined as the ability of a substance to resist deformation and as

expected this value varies greatly between different materials, for example a sample such as a

plastic that produces a larger extension for lighter load will give a shallower line on the

tensile profile and as a result give a lower Youngs Modulus.


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Figure 2: Table showing an example of a tensile profile showing

some of its key features (Instron, n.d.)

Figure 3: Necking of Polyethylene during testing

(Powell, 2012)

Once the elastic region has been exceeded the sample behaves plastic or nonelastic which

means that further increases in extension is causing irreversible deformation. As the sample is

further being deformed the cross-sectional area decreases as mass and volume must be

conserved, this constant decrease in cross-sectional area causes it to become unstable and at a

point along its length it begins to neck. Necking begins to occur at the point of ultimatetensile strength (Figure 3.) and continues to develop afterwards which weakens that portion

of sample until it can no longer withstand the load and it fails. A visual example of necking

can be seen in Figure 2. Necking is the localised plastic flowing of a specimen caused by

increased shear stresses at a certain location along its length (Ashby,Jones,2012).

Whilst the tensile load is being applied to the samples there are also shear stresses which are

being transmitted through the material at an angle from the direction of the tensile force. This

is particularly the case with the metals which have a crystal structure which is built up of a

number of lattices. It is these lattices which slip past one another on the nearest plane to 45

(Ashby,Jones,2012) as this is the angle at which the shear stresses are at their greatest, it isthese dislocations that are the cause of the increase in the materials length.

As well as the Youngs Modulus there are a number of other key characteristicswhich can be

obtained from tensile testing. The ones important to the experiment undertaken is the Yield

Strength and the Ultimate Tensile Strength. Yield strength is the stress (Load / Cross-

sectional area) at the point of transition from elastic to plastic flow. It is important to

recognise this value as in many applications it shouldnt be exceededfor purposes previously

stated. The ultimate tensile strength of a specimen is the maximum load it can take before

failure.
http://self.close%28%29/http://self.close%28%29/


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Figure 7: Emergency stop button alongside the

switch used to control the crosshead (Powell,2012) Figure 6: Metal specimens (Powell,2012)

Method

1. For the plastic sections (Figure 5.) measure both the thickness and width along the

gauge section and then mark either side of a distance of 80mm along the samples

length. Then for the metal specimens (Figure 6.) adjust the slide on the Monsanto

Tensometer (Figure 4.) to either A or B and C depending upon the cross-sectional

area.

2. If using the plastic samples place the thick flat section inside one of the grips and

screw until closed, then repeat for the opposite side. For the metal specimens close the

thin section inside the two halves of the metal cup and fix to the top crosshead by

placing the bar through the aligning holes. Repeat for the bottom portion but when

fixing the cup to the bottom crosshead slowly bring the top one down using the

buttons (Figure 7.) whilst pushing the bar against the holes. At the instant the bar is

fully secure stop lowering the crosshead.3. Once the sample has been secured in the crosshead both the load and extension need

to be reset on the monitor to the side of the machinery.

4. When the values have been reset the test can be started by clicking begin on the

monitor, the top crosshead will then move vertically upwards and whilst this is

happening the computer records the loads, extension and time elapsed. This continues

until the specimen fails.

5. Finally for the metal samples remove them from the cups and rebuild them in the

Monsanto Tensometer to obtain the elongation values.

Figure 5: Plastic specimens (Powell,2012)Figure 4: Photograph of the Monsanto

Tesometer(Powell,2012)


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Safety

As the crosshead is continuously being moved up and down there is a risk fingers or hands

may get trapped between that and the fixed one. To reduce the risk of this happening cut out

stops can be put into use to prevent the crosshead being moved lower than a specified height.

Some of the samples are prone to shattering upon tensile failure (particularly Polystyrene)which means some of smaller parts could be expelled and cause harm to the experimenter,

therefore goggles must be worn at all times and in some circumstances it may be necessary to

place a polycarbonate screenin front of the workstation before testing (Magowan,n.d.).

Experimental Errors

Whilst undertaking the tensile test a number of experimental errors were encountered which

ranged from human error, to errors directly involving the behaviour of the materials under

testing. One issue found was the measuring of the polymer elongations had to be done

manually using rulers which meant values were only accurate to the nearest mm, then to

further decrease the validity of the results the line used to mark the 80mm section had to bedone by hand which meant accurate measuring of the extension was difficult. The polymers

were also made in-house which meant their dimensions would have varied slightly between

specimens making it difficult to compare results. Whilst testing Polystyrene it shattered at the

point of failure into multiple pieces (Figure 10.) which meant putting it back together

effectively was difficult and as a result the extension reading was possibly incorrect. Another

issue with reassembly was noticed with polypropylene which tore on failure (Figures 8.&9.).

The point where human where human error had a large effect on the results came when

testing the Mild Steel and Copper specimens. Whilst the Mild Steel was being fixed to the

crossheads the top one must have been moved upwards to the point where a tensile force inexcess 9400N was applied. This meant the yield point load had been exceed and a value had

to be estimated bearing in mind steel deforms plastically for a large portion of the graph.

When performing the test on Polyethylene it extended to the point where the crossheads

could no longer continue to separate and the specimen still hadnt failed. This meant it had to

be assumed the sample failed at the maximum possible extension to machinery was able to

produce. Such a large extension also meant measuring it was difficult and had to be done

with a thirty centimetre rule.

Figure 9: Photograph showing polypropylene after failure Figure 8: Photograph showing the torn ends after failure


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Results

Material

Gauge Length Cross

Sectional

Area(mm

2)

Yield

Point

Load(N)

Maximum

Load (N)

Final

Length(mm)

Original

Length(mm)

Width(mm) Thickness(mm)

Copper 20.00 6801.726 7350.302

Brass 20.00 5094.901 9111.645

Mild Steel 10.00 7000.000 10131.100

Polystyrene 80.00 10.00 3.95 39.50 1588.211 1653.191 82

Polypropylene 80.00 9.93 3.97 39.42 736.651 1373.585 150

Polyethylene 80.00 9.74 3.90 37.99 676.214 945.707 505

Material Yield Strength(Nmm-2

)

Tensile

Strength(Nmm

-2)

Elongation (%) Hardness(HV20)

Copper 340.086 367.515 19.0 111.9

Brass 254.745 455.582 26.0 136.1

Mild Steel 700.000 1013.11 19.0 324.6

Polystyrene 40.208 41.853 2.5

Polypropylene 18.687 34.845 87.5

Polyethylene 17.800 24.894 531.3

Ranking (Highest to Lowest)Strength Ductility

1. Mild Steel Polyethylene

2. Alpha Brass Polypropylene

3. Copper Brass

4. PolystyreneCopper and Mild Steel

5. Polypropylene

6. Polyethylene Polystyrene

Discussion

It can be seen from the shapes of the tensile profiles that all of the materials tested uponbehave differently under tension, particularly in the elastic region and all exhibited a lot of

Figure 10: Photograph showing polystyrene after

failure (Powell,2012)


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plastic deformation before failing, excluding polystyrene.

From the results it can be easily seen that the metals are by far much stronger than any of the

plastics with a difference of at least three hundred Newtons per square millimetre. In terms of

ductility the majority of the plastics show much greater ductility than the metals with the

exception of Polystyrene which shows such little extension it is noticeably less ductile thanany of the specimens.

With the values obtained from the measurements recorded using the method a number of

calculations needed to be made to make their values comparable as the samples had varying

dimensions. The calculations used can be seen below.

(Magowan,n.d.)

Many of the values had to be taken from the graph which meant their values are very

subjective and would vary from person to person particularly the value for the Yield Point

Load. A potential means of overcoming this would be the utilisation of software which was

capable of distinguishing the transition point between linear and nonlinear portions of the

graph.

Metals

After performing the tensile tests it was noticed that as the hardness value of the material

increases so does its tensile strength, this is because hardness testing is occasionally used to

give a quantitative value for a specimens strength (VanAken,2001). The reason this is the

case is because whilst the indenter is being pressed into the material, shear stresses move

through the crystals so they slip past one another allowing the indenter to press into the

sample. These shear stresses are what cause the sample to elongate during tensile loads

therefore the greater they are the easier the indenter can be inserted and the more slip which

results from tensile testing, which finally results in a soft material which easily elongates.Alpha Brass is an alloy of Copper and Zinc in the proportions 75% to 25% respectively.

Copper is widely known as being a very ductile metal which can easily be drawn into wires,

is a very good conductor of electricity but is not very strong in comparison to a lot of metals.

Zinc is a lustrous, brittle metal which is used largely in the galvanising of other metals to

prevent corrosion (Ophardt,2003). Once the two metals are alloyed they produce Brass which

is well known for being a strong, corrosion resistant, lustrous, ductile metal which can easily

be cold worked. These properties can modified by adding different proportions of Zinc and

Copper, for example the addition of greater amounts of Zinc means the Brass may need to hot

worked as it has become stronger and less ductile (Austral Wright,2008). It was difficult torate Copper or Brass in terms of strength as one has a greater yield strength and the other a


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greater tensile strength. To overcome this an average was take of both values and Brass was

found to be marginally stronger which is to be expected with the addition of Zinc.

It should have been seen on the graph of Mild Steel that there is a large section of plastic

deformation before the point of ultimate tensile strength and necking begins. This would have

meant that Steel would be very suited for large amounts of moulding, rolling etc beforebecoming unstable (Ashby,Jones,2012). Another metal that exhibits similar plastic properties

is that of Brass which can be seen to deform plastically around 5mm before reaching the

ultimate tensile strength and therefore would also be very suited for manipulation.

Plastics

Both Polyethylene and Polypropylene show very little relative elastic deformation and show a

very steep linear section as they only demonstrate elastic properties at very low strains. Then

once this fine elastic region is exceeded there is a small portion of the line which displays

plastic deformation before the point of ultimate tensile strength. This means that plastics are

quick to neck once their elastic strain has been exceeded. Polyethylene differs from the otherplastics being tested as it expresses a form of necking which does not lead to fracture as it

grows but instead work hardens so it remains stable for longer and can produce greater

extensions (Ashby,Jones,2012).

Polystyrene behaves very differently from the other two plastics being tested as it exhibits a

relatively large amount of elastic deformation which results in a shallow straight line section.

Once the yield point has been exceeded there is almost no plastic deformation before the

ultimate tensile strength point, then necking begins to occur but it is so unstable it last only

for a small strain before failure. This behaviour is very typical of brittle materials. Despite

this Polystyrene is capable of storing a large amount of elastic energy as the area under itslinear section is much larger than that of either Polyethylene or Polypropylene

(Bingham,n.d.).

Conclusions

- Metals are far stronger than plastics with greater values of both Yield strength and

Tensile strength.

- Plastics are much more ductile than the metals with the exception of Polystyrene.

- Hardness is a form of quantifying strength and therefore as the hardness of the

materials increases so does their strength.- Brass and Steel deform over large strains in the plastic region before necking which

means they can be manipulated a lot before developing weaknesses.

- Copper is quick to neck after passing the yield point.

-

Internal slippages caused by shear stresses are the cause of elongation.

- The alloying of Copper and Zinc produces Brass which in different proportions

changes its properties.

- The addition of Zinc to Copper makes it much stronger.

- Brass is well known for being a strong, corrosion resistant, lustrous, ductile metal

which can easily be cold worked.


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- Polyethylene and Polypropylene show very little relative elastic deformation (over

small strains) but extend greatly during necking.

- Polystyrene is a very brittle plastic and soon after the UTS point is passed it fractures.


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0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250 300 350 400 450 500

Load

(N)

Extension (mm)

Tensile Profile Polyethylene

0

200

400

600

800

1000

1200

1400

1600

0 10 20 30 40 50 60 70 80 90

Load

(N)

Extension mm

Tensile Profile for Polypropylene

0

200

400

600

800

1000

1200

1400

1600

1800

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Load

(N)

Extension (mm)

Tensile Profile for Polystyrene


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0

2000

4000

6000

8000

10000

12000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Load

(N)

Extension (mm)

Tensile Profile for Mild Steel

0

1000

2000

3000

4000

5000

6000

7000

8000

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5

Load

(N)

Extension (mm)

Tensile Profile for Copper

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 1 2 3 4 5 6 7 8

Load

(N)

Extension (mm)

Tensile Profile for Brass


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References

Ashby, M.F., Jones, D.R.H.. 2012.Engineering Materials. Fourth Edition. An Introduction

to Properties, Applications, and Design.[Book]. Elsevier-Oxford.

Austral Wright. 2008.Metal Alloys - Properties and Applications of Brass and Brass

Alloys.[Website]. Austral Wright Metals. Date Accessed: 19/03/2012. Available from:

http://www.azom.com/article.aspx?ArticleID=4387

Bingham, P.Not Dated.Mechanical Properties of Metals.[PowerPoint]. Sheffield Hallam

University. Date Accessed: 23/03/2012. Available from: shuspace.shu.ac.uk

Instron.Not Dated. Tensile Testing.[Website]. Instron- Materials Testing Solutions. Date

Accessed: 10/03/2012. Available from:

http://www.instron.us/wa/applications/test_types/tension/default.aspx

Magowan, S. 2012. Summary of Assessment of Risks Associated With Laboratory Practical

Work.[Hand-out]. Sheffield Hallam Faculty of ACES.

Ophardt, C.E.. 2003.Zinc, Zn.[Website]. Virtual Chembook- Elmhurst College. Accessed:

17/03/2012. Available from:http://www.elmhurst.edu/~chm/vchembook/102zinc.html

VanAken, D.2001.ENGINEERING CONCEPTS: Relationship Between Hardness and

Strength.[Website]. Industrial Heating- The International Journal of Thermal Technology.

Accessed: 16/30/2012. Available from:

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Tensile Strength and Deformation Characteristics of Engineering Materials

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