UNIVERSITY OF

LIVERPOOL

1 Aim & Objectives

The aim of this lab is to develop an appreciation of the principles of mechanical testing, in particular

uniaxial tensile testing of metals and polymers, and how it is used to measure mechanical properties

of materials.

Upon successful completion of this lab, you will be able to:

Undertake tensile testing to measure mechanical properties associated with elastic and plastic

deformation;

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●

TT Lab Script // MATS105

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●

Describe and explain qualitative aspects of mechanical properties and mechanical testing

Record and present experimental data

Plot graphs, and extract values.

Improve technical reporting skills.

The technical objective of this lab is to determine the tensile deformation behaviour of two metal

samples and two polymer samples (steel, aluminium, polypropylene and Perspex acrylic) using an

Instron tensile tester, and compare the mechanical properties measured with "text-book" values./n2 Introduction

The uniaxial Tensile Test is a common, standard mechanical test, and is widely used in engineering.

The objective of the tensile test is to measure specific mechanical properties of structural materials

so that their strength, ductility, Young's modulus etc. may be compared. The mechanical properties

measured by this test are often required by engineering designers undertaking various stress

calculations, therefore the reliability and standardization of the measurement process is essential. In

this experiment you will test four common engineering materials using an industry standard "Instron"

testing machine:

.

.

UNIVERSITY OF

LIVERPOOL

●

a steel,

an aluminium alloy,

polypropylene, and

Perspex acrylic. Note acrylic is a commonly used name for polymethyl pethacrylate (PMMA)

polymer. Perspex® acrylic is a tradename of Lucite International and is a premium acrylic.

There are British Standard (nowadays also European Standard, or "EN") procedures which specify the

precise procedure that must be adopted for tensile testing. One example is EN 10002-1 for metallic

materials which says it must be undertaken at an ambient temperature between 10 °C and 35 °C,

normally at a controlled temperature of 23 °C ±5 °C, and defines how the mechanical properties

should be determined. This standard defines the principle of the test as "straining a test piece in tension,

generally to fracture, for the purpose of determining one or more of the mechanical properties".

Definitions of some material mechanical properties have been (or will be) provided to you in your

lecture notes. You will also have the tensile test explained in detail at some point during lectures. But

resources are also provided in case you are undertaking the lab before this time.

In this TT experiment you will gradually strain (elongate) each sample incrementally, recording the

sample's elongation and applied load (force) required. You will then/nSome of these mechanical property measurements are shown in the schematic strass vs strain curve

in Figure 1.

Stress a

Yield stress

0.1% offset

strain

Modulus

of

Elasticity

0.1% proof

stress

E

Strain

Ultimate

Tensile

Strength

Ao

Ac

Failing

i Stress

Failing

Strain

Figure 1: Schematic of the stress vs strain curve showing various mechanical properties that can be

extracted using data obtained from tensile testing.

2.1 Apparatus

You will be using an Instron testing machine. The load (force applied) is accurately measured using a

load cell attached to one of the sample grips as you manually extend the samples, and displayed on

the PC in Newtons. When undertaking tensile testing in industry, the strain would normally be

measured very accurately using a strain gauge glued to the test sample surface

(https://en.wikipedia.org/wiki/Strain gauge), or a "clip-on" or "digital sensor arm" extensometer

attached to the sample being tested https://en.wikipedia.org/wiki/Extensometer. However in this lab

you will not have strain gauges or extensometers available, you so will be making a much more

inaccurate measurement of the strain by simply measuring the elongation of the whole test sample

(the increase in separation of the grips holding each end of the test sample) and dividing that by the

original gauge length of the test sample. This tends to result in measurements of strain greater than

the correct value due to various reasons (sample slippage in the grips, tensioning of the joints in the

system etc), particularly at the start of the test. A consequence can be that the values of Young's

modulus that you obtain from your stress vs. strain curves may be up to an order of magnitude too

low, whist your values of yield stress and ultimate tensile stress can be very accurate, and you will

investigate this as part of the practical./nTT Lab Script // MATS105

UNIVERSITY OF

LIVERPOOL

3

Health & Safety

You must always have a screen between you and the test samples during testing. It is possible that

the samples could fail suddenly, which can be associated with a loud noise and some fragments may

fly off. The apparatus also contains moving parts. You should carefully follow the instructions given by

the laboratory teaching assistants for loading the test samples, and follow the standard testing

procedure which will be explained to you. Students are reminded that they are required by law to

comply with the School's rules of lab safety.

4 Experimental Procedure

Each group will be given one specimen of each of the four samples: mild steel, an aluminium alloy, a

polypropylene, and Perspex acrylic. You should record by hand the readings of load and elongation in

the tables in your own lab script as the test proceeds. You will eventually have to photograph these

tables and upload them into your technical note, so ensure accurate recording and that your hand-

writing is clear.

For each specimen in turn:

Measure the starting width and thickness of the original gauge length region of the sample

be tested, using the micrometre or callipers provided. Determine the original cross-sectional

area (which you will need to convert applied load to engineering stress). Record these values

in Table 1a & 2a.

a.

b. Measure (approximately) the original gauge length of the test sample. This is the length of the

narrow central parallel region of the test sample over which most of the displacement/

elongation takes place. You will need this to convert measured elongation to engineering

strain. Record the value in Table 1a & 2a.

C. Remove any surface cover sheet from the Perspex acrylic samples.

d. Carefully mount the sample vertically and securely in the grips of the tensile testing machine,

following the instructions provided. If you don't tighten the grips sufficiently, your sample may

slip in the grips and you will have to repeat the test. Zero the load and displacement

measurements, and set the manual cross-head speed (rate of elongation) to its minimum

value. Make sure the screen is in place. Follow the instructions and guidance of the teaching

assistants at all times, and ask them if you are unsure about anything.

e. Using the manual jog control, elongate the sample in initial increments (steps) of about

0.1mm, increasing the increments gradually as per the elongation values given in Table 1b &

2b. Record in Table 1b & 2b the actual elongation (in mm) and the load (in N) on the specimen

after each elongation step, using the values displayed in the Bluehill software on the Instron's

PC screen. Record your data carefully and neatly in this lab script. (You will notice that the

ad record the valuo faicho/n4.

3.

Ultimate Tensile Strength the maximum engineering stress recorded.

1.

2.

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Strain at failure a measure of the material's ductility. Note that strain is sometimes

expressed as a % rather than a fraction (a factor of 100 different!).

5 Technical Note Instructions

You will complete a Technical Note using the template provided in Canvas, and submit this in

Canvas for marking. The assessment of this practical is mainly an assessment of the quality of the

presentation of your results and their interpretation. The technical note contains the following

sections, each worth the percentages indicated:

TT Lab Script // MATS105

Abstract

Write a paragraph about what you did, why you did it, how you did it, the

results and what you concluded (roughly 250 words)

Results

Images of original data recorded by hand during the practical in the lab script.

Images of the excel spreadsheet, showing data from both metals and from

both polymers.

Image of two excel stress-strain graphs, each containing data from two

materials i.e. showing two stress-strain curves.

Completed tables with values obtained from the stress-strain graphs

Discussion

The quality of technical writing is important - good English without any

grammatical errors

Conclusions

Submission instructions:

10%

60%

20%

10%

Download the Technical Note Template from the Submission folder in the TT: Tensile Test section of

the Year 1 Labs Canvas site. Complete the Technical Note Template by following the instructions and

answering the questions. If you are having difficulty please contact the teaching assistants by email

for help./nRepeat for each of the four samples.

You have now completed the practical part of the experiment.

// MATS100

For the mild steel sample, transcribe your recorded elongation and load data into two adjacent

columns in an excel spreadsheet, and generate stress (in MPa) and strain values in two columns

immediately to the right. Hence you should have four columns of data for each sample. The top row

(row 1) should have merged cells across all four columns and state the name of the sample material

(e.g. "Mild steel"). The second row (row 2) will contain the column header i.e. "Elongation (mm)",

"Load (N)", "Stress (MPa)" and "Strain" respectively. The rows below these will be filled with

transcribed and calculated data. Add a border around all the cells in the four columns containing data

for the sample.

For the aluminium alloy sample data, leave a blank column to the right of the steel sample's data and

then repeat as for steel, entering the aluminium data into the next four columns to the right starting

again with the name of the material ("Aluminium alloy") in row 1. It is important to use this layout so

that you can produce a single image of all the data for both metals to upload into your technical note.

Open a new sheet in excel, and repeat the data entry for the polypropylene and Perspex acrylic sample

data.

In the sheet containing metal data, create a single stress v strain scatter graph (called scatter chart in

excel) showing both mild steel and aluminium alloy curves on a single graph. To create a single stress v

strain scatter graph showing both mild steel and aluminium alloy data on a single graph, you need to plot two sets of x and

y data on common axes. There are various ways to do this in excel, which will require you to re-arrange the data in your excel

spreadsheet slightly. If you search the internet using a search term like "excel plot two sets of x and y data on common axes"

you will find out how to do it. Repeat for the polypropylene and Perspex acrylic data in the other sheet.

Make sure the axes are correctly labelled and with easy-to-read axis values. Use a different data

marker shape for each material (one square and one circle is sensible) and make sure that a legend

clearly identifies the material corresponding to each set of data. Then manually add a straight line to

give a reasonable fit to the initial linear region of each stress-strain curve where elastic-only

deformation is taking place, and estimate Young's modulus from the gradient of this line (normally

expressed in GPa). Do not use excel's trend-lines for this just use your best judgement to fit a straight line (you may

need to ignore the first two or three data values). Ensure all the data points and labels are clearly visible as you

will have to paste images of these graphs into your technical note.

For each material, obtain the following mechanical properties from the stress-strain graphs:

Young's Modulus (Modulus of Elasticity)-the gradient of the linear initial region of the stress

strain graph.

Yield stress (Limit of Proportionality) - the stress at which there is a change from elastic

(linear) to plastic (non-linear) deformation. For mild steel you may also be able to see an

Upper and Lower Yield Stress, but they are not always clearly visible - if you can see it, use

the lower yield stress. Note that if the yield stress is not clear because of a very gradual

transition from linear to non-linear behaviour, it is customary to use the 0.1% Proof Stress to

define the transition to plastic deformation (see previous figure). However this is not

necessary in this practical.

Ultimate Tensile Strength - the maximum engineering stress recorded./nUNIVERSITY OF

LIVERPOOL

Table 2a: Initial Test Sample Measurements

Thickness of gauge length (mm)

Width of gauge length (mm)

Original Cross-section Area (mm²)

Length of gauge length (mm)

Table 2b: Elongation and load data during tensile test

Indicative

elongation

(mm)

0

0.1

0.2

0.3

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Results tables for polypropylene and Perspex acrylic

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.5

4

4.5

5

POLYPROPYLENE

Actual

elongation

(mm)

0

Measured load

(N)

0

Polypropylene

PERSPEX ACRYLIC

Actual elongation

(mm)

0

O

013

016

TT Lab Script // MATS105

0,7

0,7

1/1

1,1

\/\

\r\

115

115

117

211

2,1

211

211

Perspex acrylic

2,5

1212

80

Measured

load (N) die

0

10

11,96

15,3

15,225

3814

164

267

394

550

636

753

869

976

1088

1190

1275/nTadic 19.1

Thickness of gauge length (mm)

Width of gauge length (mm)

Original Cross-section Area (mm²)

Length of gauge length (mm)

Indicative

elongation

(mm)

0

0.1

0.2

0.3

0.4

0.6

0.8

1.0

1.25

1.5

1.75

2.0

2.25

2.5

2.75

3.0

3.5

4

4.5

5

10

Table 1b: Elongation and load data during tensile test

15

20

imple ivice

25

30

35

40

45

50

ements

MILD STEEL

Actual

elongation

(mm)

0

Mild Steel

Measured load

(N)

0

0

0

O

O

O

Actual elongation

(mm)

O

O

0

013

0,3

03

113

115

118

2,2

21 2

215

2,6

26

4,0

7,9

Aluminium

1,4

ALUMINIUM

99.5

Measured

load (N)

0

11,6

25,5

158,0

314

722

1146

1532

1911

2144

2224

2268

2310

2337

2360

2364

2385

2365

2300

2355

2373