Tensile Testing of Metal Specimens

plantcalicobeansUrban and Civil

Nov 29, 2013 (3 years and 6 months ago)

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Tensile Testing
of
Metal Specimens


What you should learn from this practical



An appreciation of the change in mechanical properties resulting from the addition of
alloying elements (in this case carbon).



How the % carbon in steel changes the tensile stren
gth, hardness and microstructure of
carbon steels.


Practical Skills



Team working.



Familiarity with data
-
logging
.



Setting up a tensometer to measure tensile strength of a material
.



Use of a hardness indenter to measure hardness
.



Use of an optical microscop
e to see the microstructure of different steels
.



Use of a spreadsheet to store
,

manipulate
and display
data
.


Background

Stone and
concrete

have significant strength in compression, but are brittle
and

of little value

in
tension. Metals have excellent str
ength
both
in
compression and tension:

c
onsequently metals are
used when tensile forces will be in effect.

Materials Engineers / Scientists study materials to understand why one material has a unique set of
properties and another even a similar one has dif
ferent properties.

This experiment investigates the influence of carbon on the mechanical properties of steel.

Furthermore, physical properties are characteristic of materials and their processing history. Data
such as
that obtained by tensile testing

can

be used to characterise an unknown sample.

Steel exhibits a mixture of elastic and plastic deformation before it breaks
.


T
he presence of carbon
modifies the crystal structure and hence the mechanical properties of the steel.

You will test the following s
teels as a group and share results…



0.1% carbon


as would be used in sheet steel to make car body panels.

o

The material needs to be very tough


that is, able to absorb lots of energy when it is
shaped without any cracking.



0.4% carbon


as might be used i
n steel girders for building bridges or skyscrapers.

o

This material needs greater strength and rigidity.

o

Must still be tough


if a
building

or part of a building

was to fail, a ductile failure
would give recognisable warnings before collapsing, but a britt
le structure may fail
suddenly, killing many people.



0.54% carbon


as might be used to make railway lines.

o

Needs to be very hard and wear resistant

o

Needs to have some toughness to resist crack formation and propagation.



0.8% carbon


as might be used to

make hand tools, such a
s

saw blades, hammers etc.

Experimental
overview

You will be shown how to set up a tensile specimen in a tensometer and test it to destruction.

Numeric data will be downloaded
from a tensometer
onto a computer and from here it wil
l be
imported into an Excel spreadsheet from wh
ich S
tress

v

S
train plots will be prepared.

The extent of plastic deformation will be recorded by measuring the amount of necking (thinning of
the steel in the region where fracture finally occurs) and permane
nt elongation of the specimens.

Hardness tests will be conducted on identical
steel
specimens and that data, too, will be plotted.

The microstructures of the different steels will also be looked
examined
.

There is a lot of work involved in this experiment
;

to make the process as efficient as possible, you
will need to organise your group such that different people concentrate on different tasks.


Method

Whole Group

1.

Make sure that the tensometer and computer are switched on.


2.

Log onto the computer to record
your data in the appropriate Excel spreadsheet.

You will
be provided with log
-
on instructions separately.

a.

Load the following programmes…

i.

Excel

ii.

Hounsfield Logger

3.

You should have a labelled tensile test specimen with the tensometer. Check that you can
find

it.

4.

Click the “My Computer” icon, and use the program to find the folder


O:
\
My Documents
”.

5.

This folder will contain two files. You will need the file “Headstart Tensile Testing.xls”.

a.

Double click this file to load it.

b.

Note the warning at the top of th
e page as it opens. Only copy or type into cells that
have a
BLUE

background.

c.

There are some notes on one worksheet, but you do not need much background
information for this exercise.

6.

Decide who in your group will collect which data

and who will
enter it

into Excel. There
are several areas to get on with…

a.

Tensile test data

b.

Plastic deformation (necking and permanent elongation)

c.

Hardness

d.

Images

e.

Entering data into the computer.


Tensile test data

1.

DO NOT PRESS THE LARGE EMERGENCY STOP BUTTON UNLESS THERE
IS

AN
EMERGENCY.

2.

Mount the specimen in the tensometer making sure that all slack is taken up. If the initial
load recorded is about 100 to 200 N, this is sufficient.

3.

P
ress buttons to


a.

Zero

force
.

b.

Zero

extension

(ext).

c.

Zero auxiliary (aux).

4.

The message “
Test

1


should appear on the control panel.

5.

Press the test button


a red light appears.

6.

Press the blue triangular arrow pointing to the right



the
tensile
test starts
.

7.

Check that the output
s
for force and extension are increasing.

8.

The specimen will break af
ter a minute or two, possibly with a bang


be ready for it.

9.

Stop the tensometer and remove the specimen


use this for the plastic deformation
measurements.

10.

The Hounsfield logger program should have been loaded. If not do this now.

11.

Press the “


Print” k
ey and data is downloaded into the Hounsfield Logger Programme.

12.

In the Hounsfield Logger window, click the button “Save to Excel”. This will automatically
save to a folder called “My Documents
\
Hounsfield
\
Data

13.

Give the file a name, indicating your specimen

and your group e.g.
54C


a
2
.xls


(NO
decimal point before the 5 and NO quotes!) then save it.


Treatment of Tensile Test Data

1.

Find the file you have just saved and open it.

2.

Find the data for the experiment, headed “mm” and “N”.

3.

Highlight th
i
s data
i
ncluding the headings
.
Click to copy this data
.

4.

Open the file “Headstart Tensile Testing.xls”; look at the “Hounsfield Data” worksheet and
find the columns relating to the sample you have just tested.

5.

Click on the cell marked “mm” and paste your data into

it.

6.

Look in the worksheet “Stress
-
Strain”


you will see that the extensions and load are
automatically converted to
Engineering Stress and Strain.


7.

Repeat the above procedure for as many different metals as you can.

8.

An X
-
Y plot of

Stress
v

Strain for
eac
h
sample

recorded should be plotted automatically
.

9.

Make sure that you can identify

your own graph and print it by


a.

Click File; Print

b.

Select one of the following printers…

i.

\
\
NSMS60
\
Materials
-
tl
-
fs1010.Student Printers......

ii.

\
\
NSMS60
\
Materials
-
tl
-
fs1920.Stud
ent Printers......

c.

Both of these printers are in the computer room where you will collect your printout.

Plastic Deformation Measurements

These are made simple by the use of special tools which have already been set to the dimensions of
the specimens you
are using.

“Reduction in area Gauge”

1.

The Gauge is set by closing the gauge around the middle of the specimen and locking it a
t
the

“0”

position
.
THIS IS DONE FOR YOU.
THERE IS NO NEED FOR YOU TO
RESET THIS.

2.

After breaking, the thinnest part of the specim
en is slid down the gauge until it sticks. The
reading on the scale at this point is the percentage reduction in area for the specimen.

3.

Care
is needed to get this reasonably

accurate, as the specimens may fracture across the
narrowest
line.

4.

Record this i
n the “Headstart Tensile Tests.xls” file located in the “My Documents” folder.

“Elongation Gauge”

1.

As before, to save time, the Elongation Gauge is set and ready to use.

2.

Place the broken ends of the specimen together, in the compartment such that the ends f
it
together like jig
-
saw puzzle pieces.

3.

Move the slider to press against the specimen and read the percentage permanent elongation
directly from the scale.


Treatment of
Plastic Deformation Measurements

1.

Check each value

two or three

times
for each steel.
Calculate the average value for your
specimen.

2.

The

data must be entered into the appropriate cells of the

Headstart Tensile Tests.xls”
spreadsheet.

3.

Repeat the procedure

for other steels and enter them
in the same spreadsheet. If time is
running out, shar
e data with other groups. You should collect data for all four steels.

4.

Data
should be plotted automatically

showing
the relationship between plastic deformation
and carbon content of the steel.

5.

C
learly label
your
graph

to identify your group

and print it
.


Hardness Data

1.

There is only one hardness tester in the lab, so you will have to wait for your turn

2.

You will be shown how to use the indenter, which is not complicated, but needs several
adjustments. Collect
two

or three readings and calculate an average

value.

3.

Record this on the “Headstart Tensile Tests.xls” file located in the “My Documents” folder.

4.

Repeat the procedure for other steel samples and collect data for all four available steels.

5.

C
learly label
the
graph

in your spreadsheet to identify your gr
oup

and print it
.


Microstructure

data

1.

Use a specially prepared polished and etched steel specimen of the same steel that you have
tested to examine under an optical microscope.

2.

The microscope is expensive and the software is complicated


make sure that
you have one of
the demonstrators to show you how to use it safely.

3.

Make sure that the computer is logged on to your account.

4.

Examine the microstructure of the steel at different magnifications (demonstrator).

5.

Make sure any image you save has a scale bar a
ttached.

6.

Save an image (or images at different magnifications) to your folder, making sure that the
filename indicates...

i.

The steel carbon content

ii.

The magnification

iii.

Your group identity

iv.

E.g.
80C
-
x200
-
b1.jpg

7.

Repeat this procedure this for as many differ
ent steel specimens that you can.

8.

Images can be imported into word and printed together with captions


don’t forget to identify
your group on any printouts you have.




Write up

1.

Determine a value for Young’s Modulus for each steel. This can be obtained f
rom the slope
of the first stage (elastic region) of the Stress v Strain plot where it should be linear.

2.

Collect and collate your printouts. Write a brief account commenting on the effect of carbon
content on the physical properties and appearance that yo
u have measured.

3.

Make a final check that all of your work can be assigned to your group and leave it to be
collected on the tensometer.