Alexandria University, Faculty of Engineering
Civil Engineering Department
Second Year
2012-1013
Presented By: Ahmad Said Helmy Al-Abassy
Section: 1
Seat Number: 33
Metallic Engineering Materials
Assignment No. 1 - Research
ABSTRACT
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Some materials like cast iron, glass and some plastics which offer
considerable resistance to static load, often shatter easily when a sudden
load (impact) is applied.
The impact strength is defined as the resistance of the materials to shock.
The impact testing is to find out the energy absorbed by a specimen when
brought to fracture by hammer blow and gives a quality of the material,
particularly its brittleness. Highly brittle materials have low impact strength.
Heat treatment of metals has found to lower impact considerably.
The impact load can be applied in many ways. Allowing a standard mass to
fall on the specimen from progressively increasing heights until fracture
occurs. Charpy and Izod impact tests are used. These notched specimens are
fractured with a standard blow from a pendulum hammer and energy
absorbed is measured.
Basic types of impact testing
1. Charpy impact test
§ The specimen is supported as a simple beam with the load applied at the
center.
§ The position of latching tube is set to 140°.
§ The specimen is supported horizontally from two sides.
2. Izod test
§ The specimen is supported as a cantilever beam.
§ The position of latching tube is set to 90°.
§ The specimen is supported Vertically from one side.
Both Charpy and Izod impact testing use a swinging pendulum to apply the load.
The difference in the Charpy and the Izod techniques is in the way that the
specimens are supported in the apparatus machine. Using notched specimens the
specimen is fractured at the notch.
How to measure absorbed energy
1. The impact load is applied from a hammer that is released from position h1.
2. The pendulum with a knife edge strikes and fractures the specimen at the
notch.
3. The pendulum continues its swing, rising to a maximum height h2, which is
lower than h1.
4. The energy is calculated from the difference in initial and final heights of the
swinging pendulum. Impact energy (toughness) from the test is related to
the area under the total stress-strain curve.
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Energy = Mghinitial – Mghfinal
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PROCEDURE
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Select the test (Charpy/Izod).
Select and fit the respective striker in the hammer, first tighten the screws of
the wedge and then of strikers.
Fix the latching tube to corresponding position (for Charpy 140° position and
for Izod 90° position).
Place a specimen on the support of the block.
Bring the striker (hammer) closely to specimen and tough it lightly with the
specimen.
Pointer when touched to its carrier should read 300 J line for Charpy and 170
J for Izod. Otherwise correct it by losing and tighten the screw of the pointer
carrier.
Remove the specimen. Latch the hammer. Place the pointer as 300 J for
Charpy and 170 j for Izod.
Release the hammer. Hold back the releasing lever.
The pointer will show the frictional losses. This reading should be less than
1.5 joules for Charpy and 0.8 for Izod.
Thus the machine is ready for the test.
Note readings for different temperatures and tabulate them.
CONDUCTING THE TEST
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Place the specimen onto the support with notch facing forwards the direction
of striker of the striking direction.
Using the setting gauge, center the notch to the reference level.
Face the pointer to read 170 J. latch the hammer4) Release the hammer. The
pointer will indicate the amount of energy consumed by the specimen for its
rupture.
OBSERVATIONS
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Factors Affecting Impact Energy
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Yield strength and ductility
Notches
Temperature and strain rate
Fracture mechanism
1. Yield Strength and Ductility
For a given material the impact energy will be seen to decrease if the yield strength
is increased, i.e. if the material undergoes some process that makes it more brittle
and less able to undergo plastic deformation. Such processes may include cold
working or precipitation hardening.
2. Notches
The notch serves as a stress concentration zone and some materials are more
sensitive towards notches than others. The notch depth and tip radius are therefore
very important.
3. Temperature and Strain Rate
Most of the impact energy is absorbed by means of plastic deformation during the
yielding of the specimen. Therefore, factors that affect the yield behaviour and
hence ductility of the material such as temperature and strain rate will affect the
impact energy.
This type of behaviour is more prominent in materials with a body centred cubic
structure, where lowering the temperature reduces ductility more markedly than
face centred cubic materials.
4. Fracture Mechanism
Metals tend to fail by one of two mechanisms, microvoid coalescence or cleavage.
Cleavage can occur in body centred cubic materials. Microvoid coalescence is the
more common fracture mechanism where voids form as strain increases, and these
voids eventually join together and failure occurs. Of the two fracture mechanisms
cleavage involved far less plastic deformation ad hence absorbs far less fracture
energy.
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Ductile to Brittle Transition
Some materials such as carbon steels
undergo what is known as a ‘ductile to
brittle transition’. This behaviour is
obvious when impact energy is plotted as
a function of temperature. The resultant
curve will show a rapid dropping off of
impact energy as the temperature
decreases. If the impact energy drops off
very sharply, a transition temperature can be determined. This is often a good
indicator of the minimum recommended service temperature for a material.
CONCLUSION
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Fracture surface of specimens after impact testing
a) Ductile material. Uneven surface
b) Brittle material. Smooth surface
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Impact Energy of Mild Steel decreases with temperature. This indicates
material becomes more brittle as temperature decreases.
Aluminum is more ductile and hence has higher impact energy than mild
steel at room temperature
A higher toughness material will absorb more energy upon impact and will
therefore result in a low height to which the pendulum arm will swing to
following impact.
Specimen with lowest absorbed energy means it’s brittle and has least
toughness which can break easily and cannot withstand the sudden high
loads.
Specimen with Highest absorbed energy means it’s ductile and has highest
toughness which can withstand the sudden high loads.
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REFERENCES
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Materials science and engineering callister 7th edition Solution
http://books.google.com.eg/books?id=CfFEerBY7nEC&pg=PA326&lpg=PA326&dq=results+of+iz
od+impact+test+on+polymer&source=bl&ots=jbq9h1zJ_6&sig=rY1NBi_VUZctIcapNLws7OXRN4&hl=en&sa=X&ei=D9tQUdTNH8rGtAanpYDIAw&redir_esc=y#v=onepage&
q=results%20of%20izod%20impact%20test%20on%20polymer&f=false
Instrumented Impact Testing
http://books.google.com.eg/books?id=d3dPNq95NPIC&pg=PA74&dq=results+of+izod+impact+t
est+on+aluminium&hl=en&sa=X&ei=YxJRUZPFBsw7AavqoC4Dw&redir_esc=y#v=onepage&q&f=true
lecture 10 – fracture , Mechanical Engineering Department Concordia University
http://users.encs.concordia.ca/~mmedraj/mech321/lecture_10_fracture.pdf
http://www.azom.com/article.aspx?ArticleID=2763
http://www.scribd.com/doc/29061406/Impact-Test-Report
http://www.scribd.com/doc/18004149/Charpy-Test
http://www.scribd.com/doc/29392634/Charpy-Impact-Test
http://www.scribd.com/doc/70933843/Charpy-Impact-Test
http://www.santarosa.edu/~yataiiya/E45/PROJECTS/DBT%20experiment.pdf