Kathmandu University Civil Labs
DETERMINATION OF IMPACT ENERGY BY CHARPY TEST
OBJECTIVE:
To test the resistance of the material towards an impact load.
THEORY:
The tendency of material to break under an impact of load is called as brittleness while resistance of
material to fracture by bending, twisting, fatigue or impact of load is known as toughness.
The Charpy Impact Test was invented by Georges Augustin Albert Charpy (1865-1945). The Charpy test
measures the energy absorbed by a standard notched specimen while breaking under an impact load. The
Charpy impact test continues to be used as an economical quality control method to determine the notch
sensitivity and impact toughness of engineering materials.
The Charpy Test is commonly used on metals, but is also applied to composites, ceramics and polymers.
With the Charpy test one most commonly evaluates the relative toughness of a material, as such, it is used
as a quick and economical quality control device.
The Charpy impact test is one of the impact-testing that is used to determine the resistance towards a
sudden load or impact. Impacted specimens with small fracture or with a very little plastic deformation is
said to be in a brittle manner whereas fracture of a metal after extensive plastic deformation is said to be
in a ductile manner. Brittle fracture looks clear, shinny with surfaces while ductile fracture looks grayish
and fibrous.
Impact energy is a measure of the work done to fracture a test specimen. When the striker impacts the
specimen, the specimen will absorb energy until it yields. At this point, the specimen will begin to
undergo plastic deformation at the notch. The test specimen continues to absorb energy and work hardens
at the plastic zone at the notch. When the specimen can absorb no more energy, fracture occurs.
CHARPY TEST SPECIMENS
Charpy test specimens normally measure 55x10x10 mm and have a notch machined across one of the
larger faces. The notches may be:
V-notch – A V-shaped notch, 2mm deep, with 45° angle and 0.25mm radius along the base
U-notch or keyhole notch – A 5mm deep notch with 1mm radius at the base of the notch.
The Charpy Test consist of striking a suitable specimen with a hammer on a pendulum arm while the
specimen is held securely at each end. The hammer strikes opposite the notch. The energy absorbed by
the specimen is determined byprecisely measuring the decrease in motion of the pendulum arm.
By applying the Charpy Test to identical specimens at different temperatures, and then plotting the
impact energy as a function of temperature, the ductile-brittle transition becomes apparent. This is
essential information to obtain when determining the minimum service temperature for a material.
What Does the Charpy Test Involve?
The Charpy test involves striking a suitable test piece with a striker, mounted at the end of a pendulum.
The test piece is fixed in place at both ends and the striker impacts the test piece immediately behind a
machined notch.
Figure 1. Schematic diagram of the Charpy impact test.
PROCEDURE:
1) A Charpy V-notch specimen is placed across parallel jaws in the impact-testing machine
2) The pointer is being set up to its maximum value (300 J)
3) The hammer is released from the initial height downward towards the sample
4) Observations and the energy absorbed is recorded and tabulated.
5) Step 1-3 is repeated for another type of metal.
Determination of Charpy Impact Energy
At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated
based on the height to which the striker would have risen, if no test specimen was in place, and this
compared to the height to which the striker actually rises.
Tough materials absorb a lot of energy, while brittle materials tend to absorb very little energy prior to
fracture.
Factors Affecting Charpy Impact Energy
Factors that affect the Charpy impact energy of a specimen will include:
Yield strength and ductility
Notches
Temperature and strain rate
Fracture mechanism
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.
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.
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.
Fracture Mechanism
Metals tend to fail by one of two mechanisms, microvoid coalescence or cleavage.
Cleavage can occur in body centred cubic materials, where cleavage takes place along the {001} crystal
plane. 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.
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.
Results:
Test of the ability of a material to withstand impact, used by engineers to predict its behavior under actual
conditions. Many materials fail suddenly under impact, at flaws, cracks, or notches. The most common
impact tests use a swinging pendulum to strike a notched bar; heights before and after impact are used to
compute the energy required to fracture the bar. In the Charpy test, the test piece is held horizontally
between two vertical bars, much like the lintel over a door.
Steel Energy Absorbed (Joule)
Mild steel 299.0
Carbon Steel 21.0
Mild Steel Carbon Steel
DISCUSSION:
From the results obtain from the experiment, we can see that the specimen fracture differently. The mild
steel fracture but did not broke completely and some part of the steel still attached therefore it is said to be
in the ductile manner. The carbon steel is said to be in brittle manner although it is harder. The carbon
specimen broke completely into 2 parts and the broken surfaces looks smoother than the mild steel.
Carbon steel has lower ductility compared to the mild steel but the carbon steel is more brittle than the
mild steel. This is because carbon steel has high percentage of carbon in it (percentage of carbon in mild
steel is less than 0.2%) that lower the ability to absorb energy when sudden load is given.
From the way both specimen cracks and the ‘grayish and fibrous’ broken surfaces of the mild steel shows
that it undergoes plastic deformation and in the ductile manner. The carbon steel experience less plastic
deformation and the surfaces looks smoother and shinny.
Ductile fracture is better because of the following reasons:
a) More energy needed in the ductile fracture because it is a tough material
b) Brittle fracture happens quickly without warning while the ductile fracture took a longer time before
the whole process to happen
CONCLUSION:
From the Charpy impact test, carbon steel undergoes brittle fracture while the mild steel undergoes ductile
fracture. More energy is absorbed by mild steel shows that it is more suitable to be use in the structural
construction that expose to high load for example: car body.
References:
http://www.wmtr.com/content/charpy.htm
http://theconstructor.org/2010/02/the-charpy-impact-test/
Click the above mentioned drive lnk for clarity
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