Impact Testing of Materials

1. Impact test
The purpose of impact testing is to measure an object's ability to resist high-rate loading. The test measures the impact
energy, or the energy absorbed prior to fracture.
What is Impact Energy?
Impact energy is a measure of the toughness of a materials, i.e. materials resistance to fracture.
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 at the plastic zone at the
notch. When the specimen can absorb no more energy, fracture occurs.
The most common methods of measuring impact energy are the:
• Charpy Test
• Izod Test
The Charpy Test
While most commonly used on metals, it is also used on polymers, ceramics and composites. The Charpy test is most
commonly used to evaluate the relative toughness or impact toughness of materials and as such is often used in quality
control applications where it is a fast and economical test

2. Charpy Test Specimens
According to ASTM Standard E 23, “Standard Test Methods for Notched Bar Impact Testing of Metallic Materials”, Charpy test specimens
normally measure 55x10x10mm 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.
Figure (1) Standard
specimens of
Charpy and Izod
impact energy tests

3. Figure 3. Schematic of the Charpy specimen impact test.
Figure 2. Schematic of the Charpy impact test.

4. 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.
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
difference between initial and final heights of the swinging pendulum.
Impact energy (E) = mgh1- mgh2 = mg ( h1-h2)
Energy unit is Joule = N.m
h unit is m
g is gravity ( 9.8 N/Kg or m/s2 )
m unit (Kg)
Note: Tough materials absorb a lot of energy, whilst brittle materials tend to absorb
very little energy prior to fracture

5. The left specimen brittle—looks like it just snapped in half. The right
specimen sample is ductile and bends without breaking into pieces.
Figure 4. A- Brittle and B- ductile fracture under Charpy impact test
A B

6. The main differences between Izod and Charpy
Izod and Charpy tests are similar, but they are different in :
1. Point of Strike : Point at which the hammer strike the specimen is different for both of them. In Izod test hammer strike at the
upper tip of specimen while in Charpy test hammer strike at point of notch but in opposite direction
2- Direction of Notch: Face of specimen which faces the striker is different. The notch face in the izod test is facing the striker,
fastened in a pendulum, while in the charpy test, the notch face is positioned away from the striker.
3-Type Of Notch: In hardness testing two types of notches are used V-notch and U-notch. In the Charpy method, there are two
kinds of notches, the V-notch and the U-notch, while in the Izod method, there is V-notch is used
4-Specimen Dimensions: Even if you are testing the same material the test specimens have different dimensions for each test. The
basic Izod test specimen is 75 x 10 x 10mm, the basic Charpy test specimen is 55 x 10 x 10mm.
IZOD TEST
CHARPY
TEST

7. Factors Affecting Charpy Impact Energy
Factors that affect the Charpy impact energy of a specimen will include:
1- Yield Strength and Ductility: For a given material the impact energy will be seen to decrease
if the yield strength is increased due to the reduction of ductility
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: The higher strain rate, the lower impact energy

8. Figure 5. effect of loading rate on Imoact energy

9. Ductile to Brittle Transition for metals
Body centered cube materials such as carbon steels undergo what is known as a ‘ductile to brittle transition’. This behavior is
obvious when impact energy is plotted as a function of temperature. FCC metals do not have a ductile to brittle transition
temperature and instead remain ductile at low temperatures. This is because the stress required to move dislocations is not
strongly temperature-dependent in FCC metals, and thus failure occurs by plastic flow instead of crack propagation.
In BCC metals, at low temperature the stress required to make crack propagation is less than stress required for plastic flow,
thus failure occurs by crack propagation.
Examples of Materials which have FCC structure are: Aluminum, Nickel, Copper,
austenite (Gamma-Iron)
Examples of Materials which have BCC structure are: Pure Iron, Chromium, (alpha, delta Iron)

10. Figure 6. Ductile to Brittle transition of BCC materials at low temperature

11. Figure 7. Effect of Temperature on materials with FCC and BCC structure

12. Example:
Following is tabulated data that were gathered from a series of Charpy impact tests on a tempered 4340 steel alloy
Temperature (c) Impact Energy (J)
0 105
_25 104
_50 103
_75 97
_100 63
_113 40
_125 34
_150 28
_175 25
_200 24
(a) Plot the data as impact energy versus temperature.
(b) Determine a ductile-to-brittle transition temperature as that temperature
corresponding to the average of the maximum and minimum impact
energies.
(a) Determine a ductile-to-brittle transition temperature as that
temperature at which the impact energy is 50 J

13. 0
10
20
30
40
50
60
70
80
90
100
110
120
-250 -200 -150 -100 -50 0
energy
J
temperature C
B
Solution:
(b) Average maximum and minimum impact energies = 24+105 / 2 = 64.5 J
So, temperature at 64.5 J is equal to about - 99 C
( c ) a ductile-to-brittle transition temperature as that temperature at which the impact energy is 50 J is about
T= -110 C
C