Mechanical Properties of Materials: Definition, Testing and Application
International Journal of Modern Studies in Mechanical Engineering (IJMSME) Page | 30
material to withstand cold deformation without fracture. Ductility of a material is to stretch under the
application of tensile load and retain the deformed shape on the removal of the load. If subjected to a
shock load the material would yield and become deformed. Ductile material can be worked into a
shape without loss of strength. All materials which are formed by drawing are required to be ductile,
e.g.- drawing into wire form.
2.6. Brittleness
Breaking of a material with little permanent distortion simply states the property of brittleness. Brittle
materials when subjected to tensile loads snap off without giving any sensible elongation [5]. Usually
the tensile strength of brittle materials is only a fraction of their compressive strength. Examples of
brittle materials are glass, bricks, cast iron etc… It is also a tendency of a material to fracture when
subjected to shock loading or a blow. Material that shatters is also a brittle material.
2.7. Malleability
It is the ability of materials to be rolled, flattened or hammered into thin sheets without cracking by
hot or cold working. Malleable material should be plastic but it is not essential to be strong and
malleability is considered as a compressive quality. Examples for malleability Al, Cu, Sn, Pb, soft
steel, wrought iron. This is the property of a material to deform permanently under the application of a
compressive load. A material which is forged to its final shape is required to be malleable. Forging,
Rolling processes are malleability.
2.8. Toughness and Testing
It is the ability of a material to withstand bending without fracture due to high impact loads.
Toughness of material decreases when it is heated [16]. It is also measured by the amount of energy
that a unit volume of the material has absorbed after being stressed up to failure point and is the area
under stress strain curve. For example, if a load is suddenly applied to a piece of mild steel and then to
a piece of glass, the mild steel will absorb much more energy before failure occurs. Thus mild steel is
said to be much tougher than a glass. This property is desirable in parts subjected to shock and impact
loads. Notch toughness is the measure of the metal‘s resistance to brittle fracture in presence of flaw
or notch and fast loading conditions [17]. Examples are Mn-steel, wrought iron, MS, etc…it can be
also defined as property of absorbing energy before fracture. To the opposite of brittleness, the ability
of a material is to resist fracture under shock loading. Basically, two main impact tests for measuring
the toughness of material in Joule are available namely Izod and Charpy test. Figure 2 shows the three
types of Notches used for fracture study. U type notch specimens can also be used for testing. In case
of ductile materials, when the material is stressed, it plastically deforms by absorbing high energy and
then the material fractures. But in the case of brittle materials, the cohesive strength of the material
exceeds before getting plastically deformed and hence absorbs less energy before getting fractured.
There are factors responsible for brittle behaviour; they are notch, low temperature, thickness and
microstructure. When temperature falling, the failure mode of certain materials changes from ductile
to brittle. For FCC materials, if the temperature increases, the energy absorbed also slightly increases.
The factors responsible for the Charpy impact test are ductility, yield strength, notch, temperature, and
fracture mechanism. Figure 3, shows the working procedure of impact testing. The pivoting arm is
raised to a specific height, which is the potential energy and then this arm gets released. The arm
swings down hitting a notched sample, available on the specimen holding vise, and breaking the
specimen. The energy absorbed by the sample is measured from the height the arm swings to after
hitting the sample. The fracture energy (Joule) is determined from the swing-up angle of the hammer
and its swing-down angle. A notched sample is generally used to determine impact energy and notch
sensitivity. Some of the standards are followed worldwide for the test they are ASTM D6110, ASTM
E23, and ASTM D256 etc..., Figure 4 depicts the difference between Izod and Charpy test. In Charpy
test (figure 4 a), a test specimen having a V-shaped notch (figure 5) is placed on the holder in such
position that the notched section is in the center of the holder and the specimen is broken by striking
the back of the notched section with the hammer. The Charpy impact value (kJ/m
2
) is calculated by
dividing the fracture energy by the cross-section area of the specimen. If a test specimen having a V-
shaped notch is fixed vertically, and the specimen is broken by striking it from the same side as that of
the notch by the use of the hammer, this is called Izod test (figure 4 b). The Izod impact energy value
(J/m,) is calculated by dividing the fracture energy by the width of the specimen.