During the process
of selecting a material for engineering applications, their mechanical
properties play a major role, which depend upon the components being
manufactured and their utilization.
Mechanical
properties are characteristics of materials that are revealed when that
material is subjected to mechanical loading. There are standard tests and
procedures that are performed in a materials laboratory in order to determine
the mechanical properties
Tensile
properties indicate how the material will respond to forces being applied in
tension. A tensile test is an essential mechanical test where a thoroughly
prepared specimen is loaded in a controlled technique while measuring the
applied load and the elongation of the specimen over some distance. The diagram
here shows the results of a tensile test. Diagrams like this are used to
determine the modulus of elastic limit (Young's Modulus), elasticity,
proportional limit, elongation, reduction in area, tensile strength, yield
strength, yield point, and other tensile properties.
Yield Strength
The yield
strength or yield point of a material is defined in materials and engineering
science as the stress at which a material starts to deform plastically - meaning
the material deforms permanently. Preceding to the yield point, the material
will deform elastically and will come back to its original shape when the
stress is removed. The slope of the elastic deformation region is actually
the Young's Modulus of the material, a measure of its
elastic deformation. When the yield point is crossed some part of the
deformation will be permanent and non-reversible. Details regarding the yield
point is important when designing a component since it normally represents an
upper limit to the load that can be applied. It is also important in many
materials production techniques such as rolling, forging, or pressing. In
structural engineering, this is a soft mode of failure which is not usually a
source of catastrophic failure or ultimate failure.
Ultimate Tensile Strength
It is the
maximum stress that a material can undertake. It measures the magnitude of
stress applied to a material at its breaking point or the point at which it
fails. The tensile strength of a material is the point at which a material,
under the stress of an applied force, breaks or can no longer maintains its
structural reliability. It is, in other words, the extent of force the material
can resist without breaking.
Necking
Under
constant tensile deformation, some materials (especially polymers) may deform
disproportionately, forming a "neck".
Fracture
After
necking, the applied stress will result in the breaking of the material (point
of fracture).
Ductility
It is the
ability of a material to undergo large plastic deformation without cracking or
shattering. This is the property that allows the material to be drawn out into
a thin wire.
Resilience
It is the
ability of a material to store elastic energy, which is also called modulus of
resilience. This property is important when selecting a material for spring
manufacturing.
Toughness
It is the
ability of a material to withstand sudden loading or to absorb energy until it
breaks and it is measured in terms of modulus of toughness. Machine components
that are subjected to sudden loading must be made from materials that possess
high toughness.
Hardness
It is the
ability of a material to resist scratching, abrasion or indentation. It is one
of the easiest criterion or measurement in acceptance tests, and quality
control of raw stock as manufactured products. Common hardness testing methods
are Brinell and
Rockwell hardness tests.
Impact Strength
It is a
measure of the ability of a material to withstand shock loading, and can also
be described as the energy required to fracture a given volume of material. It
is measured by an impact machine. Types of impact tests are Charpy and
the Izod.
Metal Fatigue
The
number of cycles for which a component or a specimen can endure an alternating
load mainly depends upon the magnitude or amplitude of that load. The greater
the magnitude of the alternating load, the lesser the number of cycles after
which it fails. Magnitude of the load that is the basis of failure, is much
less than the yield stress of the material, indicating that the mechanism of
failure is different from one in which the specimen is subjected to uniaxial
tension. This phenomenon is called metal fatigue, and the failure is due to
initiation and then propagation of a crack within the part.
Plasticity
It is the
ability of the material to deform plastically at elevated temperature under
constant load. It occurs even though the applied stress is less than the yield
stress at that temperature. Creep characteristics are determined by subjecting
the specimen to different constant stresses at elevated temperatures and
observing the corresponding strain.
Brittleness
It is the
ability of the material to shatter or break before deformation (eg. iron,
glass). This property is actually the opposite of plasticity.