Introduction to Basic Manufacturing Processes and ... - always yours

Porperties and Testing of Metals 119
structures. Theses properties are structure sensitive in the sense that they depend upon the
crystal structure and its bonding forces, and especially upon the nature and behavior of the
imperfections which- exist within the crystal itself or at the grain boundaries. The mechanical
properties of the metals are those which are associated with the ability of the material to resist
mechanical forces and load. The main mechanical properties of the metal are strength, stiffness,
elasticity, plasticity, ductility, malleability, toughness, brittleness, hardness, formability, castability
and weldability. These properties can be well understood with help of tensile test and stress
strain diagram. The few important and useful mechanical properties are explained below.
1. Elasticity
It is defined as the property of a material to regain its original shape after deformation
when the external forces are removed. It can also be referred as the power of material to
come back to its original position after deformation when the stress or load is removed. It
is also called as the tensile property of the material.
2. Proportional limit
It is defined as the maximum stress under which a material will maintain a perfectly
uniform rate of strain to stress. Though its value is difficult to measure, yet it can be used
as the important applications for building precision instruments, springs, etc.
3. Elastic limit
Many metals can be put under stress slightly above the proportional limit without
taking a permanent set. The greatest stress that a material can endure without taking up
some permanent set is called elastic limit. Beyond this limit, the metal does not regain its
original form and permanent set will occurs.
4. Yield point
At a specific stress, ductile metals particularly ceases, offering resistance to tensile
forces. This means, the metals flow and a relatively large permanent set takes place without
a noticeable increase in load. This point is called yield point. Certain metals such as mild steel
exhibit a definite yield point, in which case the yield stress is simply the stress at this point.
5. Strength
Strength is defined as the ability of a material to resist the externally applied forces with
breakdown or yielding. The internal resistance offered by a material to an externally applied
force is called stress. The capacity of bearing load by metal and to withstand destruction
under the action of external loads is known as strength. The stronger the material the
greater the load it can withstand. This property of material therefore determines the ability
to withstand stress without failure. Strength varies according to the type of loading. It is
always possible to assess tensile, compressive, shearing and torsional strengths. The maximum
stress that any material can withstand before destruction is called its ultimate strength. The
tenacity of the material is its ultimate strength in tension.
6. Stiffness
It is defined as the ability of a material to resist deformation under stress. The resistance
of a material to elastic deformation or deflection is called stiffness or rigidity. A material that
suffers slight or very less deformation under load has a high degree of stiffness or rigidity.
For instance suspended beams of steel and aluminium may both be strong enough to carry
the required load but the aluminium beam will “sag” or deflect further. That means, the steel