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

120 Introduction to Basic Manufacturing Processes and Workshop Technology
beam is stiffer or more rigid than aluminium beam. If the material behaves elastically with
linear stress-strain relationship under Hooks law, its stiffness is measured by the Young’s
modulus of elasticity (E). The higher is the value of the Young’s modulus, the stiffer is the
material. In tensile and compressive stress, it is called modulus of stiffness or “modulus of
elasticity”; in shear, the modulus of rigidity, and this is usually 40% of the value of Young’s
modulus for commonly used materials; in volumetric distortion, the bulk modulus.
7. Plasticity
Plasticity is defined the mechanical property of a material which retains the deformation
produced under load permanently. This property of the material is required in forging, in
stamping images on coins and in ornamental work. It is the ability or tendency of material
to undergo some degree of permanent deformation without its rupture or its failure. Plastic
deformation takes place only after the elastic range of material has been exceeded. Such
property of material is important in forming, shaping, extruding and many other hot or cold
working processes. Materials such as clay, lead, etc. are plastic at room temperature and steel
is plastic at forging temperature. This property generally increases with increase in temperature
of materials.
8. Ductility
Ductility is termed as the property of a material enabling it to be drawn into wire with
the application of tensile load. A ductile material must be strong and plastic. The ductility is
usually measured by the terms, percentage elongation and percent reduction in area which
is often used as empirical measures of ductility. The materials those possess more than 5%
elongation are called as ductile materials. The ductile material commonly used in engineering
practice in order of diminishing ductility are mild steel, copper, aluminium, nickel, zinc, tin
and lead.
9. Malleability
Malleability is the ability of the material to be flattened into thin sheets under applications
of heavy compressive forces without cracking by hot or cold working means. It is a special
case of ductility which permits materials to be rolled or hammered into thin sheets. A
malleable material should be plastic but it is not essential to be so strong. The malleable
materials commonly used in engineering practice in order of diminishing malleability are
lead, soft steel, wrought iron, copper and aluminium. Aluminium, copper, tin, lead, steel, etc.
are recognized as highly malleable metals.
10. Hardness
Hardness is defined as the ability of a metal to cut another metal. A harder metal can
always cut or put impression to the softer metals by virtue of its hardness. It is a very
important property of the metals and has a wide variety of meanings. It embraces many
different properties such as resistance to wear, scratching, deformation and machinability etc.
11. Brittleness
Brittleness is the property of a material opposite to ductility. It is the property of
breaking of a material with little permanent distortion. The materials having less than 5%
elongation under loading behavior are said to be brittle materials. Brittle materials when
subjected to tensile loads, snap off without giving any sensible elongation. Glass, cast iron,
brass and ceramics are considered as brittle material.