Monografia

1.3 principal mechanical properties
Those characteristics of the materials which describe their behavior under external loads are known as mechanical properties. The most important and useful mechanical properties are:
1.3.1 Strength
It is the resistance offered by a material when subjected to external loading. So, stronger the material the greater the load it can withstand.
Depending uponthe type of load applied the strength can be tensile, compressive, shear or torsional.
The maximum stress that any material will withstand before destruction is called it is ultimate strength (point d).
1.3.2 Elasticity
Elasticity of a material is it is power of coming back to it is original position after deformation when the stress or load is removed. Elasticity is a tensible property of itis material.
The greatest stress that a material can endure without taking up some permanent set is called elastic limit (point a)
1.3.3 Stiffness (rigidity)
The resistance of a material to deflection is called stiffness or rigidity. Steel is stiffer or more rigid than aluminium.
Stiffness is measured by young is modulus E. the higher the value of the young is modulus, the stiffer thematerial E is the ratio of stress over strain and is given by the slope of line 0-a.
1.3.4 Plasticity
The plasticity of a material is it is ability to undergo some degree of permanent deformation without failure. Plastic deformation will take place only after the elastic range has been exceeded, beyond point b.
Plasticity is an important property and widely used in several mechanical processes likeforming, shaping, extruding and many other hot and cold working processes. In general, plasticity increases with increasing temperature and is a favourable property of material for secondary forming processes.
Due to this various metal can be transformed into different product of required shape and size. This conversion into desired shape and size is effected either by the application of pressure,heat or both.
1.3.5 Ductility
Ductility of a material enables it to draw out into thin wire application of the load. Mild steel is a ductile material. The wires of gold, silver, copper, aluminium, etc. are draw by extrusion or by pulling through a hole in a die due to the ductile property. The ductility decreases with increase of temperature.
The per cent elongation and the reduction in areain tension are often used as empirical measures of ductility.
1.3.6 Malleability
Malleability of a material is it is ability to be flattened into thin sheets without cracking by hot or cold working. Aluminium copper, tin, steel, etc. are malleable metals.
Lead can be readily rolled and hammered into thin sheets but can not be draw into wire. Ductility is a tensile property, whereasmalleability is a compressive property. Malleability increases with increase of temperature.
1.3.7 Brittleness
The brittleness of a material is the property of breaking without much permanent distortion. There are many materials, which break or fail before much deformation take place. Such materials are brittle e. g…, glass, and cast iron.
Therefore, a non-ductile is said to be a brittle material.Usually the tensile strength of brittle materials is only a fraction of their compressive strength. A brittle material should not be considered as lacking in strength. It only shows the lack of plasticity. On stress-strain diagram, these materials do not have yield point and value of E is small.
1.3.8 Toughness
The toughness of a material is it is ability to withstand both plastic and elasticdeformations. It is a highly desirable quality for structural and machine parts to withstand shock and vibration. Manganese steel, wrought, iron, mild steels are tough materials.
For Ex : 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....