basic mechanical engineering

Q.8 Define following mechanical properties-

निम्न मेकेनिकल गुणों को परिभाषित करे।

i) Tensile strength ii) Hardness iii) Ductility iv) Toughness

Ans. (i) Tensile strength - Tensile strength of a material can be defined as the maximum value of tensile stress, under a steady load which a material can withstand before fracture. It is calculated by dividing the maximum load by the original cross-section area, i.e.

Tensile strength = Maximum tensile load / Original cross-section area

Tensile strength is also called as ultimate tensile strength or ultimate strength. It is taken as a basis for fixing the working stresses, especially in brittle materials. Its units are same as those of stress, i.e. kg/cm2 or N/mm2 or Pa.

(ii) Hardness – Hardness is the property of material by virtue of which it is able to resist wear and plastic deformation usually by indentation. Hardness also means the ability of a material to cut another material. The hardness of the material depends upon the type of bonding forces between atoms, ions or molecules and increases with the magnitude of these forces. Thus molecular solids such as plastics are relatively soft, metallic and ionic solids are harder and covalent solids are the hardest materials known.

Diamond is the hardest substance known, while other hard substances are corundum, topaz, quartz, high carbon iron, etc.

(iii) Ductility – It is the property of a material which enables it to draw into the shape of wire. Mild steel is an example of ductile material. The percentage elongation and the reduction in area in tension is often used as empirical measure of ductility.

%Elongation = (Final gauge length – Original gauge length) × 100 / (original gauge length)

%Reduction in area = (Original area – Final area) × 100 / (Original area)

Original area A material is generally said to be ductile if the percentage elongation is more than 5 in a gauge length of 50 mm. Mild steel, copper, aluminium, zinc, nickel, tin, etc., are ductile metals.

(iv) Toughness – Toughness is the strength with which the material opposes rupture. It is a measure of the amount of energy, a material can absorb before actual fracture or failure takes place. Mild steel is tougher than glass.