21-11-2012, 02:40 PM
Mechanical Properties of Metals
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The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. These mechanical properties of the metal include…
1.Strength: It is the ability of a material to resist the externally applied forces without breaking or yielding…. The internal resistance offered by a part to an externally applied force is called stress.
2. Stiffness: It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.
3. Elasticity: It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines.It may be noted that steel is more elastic than rubber.
4. Plasticity: It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.
5. Ductility: It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead.
6. Brittleness: It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material.
7. Malleability: It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. malleable materials commonly used are lead, soft steel, wrought iron, copper and aluminium.
8. Toughness: It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. This property is desirable in parts subjected to shock and impact loads.
9. Machinability: It is the property of a material which refers to a relative case with which a material can be cut. brass can be easily machined than steel.
10. Resilience: It is the property of a material to absorb energy and to resist shock and impact loads. This property is essential for spring materials.
11. Creep: When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.
12. Fatigue: When a material is subjected to repeated fluctuating stresses, it fails at stresses below the yield point stresses. This property is considered in designing shafts, connecting rods, springs, gears, etc
13. Hardness: It can be defined as the resistance of a metal to attempts to deform it. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal.
Simple Stresses in Machine Parts
Load: It is defined as any external force acting upon a machine part. The following four types of the load are important…
1. Dead or steady load: A load is said to be a dead or steady load, when it does not change in magnitude or direction.
2. Live or variable load: A load is said to be a live or variable load, when it changes continually.
3. Suddenly applied or shock loads: A load is said to be a suddenly applied or shock load, when it is suddenly applied or removed.
4. Impact load: A load is said to be an impact load, when it is applied with some initial velocity.
Stress: When some external system of forces or loads act on a body, the internal forces (equal and opposite) are set up at various sections of the body, which resist the external forces. This internal force per unit area at any section of the body is known as unit stress or simply a stress. It is denoted by a Greek letter sigma (σ).
Strain: When a system of forces or loads act on a body, it undergoes some deformation. This deformation per unit length is known as unit strain or simply a strain.