STRENGTH OF MATERIALS- Mechanical Properties of Metals
MECHANICAL PROPERTIES OF METALS
When external forces are applied on a body, the external forces tend to deform the body while the molecular forces acting between the molecules offer resistance to deformation. The deformation of the particles continues till full resistance to the external forces is set up. If the forces are now gradually diminished, the body will return, wholly or partially to its original shape.
Elasticity is the property by virtue of which a material deformed under the load is enabled to return to its original dimension when the load is removed.
Steel, aluminum, copper etc. may be considered perfectly elastic within certain limits.
Plasticity is the reverse of elasticity. It is the property that enables the formation of a permanent deformation in a material. A plastic material will retain exactly the shape it takes under load even after the load is removed.
During plastic deformation there is the displacement of atoms within the metallic grains and consequently the shapes of metallic components change.
Gold and lead are highly plastic materials.
Plasticity is made use of in stamping images on coin and ornamental work.
Ductility is the characteristic which permits a material to be drawn out longitudinally to a reduced section, under the action of tensile force. Due to this property, wires are made by drawing through a hole. During ductile extension, a material shows a certain degree of elasticity, together with a considerable degree of plasticity. This is a valuable property in chains, ropes etc. because they do not snap off, while in service, without giving sufficient warning by elongation.
Brittleness implies lack of ductility. A material is said to be brittle when it can not be drawn out by tension to smaller section. In brittle material, failure takes places under load without significant deformation. Brittle fractures take place without warning and the property is generally highly undesirable.
Cast iron, high carbon steel, concrete, stone, glass etc. are common examples of brittle material.
Malleability is the property of a material which permits the materials to be extended in all the directions without rupture. A malleable material possesses a high degree of plasticity, but not necessarily great strength. This property is utilized in many operations such as forging, hot rolling, drop-stampings etc.
This property generally increases with increase of temperature.
Gold, silver, copper, aluminum, lead, zinc etc. are common examples of malleable material.
Toughness is the property of a material which enables it to absorb energy without fracture. This property is very desirable in components subjected to cyclic or shock loading.
Toughness is measured in terms of energy required per unit volume of the material, to cause rupture under the action of gradually increasing tensile load.
A common comparative test is the bend test in which a material is expected to sustain angular bending without failure.
Hardness is the ability of a material to resist indentation or surface abrasion. Hard materials resists scratches or being worn out by friction with another body.
Tests on hardness may be classified into
(i) Scratch Test (ii) Indentation Test
The scratch test consists of pressing a loaded diamond into the surface of the specimen and then pulling the diamond so as to make a scratch. The hardness number is then determined on the basis of (i) load required to make a scratch of a given width, or (ii) the width of the scratch made with a given load.
The indentation test consists of pressing a body of standard shape into the surface of the test specimen In the commonly used Brinell Hardness Test a hardened steel ball of a given diameter is squeezed into the surface of test specimen, under a fixed standard load and then surface area of the indent is measured.
The converse of hardness is known as softness.
The strength of a metal is its ability to withstand various forces to which it is subjected during a test or in service. The strength of a material enables it to resist the fracture under load. It is usually defined as tensile strength, compressive strength, proof stress, shear strength etc.
Materials of all kinds owe their strength to the cation of the forces residing in and about the molecules of the bodies but mainly to that ones of these known as cohesion.