Mechanical properties of metals

What are different Mechanical properties of metals?

Following are the Mechanical Properties of Metals

Brittleness

It is the property of breaking without not too much permanent distortion. It may be due to the fragility of the grain boundaries or the crystals themselves. Cast iron is brittle because its structure is broken up by chips of graphite, which is a brittle material. Frailty is often referred to as brevity. Hot, or lack of red in steel, is when it is brittle in the red-hot state.

It is caused by excess sulfur which is present as iron sulphide and forms a brittle membrane around the steel crystals. The lack of cold means that the metal is brittle when cold. In steel, the cold fault is produced when the phosphorus content is too high.

Ductility

A metal is ductile when it can be stretched in tension without breaking. Wire drawing depends on ductility for its successful operation. A ductile metal must be both strong as well as plastic, e.g. the lead wire is difficult to stretch because the resistance of the lead is low.

Elasticity

A metal’s elasticity is its power to return to its original shape after deformation by force. The material can be stretched, compressed or its volume changed by pressure on all sides (eg immersion in a liquid). Many materials behave to some extent like powerful elastics and, within limits, regain their shape when the load on them is removed.

The elastic limit is the elastic limit of a material and is expressed in newtons per unit area (mm2, cm2 or m2). For example, if the elastic limit of the material were 23 kilonewtons per square centimetre, then with a bar of material 1 square centimetre in area, the material would return to its original length from a load of 23 kilonewtons. If this loading intensity were exceeded, the bar would assume a permanent stretch, often called a permanent set.

Elongation

When a material is pulled through a testing machine for the purpose of finding its tensile strength, stretching occurs before the bar fractures. Elongation is the amount of that stretch and is usually expressed as a percentage of the original length.

Hardness

The hardness of a metal is a measure of its ability to resist scratches, wear and abrasion, indentation by harder bodies, marking by a file, etc. Machinability and cutability are also important hardness properties in the shop. A rough but often reliable test of the hardness of a hardened tool is to see if the edge of a thin file will touch it.

The Brinell hardness of a metal is found by pressing a ball onto the surface of the metal, the hardness number is found by dividing the charge on the ball by the surface area of the print. To test the steel, the ball is 10 mm in diameter and the load is 3000 kilograms. Brinell machines are usually supplied with a chart that gives the hardness number when the diameter of the ball print is known.

Malleability

This is the property of permanently extending in all directions without breaking by pressing, hammering, rolling, etc. It requires the metal to be plastic, but it doesn’t depend as much on strength, for example. Lead is a very malleable metal.

Plasticity

This is a property very similar to malleability and involves permanent deformation without rupture. It is the opposite extreme of elasticity, as can be demonstrated by comparing the behaviour of a piece of elastic rubber and a piece of plasticine under a deformation force.

Plasticity is necessary for forging, and metals can be made plastic by heating, for example. steel is plastic when in a bright red heat.

Strength

The strength of a metal is its ability to resist the application of force without breaking. In service, a material may have to withstand tensile, compressive, or shear forces. The strength of a material is measured by loading it into a testing machine.

The ultimate strength is the load required to fracture the unit area (mm2, cm2 or m2) of a metal cross-section. Tenacity is the maximum strength in tension. Maximum strength and toughness are always expressed in newtons per unit area (usually in newtons per square millimetre).

Toughness

It is the amount of energy a material can absorb before it fractures. A measure of a metal’s toughness can be obtained by cutting it, placing it in a vice, and tapping the end with a hammer. Certain woods are very strong, which is why walnut is a good material for a sledgehammer.