Basic Mechanical Properties of Engineering Materials

Introduction to Mechanical Properties

Materials are defined by their qualities. They could be hard, ductile, or hefty. Alternatively, they can be soft, fragile, or lightweight. Mechanical properties define how materials behave under external forces. They often refer to the elastic and plastic properties of a material. Material selection for structural machine components relies heavily on mechanical qualities.

The important properties from design point of view are:
(a) Elasticity
(b) Plasticity
(c) Hardness
(d) Ductility
(e) Malleability
(f) Brittleness
(g) Resilience
(h) Toughness
(i) Creep

(a) Elasticity

This is the property of a material to regain its original shape after deformation when the external forces are removed. All materials are plastic to some extent, but the degree varies, for example, both mild steel and rubber are elastic materials, but steel is more elastic than rubber.

Engineering metals are elastic, although their degree of elasticity varies. Steel has ideal elastic properties within a particular limit. Metals can only experience minimal elastic deformation. Elastic deformation causes displacement of metal atoms from their original locations, but not to new places. When an external force is removed, the metal’s atoms return to their normal places, restoring the metal’s shape.

(b) Plasticity

This is associated with the permanent deformation of material when the stress level exceeds the yield point Under
plastic conditions materials ideally deform without any increase in stress.

A typical stress strain diagram for an elastic perfectly plastic material is shown in the figure1. Mises Henky criterion gives a good starting point for plasticity analysis. The criterion is given as where σ 1 σ 2, σ 3 and σy are the three principal stresses at a point for any given loading and the stress at the tensile yield point, respectively.

A typical example of plastic flow is the indentation test where a spherical ball is pressed in a semi infinite body where 2a is the indentation  diameter. In a simplified  model we may write that ifplastic flow occurs, where,Pm is the flow pressure This is also shown in figure.

(c) Hardness

Property of the material that enables it to resist permanent deformation, penetration, indentation etc. Size of indentations by various types of indenters are the measure of hardness e.g. Brinnel hardness test, Rockwell hardness test, Vickers hardness (diamond pyramid) test These tests give hardness numbers which are related to yield pressure MPa

External forces distort the metal, preventing it from entirely recovering its original proportions. Plastic deformation causes metal atoms to be permanently displaced and take up new locations. Metals can be severely deformed in the plastic range without fracturing, making them ideal engineering materials. Low carbon steels have substantial plastic deformability, making it possible to stamp automobile pieces such as the body, hood, and doors without fracture.

(d) Ductility

This is the property of the material that enables it to be drawn out or elongated to an appreciable extent before rupture occurs. The percentage elongation or percentage reduction in area before rupture of a test specimen is the measure of ductility Normally if percentage elongation exceeds 15 the material is ductile and if it is less than 5 the material is brittle Lead, copper, aluminium mild steel are typical ductile materials.

In other words Ductility refers to the permanent strain caused by fracture during a tension test. Ductile materials stretch significantly before breakage during tension testing. Mild steel, copper, and aluminum are ductile materials. Ductile metals can be molded, pulled, or bent by applying tension to shape them. Machine components with ductility are ideal for handling unexpected overloads or impact loads. Ductility is assessed as % elongation or reduction in area after a tension test. Metal’s ductility reduces with rising temperature due to its weakening.
While all ductile materials are malleable, the opposite is not always true.

(e) Malleability

It is a special case of ductility where it can be rolled into thin sheets but it is not necessary to be so strong
Lead,
Soft
steel,
Wrought
iron,
Copper
and
Aluminium
are some materials in order of diminishing malleability.

The term ‘malleability’ is derived from the word ‘hammer’ and refers to the ability to be hammered into thin portions. Malleable metals can be rolled, forged, or extruded using compressive force. Low carbon steels, copper, and aluminum are malleable metals. Malleability typically rises with temperature. Forging and rolling involve shaping hot ingots or slabs.

(f) Brittleness

This is opposite to ductility Brittle  materials show little deformation before fracture and failure occur suddenly without any warning Normally if the elongation is less than 5 the material is considered to be brittle e.g. cast iron, glass, ceramics are typical brittle materials.

(g) Resilience

This is the property of the material that enables it to resist shock and impact by storing energy The measure of resilience is the strain energy absorbed per unit volume. For a rod of length L subjected to tensile load P, a linear load
deflection plot is shown in figure

(h) Toughness

This is the property which enables a material to be twisted, bent or stretched under impact load or high stress before rupture. It may be considered to be the ability of the material to absorb energy in the plastic zone. The measure of toughness is the amount of energy absorbed after being stressed upto the point of fracture.

(i) Creep

When a member is subjected to a constant load over a long period of time it undergoes a slow permanent deformation, and this is termed as “ Creep”. This is dependent on temperature.

References

• ABDULLA SHARIF, Design of Machine Elements, Dhanpat Rai Publications (P) Ltd, New Delhi, 1995
• V. B. Bhandari, Design of Machine Elements, Third Ed., The McGraw-Hills Companies, New Delhi
• R. S. KHURMI and J.K. GUPTA, A Text-Book of Machine Design, S.Chand and company ltd., New Delhi, 2000.
• Design of Machine Elements https://nptel.ac.in/courses/112/105/112105125/

Q & A

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