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## Introduction

The strength of material may broadly be defined as that branch of Engineering-Science which deals with the ability of various types of materials to resist its failure and their behaviour under the action of the forces.

## Concept of Rigid and Non-Rigid Bodies

All physical bodies, such as engineering structures and machine part, can be classified either as rigid bodies or non-rigid bodies.

1. Rigid Bodies:- A body is said to be a rigid body if there is no deformation occurs in the body. Like Piston, Connecting Rod, Engine Cylinder, etc.
2. Non-Rigid Bodies: A body is said to be a non-rigid body if there is some deformation may occurs in the body. Like. Plastic, Wood, Paper, etc.

Load may be defined as any force, which is applied on a physical body, and which may causes some amount of deformation in the body. The force that applied on a the body may be in the form of dead weight, friction force or an internal force generated within the body due to heating or cooling of the body

1. a) Tensile Force
2. b) Compressive Force
3. c) Shear Force
4. d) Torsion Force
5. e) Bending Force

The following points are worth to be noted:-

1. 1. A tensile force always acts along the longitude of the specimen and brings about an increase in the length of the specimen and a decrease in the cross-section area of the specimen.
2. 2. A compressive force always acts along the longitude of the specimen and brings about a decrease in the length of the specimen and an increase in the cross-section area of the specimen.
3. 3. A shear force act along the surface of the specimen.
4. 4. A torsion force acts about the axis of the specimen and brings about a resultant twist in the specimen.
5. 5. Bending force brings about a resultant sag in the specimen, about its transverse sections.

Note: Forces generated due to heating or cooling of the specimen can either be tensile or compressive in nature, depending upon the type of supports associated with the specimen.

### Elastic and Plastic Bodies

All physical bodies can also be classified as elastic or plastic bodies. Consider a body, upon which a certain load is being applied. Either of the following two cases can occur when the load is removed from the body.

1. i) The body regains its original shape, after removal of the load. In this case the body will be termed as elastic body. Hence a perfectly elastic body is defined as a body which does not undergo any permanent deformation upon removal of the load on the body.
2. ii) The body undergoes some amount of permanent deformation, after removal of the load. In this case, the body will be termed as plastic body. Hence a plastic body is defined as a body which undergoes a permanent set or deformation, after the removal of the load on the body

Elastic bodies, when loaded beyond the certain limit, commonly known as the elastic limit, loses its elasticity and became plastic in nature. I can be said that a permanent deformation will occur in an elastic body, when it is loaded beyond the elastic limit. The elastic limit of the body depends upon the material of the body.

### Ductile and Brittle Materials

Mild steel and cast iron are two important forms of iron from an engineering analysis point of view. While mild steel is a ductile material, cast iron is a brittle material.

1. Ductility:- Ductility is the property of a material by virtue of which material can be elongated or drawn out into thin wire.
2. Brittleness: Brittleness can be termed as the lack of ductility of the material. Materials that show negligible deformation on loading, generally come under the category of brittle materials. The compressive strength of material plays a very important role in case of brittle materials.

Some Other Important Mechanical Properties:-

1. (i) Malleability:- It is the property by virtue of which the material can be drawn into thin sheets without rupture. The malleability of material generally increases with the rise in temperature.
2. (ii) Toughness (or Tenacity):- It is that property of which by virtue of which the material can be twisted, bent or stretced easily under the repective forms of loading.
3. (iii) Hardness:- It is that property of a material by virtue of which it can resist indentation, abrasion, machining or scratching. Hardness of any material can be measured by Rockwell Hardness or Brinell Hardness. The hardness of any material generally decreases with the rise in temperature.

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