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A solid body has a definite shape and size, and we need to apply force on the body if we want to change its shape. The direction and magnitude of force applied to the body will develop stress – different types of strain act on the body depending upon the stress that is induced. The strain produced in the body leads to the change in the shape of the body. Stress and strain are interrelated, and according to Hooke’s law, the amount of stress-induced in the body is directly proportional to the strain produced by it. Thus the higher the stress-induced, the greater will be the strain produced in the body.
Elasticity and Plasticity
When no force is applied to the body, it maintains its original shape and size. For example- when no force is applied to a helical spring, it maintains its original shape and size. But when the ends of the spring are pulled, the length of the spring increases and when the ends of the springs are released from the pull action, it regains their original shape. This tendency of the spring to regain its original shape on the removal of force is called elasticity, and the deformation produced due to the application of the force is called elastic deformation.
Plastic behaviour is the opposite of elastic behaviour. To understand plasticity, you can take the example of mud. Mud doesn’t have the tendency to regain its original shape. When a force is applied to mud, the mud gets permanently deformed. Thus, it cannot get back to its original shape and hence it achieves a plastic nature. This tendency to not achieve the original shape again is called plasticity.
The elastic nature of materials plays a vital role in design engineering. The knowledge of elastic behaviour of materials helps the engineer construct a structure with suitable materials that withstand the forces acting on them and have elasticity up to a certain limit.
Elastic Behaviour of solids
We are aware that the atoms and molecules are closely packed against each other in solids. These atoms and molecules are bonded together with the help of interatomic forces. They maintain an equilibrium position until an external force is applied that causes the molecules to get disturbed. To understand the elastic behaviour of solids, you can consider a system consisting of a spring-ball mechanism. In this system, the balls represent the molecules or atoms, and the spring represents the interatomic forces.
When you try to remove one ball from the system, the spring tries to get it back into its original position. This behaviour of getting back to the original shape is called elasticity.
Between 1635 and 1703 AD, English Physicist Robert Hooke identified this phenomenon in the materials and concluded that the elongation produced in a body is directly proportional to the force or load applied to it. In 1676 Robert Hooke presented this law to the people, and it was named Hooke’s law.
Types of Strain
Strain is defined as the ratio of change in dimension to the original dimension when a force is applied to the body. There are different types of strain, such as longitudinal or normal strain, lateral strain, shear strain and volumetric strain.
Longitudinal Strain
When tensile or compressive stress is induced on a body, the length of the body changes. Thus the change in the length of the body to the original length is known as longitudinal or normal strain.
Lateral Strain
When compressive or tensile stress is induced on a body, the cross-section area of the body changes due to the application of compressive or tensile forces, respectively. Thus, lateral strain is the ratio of the change in the lateral dimension, such as the diameter, to the original dimension.
Shear Strain
When a tangential force is applied to the surface of the body, the surface moves away from its original position. Thus, shear strain is defined as the ratio of the perpendicular distance between the object’s normal and surface to the actual length of the object. Shear strain gives the angular displacement of the object from the original position.
Volumetric Strain
When pressure is applied to the body, hydraulic stresses are produced in it. Due to the development of hydraulic stresses, a strain is produced in the body. This strain is defined as the ratio of change in volume of the object to the original volume and is called the volumetric strain.
The different types of strain don’t have any dimension because they are the ratio of similar quantities.
Conclusion
Strain is a concept closely related to stress which causes changes in the shape and size of objects. Any solid object has a specific shape and size, which remain unchanged till some external force is applied. The external force applied to the solid body is called the stress, and the strain which is produced due to this stress changes the shape or size of the object. The different types of strain are longitudinal, lateral, shear and volumetric. The concept of strain is in turn related to the concepts of elasticity and plasticity – elasticity is a characteristic which allows a body to regain its original shape and size once the stress is removed, while plasticity is a trait which does not allow the object to get back to its original shape and size.
