Key Concepts of Mechanical Properties of Solids Explained

Mechanical properties of solids encompass what has been described as the ability of solids to resist deformation and strength. Strength is defined as the ultimate limit to which an object can carry a load, to what extent it can bear the load. Resistance to deformation originates from the ability of any object to withstand change of shape. This means that if the object offers a low resistance to deformation, then it easily changes shape and vice-versa.

Solids have various mechanical properties. They are:

• Elasticity: If we pull an object, its size alters and when we leave it relaxes back to its original size. Or we can say it is the ability to return to its former shape upon the removal of an external force. Example: Spring
• Plasticity: If an object undergoes a change of shape and does not regain its original shape even if the forces which deformed it are withdrawn. It is also known as the property of permanent deformation. Example: Plastic materials.
• Ductility: A material possesses the ductile properties if it can be pulled in thin sheets, wires or plates. The property that is drawn into thin wires/sheets/plates is referred to as ductility. Example: Gold or Silver
• Strength: The capability to hold up under the actual application of force or other load without fracturing or breaking. Strength values differ quite significantly among the many categories of the objects.

What do you understand by deforming force?

A deforming force is a force used to deform an object, meaning a force that alters the dimensions of the object either in size or in its shape. When an external force is applied to an object it may lead to deformation and it means any change in the form and shape of the body. This force can in turn be pull force, or push force depending on the type of force and its direction of working e.g. an object can be stretched, compressed, or bent.
Distorting forces which are generated by such influences as pressure, tension, or an impact are often fundamental in commanding the physical behavior of a material. It is widely applied in engineering, physics, and material science in order to make prediction and analysis of the deforming force acting upon an object within various conditions.

What do you understand by the term stress?

Stress refers to the force of restoration per unit of area of the cross-section of an object under deformation. It is also defined as the force put on by divided by area. It is the restoring force that comes into existence in an object; it is measured per unit area. For instance, when a rubber ball is subjected to an external force through the hands to squash it simultaneously, the ball generates an opposite force capable of restoring it. This restoring force created by the object or ball is referred to as stress.

Stress = Force/Area 
The S.I. unit of Stress is N/m square or Pascal (Pa)

What are the different types of stress?

There are 3 different types of stress:

1. Longitudinal Stress- This stress acts normal to the cross-section of the object. Longitude means lengthwise; hence, it can be defined as the force restoring force per unit area given if the force applied is perpendicularly along the cross-sectional area of the cylindrical body. It is responsible for causing either tensile stress or compressional stress.
 Longitudinal stress is further subdivided into two subcategories-

• Tensile stress- Tensile stress occurs when force is applied to elongate the cylinder.
• Compressive stress- When force is applied on the object in such a way that the object gets compressed.
   

2. Tangential or Shearing Stress- Responsible for causing a change in shape by acting tangentially to the surface of the body. Shearing or tangential stress is the restoring force per unit area when the force applied is parallel to the sectional area of the body. This is a sort of relative displacement that takes place between two diametrically opposite faces of the body.

3. Hydraulic Stress- Outcomes derived from a liquid that exerts pressure from all directions. In other words, it is the restoring force per unit area that comes up when the force is in the fluid form powerful enough to act on the body or an object.

What do you understand by the term “strain?"

Strain is defined as the change of dimension of an object per unit of its dimension under the influence of a deforming force. This is a measure of the deformation that can give a displacement between particles in the body with respect to a reference length. 
 

Strain is the measure of dimensionless quantity. 

What are the different types of strain?
 

There are 3 different types of strain. They are:

1. Longitudinal Strain- This is defined as the alteration on the original length of the body or; how much the length of an object has been extended to its original length due to the impact of the longitudinal stress.

2. Volumetric Strain-  It is defined as the change in the original volume. Due to hydraulic stress, it is defined as the ratio of change in volume to the original volume of the body.

3. Shearing Strain- It is defined as the tangential applied force by the area of force. Shearing strain can be expressed simply as tan Θ.

Describe Hooke’s law

It was named after Robert Hooke, a scientist. Stress developed is directly proportional to the strain produced in an object, provided the object is made up of an elastic material and below the elastic limit (according to Hooke's Law).
The ability of an object to regain its original shape once deformed is defined as elasticity. Therefore, Hooke’s law should apply to elastic objects. However it cannot be applied in the context of the plasticity property of solids. 
Hooke’s law can be represented as Stress = k * Strain, where k is the modulus of elasticity

What do you understand about the limit of elasticity?

The limit of elasticity is meant to describe the largest amount of force that may be placed on a material so that once that force has been removed the material may resume its original shape. In other words, if the stress applied is beyond this value the material will experience a permanent deformation.

Define Young’s Modulus of Rigidity

Young’s modulus of rigidity is represented by the symbol Y, which stands for the stiffness of a material. It measures the degree or level of stress to strain in a material, which is tested under tensile and or compressive forces along its length.
Young’s modulus is a measure of the material resistance to deformation that is due to tensile or compressive stress. 
A material which has a high value of Young’s moduli is regarded as stiff; it requires more force to be deformed while a material with lower value of Young's modulus is flexible and deforms easily under force.

FAQS:
1. What are the different types of strain? 

The 3 different types of strain are: Longitudinal strain, Shearing strain, and Volumetric strain. 

2. What do you mean by the Elastic Modulus (K)? 

Elastic Modulus is defined as the ability of a material to resist the force of deformation under stress. Generally, different materials have different elastic moduli. 

3. What is the stress-strain curve? 

Stress-Strain curve describes the curves that are drawn between stress and strain. A linear graph is formed when stress is drawn along the y-axis and strain along the x-axis. This curve gives a detailed idea about the elastic and plastic behavior of a material.