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Introduction:
Rigid bodies are present everywhere around us. Almost all the physical bodies that we can observe are to a great extent rigid in nature. Understanding the properties of such rigid bodies has great application in the analysis of mechanical systems and other related concepts of physics.
In every rigid body, the forces applied cause stress and strain to develop. Stress is defined as any force that is applied to a rigid body per unit area. Stress may or may not cause deformation in the shape of the rigid body. The effect that is viewed on a rigid body after a certain amount of stress is applied is called strain. An important aspect of stress is known as breaking stress. Breaking stress is one that, after a certain threshold value, can cause some kind of special strain in the body.
What is Stress?
Stress, as mentioned earlier, is a force that is applied to the unit area of a rigid body that can induce a certain effect on the body. Stress is defined as the force that is applied per unit area. Stress causes deformation whenever applied to a rigid body. There are different types of stress that cause different types of deformation.
If the stress that is being applied causes the elongation of an object, it is called tensile stress. Tensile stress causes deformation by increasing the length of the rigid body. If the stress that is being applied causes the body to decrease in its length, it is called compressive stress. Compressive stress is only applied on one side to decrease the length.
When the stress that is being applied from all directions causes the body to lose its shape due to compression from all sides, it is called volume stress, or bulk stress. When the stress force that is applied to a rigid body is tangential to the surface of the body, it is called shear stress.
What is Strain?
Strain is defined as the effect that is observed on a rigid body when stress is applied to it. Strain is characterised by some sort of deformation in the rigid body. It is also defined as the quantity that measures the deformation caused by stress.
Strain is mathematically given as the fractional change that is observed in either the length, volume, or geometry of the rigid body. Similar to stress, strain is also divided into different types. The strain that is observed when tensile stress is applied, that is, the body elongates, is called tensile strain.
If deformation is in a manner such that the volume of the rigid body changes, then it is called volume strain. In other words, volume strain is the effect that is observed when a rigid body is subjected to volume stress or bulk stress. For example, a vacuum flask when subjected to high atmospheric pressure gets compressed. This causes the volume that the flask can store to change. The fractional change in volume that is observed due to volume stress is called volume strain.
When a rigid body is subjected to shear stress, then the body undergoes a deformation that is known as shear strain. Shear stress changes the geometry of the object and, therefore, shear strain measures the fractional change in the geometry of the rigid body.
What is breaking stress?
No rigid body can undergo an infinite amount of stress. The stress that is applied to a body is limited by the breaking stress of the body. Breaking stress is defined as the maximum stress that can be applied to a body that it can withstand without enduring permanent deformation, such as breaking.
Every rigid body has some elastic properties that allow it to regain its shape after being subjected to stress. But breaking stress gives the final value of stress, after which a rigid body loses all its elastic properties and undergoes permanent deformation. Breaking stress can also be expressed as the minimum amount of stress that will be required to split a material apart.
For example, let us consider that we have a metal cable that can withstand 2,000 pounds of stress and is used in a bridge. The cross-section of this metal cable is 2 square inches. The metal cable is a rigid body, but it can withstand stress only up to 2,000 pounds. Any stress that is within these limits will elongate the cable or cause some strain, but not permanently deform it.
Now, the breaking stress of the metal cable is calculated by dividing the stress capacity by the cross-section area. This leads to 1,000 pounds per square inch. Therefore, if the applied stress is greater than 1,000 pounds per square inch, the metal cable will lose its elastic property of regaining its shape and break into two pieces.
Breaking stress when applied results in two types of tensile failures.
- Ductile Failure: The section of the rigid body that undergoes ductile failure is usually substantially distorted and deformed. The cross-section of the deformed section constantly reduces until the section finally breaks into two.
- Brittle Failure: Brittle failure is caused when the breaking stress causes the rigid body to break into two or more pieces in a sudden manner. But the difference between ductile and brittle failure is that brittle failure can occur at a low-stress state.
Conclusion
Stress is described as any force that is applied to a rigid body per unit area. Every rigid body that is subjected to stress undergoes some form of deformation. The measure of this deformation is called strain. Both stress and strain are differentiated based on the different methods in which they are applied.
Breaking stress is described as the final amount of stress that will result in causing a tensile failure in the body. It can also be defined as the stress that causes a rigid body to lose its elastic property and result in a permanent deformation in the rigid body. The knowledge of breaking stress is important while building a structure,bridge etc.
