Theory of failure

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Failure -

When a force is working on a object continuosly, the object will get fractured after sometime, that is known as failure. For example, if we apply 10 Newton on a body and it is not a fracture, then we said that there is no failure. But if we apply force more than hundred Newton, then the object will get fractured, then that point is known as the fracture point.

Theory of failure -

There are various theories of the failure that explains how the object get fracture or how they get excessive elastic deformation. There are various modes of failure, first one is the excessive elastic deformation, second is the only sudden fracture. If we apply the excessive force continuously, then it may be possible that the object get excessive elastic deformation or there may be fracture of the object.

There are various types of theories of the failure.

I) Rankine's theory

ii) Saint venant's theory

iii) Guest theory

iv) Haigh's theory

v) Von mises theory

.i )Rankine's Theory of failure -

According to this theory, the fracture takes place, when maximum principal stress reachs its maximum limit of its principal stress.

Rankine theory of failure

Disadvantages of Rankine's Theory of failure

There are various disadvantages of the Rankine theory. This theory illustrates only principal stress. It does not consider shear stress.

ii) Guest's Theory of failure

According to this theory, failure occurs when the maximum shear stress reaches its limits of maximum shear stress in elastic mode.

The maximum shear stress can be obtained by subtracting minimum stress from maximum stress and dividing them by two.

Disadvantages of Guest's Theory of failure

It is applicable only for the brittle object like cast iron.

iii) Saint Venant's Theory of failure

This theory explains that the failure occurs when the maximum principal strain crosses or reaches its maximum principal strain in elastic mode.

Disadvantages of Saint Venant's Theory of failure

There are some limitations of this theory. According to this theory, the tensile stress is greater than the yield stress, but in experiment, it is found that the yield stress is greater than the tensile stress. And also it over estimates the elastic deformation of the material.