ABOUT SELECTION AND USE OF THEORIES OF FAILURE

WHAT IS SELECTION AND USE OF THEORIES OF FAILURE?

• The commonly used three theories of failure are :

1. Maximum principal stress theory;
2. Maximum shear stress theory and ;
3. Distortion energy theory.

• The rectangle shows a safe zone for a maximum principal stress theory, the hexagon shows a safe zone for maximum shear stress theory and the ellipse shows a safe zone for distortion energy theory.

* Selection of Theories of Failure for Design of Given Mechanical Component :

• The following points are considered while selecting the theory of failure for the design of a given mechanical component :

1. For the design of a mechanical component made of brittle materials, the maximum principal stress theory is used.
2. For the design of a mechanical component made of ductile material, maximum shear stress theory, and distortion energy theory are used.
3. The hexagon of the maximum shear stress theory is inside the ellipse of distortion energy theory. Hence, for ductile materials, the use of maximum shear stress theory results in design on the safer side.
4. For ductile materials, the use of distortion energy theory results in a more accurate design.
5. For ductile materials, the choice of the theory of failure depends upon the objective of the design. If there are uncertainties in the calculation of loads and the designer wanted to be on the safer side, the maximum shear stress theory is used. However, if the load estimation is fairly correct and the designer wants the design to be fairly accurate, the distortion energy theory is used.