π **Write short notes on –
1. Principal plane
2. Principal stress**

At any point within a strained material, no matter how complex the state of stress may be, there are alWays three planes, mutually perpendicular to each other along which resultant stress is a normal stress and no shear stress. These three mutually perpendicular p anes which carry no shear stress r are known as **principal planes** and the resultant normal stresses corresponding t to these planes are known as **principal stresses. **

In a two-dimensional stress system one of the principal stresses is zero I! and out of the other twoone is the maximum and the other is minimum. The i plane carrying the maximum normal stress is called the major principal plane it and the corresponding stress the major principal stress. Similarly, the plane carrying the minimum normal stress is called **minor principal plane **

**πDefine the term obliquity. **

When any oblique plane of a body is subjected to a normal stress and a tangential stress , then the angle that the line of action of the resultant stress makes with the normal to the plane is called **obliquity or angle of obliquity **

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