Why is the direction of shear stress important in pure shear stress?

[chetzread]

Homework Statement

In my book, I was told that when an element is subjected to pure shear stress, equalibrium required that equal shear stress must be developed on four sides of the element...these stresses must be direct toward or away from opposite corners of element..

Homework Equations

The Attempt at a Solution

i think that is wrong... Why must them direct toward or away from opposite corners of element..?
there's no reason why they must be direct toward or away from opposite corners of element..

 

[PhanthomJay]

From equilibrium considerations, which way would you have them point?

 

[chetzread]

From equilibrium considerations, which way would you have them point?

According to the book, the shear stress should have pointed this way(above), IMO, they can also point as this way (below) , which means it can be anticlockwise or clockwise direction

 

[PhanthomJay]

The above is the convention for positive shear. Reversing it is negative shear.

 

[chetzread]

The above is the convention for positive shear. Reversing it is negative shear.

do you mean when the orientation is clockwise or anticlockwise . they are considered as negative shear?

 

[Nidum]

No .

The shears have to be in pairs pointing to opposite corners of the square to maintain equilibrium . If they are all going clockwise or all anticlockiwise then they would generate a torque which would rotate the square about it's centre .

 

[chetzread]

No .

The shears have to be in pairs pointing to opposite corners of the square to maintain equilibrium . If they are all going clockwise or all anticlockiwise then they would generate a torque which would rotate the square about it's centre .

that's weird . I have an example here , stating that shear stress in clockwise direction is positive ...

 

[Nidum]

https://en.wikipedia.org/wiki/Pure_shear

 

[chetzread]

https://en.wikipedia.org/wiki/Pure_shear

So, the notes above is wrong?

 

[Nidum]

The notes are correct but all they do is tell you the direction conventionally taken as positive for shear stress .

Perhaps you have not understood that pure shear stress that you have asked about is a special case ? Note word irrotational in the above Wikipedia article .

 

[chetzread]

The notes are correct but all they do is tell you the direction conventionally taken as positive for shear stress .

Perhaps you have not understood that pure shear stress that you have asked about is a special case ? Note word irrotational in the above Wikipedia article .

Yes, but the case that I attached earlier inpost ＃7 is rotational , right? How could that be?