Why does shearing strain increase when one angle decreases in a parallelogram?

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chetzread
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Homework Statement


In the notes , refer to the circled part . It 's stated that the shearing strain is positive when the angle formed by two surfaces oriented along positive y and x axes decreases ..

Homework Equations

The Attempt at a Solution


. As we can see from the figure( paralleogram ) , one of the angle increases , while another angle decreases , am i right ? Why the author said angle decreases ?
 

Discussion

Here it is
 
Attachments
  • Photo of a printed textbook page showing a 3D deformed element with axes x, y, z, shear stresses labelled tyx and txy, corner angles pi/2 + gamma_xy and pi/2 - gamma_xy, followed by text on shearing strain sign convention.
    IMG_20161015_102808.jpg
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chetzread said:
...angle formed by two surfaces oriented along positive y and x axes decreases ..
I think that could be stated more clearly. Obviously there are two surfaces oriented along each axis. To match the description to the diagram, try rewording it to:

"...angle formed by two surfaces closest to the origin and oriented along positive y and x axes decreases .."
 
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Tom.G said:
closest to the origin
what do you mean by it ? Can you explain further ?
 
Tom.G said:
"...closest to the origin..."

closest to the origin of the graph as shown in the image of your post #2.
-or-
the two faces closest to and facing the X-Z and Y-Z planes.
https://www.physicsforums.com/attachments/img_20161015_102808-jpg.107454/
if the downwrads plane is sheared to the right , and the upper planes sheared to the left , then the angle formed by two surfaces closest to the origin and oriented along positive y and x axes will increase , right ?.