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If one looks up the meaning of shear strain, most sources talk about this situation:

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That is, a force acts along a surface of an element at it is elongated, turning a rectangle into a rhombus. Apparently this is called "simple shear".

But then I found out there is something called "pure shear", which is illustrated in this picture:

enter image description here

So pure shear, on the other hand, is a "flattening" of a body; the element is squeezed flatter on one axis and elongated on another. There is no change in angle between the lines, as there was in the simple shear case.

My question is: Why are these two things both called "shear strains"? What do they have in common, besides that neither change the area of the element? What is "pure" about "pure shear" and "simple" about "simple shear"?

Simple shear is caused when the force is acting along the surface of the element, without normal forces. But what about pure shear? It involves squeezing in one axis and elongating on other, so it must be caused by normal forces, right? But if this is right, then it wouldn't make sense to put this in the same category ("shear") as simple shear, as one would be caused by normal forces that act directly on the surfaces of the element, and the other by forces along the surfaces of the element.

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