Why do compressed materials fail in shear?

[phyzzy_physh]

Why when you compress materials like concrete do they fail in shear?

Can the thing you're compressing be too small to fail in this way?

 

[Studiot]

Direct stresses (tension or compression) induce shear stresses on any plane they are not exactly perpendicular to. These shear stresses vary from zero on a plane at right angles to the compressive stress to a maximum on a plane at 45° to the applied compression.

The ratio of the strength of concrete in shear to the strength in compression is such that the concrete will fail in shear along these planes before it fails in compression.

This is the simple answer for one dimensional applied stress.