Physical reason behind Plane Stress

[FAlonso]

I can now although do the maths related to plane stress etc but I have struggled to grasp the logic behind this concept. It says changing the way you see the stress by rotating the view, stress changes. My question is why ? Why is this so that we see stress with one value when standing at let's say 0 degrees with the axis of pipe under stress and different when we see it from 45 degrees rotated ? Is it some inherent material property or the force isn't really acting the way as we think and show in free body diagrams ? Can someone put it up simple ?

 

[rsk2mc]

It's a function of mohrs circle. Plot the mohrs circle.you will see at different angles on the circle there is different stresses. In a real scenario if you put.steel in a tensile machine and pull it , it will shear at 45 degrees, this is because its maximum shear angle is 45 degrees