By the end of this tutorial, you should have an understanding of:
In materials such as metals, permanent (or plastic) deformation occurs at stresses much less than the ideal strength, through the movement of dislocations. The dislocations cause the material to deform by the process of SHEAR.
Yield occurs by shear stress because tensile and compressive stresses create shear stresses. The shear stress is at a maximum at 45 degrees to the axis of tension or compression.