Now for Pure Bending. A couple of notes first:
- Bending never occurs without shear, they are just intertwined.
- Most structures will be experiencing a combination of the 3 forces at once.
Pure bending is simple when an element is experiencing bending ONLY (with it's associated shear) without any of the other 2 forces. The reason I'm excluding shear here is that bending causes internal shear forces by default, but in pure bending, there is no external force causing shear of it's own.
So breaking a pencil by twisting both ends upwards is pure bending, but breaking it by supporting it over 2 books and pressing down the middle induces shear, thus, NOT pure bending.