Let's continue with our previous discussion. We learned that internal forces are something inherent in an object or system due to external forces.
Now, we'll discuss it more - specifically, the types of internal forces. To illustrate, say we have a french fry in your hand - don't eat it! Let's apply some external forces to it and investigate what happens to it.
Axial Forces
Say we apply a pulling force at the ends of the French Fry, like stretching it out. Observe that it will elongate along its length. Apply too much force, and it will eventually break at some point! We just applied a pulling external force that caused internal tension forces.
Instead of pulling it, try to push its ends toward each other. We say that the French fry is now shorter or being compressed. Apply too much force, and the object tends to lump at some point. When we applied such pushing force, we made the French fry experience internal compression force.
Both of these forces, tension, and compression, are subtypes of what we call axial forces (or normal forces). These are internal forces in which the force is perpendicular to the surface. In our example, that surface would be the cross-section of the French Fry.
Shear Forces
Now, say we have a piece of scissors and cut the food slowly. As we apply such external force, we're creating a tear in the French Fry in the same cutting plane of our scissors. Entirely cut the food, and you would have two pieces of French Fry (yay!). The other piece slides away from the original piece.
When we cut the French Fry, some resisting internal force prevents it from tearing, which we call the shear force.
If axial forces are perpendicular to the surface, shear forces are parallel. In our example, our surface was the plane where we cut it - the cutting plane of the scissors. The external force from the scissors' blades lies in the plane parallel to it.
Bending
Get another piece of French Fry, and try to bend it this time. One can do that by holding the food's ends and rotating one's wrists to bend it. This action that enables an object to rotate about a fixed point or axis is what we call a moment. In our example, the fixed points are the ends of the French Fry.
One can bend French Fries to make its shape concave, convex, or simultaneously along specific points. Apply too much moment, and it will eventually break at some point. The French Fry offers an internal force that counteracts this external action and is the bending moment.
A critical aspect of the bending moment is the reference. When we bend the French Fry, the axis of rotation of your wrists is NOT along the length of the French Fry but perpendicular to it.
Torsion
Take another piece of French fry and try to twist it at its ends as if you're wringing a towel. You're applying a rotational action called torque that causes it to twist along its length. The French Fry offers resistance to this action and is called torsion.
If the axis of rotation of bending moments is perpendicular to the plane of bending, the reference for torque is along the length of the French Fry. This reference makes it different from bending moments. These two are similar since they require a rotating moment or torque action.
Summary
There are four types of internal stresses: (1) axial, (2) shear, (3) bending moment, and (4) torsion.
Axial forces provide resistance for objects to elongate or shorten. These forces are perpendicular to the surface being investigated and can be tension (pulling force) or compression (pushing force).
Shearing forces provide resistance for objects to tear or slide at specific points. These forces are parallel to the surface.
Bending moments provide resistance for objects to bend about axes perpendicular to the plane where bending occurs.
Torsion provides resistance for objects to twist about their length.
