the internal force is bending

Yes,torsion(Twist) is an internal force,along with tension(stretch),shear(cut,bend),compression(squeeze).An internal force acts between two parts of something.

A baseball is a great example of an internal force. The particles holding the baseball together is an internal force. But a baseball bat hitting the baseball is not an internal force, because an internal force is a force exerted by one part of a structure on another. The bat hitting the ball is an external force.

Another example is a car. The pistons pushing the rods, the axle pushing the wheels, the wheels moving the car...etc. All these things are internal forces working on the car because the these things are all inside the car. But the friction created by the wheels of the car on the ground is an external force, as is the force holding the car up. Hope this helps!

A force internal to the system under consideration. Whether a force is regarded as internal or external depends on how the system is defined and classified, and is largely a matter of convenience. In biomechanics, the human body is generally regarded as the system and any force exerted by one part of the body on another is an internal force. Muscle contractions produce internal forces, which move a joint. Compare http://www.answers.com/topic/external-force.

external force and internal force