Consider a close coiled helical spring which is acted upon by a torque about its longitudinal axis as shown. I'm interested in knowing the effect of this torque on the spring wire.
Here's my take: I made an imaginary cut via a plane (shown in red) to expose the cross section of the wire as below.
The upper part of the spring (unshaded) has to remain in equilibrium, hence the torque from the support (ceiling) about the longitudinal axis has to be balanced by another torque developed about the longitudinal axis. This would happen if on the cross section internal resistive forces are developed in such a way that they cancel out producing no net force but producing a net moment about the point A. This moment of the internal resistive forces developed on the c/s about A, will balance the torque T from the upper end and bring the upper part of the spring in equilibrium.
In effect of the arguments above, the spring wire should experience bending. I do know that the correct answer to this question is bending, but is my reasoning correct?