When a spring twists, the material is in a state of tension or shear, as discussed in the torsional calculation. Therefore, the limitation on the torsional angle is ultimately caused by material failure; when a material undergoes plastic deformation, it cannot return to its original state.
Therefore, it ultimately comes down to the material's tensile strength and shear strength. The Von Mises stress calculated using the fourth strength theory represents the maximum strength the material can withstand. Combining this with the spring's length, the maximum permissible torsional angle can then be calculated.
