A spring having seven coils and a coil-to-coil diameter of 0.5 in. is to be made of cold-drawn steel wire. When a tensile load of 15 lbf is applied the spring is to deflect no more than 0.60 in. The colddrawing operation will, of course, increase the shear yield strength of the wire, and it has been observed that (in ksi) depends on wire diameter d (in in.) according to ty=63/D^0.2
If the shear modulus for this steel is11 × 10^6 psi, calculate the minimum wire diameter required such that the spring will not plastically deform when subjected to the preceding load.