A thin cylindrical shell of mean diameter 750 mm and wall thickness 10 mm has its ends rigidly closed by flat steel plates. The shell is subjected to internal fluid pressure of 10 N/nm2 and an axial external pressure p1. If the longitudinal stress in the shell is to be zero, what should be the approximate value of p1?
(a) 8N/nm2 (b) 9N/nm2 (c) 10N/nm2 (d) 12N/nm2