The aluminum rod (E_1 = 75 GPa) is reinforced with the firmly bonded steel tube (E_2 = 209 GPa). The diameter of the aluminum rod is d = 28 mm and the outside diameter of the steel tube is D = 42 mm. The length of the composite column is L = 729 mm. A force P is applied at the top surface, distributed across both the rod and tube. Determine the normal stress sigma in the aluminum rod when P = 97 kN:
