Q. What do you mean by section modulus (Z)? What is value of bending moment in the terms of section modulus?
Sol.: Section modulus is ratio of M.I. about neutral axis which is divided by the outer most point from the neutral axis.
Z = I/ymax .
For the Circular Shaft (Z) = I/y = ( ? D4/64)/D/2 =( D3/32)
For the Hollow Shaft (Z) = I/y = {? (D4 - d4)/64}/D/2 =( ? /32)( D4 - d4)/D
For the Rectangular section (Z) = I/y = (bd3/12)/d/2 =bd2/6
Section modulus represents strength of the section of the beam.
As; M = σ .I/y = .Z
Stress at outer fiber will be highest. That is,
M = max.(I/ymax) = max.Z