Theory of Structures - Engineering

Q1:

A steel bar 20 mm in diameter simply-supported at its ends over a total span of 40 cm carries a load at its centre. If the maximum stress induced in the bar is limited to N/mm2, the bending strain energy stored in the bar, is

A 411 N mm

B 511 N mm

C 611 Nmm

D 711 N mm

ANS:C - 611 Nmm

Does anyone know the Formula of bending strain energy? Please explain.