Flexural Stress for Beams Tested at Large Support

Spans (s f)—

If support span-to-depth ratios greater than 16 to

1 are used such that deflections in excess of 10 % of the

support span occur, the stress in the outer surface of the

specimen for a simple beam can be reasonably approximated

with the following equation 

stress=(3PL/2bd^2) [1+6 (D/L)^2-4(d/L)(D/L)]

if your case is short beam (span-to-depth) less than 16 to 1

you can use the nominal equation drived in strength of materials books

stress= 3PL/2bd^2

P=load

L=support span

b= width of tested beam

d=depth of tested beam

D = deflection of the centerline of the specimen at the

middle of the support span,