In the following we will see the measurement of flexural modulus and maximum stress on the outer surface of the beam.

Flexural strength: This is the stress on the surface of the specimen at failure, which should be accompanied by the breaking of fibers, rather than inter laminar shear.

In the three point bending method the flexural modulus  is given as

(8.34)

where,    is flexural modulus,    is the support span,    is the slope of the load-deflection curve, and    are the width and thickness of the specimen, respectively.

In case of four point bending there are two options according to ASTM D790 standard. In the first option the loading span is one third of the support span. For this case the flexural modulus is given as

(8.35)

In the second option the loading span is half of the support span. The flexural modulus for this case is given as

(8.36)

where, the parameters in these two equations are as defined earlier.       
      
The maximum stress on outer surface of the beam is given below for all the cases.

(8.37)

It is important to note that the measurement of width and thickness of the beam is important for accurate measurement of flexural modulus and maximum stresses.

For more details on these tests one can refer to ASTM D790-92 and ASTM D790M-93.