Tensile testing curve and fracture toughness relation

Hello everyone,
I am studying fracture of polymer composites.
I had a question I want to relate fracture toughness of composite to the composite stress-strain curve found from simple tensin test(on samples with no notch or pre-existing crack), can anyone give me a hint , how to do it?
I noticed in some of my samples (neat epoxy and UD glass epoxy),if the value of strain energy density(KJ/m3) is multiplied by the crack length found from the following relation, the result is close to the value of strain energy release rate (KJ/m^2) found from fracture tes (using CTS sample).
if a=KIc^2/((sut/2)*pi))  & SED=area under stress-strain curve
where sut is failure stress of the material
then SED*a=GIc (approximately equal)
I am not sure if the above equation is correct or not?
If I calculate SED(critical) from KIc using Sih's SED paper, can I relate it the SED I get from stress-strain curve?
Thank you,
Jamal