Anyone know the Garwood method which is used to determine the fracture energy of concrete (composite materials)?

Garwood method: with a load - load line displacement curve of an three point bending fracture test (or SENB), we have firstly:

J0=2U0/B(H-a0)

After, an iterative relation is used:

Jn=Jn-1(H-an)/(H-an-1)+2Un/B(H-an-1)

B,H : width and height of the beam respectively.

a : crack length 

In fact, it's not clear the determination of the initial value Jo. 

Please tell me references for this method.

Hi everyone,

Anyone can tell me how to determine the fracture energy for crack propagation in a 4 points bending notched beam (mode I)?

In fact, in a 4 points bending, there are different displacements: displacement mesured at the upper supports (u), displacement mesured at the midle of the beam (deflection of the beam, f), crack opening displacement (COD), and others ... Among these first two displacements, it's u which corresponds to the mesured force. However, this one includes also bias effect (punching effect = "effet de poiçonnement" in french). So my question is which one of these two displacements must be used to calculate fracture energy?