Calculation of Strain Energy Release Rate from Load — Displacement — Crack Area Equation

For a certain experiment, Gurney and Ngan (Proc. Roy Soc. Lond., Vol. A325, p. 207, 1971) expressed the load-displacement-crack area relation in the form $$ P = \sum\limits_m {\frac{{{C_m}{u^m}}}{{{A^{\left( {m + 1} \right)/2}}}}} $$ (1) where Cm (m = 1, 2, 3, ...) are constants.