Derivation of the strain energy release rate G from first principles for the pressurized blister test

The strain energy release rate, G, is derived from first principles and is shown to be consistent with the exact numerical solution by Cotterell and Chen (Int. J. Fract. 86 (1997) 191). Comparison with the classical Rayleigh-Ritz energy method and other published methods shows that the gradual change in blister profile over the entire loading process must be considered for the correct G to be calculated.