The strain energy release rate (SERR) is widely used to study delamination growth in composites and adhesively bonded structures. Both the maximum SERR (Gmax) and the range of SERR (ΔG) are commonly used to characterize delamination growth rate. The present paper discusses the appropriateness of using the SERR range to characterize delamination growth in fibre reinforced plastic (FRP) composite structures, and several inconsistent results associated with fatigue tests reported in the open literature will be presented. To this end, the paper focuses on the question of ‘similitude’ and the potential for using the terms ΔG' and Δ√G as alternative methods for characterizing Mode I, Mode II and Mixed mode I/II delamination growth.