A proposal for similitude in characterizing fatigue delamination behavior with fibre bridging of carbon-fibre reinforced polymer composites

Methods based on fracture mechanics have been widely used in fatigue delamination growth (FDG) characterization of composite laminates. These methods are based on the similitude hypothesis. It is therefore important to have appropriate parameters to well represent the similitude, which is useful for fatigue delamination test standard development aimed by Technical Committee 4 of the European Structural Integrity Society (ESIS TC4) and the ISO/TC61/SC13. In the present study, discussions on similitude parameters for fibre-bridged fatigue delamination interpretation have been conducted via fatigue data with fibre bridging at different R-ratios. The results clearly demonstrate that the strain energy release rate (SERR) indeed applied around the crack front, rather than the total applied SERR, should be employed to represent the similitude for FDG interpretation with large-scale fibre bridging. Particularly, the use of Δ√G_tip can well determine fibre-bridged delamination behavior of a given R-ratio, but it is not valid for FDG at different R-ratios in accordance with the similitude principles. A new similitude parameter, in terms of both Δ√G_tip and the maximum SERR G_{max_tip}, was therefore proposed to appropriately represent FDG behavior with fibre bridging at different R-ratios. This study can not only provide database, but also give important insights for the development of mode I fatigue delamination test standard of composite laminates.