Temperature can significantly affect fatigue delamination growth (FDG) behavior in composites, while fiber bridging has been frequently reported during FDG. The focus of this study was therefore on investigating temperature effects on FDG behavior with fiber bridging. Mode I fatigue delamination experiments were conducted on a thermoset composite laminates M30SC/DT120 at different temperatures. The Paris relation and fatigue resistance curve (i.e. fatigue R-curve) were used to interpret bridging effects on FDG behavior and to explore temperature effects on fiber bridging development. A modified Paris relation was employed to determine the effects of temperature on the intrinsic FDG behavior at the crack front excluding fiber bridging. The Paris interpretations clearly demonstrate that fiber bridging can significantly retard FDG behavior at different temperatures. Temperature can have different effects on fiber bridging development and the intrinsic FDG behavior. Particularly, elevated temperature can promote more bridging fibers, whereas decreased temperature has negligible influence on fiber bridging. When looking at the intrinsic delamination resistance, mode I FDG can accelerate at elevated temperature but decrease at freezing temperature. Fractographic examinations indicate that fiber/matrix interface debonding is the dominant damage mechanism in mode I FDG at different temperatures. Elevated temperature can lead to the weakening of interface adhesion, contributing to faster intrinsic mode I FDG behavior and more fiber bridging development. And a semi-empirical fatigue model based on normalization was finally proposed to determine mode I intrinsic FDG behavior at different temperatures for engineering applications.

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.

This study provides an investigation on mode I fatigue delamination growth (FDG) with fibre bridging at different R-ratios and temperatures in carbon-fibre reinforced polymer composites. FDG experiments were first conducted at different temperatures of R-ratios 0.1 and 0.5 via unidirectional double cantilever beam (DCB) specimens. A fatigue model, employing both the strain energy release rate (SERR) range and the maximum SERR around crack front as similitude parameter, was proposed to interpret FDG behavior. The use of this model can collapse FDG data with fibre bridging at different R-ratios into one master curve, obeying well with the similitude principles. Accordingly, it was found that FDG can accelerate with elevated temperature, but decrease at sub-zero temperature. Furthermore, there are strong correlations between the fatigue model parameters and temperature using this model in FDG interpretations. Taking these correlations into account can extend the model to accurately predict FDG behavior of other temperatures. Fractographic examinations demonstrated that temperature has effects on the FDG damage mechanisms. Both fibre/matrix interfacial debonding and matrix brittle failure were observed in FDG of −40℃. Fibre/matrix interfacial debonding becomes the dominant failure in FDG of RT and 80℃. No obvious difference on the fracture morphology was identified for FDG at different R-ratios of a given temperature.