Fatigue delamination in multidirectional composite laminates was experimentally investigated in present study. Both the Paris relation and a modified Paris relation (with a new similitude parameter) were employed to interpret fatigue delamination with significant fibre bridging. The results clearly demonstrated that fatigue delamination was independent of fibre bridging, if a reasonable similitude parameter was used in data reduction. As a result, a master resistance curve can be fitted to determine fatigue crack growth with different amounts of fibre bridging. The energy principles were subsequently used to provide physical interpretation on fatigue delamination. The results indicated the energy release for the same fatigue crack growth remained constant with fibre bridging. Bridging fibres in most cases just periodically stored and released strain energy under fatigue loading, but had little contribution to real energy release. The master resistance curve was finally applied to predict fatigue delamination with fibre bridging. Acceptable agreement between predictions and experiments was achieved, demonstrating the validation of the modified Paris relation in fibre-bridged fatigue delamination study.

The aim of present research is to determine fatigue delamination with fibre bridging in composite laminates. Both the Paris relation and the Hartman-Schijve equation were employed to explore fatigue delamination behavior. The use of the Paris relation can result in fatigue delamination growth being crack scale dependent. This dependence was significantly reduced in case of using the Hartman-Schijve relation in data reduction. This difference can lead to controversies on fatigue delamination behavior in composite laminates. To address this dispute, a new parameter, which was consistent with the hypothesis of similitude as well as damage mechanisms, was introduced to represent the similitude in fatigue delamination growth. A modified Paris relation based on this parameter was proposed and used to determine fatigue delamination. And a master resistance curve was obtained to determine fatigue delamination growth with different amounts of fibre bridging. Thus, fatigue delamination is crack scale independent, if the similitude is well characterized. And in the Paris region, the modified Paris relation can provide good predictions in fatigue delamination growth with fibre bridging.

Fatigue delamination with fibre bridging in composite laminates with different thicknesses was investigated. The experimental results clearly demonstrated fibre bridging had significant retardation effects on fatigue delamination behavior, making it insufficient to use a single Paris resistance curve to determine fatigue crack growth. To address this problem, the coefficients of the Paris relation were correlated to the normalized crack extension (a − a₀)/L_pz. It was found that the exponent n was independent on the normalized crack extension and specimen thickness, whereas the parameter log(c) bi-linearly decreased with the normalized crack extension and kept constant once fibre bridging became saturation. And the magnitude of log(c) was independent on specimen thickness. Thus, it was concluded that fatigue delamination behavior and fibre bridging significance were independent on specimen thickness at a given normalized crack extension (a − a₀)/L_pz. With substitutions of these correlations into the Paris relation, an empirical power law relation was developed to characterize fatigue delamination behavior. And its validation was verified by a comparison between predictions and experiments.