Ageing is known to have significantly detrimental effect on mode I fatigue delamination growth (FDG) in unidirectional (UD) composite laminates. However, composite structures are usually designed with multidirectional (MD) layups, which raises the question that is it enough to only conducted fatigue delamination experiments on specimens with a UD layup. The aim of this study is therefore to explore mode I FDG in MD composite laminates with 45//45 interface after different ageing, i.e. at 70 °C 85 % relative humidity (RH) and immersion in 70 °C water bath. Fatigue delamination experiments were conducted at stress ratios R = 0.1 and 0.5. The fatigue data, interpreted via Paris-type fatigue laws, demonstrated that: (1) the change of ageing severity has no influence on mode I FDG in MD composite laminates; (2) FDG remains the same in composite laminates after different ageing, regardless of layups. In all cases, the same master resistance curves can be obtained to determine the intrinsic mode I fatigue delamination resistance of UD and MD composite laminates after different ageing. The physical reasons for these findings were discussed based on the moisture content analysis, Fourier transform infrared (FTIR) analysis, dynamic mechanical thermal analysis (DMTA), and fractographic examinations. It was found that material degradation and delamination mechanisms remain the same for UD and MD layups, as well as for 85 %RH and water bath conditioning.

Hygrothermal ageing has detrimental effect of the fatigue delamination growth (FDG) in carbon fibre reinforced polymer laminates, and may increase the crack growth rate by a factor of ∼5. The paper examines, how this degradation for Mode I fatigue delamination is affected by the severity of the ageing conditions. Fatigue delamination tests for R = 0.1 and R = 0.5 are conducted after ageing (1) at 70 °C 85 % relative humidity (RH) and (2) immersion in 70 °C water bath (WB). Paris-type FDG characterisation is derived, in the form, which accounts for the effect of fibre bridging. It is demonstrated that parameters of FDG degradation do not differ for these two types of hygrothermal ageing. The physical reasons for this are examined using dynamic mechanical thermal analysis (DMTA) and fractographic analysis, which revealed similar irreversible degradation of the material near the fibre/matrix interface and in the matrix itself, and the similar damage mechanisms in fatigue delamination. Furthermore, this study can highlight the importance of obeying similitude principles in FDG characterisation, and provide extra information for the ISO standard development for mode I fatigue delamination in unidirectional carbon fibre reinforced polymer composites.