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.

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.

Permeability measurements of engineering textiles exhibit large variability as no standardization method currently exists; numerical permeability prediction is thus an attractive alternative. It has all advantages of virtual material characterization, including the possibility to study the impact of material variability and small-scale parameters. This paper presents the results of an international virtual permeability benchmark, which is a first contribution to permeability predictions for fibrous reinforcements based on real images. In this first stage, the focus was on the microscale computation of fiber bundle permeability. In total 16 participants provided 50 results using different numerical methods, boundary conditions, permeability identification techniques. The scatter of the predicted axial permeability after the elimination of inconsistent results was found to be smaller (14%) than that of the transverse permeability (∼24%). Dominant effects on the permeability were found to be the boundary conditions in tangential direction, number of sub-domains used in the renormalization approach, and the permeability identification technique.

In-plane permeability of small area (100 × 50 mm) alumina fiber woven fabrics grafted with aligned carbon nanotubes (CNT) was quantified by placing them in series with a glass mat of known permeability during a flow experiment. The methodology was first validated on a reference woven textile. Permeability values matched those obtained by a direct method within a margin of ±15%. Permeabilities of radial-aligned (short CNT, SCNT) and so-called ‘Mohawk’ (long CNT, LCNT) morphologies of the CNT-grafted samples were then measured and compared to the non-grafted alumina, showing a decrease attributed to a change in local textile structure as assessed in previous studies. Unsaturated permeability decreased by 77% after SCNT- and 88% after LCNT-grafting, while saturated permeability further decreased by 90% and 93%, respectively. The high ratio of unsaturated to saturated permeability (in the range of 1.14 – 2.89) implies that capillary wicking contributes largely to the impregnation of CNT-grafted fabrics.

This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions. All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38% to 58%. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress. Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter.