The effects of pre-bond contamination with de-icing fluid on the mode-I and mode-II fracture toughness of composite bonded joints

Damaged carbon fibre reinforced plastic (CFRP) aircraft parts may have been subjected during service to a range of hostile chemicals before being repaired by adhesively bonded patches. One chemical that could contaminate aircraft parts is the de-icing fluid used to maximize the runway friction during all plane movements at airports in winter. The scope of the present work is to experimentally investigate the effect of pre-bond contamination with de-icing fluid on integrity of CFRP bonded joints by conducting mode-I and mode-II fracture toughness tests on both reference and contaminated specimens. The de-icer used to contaminate the surface of one adherent was diluted with demineralized water to obtain solutions with the following concentrations in vol%: 2% (low level contamination, DI-1), 7% (medium level contamination, DI-2) and 10% de-icer (high level contamination, DI-3). Then, it was applied on the surfaces by dip coating and dried in the oven for 2h at 40°C. Afterwards, acclimatization at RT was allowed for at least 24h. The dip coating results were controlled by XPS measurements. Since the de-icer contains potassium formiate, the potassium content on the surface was taken as a measure for the degree of de-icer contamination. XPS-results showed mean values and standard deviations from two dip coated samples with three measuring positions each: DI-1: 6.4 (±1.8) at% K, DI-2: 10.9 (±2.3) at% K, DI-2: 12.0 (±1.4) at% K. The experimental results revealed a detrimental effect of de-icing fluid on the integrity of the joints. Specifically, for DI-1 a reduction of 30% and 56% is observed for G_IC and G_IIC, respectively, with regard to the reference values (non-contaminated joints), for DI-2 the corresponding values are 37% and 62% and for DI-3 it is 56% and 83%. Regarding the fracture surfaces, it was observed that with increasing the contamination level the presence of light-fiber-tear failure mode increases, which indicates that the de-icing fluid has a deleterious impact on the composite material.