Issue 45

C. Bellini et alii, Frattura ed Integrità Strutturale, 45(2018) 173-182; DOI: 10.3221/IGF-ESIS.45.15 179 apparent resistance of the aged joints than the non-degraded joints. Chadegani et al. [14] and Kumar et al. [15] state that the stress field that is generated inside a single-lap joint has a singularity to the adhesive/adherend interface near the extreme edges, so that the ends of the bonding tend to be stressed more respect the central area of the interface. A greater ductility of the adhesive localized to the extremes allows to attenuate the presence of this singularity and in this way generate a more uniform load, allowing an increase in the apparent resistance of the joint, as noted by da Silva et al. [16]. It is possible to observe the chromatic variation on the green that occurs in the joints aged in water (Fig. 6c and Fig. 6d) compared to the other joints tested (Fig. 6a and Fig. 6b), which tends to concentrate near the edges of the bonding area. From the analysis of the failure surfaces, it is possible to observe how the failure mode passes from adhesive (Fig. 6a) to mixed adhesive/light fibre tear (Fig. 6b, Fig. 6c and Fig. 6d): this is probably due to an improvement of the resistance of the adhesive/adherend interface due to ageing. Source Contribution Mean Square F-value P-value Ageing 83.00% 13.52 16.28 0.000 Error 17.00% 0.83 Total 100.00% Table 5 : One-Way ANOVA for EA 9309NA. a b c d Figure6: Failure surfaces of EA 9309NA specimens: a) without ageing; b) with air ageing; c) with distilled water ageing; d) with salt water ageing. The results of lap shear test obtained with EA 9309NA have shown a strong influence from ageing. DSC analyses were carried out to analyze how much the ageing has influenced the properties of the adhesive. The heating ramp used is 10°C/min. and the observation temperature range is from 25 ° C to 145 ° C. It is possible to state that, observing the curve relative to the unaged reference (Fig. 7), the polymerization at room temperature does not allow the total cure of the adhesive. In fact, the transition temperature is equal to 59 ° C, according to the technical data sheet, and it presents an endothermic peak probably due to some unreacted components. To be noted that there is no sensible difference between the behavior of the edge and the bulk of the bonded area of the adhesive. By analyzing the results obtained on specimens aged in air (Fig. 8), the adhesive no longer presents an endothermic peak. This probably occurs because this treatment has entailed a complete polymerization of the adhesive: in fact, the lack of peaks is compatible with a post-cure effect of the ageing. The DSC analysis of the samples tested in water showed that there are different behavior between the edge and the bulk of the adhesive. In particular, the edge of the adhesive (Fig. 9, curve A) presents an endothermic peak similar to unaged specimens, while the bulk of the adhesive (Fig. 9, curve B) does not present peaks. The adhesive has absorbed water only in the edges and probably this did not allow cross-linking of unreacted components during maintenance at the higher