The effects of manufacturing-induced and in-service related bonding quality reduction on the mode-I fracture toughness of composite bonded joints for aeronautical use

The scope of the present work is to investigate the possible effects of manufacturing and in-service induced reduction of bonding quality on the mechanical performance of composite bonded joints appearing in aeronautical applications, due to defects that cannot be detected sufficiently by means of conventional NDT. To this end, an experimental program has been conducted to measure mode-I interlaminar fracture toughness of adhesively bonded CFRP laminates. Five different representative scenarios have been considered; namely, poor curing of the adhesive, release agent contamination and moisture uptake by the composite substrates, which may occur during manufacturing of the joints, as well as Skydrol contamination and thermal degradation of the composite substrates, which are related to in-service life and could affect an adhesively bonded repair. Eight specimens for each scenario were tested. To assess the effects of each scenario, the experimental results have been compared with relative measurements taken from a set of reference specimens. Prior to mechanical testing, and the conventional NDT techniques, ultrasound C-scan and X-ray tests have been conducted to assess the quality of the bondline. All scenarios considered in the present study have been found to have an effect on the mode-I fracture toughness of the composite bonded joints. It was also shown that conventional NDT, such as ultrasonic and X-ray inspection, are not capable to sufficiently detect the defects resulting from the specific scenarios considered and being responsible for the reduction of joint’s performance.