Investigation into Fatigue Behaviour of Metal-Composite Glue Connection

The paper presents the results of experimental investigations into fatigue properties of metal and CFRP-composite glued joints. The object of investigation was adhesive joints between metal sheets made from iron alloy 30HGS (popular steel in the Polish aviation industry) and CFRP orthogonal reinforced sheets made from 10 layers of SIGRATEX CE3208 pre-preg. The main adhesive was the WK-3 glue (applied in the form of 0.2 mm thin foil) while the DP-490 - liquid epoxy glue was used as technological aid for specimen manufacturing. Two kinds of specimens were prepared and tested: single-lap joints and double-lap joints. The paper presents S-N curves for both kinds of those specimens. Besides fatigue life of the glued joints, the interest was focused on the NDT tests regarding damage propagation inside the glue-connection. The paper presents several C-scans made at different stages of fatigue testing. It has been found that the damage occurs almost equally in the whole volume of the glue layer (contrary to the expectation following from the physical model of the glue-connection internal loading). The results of experiments regarding thermal effect of cyclic loading of the glued joints are also described. It has been found that this effect could be used as an important parameter for diagnostics purposes of the glued joints.