Issue 47

F. Moroni et alii, Frattura ed Integrità Strutturale, 47 (2019) 294-302; DOI: 10.3221/IGF-ESIS.47.22 302 A reason of the different behavior in mode I and II can be found in the competition between composite resin and adhesive for the determination of the failure behavior. In fact, both TRAZ_CI and TRAZ_BJ tests yielded interlaminar failure of the CFRP. Therefore, under mode I loading the weakest region is the ply-to-ply interface and the adhesive cannot fully exploit its higher strength and (probably) fracture toughness with respect to the composite epoxy resin. On the other hand it is not yet fully understood why mode I CFRP delamination in bonded joint (see Fig. 14) occurs at G I slightly lower than in the co-cured CFRP. A first guess hypothesis is that the stress state in the delaminating plies at the crack tip region in the co- cured and co-bonded specimens are not exactly the same, due to presence of a compliant adhesive layer. The presence of a small resin excess in co-cured CFRP and the possible mixing of pre-preg resin and adhesive during cure cycle in bonded joints are also factors that may contribute to that difference. Further investigation, including micrographic analysis is foreseen to highlight reasons of this behavior. C ONCLUSIONS hen comparing to mode I and mode II fracture toughness co-cured and CFRP joints bonded with a structural adhesive film, the following conclusions can be drawn: - tensile tests in the direction normal to the joint demonstrated that the tensile strength of the adhesive used here is higher than the ILTS of the composite; - mode I fracture toughness and the R-curve of co-cured joints are higher than those of co-bonded joints. In both cases the increasing R-curve can be related to the development of multiple delaminations. - the competition between composite resin and adhesive in determining the mode I failure behavior is determinant. From the tensile tests (i.e. under mode I loading), the weakest region can be located at the ply-to-ply interface and, therefore, the adhesive cannot improve the fracture toughness of the joint with respect to the co-cured joint since the crack skips the adhesive layer soon after the beginning of propagation; - mode II fracture toughness of co-bonded joints is more than twice that of co-cured ones since the crack in this case moves within the adhesive layer, differently from mode I loading. R EFERENCES [1] Vassilopoulos, A.P., Ed. (2015). Fatigue and Fracture of Adhesively Bonded Composite Joints, Cambridge (UK), Woodhead Publishing. [2] Floros, I.S., Tserpes, K.I., Leobel, T. (2015). Mode-I, mode-II and mixed-mode I-II fracture behavior of composite bonded joints: Experimental characterization and numerical simulation, Composites Part B, 78, pp. 459-468. [3] Li, J., Ying, Y., Zhang, T., Liang, Z. (2015). Experimental study of adhesively bonded CFRP joints subjected to tensile loads, International Journal of Adhesion & Adhesives, 57, pp. 95–104. [4] O’Dwyer, D.J., O’Dowd, N.P., McCarthy, C.T. (2013). Micromechanical investigation of damage processes at composite-adhesive interfaces, Composites Science and Technology, 86, pp. 61–69. [5] Soykok, I.F. (2015). End geometry and pin-hole effects on axially loaded adhesively bonded composite joints, Composites Part B, 77, pp. 129-138. [6] Camanho, P.P., Tong, L., Eds. (2011). Composite Joints and Connections, Cambridge (UK), Woodhead Publishing. [7] Ascione, F. (2009). Ultimate behaviour of adhesively bonded FRP lap joints, Composites: Part B, 40, pp. 107–115. [8] Renart, J., Costa, J., Sarrado, C., Budhe, S., Turon, A., Rodriguez-Bellido, A. (2015). In Vassilopoulos, A.P., Ed., Fatigue and Fracture of Adhesively Bonded Composite Joints, Cambridge (UK), Woodhead Publishing. W