PSI - Issue 2_A

F. Musiari et al. / Procedia Structural Integrity 2 (2016) 112–119

119

Author name / Structural Integrity Procedia 00 (2016) 000 – 000

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initiation, but suddenly the crack is obstructed by the nanofiber, making the damage path dissipate more energy to propagate. This could be probably due to the nanomat capability, already observed in the interfaces of composite laminates, to enable a ‘bridging effect’ through the opposite sides of the bonding interface. Therefore, the tendency of the nanomat to behave like a net-like reinforcing web, exhibited if used in combination with composite materials, seems to be confirmed also when using the nanomat like an adhesive carrier, even if limited only for one of the two tested specimens with nanofiber. 4. Conclusions In the present paper, the use of an electrospun nanofiber mat as adhesive carrier in bonding joints was evaluated. An experimental campaign was performed by means of the manufacturing of 2024-T3 aluminum alloys bonded with an epoxy impregnation resin for woven pre-pregs and the subsequent comparison between simple bonded joints and joints with a Nylon 6,6 nanofiber mat interleaved in the interface and impregnated by the epoxy resin. A 40 mm thick crack was artificially induced through the insertion of a polyester sheet. Some 60 μm thick metal sheets were used to calibrate the thickness of the interface and to make it uniform in both the virgin and the nanomodified specimens. The curing cycle was performed under vacuum in order to increase the air ejection from the adhesive. Two specimens for each configuration (virgin and nanomodified) were tested. The results of the DCB tests were diversified. The first nanomodified specimen exhibited a trend of the Mode I fracture toughness which remained always lower than the virgin ones. The second nanomodified specimen instead presented a value of critical fracture toughness G C significantly lower than the virgin one, but on the other hand the fracture toughness kept on increasing monotonically, until it reached the peak of 0.25 N/mm when the crack length was approximately 124 mm, exceeding by far the corresponding virgin values for the same crack length. This could mean that the presence of the nanomat does not succeed to tough the interface with respect to the damage initiation, but it can raise the amount of damage dissipated energy, slowing down the crack propagation and the catastrophic breakage. This properties could be used to make the nanomat being successfully used to act as an adhesive carrier in those applications in which an increase of the dissipated energy is involved and it is more desired than an high strength of the bonding to the damage initiation. Future development of the works will be represented by further tests to validate the trend shown by the second nanomodified specimen, combined with microscopic and SEM analysis to better examine the fracture surface and to understand the reason why the two nanomodified specimens show a mismatching behavior during the crack propagation phase. Other tests will be performed for both investigating the effects leading by the nanomat to other failure modes and evaluating the properties of the adhesive and the influence that the mechanical parameters has over the bonding toughening with nanofiber mats. da Silva, L. F. M., Ochsner, A., and Adams, R. D., Handbook of Adhesion, Technology, (Springer, Berlin, 2011). Giuliese, G., Palazzetti, R., Moroni, F., Zucchelli, A., Pirondi, A., Minak, G., Ramakrishna, S. 2013. Experimental and numerical study of the effect of Nylon 6,6 electrospun nanofibrous mats on the delamination of CFR-epoxy composite laminates. 19th International Conference on Composite Materials. Huang, Z. M., Zhang, Y. Z., Kotaki, M., and Ramakrishna, S., Compos. Sci. Technol. 63, 2223 – 2253 (2003). Krenk, S. 1992. Energy release rate of symmetric adhesive joints. Engineering Fracture Mechanics, vol. 43, pp. 549-559. Musiari, F., Giuliese, G., Pirondi, A., Zucchelli, A. 2015. Development of a workflow for the virtual optimization of a nanofiber-interleaved composite laminate subjected to impact loading. 20th International Conference on Composite Materials. Oh, H. J., Kim, H. Y., Kim, S.S. (2014) Effect of the Core/ Shell-Structured Meta-Aramid/Epoxy Nanofiber on the Mechanical and Thermal Properties in Epoxy Adhesive Composites by Electrospinning, The Journal of Adhesion, 90:9, 787-801. Palazzetti, R., Zucchelli, A., Trendafilova, I. 2013. The self-reinforcing effect of Nylon 6,6 nano-fibres on CFRP laminates subjected to low velocity impact. Composite Structures, vol. 106, pp. 661-671. Zucchelli, A., Focarete, M.L., Gualandi, C., Ramakrishna, S. Electrospun nanofibres for enhancing structural performance of composite materials. Polym Adv Technol 2011;22(3):339 – 49. References