Lightweight structures are more and more used in all kinds of transportation vehicles. Due to their high production costs, monolithic carbon fiber reinforced polymer (short CFRP) structures are used exclusively for highest quality applications like aeronautics. For day-live usage, hybrid structures combine the properties of metals with FRP and open access to lightweight structures at lower cost. This is possible through functional integration, which reduces the assembly’s complexity and therefore, compensates for the high production cost of FRP. However, these hybrid components rely on successful adhesive joining strategies between metal and FRP [Reitz et al.]. The aim of this work was to investigate the influence of pre-treatment strategies as well as adhesive composition on the aging behavior of hybrid single-lap-joints.
The test specimen were manufactured in accordance to the DIN EN 1465 standard and are based on EN AW 5754 aluminium and aircraft grade HexPly® M21/T800s CFRP by Hexcel. Prior to the particular joining process, two different laser sources – near-IR and UV – with different energies were applied to clean, activate and structure both materials. As reference we used a solvent based cleaning process. The surface topology was characterized quantitatively on micrographs, showing on the one hand that the ablation depth in the matrix (initial median depth of 22 µm) of the CFRP could be adjusted in a range from 0 – 35.2 µm and on the other hand a structure in the aluminium was introduced allowing a significant surface enlargement. Subsequently, the pre-treated specimen were bonded with two 2k epoxide adhesive from Henkel. Hygrothermal ageing was performed following the PV 1200 standard of Volkswagen AG. Finally, the mechanical shear strength was determined and compared to unaged samples.
After the first 26 cycles corresponding to 13 days of ageing the shear strength decreases significantly by a relative value of 12%. By doubling or quadrupling the aging period to 50 and 100 cycles the shear strength value shows no significant change anymore. Fibre exposure through high energy laser pretreatment did neither increase the bonding strength, nor did it accelerate the ageing effects. Even higher energy densities weaken the CFRP material to be the weakest link of the bond.