Issue 31

J. Lopes et alii, Frattura ed Integrità Strutturale, 31 (2015) 67-79; DOI: 10.3221/IGF-ESIS.31.06 70 A standard surface treatment for stainless steel before adhesive joining is the ‘Boeing sol-gel process’. Following this process, the material is degreased and then deoxidized by using a wet or dry grit-blasting method. Finally, an aqueous sol- gel system [13], a dilute solution of a stabilized alkoxy zirconium organometallic salt and an organosilane coupling agent, is applied. An adhesive coating is then applied to the treated surface to generate a durable bond [14]. This treatment is conducted in this work and regarded as a reference. The surface treatment process can be subdivided into surface pre- and post-treatment. The pre-treatments are further differentiated into mechanical, physical, chemical and electrochemical treatments. The aim of the surface post-treatment process is to increase adhesion with the help of a coupling agent and to conserve the surface activity achieved by the pre- treatment. A surface treatment process always consists of a particular pre-treatment in combination with a particular post- treatment. As shown in Fig. 3, the sol-gel system is used in each approach, but an additional epoxy primer is applied only on the reference ‘Boeing sol-gel’-specimens. Figure 3 : Schematic representation of the surface treatment process Then different categories of pre-treatments for the steel foils are investigated. At first, grit-blasting is regarded and parameters as time, pressure, grit material and grit size are varied. After degreasing, the aqueous sol-gel is applied. As second category, vacuum blasting is examined, varying grit material as well as grit size. Finally, sol-gel is applied. As pickling is the most prevalent chemical pre-treatment for stainless steel [15], different pickling processes were examined to replace mechanical treatment of the thin steel foils. Specimens for the evaluation of the adhesion performance were fabricated using nitric-hydrofluoric and nitric-phosphoric-hydrofluoric acid as well as a nitrate-free solution consisting of hydrofluoric acid and hydrogen peroxide as oxidizing agent [16, 17]. Acid concentrations and pickling durations were varied and the pre-treated metal foils were rinsed with deionized water before drying in an oven. After drying, the foils were also post-treated with the aqueous sol-gel system. Foils are laminated with prepreg within one hour after treatment to prevent environmental influences. However, as the manufacturing process may be constricted by this limitation, for one vacuum blasting configuration the foils were stored one day at room temperature with 50% relative humidity after treatment before positioning in the lay-up. 600 specimens were tested in total, whereas is was ensured by a second manufacturing and testing loop that 10 valid test results are existent for the most promising surface treatment configurations of each category, vacuum blasting, grit blasting, pickling and Boeing sol-gel process. Specimens manufactured utilizing these configurations and pure CFRP specimens were then additionally tested at different conditions. Dry specimens were tested at -55°C as well as 120°C and moisture saturated specimens were tested at room temperature. Cure temperature is 180°C, therefore, comparing the hybrid specimens with pure CFRP-UD reference specimens it has to be considered that the curing shear stresses act on the same plane as the inter-laminar shear stresses. As curing temperature is above testing temperature, the differing coefficients of thermal expansion inevitably lead to residual stress in the laminate which can cause deformations. These inter-ply stresses may significantly lower the mechanical properties of the hybrid laminate, especially the residual inter-laminar shear strength. Different investigations have been performed to reduce residual stresses in pure composite as well as in fibre metal laminates, utilizing modified curing processes, an additional clamping tool to reduce thermal mismatch or post-stretching to reduce residual stress level of an already cured laminate [7]. However, these approaches are regarded in research only and the measurement of the residual stress level is