Sandwich elements are often used to gain significant weight reductions in aerospace applications, but they can also be used in terrestrial vehicles. As part of internal research, the materials corrosion and corrosion protection experts at Steinbeis-Transferzentrum Werkstoffe Korrosion und Korrosionsschutz GmbH, a Steinbeis Enterprise based in Friedrichshafen, have developed a system for testing the corrosive properties of sandwich compounds made of CFRP aluminum honeycombs.
Continuous carbon fiber reinforced polymers (CFRP) have an edge on metal materials thanks to their extraordinary specific strength and rigidity. Aluminum honeycomb structures are a classic kind of lightweight material that have been tried and tested in engineering for decades. They offer excellent mechanical compatibility between the CFRP laminate top coats and the aluminum hexagons thanks to the matching stiffness of both materials. However, in electrochemical terms, combining CFRP and aluminum can be extremely problematical.
Inter-laminar shear testing is absolutely essential if engineers want to understand the qualities of a CFRP laminate. Ideally, testing should be supplemented with laminate data by conducting tests both lengthways and crossways. With aluminum honeycombs, sufficient compressive strength is required and the shear modulus has to be appropriate for the sandwich design, depending on directional forces. The sandwich samples tested by the Steinbeis experts in Friedrichshafen were produced by using an autoclave process with a twin-component epoxy adhesive. Current density (short circuit current) was measured between the “more pure” CFRP laminate and the “less pure” honeycomb alloy (AlMg5) and this served as a basis for electrochemical corrosion testing. The tests conducted for the research project were carried out in accordance with DIN standard 50918. As such, the cut or sawn edges of the components used in the research were left exposed. Corrosive testing on the sandwich elements was carried out over the course of 336 hours. This involved two approaches. The first was based on VDA233-102 methods. This allows for cyclical corrosion testing of materials used in the automotive industry because it creates reproducible corrosion profiles, although in this case the Steinbeis team deviated from VDA standards and used a 5% sodium chloride solution. The second method that was selected was ISO 9227 salt spray testing, using 5% sodium chloride solution with a pH value of between 6.5 and 7.2. This was in order to examine the weaknesses caused by corrosion on the sandwich elements. The specialists then carried out a mechanical assessment of residual load-bearing properties after corrosion, based on a four-point bending test according to DIN standard 53293.
Four-point bending strength testing ascertained that the specific mechanical properties of the sandwich samples suffered particularly strongly with the samples exposed to salt spray testing (ISO 9227). Indeed, these differences were significant compared to reference samples. Creating lengthways cuts on treated samples breaches the surface of the laminate and the honeycomb structure. By the influence of the cutting process a direct contact between the carbon fiber fragments and the AlMg5 has been created. Electrochemical measurements showed that corrosion is mainly promoted through material contact and not through intrinsic corrosion. This also explains why there was more corrosion on the honeycomb structure subjected to salt spray testing than with the VDA 233- 102 approach, primarily due to the long-term effect of electrolytes. The ISO 9227 salt spray test resulted in corrosion along the entire sides of the honeycomb, while the VDA233-102 samples only had localized corrosion near the CFRP top coat. Despite the high levels of magnesium (5% of the honeycomb alloy), there was more contact corrosion on the CFRP.
The Steinbeis-Transferzentrum Werkstoffe Korrosion und Korrosionsschutz GmbH looks back on more than two decades of advanced knowhow, both in the field of lightweight construction materials and in hybrid construction methods. The testing methods developed at the center have proven immensely useful in assessing the properties of corroded hybrid lightweight components, and the methods will now be used for development projects involving assessment of sandwich structures.
Benjamin Kröger is the director of the Steinbeis Transfer Center Werkstoffe Korrosion und Korrosionsschutz GmbH. The work carried out by the experts at the Steinbeis Enterprise includes
consulting and support in the selection of raw materials, applied research and development in the
fields of materials analytics and testing and damage assessment. Additionally expert reports are published and research findings, education, and training courses on materials, corrosion, surface technology as well as lightweight construction are carried out.
Steinbeis-Transferzentrum Werkstoffe Korrosion und Korrosionsschutz GmbH (Friedrichshafen)