Optimizing fasteners for CFRP automotive parts

Dutch automotive fastener supplier Nedschroef summarizes development test results, identifying solutions for aluminum, steel and stainless fasteners which meet industry standards and resist galvanic corrosion.

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After significant testing using CFRP plates (laminates representative of those in the BMW i3 shown here), Nedschroef has developed fastener solutions which include coatings for an aluminum bolt (center fastener, top photo) and for steel bolts (left and right pairs in bottom two photos) which survived more than 1,000 hours of corrosive salt spray testing.
SOURCE: Nedschroef.

This is my summary of research findings sent to me by material engineer Max Wagner, for global automotive fastener supplier Nedschroef (Helmond, The Netherlands), based on a large development program conducted at its Techno Centre, also in Helmond.

Why research fasteners for CFRP
Because only serious weight reductions will result in extended driving range for electric vehicles, the future of e-mobility depends on progress in lightweight solutions. Though carbon fiber reinforced plastic (CFRP) plays a major role in the next generation of these lightweight solutions, until recently, it was only used in exclusive, high-end models. BMW is the first OEM to use CFRP for larger volume production, with both i3 and i8 models using CFRP for most internal structure and body components to compensate for the added weight of electric engine,  battery and control systems.

One issue for continued expansion of lightweight CFRP in automobiles is attachment. To provide easy exchangeability of single components like seats or doors, fasteners must be used, regardless of whether the vehicle is made from steel, aluminum or CFRP. As a leading supplier of automotive fasteners in Europe for over 100 years, Nedschroef  realized a need for development of mass produced fasteners for CFRP automotive parts. This led to a project at its Techno Centre R&D facility.

CFRP test plates
To gain more experience with CFRP, tests on the laminates — including structural analysis and tensile testing — were performed. These showed that despite vast differences in the test plates from OEM and supplier sources, the product properties were comparable.  The CFRP products were quite disparate, including:

  • A wide variety of layups;
  • Thermosets as well as thermoplastics;
  • Unidirectional, multi-axial, and woven fabric laminates.

Fastener-related investigations on the CFRP plates were also performed.

Fastener testing
A surface pressure test was developed to investigate the CFRP plates’ resistance to this type of loading. During the test a cylinder made from ultra-high strength NT16 steel is pushed with a defined force into the CFRP plate until indentation is visible. Results showed that the higher the fastener grade, the more geometrical adjustments are required. This means that as stronger fasteners are used, they must be designed with larger bearing surfaces in order to avoid indentations in CFRP counter plates.

Because friction is a critical parameter for proper fastener function, friction tests were also performed. For initial tests, bolts were tightened until yield and the average friction coefficient was determined at 75% of this value, according to European and international standard test method EN ISO 16047. Next, an average torque was calculated by using the determined friction coefficient and the average value from the surface pressure test. Another friction test using this calculated torque was performed. Results for the above CFRP plates showed an average friction coefficient of 0.07 with very small deviation. According to Wagner, this means that friction of automotive fasteners on CFRP plates is roughly 40% to 50% lower than on metal plates.

Wagner explains that with this value it is possible to calculate the preliminary tightening parameters for joints in CFRP assemblies and to ensure their proper function — e.g., prevent loosening of bolted joints. However, for calculations with final products it will be necessary to perform friction tests according to specific customer demands. For example, some specific applications require several tightening and untightening operations before final measurement to define a friction value.

Wagner concludes these test results should allow Nedschroef to modify its current portfolio of products to provide faster and more cost-efficient solutions for upcoming CFRP applications. In addition to these adjusted products, Nedschroef will also introduce completely new fasteners for CFRP components to the market.

Corrosion testing
An additional challenge for fastening to CFRP is galvanic corrosion, which results from the large difference in electrochemical potentials between carbon fiber and the standard metals used in fasteners, for example, boron and chromium steels. Stainless steel grades 304 and 316 are also used in small amounts as well as aluminum alloys EN AW 5xxx, EN AW 6xxx and EN AW 7xxx series. The material used has a significant economic impact on the fastening solution: raw material costs alone are approx. $1,000/ton for steel, $2,000/ton for stainless steel and more than $5,000/ton for aluminum.

In addition to exploring more complex materials for fasteners — including titanium, copper and different grades of stainless steel and aluminum — the Techno Centre dove deep into coatings. As the interface between the fastener material and the CFRP, coatings should allow standard materials to provide a better price performance ratio than expensive solutions like titanium. To explore this ratio, coatings commonly used in the automotive industry were tested first. Metallic and non-metallic coatings — with and without organic or inorganic topcoats — were investigated and can be divided into electrolytic, mechanical or thermic applied and lacquer systems.

Due to the failing of all tested coatings in corrosion testing, non-automotive coatings like ceramic, insulation and nanocoatings were also investigated. Some of these coatings were new developments, supplied by local partners, which has expanded Nedschroef’s coating technology knowledge. For example, the best coatings solutions were identified as well as optimized coating thickness.

To investigate galvanic corrosion behavior between different CFRP plates, several insulation coatings were chosen for a salt spray test with test plates from OEMs and suppliers. The salt spray test is required by the automotive industry to accelerate corrosion, and results allow predicting coating behavior in an environment with aggressive media. No difference in galvanic corrosion behavior was observed. Coatings which resisted galvanic corrosion did so for all CFRP plates. Observed corrosion damage on failed parts had comparable quantity.

CFRP fastener solutions
Solutions resulting from this extensive research include:

  • A new insulation coating for EN AW 6056 aluminum fasteners;
  • Two new coatings for steel fasteners;
  • A special heat treatment for stainless steel grade 316 fasteners.

Tested samples achieved promising results after more than 1,000 hours inside the salt spray chamber. The stainless steel and coated aluminum test samples showed no sign of corrosion. The steel test samples showed very little sign of corrosion, which can’t be avoided due to the sacrificial characteristics of these coatings.

At the end of this challenging project, Nedschroef now offers different solutions for fastening to CFRP components. These bolts and screws, which are directly connected with CFRP, reportedly allow transfer of higher forces than other fastening solutions and the best possibility for success in exploiting the benefits of lightweight CFRP for automotive components.