The project partners are investigating joining technologies – separately and in combination – that fall under three main categories:
- Mechanical fasteners, such as rivets and flow drill screws
- Chemical joints, including adhesives and bonding materials
- Metallurgical joints, such as spin welding and resistance welding
The project is examining joining for nine different materials, including CFRP and GFRP, and 19 combinations of materials.
“Once the database of joints is done, we’re going to conduct coupon testing and computer modeling so we can make that leap of logic about additional joints that haven’t been tested,” says Pete Czech, director of government programs at LIFT.
The goal for LIFT and CAR is to provide OEMs information for various material combinations for specific applications, such as underbodies, body sides and closures. “The automotive industry tends to want all joints to meet all requirements or they won’t consider them. We have to consider which ones work best for impact crash applications on front ends or static localized crushing,” says Vadhavkar. “We may have a lot of technologies just sit there on the shelves if we ask every joint to meet every requirement. That’s just not realistic.”
Ultimately, the project partners hope their study leads to technology transfer. “We want OEMs to feel comfortable [with various joining technologies] and know what to expect,” says Joe Steele, LIFT communications director. “Our goal as an institute is to take these innovations and get them commercialized.”
Addressing Bonding Issues
For large-scale commercialization to occur, there are some key issues to consider. “Probably the largest consideration for mixed-substrate applications is accounting for mismatched coefficient of thermal expansion (CTE). One material expands at a larger degree than the next, and this happens every day as the temperature varies – not just through the seasons or during manufacturing,” says Michael Barker, a research fellow at Ashland, a global specialty chemicals company. “Every time the temperature cycles, you get a fatigue cycle in terms of stress on the bondline.”
Adhesives suppliers offer solutions to account for CTE. For instance, Ashland created a higher elongation, lower modulus family of structural adhesives called Pliogrip™. “A lower modulus adhesive will yield a lower stress on the bond for any given unit of expansion or contraction in the substrate,” says Barker.
Another consideration is bonding to low surface energy (LSE) polymers, such as polypropylene, polyethylene and powder-coated paints. “Because surface damage and abrasion is always an issue with composites, there’s a need to protect those surfaces,” says Christian Brekke, application engineer with 3M’s industrial adhesives and tapes division. “Low surface energy plastics are a really good solution.”