In-Situ Pin-Caulking: Effect of Self-Piercing Geometry and Pin Arrangement on Metal/CFRT Joints

Modern engineering increasingly relies on mixing different materials to build lighter, stronger structures. A common strategy involves pairing metals, which are tough and ductile, with advanced composites made of continuous fibers embedded in plastic. These fiber-reinforced sheets are incredibly strong in the direction of the fibers but can be brittle and difficult to join with traditional methods. Gluing them requires long waiting times and perfect surface preparation, while drilling holes to insert bolts or rivets cuts the load-bearing fibers and creates weak points where cracks can start. To solve this, engineers have turned to a technique where small metal pins are formed directly from the metal sheet and pushed into the composite. Instead of just sitting in a hole, these pins are designed to spread out or flare at the bottom, creating a mechanical hook that locks the two materials together without needing any extra parts. This method, known as in-situ pin caulking, combines the act of piercing a hole, inserting the pin, and flaring it into a head all in a single, rapid motion.

The challenge lies in the fact that the pin changes shape during this process. It starts as a solid piece of metal but is forced to deform as it enters the plastic composite. Researchers at Friedrich-Alexander-Universität Erlangen-Nürnberg set out to understand how the initial shape of the pin and the arrangement of multiple pins affect the final strength of the joint. They compared two specific designs: a standard solid cylindrical pin and a "self-piercing" pin that has thin walls designed to spread outward like a flower petal when pushed. They tested these pins in various patterns, from a single pin to groups of four, joining them to a sheet of glass-fiber reinforced plastic. The goal was to see if the way the pins were arranged changed how much force was needed to make the joint and how much weight the final connection could hold.

The researchers found that the self-piercing pin was significantly more efficient than the solid one. Because the thin walls of the self-piercing pin could bend outward easily, it required less force to push through the material and form the locking hook. In the largest test group with four pins, the self-piercing design needed twenty-five percent less force to create the joint than the solid pin. Despite using less energy to make the connection, the self-piercing pins created a hook that was just as wide, if not slightly wider, than the one made by the solid pin. This means the joint was not weaker; in fact, it was stronger. When the researchers pulled the joints apart, the self-piercing pins held much more weight. In the four-pin arrangement, the self-piercing joint could carry nearly three times the normal pulling force and one and a half times the shearing force compared to the solid pin joint.

A crucial discovery was how the pins behaved when placed close together. For the solid pins, the strength of the joint depended heavily on how they were arranged relative to the direction of the fibers in the plastic sheet. If the pins were lined up along the fibers, the joint was weaker than if they were placed side-by-side across the fibers. This happened because the solid pins relied on friction and the way the plastic squeezed around them, which was influenced by the surrounding material. The self-piercing pins, however, behaved differently. Their strength remained consistent regardless of how they were arranged. This suggests that the locking mechanism for the self-piercing pin is determined primarily by the shape of the pin itself after it has been formed, rather than by the complex interaction with the surrounding fibers. The thin walls of the self-piercing pin spread out to create a hook that grips the material effectively without being as sensitive to the local arrangement of the fibers.

The study concludes that the self-piercing pin offers a superior solution for joining these mixed materials. It achieves a stronger, more reliable connection while requiring less force to manufacture. This efficiency is particularly important for scaling up to larger structures, where using many pins is necessary. The solid pins became less efficient as more were added, requiring more force for each additional pin without a proportional gain in strength. The self-piercing pins maintained their efficiency, delivering more strength for every unit of force used to create them. This makes them a promising candidate for industrial applications where lightweight, multi-material components need to be assembled quickly and reliably, such as in the automotive industry. The research highlights that the geometry of the pin after it has been formed is what truly matters for the strength of the joint, not just the shape of the pin before it is pushed into the material.