Enhanced tensile strength of composite joints by using staple-like pins: Working principles and experimental validation

In this study, z-pinning by implementing staples as load-bearing elements for CFRP double-lap joints is presented. Both conventional staples and purpose-made stainless steel wire staples were used as fastener elements. Two different inserting methods were investigated. In addition, the influence of the element’s diameter was examined. An overall increase of 28% in tensile strength was determined for stand-alone staples in comparison to bolted joints. A patented principle of edge-staple showed an increase of 23% tensile strength in comparison to purely bonded joints. The geometrical form of staples provides an effective way in terms of axial fixation, since pull out did not occur. Furthermore, the staple principle provides the opportunity to use the elements as supporting entities for bonded joints by reducing peak stresses and suppressing peel. Nevertheless, the test results showed a decrease of the in-plane strength with increasing number of elements, which should be considered for joint design. The study represents a first step in technology evaluation and demonstrates the potential of stapled joints.