Mechanical Joining Technology

Slip-resistant bolted connections shall be used wherever the mechanical joints are subjected to impact, vibration and/or load reversal to prevent effectively movement of the connected parts if loaded mainly in shear.
The load-bearing capacity is activated by friction that is a result from the preload of the bolt and the slip factor µ in the slip planes. Commonly, the slip factors that can be achieved for typically anti-corrosion coatings in steel structure are low despite the complex pre-treatment of the faying surfaces. By applying a suitable structural adhesive on the slip planes, significantly higher slip factor and consequently, increases in load capacity, can be achieved. As part of the ongoing research project, the combination of slip-resistant connections and structural adhesives are under deep Investigation.

For fastening attachments in „thin-walled“ sheet metal structures, functional elements inserted by forming technology offer an extremely economical alternative to welded bushings. In addition to the static strength and the load-bearing capacity under crash loads, their fatigue resistance is always a relevant issue. The research project aims to generate a method of proving fatigue resistance by means of using the local stress theory for sheet metal sections influenced by forming technology. That for example occur as a result of a process sequence during the insertion of functional element. The local stress theory is also compared with the nominal S-N curves of the detail category examined on the formed sheet metal section in S-N-Tests.

In the field of steel and lightweight metal construction, there has been a trend to the use of high-strength steels in recent years. Over time, these constructions are subjected to predominantly (static) and not predominantly (fatigue) loads.
The use of blind riveting technology in these constructions raises questions from the structural engineer‘s point of view, which are addressed in the current research projects at Fraunhofer IGP. The aim is the development of design rules, which consider the higher material strengths already during planning stage. With these design concepts a more economical design should be possible.

Bolted non-slip connections are traditionally used in steel and plant construction whenever slip and deformation in the bolted connections must be minimised. The current test procedure of DIN EN 1090-2, Annex G is limited to the basic load-bearing behaviour under laboratory conditions. The aim of the research project is to record the influence of production and assembly-related imperfections as well as operation-related influences on the load-bearing behaviour of non-slip bolted connections in steel construction in order to be able to reliably guarantee the load-bearing capacity and thus the load-bearing safety of GV connections over their service life on the one hand and to avoid unnecessary repairs and costs on the other.

A scientific verification concept is to be determined for the use of lockbolt systems under combined shear and tensile loading. For this purpose, the linear interaction hypothesis, which has been very conservative so far, will be tested with the help of experimental and numerical investigations. The analysis of the load-bearing behaviour as a function of the design of the lockbolt system, the nominal diameter, the material pairing, the strength class and the pretension are in the foreground. Based on the investigations, the determined fracture interactions under combined shear and tensile loading are interpreted and, if necessary, the interaction verification for lockbolt connections is revised to enable a more economical design.