High strength steels (HSS) offer a unique opportunity to reduce weight in heavy-duty machinery such as trucks, trailers and agricultural machinery. These types of applications require significant fatigue resistance. It is well-known that fatigue failure typically originates at stress concentrations caused by the joining procedures. Indeed, fatigue cracks occur predominantly in the vicinity of joints which exhibit a tensile residual stress pattern. In addition to the interest in designing lighter, less energy consuming heavy-duty machinery, safety during operation is of course also of paramount importance, and this in turn is also related to the fatigue resistance of the assembly. Although welding is one of the most frequently applied joining techniques, specifically for HSS special weld-improvement techniques are required to satisfy the demands with respect to fatigue strength. These improvements techniques were recently explored in the RFCS sponsored FATWELDHSS project. In the DURAMECH project, we want to investigate the possibilities to convert a welded assembly to a bolted one, whilst increasing the fatigue strength.

For thin sheet metal (< 3-4 mm), it is generally accepted that mechanical joining techniques have better fatigue properties compared to resistance spot welds (RSW) and arc line welded joints. Without taking proper care of the welding process, HSS welded joints have only equivalent fatigue properties as those of lower strength steel grades. However, the thickness range of material used in the targeted heavy-duty machinery can cover a range of 3 to 10 mm, and in some cases even up to 20 mm. The generally accepted superiority of mechanical joining techniques compared to welding for these medium to thick plate materials can be questioned. In any case, it has not been demonstrated nor quantified. Therefore, the main objective of the proposed research is to investigate the possibilities to use conventional mechanical joining techniques for HSS and to assess the fatigue properties of these joints in comparison with welded solutions. At the same time, design guidelines for these types of connections will be derived. The comparison between bolted assemblies in standard structural steels and bolted assemblies in high strength steels (with similar, or improved fatigue strength) is of industrial interest as well.

As the DURAMECH project focuses on bolted connections which have a pronounced local character, several bolted joints will be necessary in most cases to withstand the required loading. The configuration of the bolted joints with respect to the specific loading condition will be a focal point in the proposed research. Such design rules exist for bolted connections under static loads, but not for the steel grades and thicknesses considered, and certainly not under fatigue loading. Since the strength of the base material is now much closer to the bolt yield strength, it affects much more the stiffness of the joint than is the case with lower strength grades. Also, the thickness of the base material considered in this proposal no longer justifies a simple plane stress approach which is used to derive many of the existing design rules in the first place. Although there have been very few publications on this topic, it is known e.g. that existing design rules (e.g. EN 1993-1-8) on the distances of bolts to the edge of the plate and in between separate bolts are too conservative and that the bearing strength of the connection is significantly higher than what is calculated from EN 1993-1-8. DURAMECH aims to develop a guidance document with respect to design rules for bolted connections subjected to fatigue loading.