# Influence of Lubrication Systems on the Fatigue Strength of Bolted Joints
## 1 Introduction  
In joining technologies, the bolted connection is a frequently chosen technical solution used to establish a detachable and high-strength connection. Bolted joints are often used in applications that are subject of cyclic loads, which is why fatigue failure is one of the most frequent causes in the event of damage [1]. As a result of high notch stresses, given by the function-related shape, screws can only transmit low cyclic loads. The durability under cyclic loading is reduced to less than 10% of the static strength and is determined by high local stresses in the thread root [2]. The fatigue strength and the lifetime of bolted joints depend on a large number of overlapping factors [2; 3; 4; 5; 6; 7; 8; 9; 10], which are insufficiently considered in the relevant design methods and guidelines. In addition to the design parameter, such as the depth of engagement, the nut material and also the manufacturing process for the nut thread itself [3], the surface system, consisting of corrosion protection and a lubricant, has a significant influence on the fatigue strength [11; 12; 13; 14]. Depending on the choice of coating system, there is a significant influence on the lifetime [15; 16]. In mechanical engineering and plant construction, the design process is carried out in accordance with the VDI guideline 2230 (Verein Deutscher Ingenieure, Association of German Engineers) [17]. In this guideline, the fatigue strength is evaluated only by the notch size in the form of the bolt diameter. With the exception of hot-dip galvanizing, for which the VDI guideline proposes a reduction of 20%, no consideration is given to the surface influence. In steel construction, where the design is carried out in accordance to Eurocode 3, there has been no consideration of the influence of the corrosion protection system, which is usually represented by hot-dipgalvanizing [18]. Hence, the relevant corrosion protection systems, such as galvanized coating systems according to DIN EN ISO 4042 [19] and non-electrolytically applied zinc flake coating systems according to DIN EN ISO 10683 [20], are not taken into account in present guidelines for the design process for bolted joints. In ref. [16], the effect on fatigue strength comparing galvanized zinc coated bolts against zinc flake coated bolts was investigated. The zinc flake coatings resulted in an improved lifetime. In industrial practice, screws are installed in a defined state of friction in order to achieve a reproducible preload for reliable connections [21]. Therefor dry or liquid lubricants are applied to the various coatings. Lubrication not only determines the friction in the contact zones, but also has a significant influence on the durability and the lifetime under fatigue loading. There is a need to investigate in more detail the influence of surface systems with additionally applied lubricants with a focus on the mechanisms in order to understand the failure development, beginning with crack initiation. Test results from various research projects show that not only does the corrosion protection of hot-dip galvanized components have an influence on the fatigue strength, but the presence of a coating system in principle also has an adverse effect on the lifetime [4; 10; 11; 12; 13; 14; 15; 16]. In ref. [22], the effects of both the thread pitch and the thread style ISO or American Unified bolts on the fatigue strength were investigated. Geometric influences appear more obvious and are also easier to quantify numerically, for example by means of stress concentration factors. The influence of coatings systems and lubricants which affect the contact area or rather the load introduction is more difficult to assess. Evaluating only the total number of cycles to failure is not sufficient. The inherent failure mechanisms have to be identified.