# Influence of Lubrication Systems on the Fatigue Strength of Bolted Joints
## 4 Summary and Discussion  
In order to evaluate the influence of corrosion protection layer and lubrication separately, a bolt M8x5-8.8 with non-electrolytically applied zinc flake coating was selected [18] and investigated with a selection of different lubricants in the fatigue test. The reference-forming variant without lubrication was compared with a sealant with lubrication integrated into the topcoat, two bonded coating systems, two lubricating greases and one lubricating oil. For initial characterization, thread friction coefficients were determined for the selected variants in the torque/preload test according to DIN EN ISO 16047 [23]. The friction coefficient was from \(\mu_{\text{th}}=0.085\) (sealing) to \(\mu_{\text{th}}=0.200\) (without lubrication). Subsequently, the fatigue strength was evaluated in the axial fatigue test according to DIN 969 [21]. For this purpose, the bolts provided with different lubrication were paired with standard steel nuts according to DIN EN ISO 4032 [26] on a resonance test system preloaded to a constant mean load and a sinusoidal operating load with constant amplitude was superimposed. Five individual tests were performed for each variant at three load levels, so that failure due to fracture occurred as a function of thread lubrication in the range of fatigue strength  
Figure 14: Dark covered surface on the crack edges and inside the opened crack. from N\({}_{\rm B}\)\(\approx\) 50,000 cycles to N\({}_{\rm B}\)\(>\) 10\({}^{6}\) cycles, or the limited number of cycles N\({}_{\rm G}\) = 5 \(\times\) 10\({}^{6}\) was reached. The lubrication in the thread contact significantly influences the lifetime as well as the fatigue strength. Regardless of the type of lubrication system, lubrication increased the lifetime and durability compared to the unlubricated bolted joint. For the standard surface system, consisting of a basecoat and lubricant additive integrated with the topcoat, the smallest improvement in lifetime was achieved, although this variant has the lowest thread friction with one-time tightening in the torque/preload test. In comparison, bonded coatings and grease lubricants lead to longer lifetimes despite slightly higher thread friction in the assembly test. The greatest improvement was achieved by the oiled bolt variant, which at the lowest test level of F\({}_{\rm A}\) = 2.9 kN was the only variant where five out of five specimens were able to reach the fatigue limit without failure due to fracture. Consequently, not only is the lifetime increased, but also the fatigue strength. Based on the determined load cycles to fracture, the greases and bonded coatings rank between the sealed and oiled variants. Two hypotheses were formulated to account for the improved lifetime due to lubrication and the increased fatigue strength due to the properties of lubrication. The first hypothesis is based on the assumption that low thread friction reduces the local stress in the bolt, thereby shifting the initiation of cracking to higher load cycles. Validation was performed via the numerical simulation of local stresses for different thread friction coefficients and by evaluating the crack initiation times via the frequency signal of the resonance test system. The evaluation of the damage parameter P\({}_{\rm SWT}\) confirms that a lower thread friction coefficient reduces the resulting local stress-strain hysteresis and shifts the crack initiation times to higher load cycle numbers. The numerical study conducted was used to verify hypothesis I. It represents a simplification compared to the real conditions, as no wear or long-term conditions of the lubricant was taken into account. In particular, the resistance of the friction conditions under cyclic loading influences the crack initiation. Furthermore, the greases also have a damping property due to the present lubricant film thickness. In order to evaluate and take the specific lubricant properties into account, the crack initiation time was determined experimentally.