# Influence of Lubrication Systems on the Fatigue Strength of Bolted Joints
## 3 Experimental and Numerical Investigations
### Fatigue Tests  
Determining the fatigue strength requires a very large experimental effort. In order to reduce the scope of testing and still be able to make a statistical observation for the comparison of the selected lubricant portfolio, the testing, consisting of five individual tests each, is carried out at three selected load levels above the fatigue strength \(\text{F}_{\text{ASV50\%}}\). The load amplitude is selected in the range of \(\approx\)1.24-2.14 \(\times\) F\({}_{\text{ASV50\%}}\). This results in fracture load cycles in the upper high cycle fatigue range (HCF, N\({}_{\text{B}}\)\(\approx\) 50,000 cycles) depending on the lubrication, up to the transition range where specimens without failure reach the limited number of cycles N\({}_{\text{G}}\) = 5 \(\times\) 10\({}^{6}\). The individual test results and the 50% regression line through the mean values of the three load levels tested in each case are shown as a Wohler diagram (Figure 6). The comparison of the fracture load cycles shows that the bolted joint can fail over a very wide range of cycles depending on the applied lubricant. Consequently, the lubrication in the thread contact significantly influences the lifetime to failure, as well as the fatigue strength. In comparison, without lubrication, the lowest lifetime was observed by failure due to fracture. Regardless of the type of lubrication, failure with lubricated threads occurs at higher numbers of cycles. Minimal improvement is achieved by sealing with lubricant additive integrated in the topcoat. Here, solid lubricants are rigidly anchored in the glass-hard thin sealant, which resulted in very low thread friction during the one-time assembly in the torque-preload test. In comparison, screws lubricated with bonded coatings and grease lubricants show significantly higher lifetimes despite slightly higher thread friction in the tightening test. Depending on the stress level, the lifetime is improved by a factor of five to 17 compared with the unlubricated screw with a zinc flake coating only. The oiled variant, with high creep capacity of the lubricant, shows not only a shift to higher numbers of cycles to failure, but also an increase in fatigue strength. For the lowest test load level with F\({}_{\text{A}}\) = 2.9 kN, five out of five specimens for this variant reach the fatigue limited number N\({}_{\text{G}}\) without failure due to fracture. If higher fatigue loads were applied to the oiled variant, cracks in several threads were observed by means of metallographic micrographs and scanning electron microscope (SEM), (Figure 7). As the crack progresses in the first load-bearing thread, it is relieved, which changes the load distribution and consequently increases the local stress in the subsequent threads. With the exception of the oiled variants, the crack and the subsequent failure due to fracture only occur in the first load-bearing thread for all other variants. The fracture surface is characterized by circumferentially distributed incipient cracks that converge to a main fatigue crack. The microstructure of the fatigue fracture is transgranular and ends in a ductile fracture. Two hypotheses are proposed to explain the extended lifetime and the increased fatigue strength, as shown in Figure 8.  
The load transmission via the 60\({}^{\circ}\) flank angle of the metric thread results in a cyclic radial force in the thread contact due to the changing direction of the operating load. This leads to a permanent alternating movement between the screw and nut threads. Hypothesis I is based on the assumption that lower thread friction facilitates relative movement between the bolt and nut threads in the form of alternating sliding, thus influencing local stress and crack initiation. Figure 6: Fatigue tests for M8x55-8.8 with basecoat KL100 and different lubricants. Figure 7: (**a**) Cross-section of the oiled variant with multiple cracks in different threads; (**b**) macroscopic detailed SEM view.