# Influence of Lubrication Systems on the Fatigue Strength of Bolted Joints
## 4 Summary and Discussion
In this way, the influence on crack initiation and crack growth depending on the lubricant could be differentiated. For the experimental evaluation of the crack initiation times, a method using frequency analysis was developed. Due to the oscillating properties of the test system, an increase in compliance, which occurs with the onset of crack initiation and increasing crack growth, can be assigned to a frequency decrease. Using the developed analysis method, a reproducibly damaged area could be detected for a critical frequency change rate. Via stopped tests and by means of heat-tinting marked cracking surfaces, the frequency change rate d\(t\)/dN = \(-\)2 \(\times\) 10\({}^{-6}\) Hz/cycle was defined as a load-dependent cracking criterion. Comparison of the crack initiation cycles shows that regardless of lubricant selection, with thread lubrication compared to the unlubricated reference, the crack initiation time is shifted to higher load cycles. Thread lubrication facilitates an alternating sliding motion between the screw and nut threads. This relative movement is forced on the thread flanks by the radial force resulting from the alternating axial operating load. If the alternating sliding is continuously facilitated by consistent lubrication properties under fatigue loading, the local bolt stress can be reduced and the crack initiation point can be shifted. On the basis of the test results, the greases in particular, with the comparatively greatest lubricant film thicknesses, are assigned the greatest resistance to the friction conditions for the comparison of lubricants carried out here. Compared with the standardized tightening test, which only records the initial thread friction during one-time tightening, wear phenomena and the resistance of the lubrication properties must be further specified for evaluation under fatigue loading. By comparing the number of cycles to crack initiation, the longest lifetime of the oiled screw variant cannot be justified. Therefore, to explain the significantly improved lifetime, the second hypothesis was formulated, which describes the influence of the mobility properties of the lubricant on the crack propagation phase. Due to a high creep ability of the oil, upon crack initiation the lubricating oil setttles at the crack front by capillary action and slows down the crack propagation. Underlying mechanisms include, for example, crack closure effects, the damping effect of the oil and the changed local threshold value which is necessary for crack growth [4; 13]. In general, crack growth in bolted joints occurs quickly. The rate of crack growth da/dN is load-dependent. The higher the local stress/strain, the greater the stress intensity factor is, and thus crack growth will be quicker. For the investigated lubricant systems, the required crack opening stress differs compared to unlubricated bolts [14]. Therefore, the crack grows more slowly. By means of the measuring methods developed, it could be proven that there is already a crack related to a specific drop in frequency respectively the rate of change in frequency. In particular, oil, based on its capability to creep, will go directly in between the crack. At the very beginning of the crack initiation, the rate of crack growth is very small. Then, the rate of crack growth may increase very rapidly depending on the lubricant, until it becomes unstable and fails by fracture. Oil is the example to be emphasized, as it enlarges the phase of crack propagation extraordinarily. If the crack initiation is recognized, it will take a longer time until failure compared to the other variants investigated. The ratio of crack-free and cracked lifetime is strongly load-dependent. The smaller the fatigue load, the greater the crack-free portion of the total number of cycles to failure is. In the given example, 5 kN is a high load amplitude. Thus, the crack initiation is going to be very early depending on the lubricant, and the portion of crack propagation can be greater than the crack-free portion.