PSI - Issue 43

Robert Szlosarek et al. / Procedia Structural Integrity 43 (2023) 41–46 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction Fasten machine elements by bolts is one of the most popular joining technology in mechanical engineering. Nevertheless, it is not free of problems. One of them is the fatigue crack initiation at the borehole. Fig. 1 exemplifies such a crack at the bolt joint of metal sheets of a car. Here, it can be seen that the crack started under the bolt head and propagated off the borehole. In case of reaching a critical crack length this can lead to a fatal failure of the joint.

Fig. 1. fatigue crack at a bolt joint of an automobile.

This phenomenon is also described in various references. Knothe et al. (2008) and Wöllner et al. (2022) describe the damage scenario between the rim and hub of utility vehicles and tractors. One typical failure type is the crack initiation in the area of the bolt connection between the hub and the rim. Zhuang (2000) depicts the fatigue crack scenario of a gas compressor disk. In this case, the crack starts at the borehole. Shanyavskiy (2013) presents the fatigue crack initiation at the borehole of an aircraft turbine disk. In addition to these practical examples exist a plenty of laboratory investigations to study the fatigue crack initiation of bolted boreholes. Szlosarek et al. (2022) investigated the fatigue behavior of boreholes bolted with a nominal bolt diameter of 22 mm. The results indicate that the number of load cycles to crack initiation as well as the location of the fatigue crack initiation strongly depends on the preload of the bolt. For low preloads the crack is initiated at the borehole. For high preloads the fatigue crack was initiated in the contact pressure zone between bolt and metal sheet. Furthermore, the load cycles to crack are about 10 to 20 times higher for high preloads. Benhamena et al. (2012) observed a similar effect for bolts with nominal diameter of 10 mm. They also observed an increase in life with increasing preload. Chakherklou et al. (2008), Chakherlou et al. (2011), and Esmaeili et al. (2013) published findings for sheets made of aluminum 7075-T6 and for bolts with a nominal diameter of 5 mm. They describe a dependency between the preload and the location of crack initiation. The mentioned references present a wide range of experimental results. The numerical investigations of Benhamena et al. (2012) and Chakherklou et al. (2008) are limited to the slip regime or the local stress distribution. An estimation of the load cycles to crack initiation by a fatigue analysis is not presented. The aim of the present research is to perform a fatigue analysis based on the result of a finite element analysis. The challenge here is the multiaxial stress situation. Due to the preloading of the bolt exists a stress in the direction of thickness of the sheet. This stress is superimposed by the stresses of the sheet due to the plain loading of the sheet. Additionally, the preload leads to a fretting between the bolt and the sheet. After an explanation of the experimental setup a finite element model will be presented. Using the results of this model a fatigue analysis was done to review if it is possible to estimate the location of crack initiation and the load cycles to crack of this combination of a multiaxial stress and fretting loading.