Interface dynamics in bolted joint connections
SCHL 275/13-1 und HO 3852/11-1
Bolted joints are the most used and most important joints in mechanical engineering and steel construction. But engineers still have to deal with failures during the tightening process and in operation. During the tightening process, failures often manifest themselves in the form of or are related to stick-slip phenomena. During operation, bolted joints may turn loose when they are subjected to dynamic loads in form of shock, vibration, or cyclic thermal loadings. In both cases, failures are initiated by microslip motion and hence by a transition from sticking to sliding. Recent findings, especially from physics and the geosciences, have shown that wave processes and fracture-like dynamics may play a decisive role in the transitions between sticking and sliding of the contact partners. Against this background, static friction can be no longer considered as a pure material constant. In fact, first modeling approaches treat static friction as a dynamical process. Especially the beginning (microslip) of the sticking to sliding transition of the interface as well as the whole process (complete failure) is determined by the stress distribution in the interface. In this project, these findings should be transferred to the dimensioning, the tightening, and the operational behavior of bolted joint connections. By the development of appropriate models, taking the friction dynamics in the interface into account, we will gain knowledge on the mechanics of microslip and complete failure of bolted joint connections. For the validation of these models, four experimental parts are included in the program. In the first part, we will study asymmetries affecting the stress distribution in the interface and thus the static friction coefficient on a disc-on-disc setup. In the second part, we will investigate how the shape of the bolt head is affecting the stress distribution in the interface and thus stick-slip motion during the tightening process of bolted joints. In the third part, we will combine the most promising principles from the first two parts by designing new types of stick-slip reducing bolts. In the fourth and last part, we will study the effect of vibrations on the self-loosening of bolted joints during operation. As a consequence of the theoretical and experimental work, we will derive a first set of guidelines for designing bolted joints which are robust to stick-slip during tightening and will not suffer from failure in operation due to vibrational motion.