Fatigue Performance of Adhesive Joints in Timber Wind Turbine Towers - A study to evaluate the fatigue performance of adhesive joints in timber connections
Examensarbete för masterexamen
Fatigue is a process which weakens a material over time under variable load. For some materials, the fatigue process is known and studied and thus the damage accumulation can be predicted and taken well into consideration when designing construction elements. For materials like steel, these properties are well known and checked whenever a steel construction is designed. For timber however, it is a rather unexplored subject and one which this thesis focuses on. The reason that the fatigue process has not been researched for timber is that it is rarely the deciding factor for the capacity of timber structures with bridges as an exception. However, for wind turbine towers, the amount of load cycles is extensive. During their lifetime, they will experience a high number of load cycles with low stress levels. This is called high cycle fatigue. This raises the question of the fatigue properties which will have a high demand regarding these load conditions. This thesis is focused on the application of glued in rod connection, which also have a lacking design regulation, even for static loading. For this connection, some research has been concluded, where some reliable design regimes have been put forward along with some information about the connections. The information available was gathered and put forward in order to present a reliable design recommendation. The available design regimes were tested using general coding software and then compared with other available lab tests done previously by others. In conjunction to this, an finite element model made in Abaqus was produced in order to provide further understanding and general knowledge into these connection types under cyclic as well as static loading. The conclusion was finally made that an adhesive layer of 2mm would be optimal for this type of connection, if fatigue as well as a static performance is to be retained. The adhesive itself should be restricted to either polyurethane or epoxy of the 2- components type. These two adhesives gave similar performance properties with small differences that could be neglected. The recommended edge and rod distance should be kept to no less than 2.5d as well as 4d respectively. The use of a laminated veneer lumber would simplify the connection with regard to load angles toward grains but glulam is a viable option, especially glulam made out of spruce or other coniferous lumber. The pullout strength was generally improved when increasing the anchorage length of the rod. Anyhow, the fatigue performance did not benefit of this to the same extent.
Fatigue , Adhesive connections , Timber structures , Wind turbine towers