Session: 09-01-16 Wind Energy: Structures 1

Paper Number: 124828

124828 - Comparative Analysis of Non-Welded Wrapped Composite Joints Versus Welded Joints Under Multi-Axial Load Conditions in Offshore Wind Turbine Supporting Structures: A Fatigue Performance Evaluation 

Offshore multi-membered structures are frequently constructed using Circular Hollow Sections (CHS). Some examples are supporting structures for wind turbines, such as jacket foundations as well as floating platforms. These structures encounter diverse loading scenarios, including axial forces and bending moments, stemming from wind, waves, and tides. Traditional welding methods for joining components introduce stress concentrations in the material, necessitating the consideration of Stress Concentration Factors (SCF) in fatigue design and the incorporation of stubs and cans to locally reinforce the joints. An innovative alternative, as proposed by TU Delft, involves the use of non-welded Wrapped Composite joints. These joints rely on the interfacial bonding between the composite wrap and the hollow section members to transfer loads. Previous pilot experiments have indicated a fatigue life improvement of up to 10-100 times for these wrapped composite joints under axial load conditions when compared to their welded counterparts. This paper presents a comparative analysis of the fatigue performance under multi-axial load conditions between the experimentally tested wrapped composite joints and the design fatigue life of their welded counterparts as those are defined based on fatigue design guidelines focused on offshore structures. The experimental evaluation extends to investigate the fatigue behavior of wrapped composite joints when subjected to multi-axial fatigue loads. These tests were conducted using a Hexapod testing machine with a capacity of 1000 kN, offering six degrees of freedom. The investigation encompasses various load scenarios, including axial tension, out-of-plane bending, axial tension combined with out-of-plane bending, and axial tension combined with out-of-plane bending and in-plane bending. In parallel, state of the art fatigue design for welded joints is showcased. Fatigue life of the welded connections is investigated under the same load scenarios. The results underscore the exceptional resistance of the wrapped composite joints to multi-axial fatigue loading in comparison to their welded counterparts. Furthermore, a dual approach incorporating Digital Image Correlation (DIC) and optical fibers embedded within the composite material was employed to meticulously monitor surface and interface strain. The principal mode of failure observed in wrapped composite joints across all loading conditions entails delamination crack propagation in close proximity to the steel-to-composite interface. The progression of these cracks is closely monitored through optical fiber technology during the experimental phase, with loading cycles terminated upon reaching a crack length equivalent to the diameter of the brace. This occurrence results in a reduction in stiffness of approximately 30%, after which a monotonic loading regime is applied to assess static resistance following fatigue. The wrapped composite joints exhibit remarkable load-carrying capacity retention, even in the presence of delamination within the joint root region.

Presenting Author: Mathieu Koetsier Delft University of Technology

Presenting Author Biography: Mathieu Koetsier
Ph.D. in Engineering Structures

Ir. Mathieu Koetsier is a distinguished researcher in the field of engineering structures, with a particular focus on the multi-axial behavior of wrapped composite joints. He earned his bachelor's and master's degrees in civil engineering from Delft University of Technology, setting the foundation for a remarkable career in structural engineering.

Mathieu Koetsier's expertise extends to the design and execution of complex multi-axial static and fatigue experiments, allowing him to gain profound insights into the behavior of hybrid joints under various load conditions. His dedication to advancing the understanding of material mechanics and joint performance has led to valuable contributions in the field.

Currently, Ir. Koetsier's research interests revolve around the static performance of bi-material bonded joints. His work not only pushes the boundaries of knowledge in structural engineering but also holds significant implications for the development of high-performance joints and innovative engineering solutions.

As a presenting author, Mathieu Koetsier brings a wealth of knowledge and experience to the forefront of academic discourse, enriching our understanding of complex structural systems and their applications.

Authors:

Mathieu Koetsier Delft University of Technology
Vasileios Mylonopoulos Tree Composites B.V.
Mees Wolters Tree Composites B.V.
Marko Pavlovic Delft University of Technology