Debonding is one of the most critical failure modes for bonded joints under fatigue loads. Numerical prediction on the fatigue debonding behaviour of bonded interfaces with complex geometry still remains a problem. This paper proposes a numerical methodology based on fracture mechanics to predict crack growth in a complex bi-material interface and illustrates the prediction procedure by a case study on wrapped composite joints. Interface coupon tests provide the fatigue crack growth properties at the composite-to-steel interface used as inputs for finite element (FE) modelling. The FE model is calibrated against fatigue tests of small scale wrapped composite joints with different steel surface roughness subject to different load levels. A sensitivity analysis is conducted to investigate the influence of key modelling parameters. The calibrated model is validated against fatigue tests on upscaled joints. Good agreements are shown between the test and modelling results in terms of crack growth and stiffness degradation, demonstrating the potential of the proposed numerical methodology for predicting fatigue debonding behaviour of complex bi-material interfaces.

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Offshore renewable energy sources, such as wind and solar require resilient support structures that are vastly loaded by cyclic loading due to wind and waves. As such, the structures built from steel circular hollow sections are structurally optimal to resist extreme loads but their design is hampered by low fatigue resistance of traditionally welded joints resulting in short lifetime, excessive use of steel material and corrosion problems. Innovative wrapped composite joints connect steel tubes by bonding and replace traditional complex welded joints of tubes by relying on excellent corrosion and fatigue performance of fibre-polymer composite material. Composite joints can reduce amount of steel needed to build supporting structures prone to fatigue up to 50% and can speed up production and assembly of towers supporting wind turbines by factor of 2. In addition, the wrapped composite joints unleash the potential of designing and building corrosion free offshore support structures completely made of composite tubes as structural members, or in combination with steel tubes. This paper presents potential of wrapped composite joints, state of development through experimental testing and numerical modelling, certification of joints and pilot projects in offshore environment. The outlook for further research and development is also given.@en

The dominant failure mode in the non-welded wrapped composite joints made with GFRP composite material wrapped around steel circular hollow sections (CHS) is characterized as interface debonding. However, in the ultimate load joint experiments, debonding process was merely inferred from the surface strain distribution obtained by the digital image correlation (DIC). A thorough understanding and explicit illustration of debonding mechanism in wrapped composite X-joints is needed with help of finite element modeling (FEM), in order to provide prediction models for design of wrapped composite joints in engineering structures. In this paper, two FE models were developed to simulate the debonding behavior of small-scale and medium-scale wrapped composite 45° X-joints in monotonic tensile tests previously conducted by the authors. A new strategy of modeling complex composite geometry using 4-node tetrahedral elements (C3D4) without defining composite lay-up was proposed. The cohesive zone modeling (CZM) approach was utilized to simulate the debonding behavior of composite-steel interface with introduction of a new four-linear traction-separation law. The generated FE models were validated by good agreement between numerical and experimental results in terms of load-displacement response and surface strain distribution throughout the failure process at two joint scales. The validated models gained good insight into the joint debonding mechanism and determined the surface strain threshold for quantifying the debonding length. Development and validation of the FE models with unique set of parameters aligned well with the experiment results at two different scales is an important step for prediction and design of wrapped composite joints.

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