Steel jacket support structures for offshore wind turbine towers made of circular hollow sections (CHS) are becoming a competitive solution compared to monopiles in deeper waters. The current limitation to improving the durability and cost-effectiveness of the CHS structures is the low fatigue endurance of their welded joints. Fatigue-driven design of such structures usually leads to thicker profiles of steel members as well as costly welding of joints. Due to excellent corrosion and fatigue endurance, high strength-to-weight ratio, easy and locally made fabrication, fibre-reinforced polymer (FRP) composite materials have been widely used to strengthen welded CHS joints. Although this strengthening technique can enhance the resistance and fatigue performance of the welded CHS joints, welding still serves as the main way for load transferring in the joints. Therefore, welds remain the source of stress concentration and brittle fatigue failure. Recently, an innovative joining technique, wrapped composite joint, has been proposed at TU Delft. In this joint, the CHS members are connected through the composite wrap. Loads are transferred at the bonded composite-to-steel interface, and welding is completely avoided. While monotonic tensile tests have shown the superior stiffness and resistance of the wrapped composite joint over its welded counterparts, the fatigue performance of the joint still needs to be investigated. As the first investigation of the fatigue behaviour of the wrapped composite joints, the present study focuses on the most unfavourable failure mode, debonding at the composite-to-steel interface. A typical joint geometry in the jacket support structures, the K-K joint, is chosen as the research object, which is simplified to be the uniplanar X-joint. The general objective is to accomplish knowledge sufficient to predict the debonding behaviour of CHS-wrapped composite X-joints under tensile cyclic loads. To achieve that goal, the present work starts from the interface level, where the fatigue crack growth (FCG) properties at the composite-to-steel interface are characterised through fracture mechanics experiments, i.e. 4-point bending end notched flexure (4ENF) tests. The steel surface of the specimens is prepared with different roughness levels, and its impact on FCG properties is investigated. The obtained FCG properties provide the basis for predicting crack growth at the joint level. In the wrapped composite joints, friction exists at the composite-to-steel interface due to the confinement by the composite wrap, which may retard the crack growth. This phenomenon is quantified in cyclic tests on joints with simple geometry, i.e. the axial splice joint (A-joint), where the debonding crack growth is monitored through the 3D digital image correlation (DIC) system. At the joint level, tensile cyclic tests are conducted on the wrapped composite X-joints with different surface roughness and at different scales. Post-fatigue static tests are conducted to check the influence of cyclic loads on the residual resistance of the joints. Using the finite element model, the methodology to predict the crack growth and stiffness degradation of the wrapped composite joints is established, which can consider the interaction between debonding on the chord and brace members. The prediction methodology is validated against the test results and used in a probabilistic analysis to explain and reproduce the scattering test results. Finally, the failure criterion of the joints under cyclic loads is proposed to establish the design S-N curves. The present study found that the surface roughness of the steel tube plays an important role in the FCG properties of the composite-to-steel interface. A minor increase of the surface roughness can significantly improve the joint’s fatigue performance, with the parameter C of the Paris curve decreasing over magnitudes. At the joint level, the wrapped composite X-joints exhibited steady stiffness degradation during the tests due to debonding propagation at the composite-to-steel interface. Joints with reduced surface roughness show deteriorated fatigue performance but still have longer fatigue life over the welded ones. By including friction at the interface, the finite element model gives reduced strain energy release rates (SERR) at the crack front as the crack grows. Thus, the main source of the crack growth retardation is explained and can be quantified. The numerical results match well with test results of X-joints considering different surface roughness, different load levels and scales, and the relationship between debonding on the chord and braces is obtained. By studying the variability of surface roughness and FCG properties, the probabilistic analysis can reproduce scattering of the test results. Finally, the design S-N curves are obtained based on the experimental and numerical results, taking 5% resistance reduction as the failure criterion. The present study provides a methodology for characterising and predicting fatigue debonding behaviour, not only for wrapped composite joints but also for other large-scale bonded joints with complex geometry, enhancing the application of bonded joints in engineering structures. @en

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.

@en

A new technology of composite wrapped joints emerges as a promising solution for improving the longterm performance of connections between circular hollow sections (CHS). The use of composite materials is shown to improve the fatigue life of structures. However, a major challenge to the implementation of this technology is to ensure the long-term performance of the composite materials subjected to the operational conditions. This work aims to evaluate the effects of temperature changes on the fatigue delamination of a glass fiber reinforced polymer (GFRP) for application in offshore structures. Specimens were manufactured by hand lay-up and series of experiments are performed to access delamination fatigue crack growth behavior in a range of operational temperatures: -10, 21 and 70 °C. Displacement controlled end-notched flexure (ENF) tests were applied to measure the fatigue behavior together with a digital image correlation (DIC) system to monitor the displacements during the tests. Results show that the delamination fatigue performance of the composite material is not significantly affected by the range of tested temperatures. The fracture behavior also remained unchanged. Standard ENF test method has limited range of crack growth to evaluate the fatigue behavior of composite laminates.@en

In this paper, mode II fatigue crack growth properties of the composite-to-steel interface are characterised through different test configurations, namely ENF and 4ENF tests. Different loading types including force control and displacement control methods are compared. An innovative shear strain based method is proposed for monitoring the mode II crack growth at the bi-material interface through Digital Image Correlation (DIC). A 3D finite element model with Virtual Crack Closure Technique (VCCT) is built and used for obtaining the strain energy release rate (SERR) to investigate the effect of geometrical nonlinearity, friction at the interface and steel yielding, as well as to verify the mode mixity. The results show that the standard 3-point bending ENF specimen can be unstable under force control and sweeps narrow SERR range by a single test under displacement control. The 4-point bending 4ENF test shows stable crack propagation and clear SERR developing trend. More pronounced geometrical nonlinearity and friction effect exist for 4ENF test which can be considered when interpreting the Paris curves by a nonlinear finite element model.

@en

Hybrid bi-material concepts in engineering structures where fibre-polymer composites are used together with steel structural members have potential to reduce material usage, extend fatigue life and improve structural reliability. The bond performance of the bi-material composite-to-steel interface is of crucial importance and highly relies on the surface preparation quality of the steel element. This paper investigates the influence of steel surface roughness on the mode II fracture toughness and fatigue crack growth behaviour of the bonded composite-to-steel joints. Glass fibre composite and mild structural steel material is considered directly bonded in a wet lay-up process. 4-point end notch flexure (4ENF) tests are conducted on specimens with the steel plates prepared with low, medium and high roughness, respectively. In addition, morphology of the fracture surfaces are characterised by a 3D profilemeter and the friction coefficient is measured by a tribometer for each roughness level. Results show that in quasi-static fracture, fibre bridging is dependent on the surface roughness. A roughness level with S_q ≥ 22 µm of the steel surface can promote significant fibre bridging thus improved fracture toughness due to enlarged effective bonding area. Under cyclic loading, no fiber bridging is observed across all roughness levels tested. However, the Paris curve parameter C is significantly affected by the roughness level, which decreases by approximately 100 times as the steel surface roughness increases from the low level of S_q = 5 µm to high level of S_q = 22 µm. The m parameter of the Paris curve remains fairly constant across all the roughness levels tested.

@en

Debonding crack propagation at the composite-to-steel interface has been found to be an important failure mechanism for wrapped composite joints under static and fatigue loads. Friction at the interface behind the crack tip may deviate fatigue debonding of the joints from the linear-fracture-mechanics behaviour. This paper presents static and fatigue tests of axial wrapped composite joints. 3D DIC and optical fiber system is employed to monitor displacements and crack propagation. A finite element model is established and validated against static and fatigue test results, where friction is considered at the cracked interface. Through FE modelling, it is proved that the friction at the interface significantly reduce the strain energy release rate (SERR) at the crack tip, leading to retardations of crack growth and stiffness degradation. Parametric study is conducted finally to investigate the influence of friction coefficient, failure modes as well as Paris relationship parameters on the predicted fatigue behaviour of wrapped composite joints.

@en

The concept of an innovative bonded joining technology where welding is not required is presented as an alternative to traditional welded connection for steel circular hollow section (CHS). Wrapped composite joints have potential to greatly improve fatigue endurance when applied in multi-membered truss structures, e.g. offshore jackets for wind turbines. This paper focuses on characterization of influence of chemical bonding resistance, fracture toughness of resins, and steel yielding on debonding of wrapped composite joints. Uniaxial splice joints (A-joints) are made with GFRP composite material wrapped around steel sections, and tested under static tensile loading conditions until failure. Different chemical bonding properties by application of bonding primer, different types of polymer resins and steel grade are used during the wrapping procedure. Debonding on the bonded interface are identified by surface strain measurements through 3D digital image correlation (DIC) technique. Testing results indicate that steel yielding limits full utilization of the resistance of the bonded interface. Wrapped composite A-joints with high-strength steel exhibits 75% larger ultimate load where yielding is prevented. Larger fracture toughness of toughened vinyl-ester resin contributes to 30% larger displacement of the joints at failure compared to regular vinyl-ester and polyester resins.

@en

Wrapped composite joint is an innovative technique which connects steel hollow sections through bonding such that the fatigue performance is improved compared to welded joint. In this paper, a DIC-based method of monitoring surface strains is proposed to quantify the debonding crack propagation within the composite wrap layers during high cycle fatigue loading. A constant strain threshold was used to obtain crack length based on strain distribution curves extracted from DIC. Sensitivity analysis of such threshold showed that within the ‘steady strain slope’ region, the influence of threshold choice on calculated crack length is insignificant, but a good choice of threshold can help obtain more stable results. During cyclic loading, it was found that stiffness degradation and crack development of the joint is arrested due to friction effect at the cracked interface. Static tests after cyclic loading showed that the joint can still sustain its original static resistance. @en

Wrapped composite joint is a novel joining technology which connects steel hollow sections through bonding, completely avoiding the welding in the load transferring mechanism. Fatigue performance of wrapped joints has been experimentally shown to be superior over their welded counterparts. Aiming to enable development of prediction methods for fatigue life of wrapped composite joints, this paper proposes a combination of 3D Digital Image Correlation (DIC) technique and FE analysis as a method for monitoring debonding crack propagation at a complex composite-to-steel interface covered by a non-uniform thickness laminate. Fatigue tests on wrapped composite X-joints under tensile load are used for the method application and to analyse crack propagation in the brace and chord, including their interaction. Variation of strain distribution on surface of composite wrap obtained in DIC is corelated to length of the debonding crack at the composite-to-steel interface by the means of 3D finite element model of such joint. Crack development obtained from the combined DIC and FEA method is correlated to strain energy release rates calculated from FEA. With the help of FEA, the failure mode is characterised by debonding on the chord at the early stage of cyclic loading, followed by debonding on the brace.

@en

Design and execution of fatigue load dominated circular hollow section (CHS) multi-membered structures, such as offshore jacket and floating structures for wind turbines, truss bridges, etc., is hinged on fatigue performance of critical welded joints. An innovative jointing technology of wrapped composite joint connects steel hollow sections by direct bonding through a fibre reinforced polymer composite wrap, which completely avoids welding and therefore has superior fatigue performance than welded joints. In this study first results on fatigue performance of the wrapped composite joints is presented. Tensile cyclic loading tests on wrapped composite X-joint specimens were carried out to characterise their fatigue performance under different constant amplitude load ranges and compare it to equivalent welded joints. In addition, the influence of surface roughness of steel tubes and re-testing (load history) on fatigue performance of wrapped composite joints, as well as the influence of fatigue loading on residual static resistance was investigated in experiments. Preliminary S-N curves of wrapped composite joints are established. The tests results showed that X45 wrapped composite joints exhibited steadier stiffness degradation and 10–100 times longer fatigue life than their welded equivalents. Through 3D DIC surface strain measurements and post-failure microscopic insights to cut specimens, different failure modes including de-bonding at glass fibre composite-to-steel interface, delamination/fracture of the composite layers and fracture of steel brace are distinguished. The relationship between joint stiffness degradation rates, crack propagation rates and nominal stress ranges in the brace are established, based on which a preliminary fatigue life prediction of wrapped composite joints can be made. The re-tested specimens exhibited superior fatigue performance than virgin ones, while specimens with poor steel surface roughness showed worse fatigue performance. After fatigue loading with 40% of stiffness degradation, the specimens were found to still have the potential to sustain its original static resistance.@en