The wrapped composite joints have been introduced as a new technology to connect steel circular hollow sections of support structures for offshore wind turbines. The design and implementation of this innovation require predicting the effects of environmental conditions on the mechanical performance of the structure. In particular, temperature changes can generate interfacial stresses along the bonded interface between the two dissimilar materials, affecting the performance of the structure. This work aims to investigate the effect of short-term changes of temperature on the mechanical behavior of a wrapped composite joint. Specimens were produced with two steel tubes wrapped by a glass fiber composite laminate. Mechanical tests were performed under fatigue and static loading conditions. Experiments are carried out at room temperature (21 ℃) as well as at non-ambient temperature using a climate chamber at –10 ℃ 50 ℃ and 70 ℃. Results revealed that lower temperatures improve the performance of wrapped composite joints under both fatigue and static loading conditions. This points to a significant contribution of the thermally-induced effects on the performance of the structure due to the different coefficients of thermal expansion of the steel and composite materials. Experimental results obtained from this work can be applied to create numerical models capable of predicting the mechanical behavior of wrapped composite joints in different temperatures.

Wrapped composite joints arise as an innovative solution for joining circular non-welded hollow sections (CHS) in jacket support structures for offshore wind, intending to enhance fatigue performance and consequently reduce weight and costs when compared to traditional welded joints. Due to combined wave and wind loads, these joints are subjected to different multi-axial loading scenarios. Therefore, it becomes fundamental to establish an interaction criterion that accurately predicts the failure behavior provided by the superposition of different load conditions. Preliminary analyses have shown that, for instance, in cases where axial loads act simultaneously with bending moments, the joint resistance is underestimated when considering a linear summation of normalized strength values. Therefore, further studies are needed to determine the optimum interaction criterion exponents. In this context, this paper aims to present the results of an ongoing numerical investigation on the multi-axial load behavior of wrapped composite joints. Based on previous standards, a multi-axial loading interaction criterion is proposed, and a finite-element (FE) model is developed using the cohesive zone model approach. Distinct load cases are applied in a medium-scale X-shaped wrapped joint to evaluate the influence of the fracture toughness parameter on interaction failure criteria exponents. It was concluded that the given exponents do not seem to be affected by the change of interfacial strength and fracture toughness, which represents a valuable finding for the development of future design guidelines.

This research investigates non-welded tubular composite joints, highlighting their potential for offshore structures such as wind turbine support jackets. These joints are subjected to complex loading from wind, waves, and tides, which simultaneously generate axial forces and bending moments. Tubular composite joints rely on the interfacial bond between the composite wrap and the Circular Hollow Section (CHS) members for efficient load transfer, with interfacial debonding and substrate debonding at the steel–composite boundary being the critical failure modes. This paper characterises the resistance of tubular composite joints against interfacial debonding through monotonic experiments. Hand-laminated glass-fibre-wrapped X90-joints, with chord and brace consisting of (Formula presented) CHS members ((Formula presented) and (Formula presented) ), are subjected to axial tension (Nx[jls-end-space/]), axial tension combined with out-of-plane bending ((Formula presented) ), axial compression combined with out-of-plane bending ((Formula presented) ), axial tension combined with in-plane bending ((Formula presented) ), and axial tension combined with both out-of-plane and in-plane bending ((Formula presented) ). A total of 12 specimens were tested. The experimental programme is complemented by numerical models employing cohesive zone modelling (CZM) to predict debonding failure. The results show that debonding is the predominant failure mode across all loading conditions. Combined loading influences the sequence and extent of debonding propagation but does not alter the primary failure mechanism. An interaction equation is proposed, combining a linear interaction between Nx and the resultant bending moment with a quadratic interaction between Mop and Mip[jls-end-space/], achieving a predictive accuracy within 10%.

Tubular composite joints offer a non-welded alternative for offshore structures by bonding a composite wrap to steel Circular Hollow Section (CHS) members, eliminating weld-induced stress concentrations and significantly improving fatigue life. This enables steel weight and cost reductions and faster fabrication for jacket structures supporting large off-shore wind turbines. In service, these joints experience complex cyclic loads combining axial forces and bending moments, which can lead to interfacial debonding and delamination, necessitating damage-tolerant design. This paper presents one of the first experimental campaigns applying combined axial and bending loads on composite X-joints using a Hexapod system, enabling realistic offshore load simulation. Fatigue tests on 1/4-scale X90 specimens cover pure axial tension, out-of-plane bending, and combined cases. Two primary failure modes were observed: interfacial debonding under compressive strain and delamination under tensile in-plane strain. A numerical methodology based on the Virtual Crack Closure Technique (VCCT) and a stepwise crack-growth model incorporating non-linear crack retardation effects, rarely considered in composite joint fatigue modelling, was developed. Calibration of the Paris-law constant C revealed variations up to two orders of magnitude due to interface quality and manufacturing variability. Despite this, results demonstrate fatigue life extensions of up to 2000 times compared to welded joints. This work introduces a design philosophy leveraging crack retardation and interface friction effects to predict fatigue life, moving beyond conservative stress-based criteria towards damage-tolerant offshore design.

Hybrid structures built with composite and steel emerge across industries (offshore, shipbuilding, bridges, etc.) due to benefits of weight optimization, fatigue and environmental resistance. Particularly, the wrapped composite joints emerge as a new method to connect steel circular hollow sections for application in supporting structures of offshore wind turbines. The implementation of this technology requires predicting the long-term performance of the bi-material interface under operational conditions of loading and environment. This work addresses the effects of temperature and saltwater aging on the fatigue crack growth behavior of the composite-steel bonded joint under mode II loading conditions. Fatigue tests were performed using a 4-point end-notched flexure (4ENF) set up with digital image correlation (DIC). A numerically based method was applied to calculate the strain energy release rate (SERR) accounting for friction effects, geometrical and material non-linearities. The consistency of the manufacturing process was evaluated by tests performed in room conditions (21 °C). The mode II fatigue behavior of the composite-steel bonded joints remained between an upper and a lower bound of the Paris curves, characterized by composite delamination and adhesive failure, respectively. Then, the effect of temperature was assessed by experiments in −10 °C and 70 °C. Short-term temperature changes showed a significant effect on the fatigue resistance of the bonded joint, followed by changes in the failure mode. Finally, a decrease in performance was observed as a consequence of the long-term aging of specimens in saltwater for up to 549 days.

The dominant failure mode was characterized as debonding in the novel non-welded wrapped composite joint made with GFRP composites wrapped around steel sections. Glass fiber composite-steel three-point end notched flexure (3ENF) and four-point end notched flexure (4ENF) specimens were utilized to experimentally investigate mode II fracture behavior of this composite-steel bonded interface. Two new methods were proposed with the help of digital image correlation (DIC) technique to quantify fracture data during the tests: 1) the “shear strain scaling method” to quantify the crack length a; 2) the asymptotic analysis method based on the longitudinal displacement distribution along the height of the specimen at the pre-crack tip to quantify the crack tip opening displacement (CTOD). To numerically simulate the mode II fracture behavior, a four-linear traction-separation law was proposed in the cohesive zone modeling (CZM) where the softening behavior with a plateau was defined by the authors between traditionally considered initiation and fiber bridging behavior. The experimental and numerical approaches were validated mutually through good matches between the test and FEA results. 3ENF test provided good insight into softening behavior while 4ENF contributed to quantification of fiber bridging. These findings contribute to a more comprehensive characterization and understanding of the ductile fracture behavior of bi-material bonded joints, especially in mode II failure scenarios.

Debonding is characterized as the governing failure mode in the innovative wrapped composite joints made with glass fiber composite material wrapped around steel hollow sections without welding. The prerequisite for predicting debonding failure of wrapped composite joints is to obtain fracture behavior of the composite-steel bonded interface. The mode I fracture behavior of the bonded interface was experimentally investigated using glass fiber composite-steel double cantilever beam (DCB) specimens. The crack length a and the crack tip opening displacement (CTOD) during the test were accurately measured by analyzing the digital image correlation (DIC) data while the strain energy release rate (SERR) was calculated through the extended global method (EGM). The cohesive zone modeling (CZM) was utilized in the finite element model with the proposal of a four-linear traction-separation law to simulate the mode I fracture process. An approach is introduced to determine the critical stages of the proposed four-linear cohesive law by combining accurate measurements of crack length a and CTOD, along with SERR values. The validity of the four-linear cohesive law and the introduced approach to determine the critical stages were confirmed by good agreement in both global and local behavior between the testing and the FEA results.

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.

This paper presents an experimental procedure for obtaining the fracture resistance (R curve) of solid wood specimens made of spruce. Double Cantilever Beam (DCB) tests were performed in order to determine energy release rate vs crack length in Mode I wood fracture (crack opening). Ten wood specimens were loaded using the Universal Testing Machine and force-displacement curves were recorded. The most important parameter - crack length was monitored as the crack propagates using Digital Image Correlation (DIC) method. In order to obtain accurate R curve results, procedure which includes calculating cumulative released energy was employed. The cohesive energy Gf was determined based on the R curves. These results can further be analysed in order to obtain cohesive law for Mode I fracture of wood.

In this paper an efficient procedure for obtaining a cohesive law for Mode I timber fracture (crack opening), based on the Double Cantilever Beam (DCB) tests is given. DCB tests were performed on ten European spruce specimens in order to determine the energy release rate vs crack length (R curves). Two crucial parameters - crack length during the experiment and the crack tip opening displacement were obtained using 2D Digital Image Correlation (DIC) technique. In order to determine accurate fracture resistance (R curve), procedure which includes calculating cumulative released energy was employed. The cohesive law for Mode I fracture of wood was obtained by differentiation of the strain energy release rate as a function of the crack tip opening displacement. This cohesive law is further implemented in the successful numerical modelling of failure modes in large-scale end-notched glulam beams which were experimentally tested in four-point bending configuration.