The durability of bridge connections is critical for the long-term performance of bridge systems with GFRP composite decks. This study investigates the shear fatigue behaviour of injected Steel Reinforced Resin (iSRR) connectors embedded in FRP composite decks, through a series of fatigue tests performed under different load ratios and exposure conditions. Twenty-four connectors are examined: twelve unaged reference specimens, eight specimens tested under varying R ratios, two connectors with deck parts submerged in water and two subjected to outdoor aging, both for a duration of one year. The results show that, despite the composite nature of the connector, the load ratio and mean load level have minimal influence on fatigue life. Instead, fatigue performance in the high-cycle regime is governed primarily by the applied load range. A unified F-N curve including all R ratios was developed, demonstrating the consistency of this trend and enabling fatigue-life prediction across different loading conditions. Environmental exposure led to measurable stiffness degradation but did not significantly alter fatigue life. These findings highlight the robustness of the iSRR connector and support its application in durable GFRP-steel hybrid bridge systems.

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.

The durability and fatigue performance of bridge components are critical factors in their long-term viability. This study investigates the experimental results of fatigue tests on injected Steel Reinforced Resin (iSRR) connectors, a promising connection technology for Glass Fibre Polymer (GFRP) composite decks in bridge construction. This research expands on previous work by examining the impact of environmental exposure on the cyclic performance of iSRR connectors. A total of twelve iSRR connectors were subjected to fatigue testing. Two connectors were exposed to outdoor conditions for one year, two connectors were fully submerged in water, and four connectors served as reference specimens without aging. The experiments aimed to evaluate the fatigue resistance and performance degradation of the connectors under varying environmental conditions. Additionally, the cyclic loading was conducted with varying R-ratios to understand the influence of load ratio on the fatigue life of the connectors. The specimens were loaded until failure, and a detailed analysis of the failure modes was conducted to determine the impact of environmental exposure and the load ratio on the failure characteristics of the connectors. The findings provide insights into the effects of environmental exposure on the fatigue performance of iSRR connectors and contribute to the development of more durable and reliable connection technologies for GFRP composite decks.

The design and effectiveness of fatigue load-dominated multi-membered tubular structures, such as offshore jackets, largely hinges on the fatigue performance of welded regions due to stress concentrations, necessitating the use of thick steel members. To address these challenges and reduce overall steel consumption, an innovative bonded joining technology known as wrapped composite joints demonstrating superior fatigue performance has been identified as a potential solution. Offshore conditions introduce combinations of different loading directions, resulting in complex stress states at the root of the composite wrap. This necessitates understanding the influence of such multi-axial loads on the static fracture resistance of the steel-composite wrapped joints. In this regard, interaction criterion exponents play a key role in understanding the correlation between different loading conditions. Ongoing research on wrapped composite joints focuses on these exponents, highlighting that delamination near the steel-composite interface stems as the main failure mechanism. In this context, this paper will develop a finite element (FE) model of X-shaped wrapped joint imposing fracture to the composite material and will study the superposition principle that exists between combined axial and bending loads. Combinations of load cases will be simulated to derive the interaction criterion exponents defining the failure envelope of such steel-composite wrapped joint. Modification to the current design recommendation is proposed with alternative an criterion with a good fit that aids in the design.

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.

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.

This work focuses on investigating the effect of short-term changes of temperature on the mode I and mode II glass fibers woven composite interleaved with layers of chopped strand mat (CSM). Existing experimental and numerical methods are critically applied to characterize and model the delamination of the woven-CSM composite laminate. Double cantilever beam (DCB) and end notched flexure (ENF) tests are performed in non-post cured and post cured specimens at room temperature (21 °C), and the operational conditions are investigated on post cured specimens tested in low (−10 °C) and high (70 °C) temperatures. The fracture behavior is characterized using the compliance-based beam method (CBBM) while crack length estimations based on the specimen compliance are compared to direct measurements from digital image correlation (DIC). Then, a failure analysis was performed using an optical profilometer and scanning electron microscopy (SEM). Temperature changes affected the preferential crack path for the woven composite delamination in mode I loading conditions. However, the crack path in mode II fracture remained independent of the testing temperature. Fractography results revealed temperature-dependent failure mechanisms, with an increase of fiber/matrix interface debonding and matrix deformation in higher temperatures. The higher matrix ductility translated into an improvement of the delamination fracture toughness in both mode I and mode II loading conditions. Finally, non-linear cohesive models directly derived from experimental results were capable to accurately reproduce the mode I and mode II delamination fracture behavior of the woven-CSM composite in different temperatures.

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.

The integration of Glass Fibre-Polymer composite (a.k.a. GFRP) deck panels in bridge infrastructure is hindered by lacking a robust connection technology. A promising bolted connection, utilising injected steel reinforced resin (iSRR) material, has demonstrated lower creep deformation and sustained significantly more shear load cycles than conventional bolts. Nonetheless, the production and testing conditions in all prior experimental campaigns followed idealized lab set-ups. This study bridges the gap between laboratory conditions and the challenges arising during connector's fabrication under representative conditions, coupled with cyclic load testing at room and elevated temperatures. The iSRR connectors design is modified and tested in actual composite sandwich web core panels, revealing excellent fatigue performance. The statistical analysis yielded F-N curves for shear performance of the connectors that can be used in the design. The slopes of the F-N curves of − 6.6 and − 5.8 were found at room and elevated temperatures, respectively. Finally, with post-cyclic static tests displaying significant connectors’ residual stiffness, resistance, and ductility, the research provides a step forward in enabling the integration of glass fibre composite deck in infrastructure.

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.

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.