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

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.

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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.

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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

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.

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This chapter discusses the mixed-mode loading of adhesive joints. The importance of mixed-mode loading is first introduced and then test methods commonly used to measure the mixed-mode fracture resistance of adhesive joints are presented and briefly discussed. The approaches to determine the fracture resistance are briefly reviewed and then the partitioning of mixed-mode fracture energies is discussed. The limitations of the local singular field and global approaches to mixed-mode partitioning are discussed and the use and application of a semianalytical cohesive zone analysis partitioning scheme is evaluated. The limitations of the global partitioning approach are further discussed in the context of developing a scheme to design and analyze adhesive joints with dissimilar adherends (a bi-material interface). A longitudinal strain criterion is proposed in addition to the matching of flexural rigidities and the approach is validated numerically. Finally, the practical issues of crack stability, failure path selection, and the use of mixed-mode failure envelopes is considered.

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In this work, the influence of a layer of glass fiber mat (GFM) inserted in a bi-material bonded joint interface is investigated as a toughening mechanism to improve the fracture performance of bonded structures. Composite-to-metal bonded specimens were manufactured by hand lay-up using two different consolidation processes: at room pressure or using vacuum bagging. The fracture behavior was evaluated under quasi-static loadings using the mixed-mode bending (MMB) test. Test results revealed an increase of the fracture toughness with the insertion of an adjacent layer of GFM. The toughening mechanism is associated with a more irregular fracture surface profile. It was verified a relationship between the fracture surface roughness and the fracture toughness. The toughening effect showed to be more evident in higher mode II fracture conditions. The insertion of a layer of GFM and the use of vacuum pressure in the consolidation of the composite increase the fracture performance of composite repairs in metal structures.

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The goal of this paper is to evaluate the strain-based partitioning method (SBM), an analytical method based on beam analysis, for the mixed-mode fracture characterization of asymmetric cracks. The fracture energy is calculated at the crack tip of mixed-mode bending (MMB) test with asymmetric geometry and asymmetric material parameters. The fracture energy obtained using SBM is benchmarked against the widely applied Williams’ partitioning method (WM) and numerical results from finite element models using the virtual crack closure technique (VCCT). Results show that WM produces inaccurate fracture modes in asymmetric conditions. However, if using SBM, the fracture mode ratio of bi-material cracks was obtained with accuracy. SBM describes a coupling function between mode I and mode II fracture that allows the mode partitioning as long the specimen complies with a simple design criterion based on the longitudinal strain equivalence between arms. Experimental results of composite-to-metal bonded joints validate the results of the parametric study. SBM proves to be an easy and reliable solution for the mixed-mode fracture characterization of bi-material cracks.

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The dissemination of composite materials introduces applications of hybrid structures with composite and metal parts. The development of reliable methodologies to evaluate the performance of these structures is required. In this work, the mixed-mode fracture behaviour of a bi-material adhesively bonded joint is investigated. A new strain-based criterion for the design of the mixed-mode bending (MMB) bi-material specimen is suggested. A new analytical partitioning method based on the ‘global method’ is proposed and tested on a composite-to-metal bonded joint and compared with a finite element model using the virtual crack closure technique (VCCT). The results show that the proposed strain-based design methodology can be successfully used in MMB test for bi-material joints. The fracture mode partitioning is accurately predicted by the analytical method. However, the absolute values of the strain energy release rate (SERR) predicted by the analytical method are only accurate if the shear deformation in the test is not significant.

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In this work, an experimental study has been carried out to evaluate the effect of salt water condition on the long-term adhesion of composite-to-metal bonded joints using peel tests. A new test configuration is tested via fully and partially immersed specimens with 500 mm length. Fracture surfaces of non-aged samples exhibited a cohesive failure within the adhesive layer, which indicates a good adhesion of the joint. Results revealed a significant difference in the interface adhesion between aged and non-aged condition after 150 days of immersion. Partially immersed specimens allow to evaluate the adhesion performance of the joint under dry and wet condition in single specimen using a simple peel test.

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The aim of this study is to assess the interlaminar adhesion of carbon-epoxy laminates under salt water condition. Carbon-epoxy laminate specimens were immersed in a salt water tank for 60 days. Some specimens were then dried at
room temperature for 280 days, until recovering their initial weight. Specimens were tested using the composite peel test, an adaptation of the floating roller peel tests for composite materials. The results showed a degradation of peel strength in some areas due to the ageing process. The drying process did not affect the test results. A scanning electron microscopic analysis carried out on the fracture surface of the specimens revealed a typical mode I failure microstructure. A mixture of matrix failure and fibre/matrix interfacial failure was observed in non-aged specimens. Finally, a chemical characterization of the fracture surfaces with energy-dispersive spectroscopy confirmed the penetration of salt water in regions near the edge of the specimens. A degradation of the fibre/matrix interface adhesion was observed in affected areas. Floating roller peel tests proved to be a fast and effective method to access the interlaminar adhesion performance of composite laminates.@en