The progression of predictive models for crack propagation in fibre-reinforced polymer composites requires the quantification of crack development induced by both static and cyclic loading. The challenge of precisely measuring crack propagation becomes more intensified when cracks are not externally visible, such as in the case of delamination crack progression in thick composites. Traditional techniques, like 3D digital image correlation (3D DIC), can be employed to detect relative changes in strain distribution at the surface. However, these methods may prove insufficient, particularly for composite components with thicknesses ranging from 10mm to 100mm. Distributed optical fibre sensing emerges as a solution, enabling precise spatial measurement of strain along the fibre's length with a remarkable resolution of 0.65mm. Embedding fibres strategically in the composite during production allows measurements in close proximity to the crack surface. Placing fibres in the direction of crack growth facilitates the observation of changes in strain distribution, markedly enhancing the precision in quantifying the delaminated area compared to conventional surface monitoring methods. This investigation offers a comprehensive overview of the design, implementation, and experimental application in simple joint geometries. It places specific emphasis on large steel-glass fibre polymer composite bonded joints, commonly referred to as wrapped composite joints. The technique is initially applied to a straightforward wrapped splice joint with 1D crack propagation. The experimental results robustly affirm the effectiveness of the proposed monitoring system and postprocessing methods subjected to both static and cyclic loading conditions. The system and methods effectively quantify the propagation of debonding cracks. In summary, these findings underscore the achievability of accurately quantifying delamination crack propagation by embedding distributed optical fibres in thick composites. This study contributes valuable insights for the development of predictive models and strongly reinforces the practical significance of employing distributed optical fibre sensing in crack monitoring.@en

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

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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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.

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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.

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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.

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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.

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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