3-D braided composites are a promising material for manufacturing tubular structures. However, a thorough understanding of their damage mechanisms under torsion is required to maximize their potential applications. The present work constructed a multiscale equivalent model, integrating mesoscopic and homogeneous structures to reveal torsional behavior of 3-D braided carbon fiber/epoxy resin composite tubes. The cumulative failure process, spatial stress distribution and interface damage were calculated to illustrate stress transfer and damage initiation and propagation. It is found that stress varies on the surface and internally within the representative unit cell (RUC). The yarns experience both axial tension parallel to the direction of torsion and axial compression perpendicular to the direction of torsion. The stress difference between them leads to damage initiation and propagation. Interfacial cracking as main damage mode hinders the stress transfer between resin and fiber bundles. The results show that the braided yarn path, axial stress dispersion in two directions and localization of damage effectively impede the torsional damage propagation.@en

In this study, a novel experimental approach was devised to investigate shear dominant and combined opening-shear planar delamination behaviours in composite laminates subjected to quasi-static out-of-plane loading. The patterns of planar delamination growth were depicted through different inspection techniques, including digital image correlation (DIC), C-scan, and microscopic observation. The artificially embedded delamination propagated in the direction parallel to the fibre orientation of the layer above the mid interface, but migrated to an upper interface in the direction transverse to the directing ply. A continuous stiffening process was recognized with increasing delamination area. Furthermore, a numerical analysis based on virtual crack closure technique (VCCT) indicated that the local mode II was dominant for delamination growth, while the local mode III triggered delamination migration.

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This paper investigates the fatigue-induced delamination growth in carbon fibre-reinforced polymer (CFRP), considering different fibre orientation combinations. The study explores the application of Artificial Neural Networks (ANN) in the simulation of fatigue delamination behaviour to reduce the number of experimental tests required for fatigue evaluation and eventual certification. The research aims to evaluate the effectiveness of ANN at different stages of data processing, including raw data simulation and final curve estimation. The results show that applying ANN at the raw data stage provides flexibility in modelling, with error < 10%. In addition, when ANN is applied directly to the final Paris curve, it minimises errors and increases reliability, allowing for a more cost-effective fatigue evaluation process. The study highlights the importance of the data processing stages in determining the accuracy of fatigue delamination predictions with AI modelling, thus informing strategies for efficient fatigue evaluation of CFRP components in structural applications.  @en

Driven by the energy transition and the reduction of carbon emissions, pultruded CFRP plates have emerged as an affordable high performance material for designing wind turbines with larger rotors. To enable the creation of thicker laminates, pre-cured plates are bonded together using an epoxy resin. The present study focuses on characterizing the fracture behaviours of these laminated bonded plates under different modes, and to generate fracture criteria for numerical simulations establishing design allowables and service life. Double cantilever beam tests, end-loaded split tests, and mixed-mode bending tests were conducted under quasi static loading for such plates. Mode I crack propagation showed brittle failure while mode II fracture tests on the other hand, presented a more stable crack growth. Finally, power law and Benzeggagh-Kenane law criteria were established to numerically describe the fracture behaviours. Overall, the results show that the available standards for fracture toughness testing are also suitable for these laminated-pultruded composite plate structures, enabling the material characterization needed for design.@en

For the past few decades, research into fatigue delamination behaviour of Carbon Fibre Reinforced Polymer (CFRP) composites has predominantly relied on standard test methods. These methods typically utilize a uniaxial delamination length to quantify the fatigue delamination process. However, this approach is inadequate to describe the nature of delamination growth in a planar manner. In this work, an experimental study was conducted to gain insights into the planar delamination behaviour of CFRP composites under mode II fatigue loading. Delamination growth of two specimen configurations, each containing an embedded initial defect, was monitored through ultrasound scanning (C-scan). During load-control fatigue testing, the growth rate exhibited an initial rise, followed by a subsequent decrease as loading cycles increased. Despite the development of a larger delamination area under fatigue loading, a consistent overall stiffness was observed. Fractography revealed the presence of various fracture mechanisms ocurring at different locations near the initial delamination front. Micro matrix cracking and fibre-matrix debonding emerged as dominant mechanisms in 2D fatigue delamination growth following the fibre direction. Matrix cracking within the laminate ply occurred at the locations where the growth direction deviated from the fibre direction, consequently triggering delamination migration. @en

Laminated pultruded composite plates are gaining interest for use in wind turbine blades due to their excellent structural performance with affordable cost. However, there is limited understanding of their fracture properties. The present work explores the interlaminar fracture behaviour of pultruded composite plates, bonded through resin infusion, to form thick CFRP structures. Mode-I, −II, and mixed-mode (I/II) tests were performed to obtain fracture properties at different mixed-mode ratios. Mode I crack propagation exhibits stick–slip behaviour, resulting in brittle failure in a few steps, while mode II provides more stable crack propagation along with cohesive failure. The mixed-mode fracture patterns follow the trend of the mode-mix ratios, in which higher mode-mix ratios (more mode II) induce more stable crack propagation. Benzeggagh-Kenane and power law criteria were compared regarding their prediction of crack initiation toughness given a mode mix ratio, and a linear relation between the mixed mode I/II fracture toughness components could exist at interfaces of laminated pultruded plates. Meanwhile, applicability of testing standards and the effect of manufacturing-induced defects on fracture properties are thoroughly discussed. The results show that existing standards provide sufficient support for characterising fracture properties of bonded pultruded plates; and that manufacturing-induced defects can be detrimental to crack propagation and cause more brittle behaviour in mode I dominant cases, while beneficial effect of defects by toughening the interface was exhibited in mode II dominant cases.@en

This article presents an overview of methods for analyzing the facture and failure of adhesives. Special attention is given to stress analysis in adhesive bonds, as the difficulty of performing an accurate stress analysis is a major limitation of many failure analysis methods. The article also covers the effect of manufacturing and operational environment, as well as long-term durability issues such as creep and fatigue.

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Delamination growth is a key damage mode threatening the structural integrity of fibre reinforced polymer composite structures. To guide design and damage management of composite structures, research efforts have been made to understand delamination behaviours and establish standardized evaluation methods based mainly on one-dimensional delamination tests. However, as most delamination growth in real structures will be planar, the question arises whether these approaches are adequate to evaluate planar delamination behaviour. In this study, a novel experimental method was developed to investigate the planar delamination behaviour under quasi-static out-of-plane loading. The planar central loaded split (PCLS) specimen was designed to investigate the planar delamination behaviour under mode II loading condition. By analysing digital image correlation (DIC) and C-scan data, the delamination progress was monitored. An acoustic emission (AE) system was used to capture the initiation of damage and to identify different damage types. The planar delamination growth was found to be dependent on the stacking sequence and interface properties. Additionally, it was found that positioning a rubber mat between the indenter and the specimen prevented the occurrence of delaminations at undesired interfaces. The artificially embedded delamination propagated in the direction to which the fibre orientation of the layer above the crack interface was parallel, but migrated initially to an upper interface at the place where the fibre was perpendicular. A constant increase in the load was observed even though the delamination propagated. The significant drop of loading seen at the end of the test was attributed to the substantial surface cracking. The research results provide a clearer understanding of the mechanisms of planar delamination under out-of-plane loading. Furthermore, combining with the experimental results, numerical simulation will be conducted to characterize planar delamination behaviour qualitatively and quantitatively, thus to establish a more reliable assessment method for planar delamination growth@en

In previous literature, a plateau phase in fatigue growth of impact delamination projected area in CFRP was found. Explaining this plateau phase still represents a knowledge gap. In the present work, echo-pulse and through thickness transmission ultrasonic scan inspections were combined with acoustic emission monitoring to explain this plateau phase. Before the onset of growth outside of projected area, growth of delamination in the central impact cone and growth of smaller delamination was observed. Low-frequency acoustic emissions localized in the impact damage were recorded even before any type of growth was measured. The provided evidence suggests that an actual plateau phase may not exist if all the damage mechanisms are properly considered.

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Quasi-static or cyclic loading of an artificial starter crack in unidirectionally fibre-reinforced composite test coupons yields fracture mechanics data—the toughness or strain-energy release rate (labelled G)—for characterising delamination initiation and propagation. Thus far, the reproducibility of these tests is typically between 10 and 20%. However, differences in the size and possibly the shape, but also in the fibre lay-up, between test coupons and components or structures raise additional questions: Is G from a coupon test a suitable parameter for describing the behaviour of delaminations in composite structures? Can planar, two-dimensional, delamination propagation in composite plates or shells be properly predicted from essentially one-dimensional propagation in coupons? How does fibre bridging in unidirectionally reinforced test coupons relate to delamination propagation in multidirectional lay-ups of components and structures? How can multiple, localised delaminations—often created by impact in composite structures—and their interaction under service loads with constant or variable amplitudes be accounted for? Does planar delamination propagation depend on laminate thickness, thickness variation or the overall shape of the structure? How does exposure to different, variable service environments affect delamination initiation and propagation? Is the microscopic and mesoscopic morphology of FRP composite structures sufficiently understood for accurate predictive modelling and simulation of delamination behaviour? This contribution will examine selected issues and discuss the consequences for test development and analysis. The discussion indicates that current coupon testing and analysis are unlikely to provide the data for reliable long-term predictions of delamination behaviour in FRP composite structures. The attempts to make the building block design methodology for composite structures more efficient via combinations of experiments and related modelling look promising, but models require input data with low scatter and, even more importantly, insight into the physics of the microscopic damage processes yielding delamination initiation and propagation.

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Impacts on carbon fiber reinforced composites (CFRP) can produce a complex internal damage comprising multiple delaminations, which is hard to detect from visual inspection. This situation is known as barely visible impact damage (BVID). Considering that every airplane faces several impacts during its operational life, and that the majority of exposed surfaces in new generation aircraft is made of CFRP, there is a high chance that some aircraft will be flying with unnoticed impact damage. For this reason, BVID damage tolerance must be taken into account in design. The FAA and EASA dictate a no-growth design philosophy for BVID. Although multiple delaminations are present, BVID fatigue growth is usually assessed by measuring only the projected delaminated area with ultrasound inspections. This is done to simplify the damage description and because of the limitations in ultrasound inspection methodologies. In the present work, we show two cases of delamination propagation that are neglected following this procedure. Our experimental monitoring of delamination propagation with different ultrasound techniques shows a) growth inside the impact cone and b) faster growth of shorter delamination. The conclusion is that the projected area description is insufficient, since a no-growth in the projected area does not necessarily correspond to a no-growth in the actual damage.@en

Although several studies have been performed, the compression after impact (CAI) failure of CFRP is still not entirely understood. It is still unclear what sequence of events determines the onset of failure in CAI tests and how the different damage modes are involved in this process. To experimentally investigate this matter, the present work relies on acoustic emission (AE) monitoring and advanced acoustic signal analysis. A series of preliminary tests was conducted to correlate damage modes with recorded acoustic waveforms. Four types of waveforms were separated and associated to different damage modes. Following the preliminary tests, AE was monitored in actual CAI tests. A damage accumulation study was conducted combining three indicators, namely wavelet packet components, sentry function and energy b-value. The results evidence different phases in the damage accumulation process that were not shown in previous literature. In all specimens, the onset of the unstable damage accumulation appeared to be triggered by an intermediate frequency acoustic event associated to a combination of matrix cracking and fiber-matrix debonding, occurring at 80% of failure displacement.

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Fibre reinforced polymer composites have found increasing use in aircraft structures. This means that fleet managers need damage assessment tools for such materials, in order to decide on appropriate sustainment strategies. Developing such tools is hindered by the difficulty of generalising from lab tests to predict the behaviour of full-scale structures. A more focussed research effort is needed to address the knowledge gaps that prevent such generalisations. This paper highlights the limitations of current non-destructive inspection techniques, and how this can lead to misinterpretation of experimental results. Furthermore, it discusses differences between coupons and full-scale structures caused by 1D vs 2D delamination growth and the effects of lay-up. New test methods to deal with these issues are discussed and some preliminary results are presented. Finally, the paper discusses the prediction of residual strength and the difficulties of determining a suitable end-point for damage propagation analyses. While the residual strength in the presence of a given damage can be accurately modelled for small components, if sufficiently computational resources are available, further development is needed to develop tools that can be implemented for sustainment of operational aircraft.@en

The fatigue behavior of an adhesively bonded joint is of key importance to its long-term structural integrity. However, the complex stress distribution inside a bonded joint does not make it easy to evaluate the fatigue behavior. This chapter discusses the various approaches that have been developed for predicting the fatigue behavior of adhesive bonds, including stress-life methods, damage mechanics, strength and stiffness wearout, fracture mechanics, and damage mechanics. It also covers experimental and numerical techniques and reviews the current understanding of the effect of joint geometry and environment.

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In this paper, the effect of interface properties on the compressive failure behavior of 3D woven composites (3DWC) is investigated by incorporating a micromechanics-based multiscale damage model (MMDM). The correlation between the mesoscopic stress of yarns and microscopic stress of constituents is established by defining a stress amplification factor. With the microscopic stresses, the fiber breakage and matrix failure can be separately evaluated at the microscale, without assuming the yarns as transversely isotropic homogeneous materials. Especially, the interfacial debonding between yarns and matrix is also a dominant damage mode within 3DWC. Given that there is still a lack of studies on the influence of interfacial properties on the compressive failure behavior of 3DWC, it is meaningful to perform numerical parametric studies to reveal how the interface properties contribute to the damage mechanisms of 3DWC under compressions. The predicted results indicate that with the increase of interface strengths and fracture toughness, the compressive resistance of 3DWC can be significantly improved, resulting in higher strength and failure strain. Additionally, the studied 3DWCs with weak, medium and strong interfaces exhibit different damage development processes.

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In this paper, a reliable progressive fatigue damage model (PFDM) for predicting the fatigue life of composite laminates is proposed by combining the normalized fatigue life model, nonlinear residual degradation models and fatigue-improved Puck criterion. To balance the accuracy of life predictions and computational efficiency, an adaptive cyclic jump algorithm is developed and implemented within the PFDM. The sensitivity of life prediction to cyclic jump parameter has been greatly reduced by correlating the cyclic jump with the increment time and viscous coefficient. Therefore, the cyclic jump parameter can be arbitrarily selected within a relatively large range to obtain convergent results. When incorporating the adaptive cyclic jump algorithm, there is no need to define a standard for determining the material failure in numerical calculations, which effectively eliminates an artificially induced uncertainty in life predictions. Two sets of experiments are conducted to validate the proposed PFDM. The numerical predictions including static failure strength and fatigue life correlate reasonably well with the available experimental data.

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A novel micromechanics-based multiscale progressive damage model, employing minimal material parameters, is proposed in this paper to simulate the compressive failure behaviours of 3D woven composites (3DWC). The highly realistic constructions of microscopic and mesoscopic representative volume cells are accomplished, and a set of strain amplification factor is employed to bridge the meso-scale and micro-scale numerical calculations. Considering that the multiple failure mechanisms of 3DWC under compression are all caused by the matrix failure from the microscopic perspective, a new method incorporating the micromechanics of failure (MMF) theory and 3D kinking model is developed to identify the micro matrix failure associated with the kinking of yarns, inter-fiber fracture and pure matrix failure. As a result, only the matrix parameters are required for the failure simulation of 3DWC, eliminating the necessity of using other material parameters such as the fracture toughness and failure strengths of fiber yarns, which are generally difficult to accurately obtain through experiments. The newly proposed damage model is numerically integrated into ABAQUS with a user-defined subroutine UMAT. The numerical predictions and the experimental results exhibit good agreement, verifying the feasibility and accuracy of the novel damage model.

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The effect of ply thickness on the notch sensitivity and bearing properties on carbon fibre reinforced polymer composites and their hybrid laminates with steel foils were studied. Laminates with ply thicknesses of 0.3 mm and 0.03 mm comprising of CFRP and hybrid laminates were manufactured and characterized using tension, open hole tension and double lap bearing tests. A 25% ply substitution was found to double the bearing load with extensive plastic deformation in the joint while maintaining high stress and maintaining constant cross-sectional thickness in the laminate. With a good agreement between the finite element predicted values and failure behaviour, the damage initiation and progression behaviour could be observed experimentally. We numerically captured (i) rapid failure of 0° plies in the thin ply CFRP hybrid and (ii) continuous delamination with significant plastic deformation for the thick ply CFRP hybrid. The numerical results significantly reduce future experimental work when designing hybrid laminates and could allow the laminate lay-up to be tailored for load cases. Both the experiments and numerical models underline the distinct size effects occurring with respect to the ply thicknesses when hybridising a very ductile metal with a brittle yet strong composite material.

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Using the slow-growth certification approach for damage tolerance of composite aircraft structures has the potential to reduce their weight. Applying this approach requires that damage growth is slow, stable, and predictable. However, currently available methods do not allow for sufficiently accurate predictions, due to knowledge gaps related to damage characterisation, prediction of damage growth, and prediction of final failure. This article highlights these knowledge gaps, discusses the limitations of the current state of the art and research approaches, and identifies possible ways forward.

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The effect of adhesive thickness on fatigue crack growth in an epoxy film adhesive (FM94) was investigated, using a combination of experiments and numerical modelling. For the range of thicknesses investigated an increased thickness led to an increased crack growth rate. It was found that the energy required per unit of crack growth did not depend on the adhesive thickness. In contrast, the energy available for crack growth does depend on the adhesive thickness. The numerical analysis confirms that the energy required per unit crack growth is not sensitive to the adhesive thickness, but that the plastic energy dissipation increases with the thickness. The experimental results imply that this increase of plasticity has an anti-shielding effect, as the crack growth rate is increased.

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