This work evaluates the ability of cohesive zone modeling-based approaches to predict delamination in composite materials that develop large process zones under complex loading conditions. The R-curve effects subjected to static and fatigue loading under multiple loading modes, considering the loading history, are analyzed. To this end, the delamination predictions of a state-of-the-art CZM-based simulation strategy are evaluated by blind simulation of a validation benchmark test. The validation test promotes a non-self-similar delamination scenario, including a process zone that evolves under different loading mode conditions with a non-straight leading delamination front. Good delamination prediction accuracy is achieved. In addition, insights into the relationship between the features of the simulation strategy and the physics of the delamination process are discussed. With regard to the limitations of the simulation strategy, particular attention should be paid to modeling the contribution of an evolving process zone based on the loading mode history.

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The main objective of this paper is to present a delamination benchmark test concept for composite materials that develop non-self-similar delamination in characterization specimens. The non-self-similar delamination is induced by rotating the loading blocks. The simplicity of the test allows for analyzing the loading mode history by concatenating different loading conditions, such as static and fatigue loading, under multiple loading modes. The methodology introduced in this paper can be particularized for any given composite material set and any sequence of loading conditions. To demonstrate the capabilities of the benchmark test, a case study is presented using AS4D/PEKK-FC thermoplastic composite material, which exhibits strong R-curve behavior. A sequence of opening and shear failure modes was applied under static and fatigue loading, providing an experimental data set that is ready to be used as a part of the validation of numerical predictive delamination models. The delamination process was monitored by X-ray radiography, and the final fracture surfaces were analyzed with scanning electron microscopy (SEM), giving a physical insight into the contribution of the fracture mechanisms to the delamination process.

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Thermoplastic composite welding is a key technology that can help to make the aviation industry more sustainable, while at the same time enable high-volume production and cost-efficient manufacturing. In this work, characterization, testing and analysis of thermoplastic composite conduction welded joints is performed while accounting for the influence of the manufacturing process. Test specimens are designed from welds of a half a meter long welding tool that is developed to weld the stiffened structures of the next-generation thermoplastic composite fuselage. In the design, special attention is paid to the weldability of the laminates, while ensuring fracture occurs only at the welded interface. Two specimen configurations are evaluated for the Double Cantilever Beam and End-Notched Flexure characterization tests. Moreover, Single Lap-Shear specimens are tested in tension and in three-point-bending. Finally, the characterized material properties are introduced in finite element analyses to demonstrate that the cohesive zone modeling approach can be used to conservatively predict the strength of these welded joints. New insights are obtained in the relation between the manufacturing process, the quality of the weld and the mechanical properties of the joints, which are significantly different compared to autoclave consolidated composites.

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The complex failure mechanisms involved in failure of interfaces requires the use of an accurate description of the cohesive law. In recent years, there have been many developments to determine the full shape of the cohesive law. However, most of the existing cohesive zone models assume a simplified shape, such as bilinear, trapezoidal or exponential, which are usually simple to model. Their accuracy is found to be rather limited, especially in the presence of a large fracture process zone due to either plastic deformation or fibre bridging. In this work, a new cohesive element description is proposed to formulate a general cohesive zone model to overcome these limitations. The benefit of the new approach is that it allows for convenient implementation of any arbitrary shape of the cohesive law obtained experimentally. The authors present a new procedure based on the superposition of n-bilinear cohesive zones to obtain an equivalent multilinear cohesive law that fits any experimental measurement. The new element formulation has been implemented in the commercial finite element software ABAQUS, using user element subroutine. Verification of the methodology is performed at the single element level and the approach is validated for different material systems (adhesives and composites) using the double cantilever beam, end-notched flexure and mixed-mode bending tests. Excellent correlation between all numerical predictions and experimental results is obtained, demonstrating the robustness of the proposed methodology.

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Through the application of innovative production processes, thermoplastic composites might help the aviation industry become more sustainable. However, there is currently not much experimental understanding on the fatigue behaviour, and validated analysis methodologies on thermoplastic composites are rather limited. In this work, the fatigue onset and propagation interlaminar properties of AS4D/PEKK-FC thermoplastic composite were characterized under mode I loading. Testing costs were reduced by using a multi-fatigue testing rig that allows loading 6 specimens at the same time. The offset in the fatigue crack growth rate curves tested at different severities was explained by the fatigue R-curve effects. The offset was modelled by means of superposed fatigue cohesive laws. Different approaches on how to consider fatigue damage were compared with experimental data, giving an insight on how fatigue damage evolves, and developing a novel modelling strategy based on a robust method for fatigue model parameter identification for fatigue delamination when several failure mechanisms interact.

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The work presented in this paper investigates the ability of continuum damage models to accurately predict matrix failure and ply splitting. Two continuum damage model approaches are implemented that use different stress–strain measures. The first approach is based on small-strain increments and the Cauchy stress, while the second approach account for large deformation kinematics through the use of the Green–Lagrange strain and the 2nd Piola–Kirchhoff stress. The investigation consists of numerical benchmarks at three different levels: (1) single element; (2) unidirectional single ply open-hole specimen and (3) open-hole composite laminate coupon. Finally, the numerically predicted failure modes are compared to experimental failure modes at the coupon level. It is shown that it is important to account for large deformation kinematics in the constitutive model, especially when predicting matrix splitting failure modes. It is also shown that continuum damage models that do not account for large deformation kinematics can easily be adapted to ensure that the damage modes and failure strength are predicted accurately.

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The capability of joining two thermoplastic composite parts by welding is a key technology to reduce the weight and cost of assembled parts and enables high volume manufacturing of future aeronautical structures made of thermoplastic composite materials. However, there is not much experimental understanding of the mechanisms involving welded joint failure, and the computational tools available for the simulation of thermoset composites have not yet been completely assessed for thermoplastic materials. In this work, a numerical and experimental evaluation is performed to investigate the strength and failure behavior of conduction welded thermoplastic composite joints. A welded single lap shear joint is designed, manufactured, tested and analyzed proposing two distinct modeling approaches. A simplified modeling strategy which only accounts for damage at the weld is compared to a high-fidelity model which can take into account the physical failure mechanisms at the lamina level. The high-fidelity modeling methodology is able to predict the experimental failure mode of the investigated welded joints with high accuracy and is used to gain new insights into the key-variables that influence the strength of thermoplastic welded joints. It is also found that the joint strength is highly influenced by the failure mechanisms not only of the welded interface but also of the surrounding plies.

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Thermoplastic composites can enable the development of new manufacturing techniques to make the aviation industry more sustainable while at the same time greatly benefit cost-efficient and high-volume production. One of the thermoplastic composite materials that can enable this transition is AS4D/PEKK-FC. In this work, the interlaminar properties of AS4D/PEKK-FC thermoplastic composite are characterized and analyzed by means of Mode I, II and Mixed Mode I/II at 50:50 tests, while considering fiber bridging and R-curve effects. In order to achieve stable crack propagation the test configurations are adjusted to account for the large fracture process zone ahead of the crack tip and an appropriate data reduction method is selected. The experimental data is reduced using an inverse methodology to extract cohesive laws based on only the load–displacement curves. Additionally, the use of this methodology provides new insights into the validity of two different mode II tests and the influence of fiber bridging on the mixed-mode interlaminar behavior. The interlaminar damage mechanisms are investigated by means of scanning electron microscopy. The resulting cohesive laws are implemented in commercial finite element software in tabular form, without the need for user-subroutines. All experimental test configurations are analyzed using a single material card and it is shown that fiber bridging and R-curve effects are well captured.

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This paper summarizes the implementation of a Virtual Testing methodology in an industrial environment to predict the mechanical behaviour of composite material through the different scales of the conventional physical testing pyramid. A robust Virtual Test Lab allows for the generation of virtual allowables, while advanced interface models ensure accurate simulation of critical interfaces up to structural level. Hybrid simulation-physical testing approaches, that can support both conventional rapid sizing and high-fidelity analysis methods, have been applied during the development of a thermoplastic orthogrid fuselage shell and will be coupled with the manufacturing process though a virtual process chain for the development of a thermoplastic fuselage for the next generation aircraft. @en