Analysis thermoplastic composites and conduction welded joints

Thermoplastic composites enable new manufacturing techniques such as conduction welding to make the aviation industry more sustainable, while at the same time, provide great benefits to cost-efficient high-volume production. One of the benefits of welding is that it reduces the amount of mechanical fasteners required. Fastener-free joining also poses new challenges, because the performance of these highly loaded structural joints relies heavily on the performance of the thermoplastic polymer matrix. Furthermore, there is currently not much understanding of the mechanisms involved in thermoplastic welded joint failure, and the numerical and experimental methodologies, originally developed and validated on thermoset composites, have not yet been fully assessed for thermoplastic composites. On top of that, the process conditions to manufacture these new structures may have a significant influence on the mechanical performance of the material and can thus play an important role in the design of thermoplastic composite structures.The objective of this research is to analyse matrix dominated failure of thermoplastic composites and conduction welded joints and to develop both experimental and numerical methodologies to support the design of thermoplastic composites structures. The research addresses important linkages between the three main pillars of Manufacturing, Experimental and Numerical analysis...

Accurate simulation of delamination under mixed-mode loading using a multilinear cohesive law

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.</p

Development of a Numerical Framework for Virtual Testing to Support Design of a Next Generation Thermoplastic Multifunctional Fuselage

This work summarizes the recent developments of a numerical framework to predict the mechanical behaviour of thermoplastic composites. It supports the design of a next generation thermoplastic multi-functional fuselage which uses advanced joining techniques such as thermoplastic welding to reduce both weight and cost by limiting the amount of mechanical fasteners required. At the lower end of the testing pyramid the framework is able to accurately predict typical preliminary design allowables such as laminate, open-hole and welded joints strength through a high-fidelity modelling approach. This information is then passed on to the structural level in a validated building-block approach to efficiently virtual test the compression strength of fuselage panels during post-buckling while also taking into account the influence of damages at the skin-stiffener interface.Aerospace Structures & Computational Mechanic

Characterization and analysis of the mode I interlaminar fatigue behaviour of thermoplastic composites considering R-curve effects

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.</p

Development of a Numerical Framework for Virtual Testing to Support Design of a Next Generation Thermoplastic Multifunctional Fuselage

This work summarizes the recent developments of a numerical framework to predict the mechanical behaviour of thermoplastic composites. It supports the design of a next generation thermoplastic multi-functional fuselage which uses advanced joining techniques such as thermoplastic welding to reduce both weight and cost by limiting the amount of mechanical fasteners required. At the lower end of the testing pyramid the framework is able to accurately predict typical preliminary design allowables such as laminate, open-hole and welded joints strength through a high-fidelity modelling approach. This information is then passed on to the structural level in a validated building-block approach to efficiently virtual test the compression strength of fuselage panels during post-buckling while also taking into account the influence of damages at the skin-stiffener interface

Skin-stringer separation in post-buckling of butt-joint stiffened thermoplastic composite panels

Two aeronautical thermoplastic composite stiffened panels are analysed and tested to investigate the buckling behaviour, the skin-stringer separation and the final failure mode. The panels are made of fast crystallising polyetherketoneketone carbon composite, have three stringers with an angled cap on one side, and are joined to the skin by a short-fibre reinforced butt-joint. The panels contain an initial damage in the middle skin-stringer interface representing barely visible impact damage. Finite element analysis using the virtual crack closure technique are conducted before the test to predict the structural behaviour. During the tests, the deformation of the panels is measured by digital image correlation, the damage propagation is recorded by GoPro cameras and the final failure is captured by high speed cameras. The panels show an initial three half-wave buckling shape in each bay, with damage propagation starting shortly after buckling. A combination of relatively stable and unstable damage propagation is observed until final failure, when the middle stringer separates completely and the panels fail in an unstable manner. The test results are compared to the numerical prediction, which shows great agreement for both the buckling and failure behaviour.Aerospace Structures & Computational Mechanic

The importance of accounting for large deformation in continuum damage models in predicting matrix failure of composites

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.Aerospace Structures & Computational Mechanic

Experimental and numerical evaluation of conduction welded thermoplastic composite joints

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.Aerospace Structures & Computational MechanicsExternenregistrati

Characterization and analysis of the interlaminar behavior of thermoplastic composites considering fiber bridging and R-curve effects

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.Aerospace Structures & Computational Mechanic