Composite sandwich panels offer high specific bending stiffness, delivering opportunities for lightweight design. However, sandwich components of complex shapes can be costly to manufacture, as machining operations on the core are required in addition to moulding processes for the skins.
In well-established high-volume applications, such as lightweight automotive interior load floor components, a comparatively low-cost composite sandwich is manufactured by pressing the core material into shape simultaneously with the skins, in a single forming operation. While the forming process causes local buckling or crushing of the core structure, the mechanical performance of the finished component has been found sufficient for this type of application [
]. Typical material combinations for press-formed sandwich structures include glass fibre composite skins, usually made from chopped strand mat with fibre lengths greater than 25 mm, hexagonal paper cores and polyurethane foaming resin. The reinforcement in the skins is wet out with the resin. As the resin expands during cure, it partially fills the open cells of the core (at the interface with the skins), which allows strong bonds between the core and skins to develop. Variants of this process exist, using different types of cores and carbon fibre textile skins, for higher structural requirements.
During manufacture of panels with variable cross-sections and complex curvature, the core can be subjected to multi-axial deformation modes including in-plane tension, compression and shear, as well as through-thickness compression and out-of-plane bending [
]. Simultaneously, the reinforcement skins are subjected to in-plane shear and out-of-plane bending while the fabric is formed. There may also be relative movement between the core and the skins during forming. Typical issues occurring in the process are wrinkling of the skins due to local fibre buckling, tearing of the core due to excessive in-plane stresses, and poor conformity of the sandwich assembly to the tool surfaces due to low through-thickness stiffness of the crushed core.
The application of simulation techniques facilitates assessing the feasibility of the process prior to implementation. However, a modelling approach to capture forming induced issues and to enable optimisation of process parameters does not currently exist. Almost all available modelling techniques address sandwich load-carrying capability including damage prediction [
]. A model would be required for simulation of the sandwich forming process in order to predict forming-induced defects. While through-thickness crushing of the honeycomb core is considered a failure mode in structural engineering [
], it is a forming mechanism in component manufacture. Also, existing finite element (FE) models are developed for sandwich structures where the skins are bonded to the core which is essential for load transfer [
]. However, the forming process relies on relative slippage between skins and core. Hence, core-skin adhesion is not an issue here.
A complete process model requires integration of three concurrent effects: forming of the reinforcement skins, multi-axial deformation of the core structure, and frictional effects (tool-reinforcement and reinforcement-core). High-fidelity predictive modelling of reinforcement forming (draping) is well advanced. Typically, woven and non-crimp fabrics can be modelled using a non-orthogonal constitutive model, incorporating multiple plies and frictional effects [
]. The multi-axial deformation behaviour of the core is complex, requiring a material model that reflects the forming characteristics arising from the distortion of the cellular structure and through-thickness crushing of the core, resulting from inelastic buckling and folding of the cell walls. The core structure can be modelled in detail at the meso-scale using shell elements to represent the cell walls at an appropriate mesh density to capture the inelastic crushing process. However, this approach is not feasible for component-scale models, as a large number of finite elements would be required, which would result in long CPU times.
As an alternative, macro-scale models for the core material can be developed, which are based on volumetric finite elements and homogenisation of the core structure to obtain effective mechanical properties. These can reduce the overall number of degrees of freedom in the model and minimise the risk of numerical instability. A disadvantage of this approach is that it requires multi-axial, non-linear material properties as input, which may be hard to characterise experimentally. In addition, the interaction between skins and core is complex.
The present work aims to develop a non-linear explicit FE model of the sandwich panel forming process and to validate it against experimental studies, including a full-scale technology demonstrator.
To simulate the process of forming sandwich panels comprising composite skins and honeycomb cores into complex shapes with variable thickness, FE models considering effects of shearing of the reinforcement skins, multi-axial deformation of the core structure, and frictional effects at the interfaces were developed. The behaviour of the core was modelled at two different scales. Predominant forming mechanisms and limits of formability were identified for different forming scenarios using a meso-scale model, which was based on measured properties of the honeycomb cell walls. Results indicate that the formability for sandwich panels with complex curvature is primarily derived from bending, if the core is free to deform, whereas local through-thickness crushing of the core becomes more important in the presence of stronger constraints. As computational costs are high, meso-scale models are only suitable for simulations of sandwich panels of limited size. A macro-scale model was developed for simulation of larger components, implementing homogenised properties determined from a series of experiments on the honeycomb structure. Here, mechanical field variables were used to correlate deformation mechanisms observed at the meso-scale. Forming of a flat sandwich panel blank into a generic 3D component was simulated at the macro-scale. The developed macro-scale FE model was employed to optimise iteratively the blank shape for net-shape forming. For the optimum blank shape, predictions from the simulations were compared with properties of a physical component. Simulation results were found to be accurate in predicting localised fibre bridging and poor conformity of the sandwich to the tool as well as fibre shear angles in the skins which are below the threshold for fabric wrinkling. This validation indicates the suitability of the proposed modelling approach for industrial application.
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