Prediction of forming effects in UD-NCF by macroscopic forming simulation – Capabilities and limitations

Abstract

Unidirectional non-crimp fabrics (UD-NCF) provide the highest lightweight potential among dry textile materials. Compared to multiaxial NCF, the fabric layers in UD-NCF enable a more targeted tailoring. Compared to woven fabrics, the fibres of UD-NCF are straight without weakening undulations. However, the formability of UD-NCF is more challenging compared to woven fabrics. The yarns are bonded by a stitching and the deformation behaviour highly depends on this stitching and on the slippage between the stitching and the fibre yarns. Moreover, distinct local draping effects occur, like gapping and fibre waviness, which can have a considerable impact on the mechanical performance. Such local effects are particularly challenging or even impossible to be predicted by macroscopic forming simulation. The present work applies a previously published macroscopic UD-NCF modelling approach to perform numerical forming analyses and evaluate the prediction accuracy of forming effects. In addition to fibre orientations and shear angles, as investigated in previous work, the present work also provides indication for fibre area ratios, gapping, transverse compaction and fibre waviness. Moreover, the prediction accuracy is validated by comparison with experimental tests, where full-field strains of inner plies are captured by prior application of dots onto the fibre yarns, by measuring them via radiography and applying a photogrammetry software. The modelling approach provides good prediction accuracy for fibre orientations, shear strains and fibre area ratio. Conversely, normal fibre strains, indicating fibre waviness, and transverse strains, indicating gapping, show some deviations due to the multiscale nature of UD-NCF that cannot be captured entirely on macroscopic scale.

Advances in composite forming through 25 years of ESAFORM

“…This work also includes specific problems as fabric-tool friction characterization [93], localization of transverse tension [86], superposition of transverse tension and shear [94], difficulties in assessing the strain fields via DIC, caused by the fabric surface distortion [95], compaction characterization [96], forming of NCF composites with continuous NCFs [97]. The special attention is on wrinkling and other local defects of draping [98][99][100][101] and its predictability during forming on part level [102]. Deformability of thermoplastic NCF sheets was studied in nearprocessing conditions in temperature and strain rate [103].…”

A unit-cell mesoscale modelling of biaxial non-crimp-fabric based on a hyperelastic approach

“…In the literature, macroscopic approaches are commonly used to predict the forming behavior of fibrous reinforcements [7][8][9][10]. However, such an approach may encounter difficulties when predicting some local defects, such as the gapping which occurs when the two adjacent yarns move away from each other and leave an empty zone without fibers.…”

Investigation of the Membrane Behavior of UD-NCF in Macroscopic Forming Simulations

“…The material parameters were inversely determined based on 30°-, 45°-and 60°-Off-Axis-Tension-tests (OATs). Its prediction accuracy was validated with qualitative [1] and quantitative [14] comparisons to component forming results. However, the complex model requires a high number of material parameters to describe the membrane behavior under multiaxial loading conditions.…”

“…Nevertheless, they can predict local forming effects, while macroscopic approaches only capture the deformations mechanisms occuring on the meso-scale in a homogenized manner to indicate areas with a high likelihood of e.g. waviness, gapping or compaction perpendicular to the carbon fibre bundles [14].…”