Fibre waviness induced bending in compression tests of uniderectional NCF composites

Compression testing of carbon fibre composites according to ASTM D6641/D3410 is limited by a maximum of 10% bending for a valid test. This allowable was exceeded in a preceding study, where all the laminates had a large out-of-plane waviness. The aim of this study is to quantify the contribution from the out-of-plane fibre waviness to bending. The fibre waviness was characterized on samples with high magnitudes of bending and the corresponding fibre misalignment angles were then mapped to a plane strain finite element model. This model represents the geometry through the thickness and in longitudinal direction. Virtual strain measurements and associated bending calculations could then be performed in Matlab from the strain field on the upper and lower surfaces, which resulted from compression loading. Virtual bending calculations have also been performed from strain measurements with an optical system (DIC). The numerical model confirms that the out-of-plane waviness has an effect on bending. The magnitudes are however lower than expected, and lower than the experimental values. Bending was in the order of 5 % with a strain gauge of 5 mm, which constitutes half of the allowed amount of bending. It was also confirmed that the length of the strain gauge has a significant effect on the measured bending and on the experimental error