Real-time non-destructive inspection and image analysis of carbon fibre orientation using polarisation vision

Fibre orientation critically affects the mechanical performance of carbon fibre reinforced composites. Aligning fibres with the loading direction improves strength and stiffness, while slight misalignments reduce performance. This thesis employs a novel optical technique, primarily a polarisation camera, to study fibre orientation and enhance the efficiency and precision of the Tailored Fibre Placement (TFP) technique. Carbon fibres reflect polarised light when illuminated by an unpolarised source, allowing the Angle of Linear Polarisation (AoLP) to indicate fibre orientation. XPL software was used to process and display the raw saturation, Degree of Linear Polarisation (DoLP), and fibre orientation images (in RGB and HSV formats) as real time video. In HSV, the hue channel encodes AoLP, and saturation encodes DoLP. The raw saturation image helps identify over- and underexposed areas, while the grayscale DoLP image shows the polarisation level from 0% to 100%. During tensile testing, images were captured and analysed using an online inspection rig equipped with a polarisation camera, enabling real-time monitoring of fibre orientation in the HSV colour space. An offline rig was employed to assess the influence of lighting and to accurately evaluate fibre orientation both with and without epoxy. Fibre behaviour under strain was analysed by fitting a Gaussian curve to the AoLP histogram and applying colour thresholding to the hue channel. This study used a polarisation camera to examine fibre orientation around reinforced holes in Tailored Fibre Placement (TFP) samples during tensile testing. Symmetric and unbalanced loop reinforcements were compared to assess the effect on fibre orientation. The influence of resin-rich areas was also evaluated by varying the machined hole diameter within a constant reinforced area. The fibre orientation of dry carbon fibre samples with different architectures was initially examined under tension using a polarisation camera. Silicon rubber was then infused to assess orientation at an intermediate stage prior to epoxy resin use. Finally, fibre orientation around machined holes of varying diameters was investigated in woven and unidirectional samples during tensile testing. The findings suggest that slight changes in the placement of supporting stitching in non-crimp fabric, (the position of the machined hole relative to the thread holding the carbon fibre), influence the strength of the composites. Furthermore, the results demonstrate that the stitching position of the reinforced hole in the TFP-reinforced hole sample and the loop-back path of the stitching have a notable impact on the sample's mechanical properties.