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.