A point in space has zero dimensions, a line has one dimension, an area has two dimensions, and a volume has three dimensions. A standard formula is N=rD, where D is the exponent and r the magnification factor. In Euclidean geometry, D has to be an integer. Mandelbrot  introduced the concept of fractals and self-similarity (e.g. Sierpiński Triangle, or Koch Snowflake), which can be quantified as Fractal Dimension (FD). FD occurs when D is a real number, and can be determined by rearranging the equation for D. FD determination is automated within ImageJ software  with the FracLac plugin . For patterns with, or without, self-similarity, either the Richardson Plot (length estimate vs length of scale is linear on a log-log plot) or the Box Counting Method (BCM) are applicable [4-7]. Figure 1 shows the boxes containing a feature, in this case resin-rich volumes (RRV), for a typical composite cross section as used in the determination of FD by BCM.
Figure 1: Representative images showing the stages of fractal data generation for a Brochier twill fabric
Pearce et al [8-9] measured reinforcement permeabilities and conducted composite tensile, compressive and interlaminar shear tests for three reinforcement fabrics. The satin weave provided the best mechanical properties, but the lowest permeability. The cross-plied Injectex satin weave, containing flow-enhancement tows (FET), exhibited a higher permeability than the standard satin but at the expense of mechanical performance. The twill fabric had significantly higher permeability than both the satin and Injectex fabrics, whilst, with the exception of ILSS, possessing slightly better mechanical properties than the Injectex. Image analysis to cross-sectional images revealed that FD was ranked in the same sequence as the tensile and compressive moduli and strengths. Subsequent analysis [9-11] considered the FD and measured permeabilities of the above, and new concept Carr FET, reinforcement fabrics (Figure 2).
Figure 2: Permeability plotted against fractal dimension
for Brochier and new concept Carr fabrics.
Summerscales et al  analysed 150 cross-sectional slices of x-ray computed tomography images of plain weave E-glass fibre-reinforced epoxy resin composites using fractal dimensions. The fractal dimension analysis returned a consistent numerical value for each of the slices in the two similar orthogonal planes.
Mahmood [13, 14] investigated RRV in fabric-reinforced composites. Fibre distribution, quantified from cross-sections as FD, was used as a parameter to characterise the static and fatigue properties of the composites tested in four-point bend. The ultimate flexural strength of the composites showed a clear dependence on the fibre distribution. Interlaminar shear strength (ILSS) also correlated with the fibre distribution albeit with lower significance.
Labrosse et al  used FD as a novel procedure for the assessment of the surface finish of in-mould gel-coated fibre-reinforced polymer matrix composites. The technique involves photographing the reflection from a coated surface on an opaque screen and analysing the image using fractal dimensions. The ranking of the quality of surface finish from this technique sensibly aligned with that obtained by human observation and by the commercial Wavescan DOI system. The investment required for the new system was primarily that required for a high-resolution digital camera, and hence is less than 5% of the cost of the commercial instrument.
Piasecki and Summerscales  reported a novel system to ensure use of the correct reinforcement fabric during composites manufacture. Three carbon fibre reinforcement fabrics (plain weave, single-tow twill and double-two twill) were characterised using a high-resolution scanner, images converted into binary, and then analysed using ImageJ/FracLac software to determine FD. The three fabrics each had a distinct FD value, in the undeformed condition and when sheared up to 30° (the locking angle). Implemented in a quality management system this analysis could eliminate the risk of an operator using fabric from the wrong roll leading to compromised mechanical properties in the component.
A summary review is also available .
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12. J Summerscales, PM Russell, S Lomov, I Verpoest and RS Parnas, The fractal dimension of X-ray tomographic sections of a woven composite, Advanced Composites Letters, 1 March 2004, 13(2), 113–121.
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14. AS Mahmood, Processing-performance relationships for fibre-reinforced composites, PhD thesis, University of Plymouth, Plymouth, 2016.
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