# Effect of Unit-Cell Size on the Barely Visible Impact Damage in Woven Composites  
[PERSON]  
1 Composite Centre, AMRC with Boeing, University of Sheffield, Rotherham 560 STZ, UK; [EMAIL_ADDRESS] (H.M.E.-D.); [EMAIL_ADDRESS] (M.S.A.) 1  
[PERSON]  
4 Structural Integrity & Composites, Faculty of Aerospace Engineering, Delft University of Technology, Delft 2628 CD, The Netherlands 4  
[PERSON]  
5 Henry Moseley X-ray Imaging Facility, School of Materials, University of Manchester, Manchester M13 9 PL, UK; [EMAIL_ADDRESS] 5  
[PERSON]  
1 Composite Centre, AMRC with Boeing, University of Sheffield, Rotherham 560 STZ, UK; [EMAIL_ADDRESS] (H.M.E.-D.); [EMAIL_ADDRESS] (M.S.A.) 1  
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enhance the impact resistance and minimise the damage to composites [2; 3; 4; 5]. For instance, [PERSON] and [PERSON] [6] studied the behaviour of low-velocity impact on three glass fibre laminates (multiaxial warp-knit, plain woven, nonwoven mat) with different thicknesses. The results showed that the dominant failure in thick laminate is the fibre fracture, while the thin laminate suffers from delamination mostly. In addition, by considering the fabric structure, they reported that noncrimp fabric (NCF) was more efficient in the impact resistance among composites laminate. [PERSON] and [PERSON] [7] analysed the low-velocity impact on a thin unstitched and stitched cross-ply laminate as a unidirectional carbon/epoxy tape. Two different cross ply lay-ups were used: [0\({}_{2}\)/90\({}_{2}\)]s and [90\({}_{2}\)/0\({}_{2}\)]s. It was found that the chosen combination of the lay-up is an important factor in stitching improvement. The results suggested that there was an increase in the postimpact residual properties in the stitched samples under high energy impacts. It was also reported that by increasing the stitch density the damage area was decreased by 33% and 16% for stitching densities of 25.4 and 12.7 mm, respectively [8]. [PERSON] et al. [9] and [PERSON] et al. [10] conclude that the importance of the thicker stitch thread becomes obvious as the impact energy becomes higher. [PERSON] and [PERSON] [11] studied the low-velocity impact on three different fibre architectures in terms of two factors: the energy absorption and the damage resistance. Those preform under investigation were unidirectional (UD), NCF and 3D woven architecture. They found that the 3D woven composites resist the most damages between the other samples while the UD laminate is the highest structure to absorb the energy among all the tested samples. [PERSON] et al. [12] studied the energy absorption in three different 3D architectures (orthogonal, angle interlock, layer-to-layer), and noticed that under 20 J, all the samples absorbed approximately 50% of the impact energy. As the impact energy increased to 40 J, the angle interlock and layer-to-layer absorbed almost all the impact energy, while the 3D orthogonal structure absorbed the least energy. A twofold improvement in energy dissipation was reported [1; 11; 12; 13], owing that to the existence of the z-binding yarns in 3D orthogonal structure. Beside improving the energy absorption, it acts as a restraint to the delamination and structure failure [14; 15]. [PERSON] et al. [16] explained, utilising the experimental and numerical approaches, that the preventions of the delamination depend on the zig-zag formation of the damages through the composite structure. Nevertheless to the authors' knowledge, no research in the open literature has investigated the effect of varying the unit-cell size of 3D woven composites on the impact damage resistance.