- 2018 Plausible mechanisms of transverse crack formation in unidirectional (UD) composites under applied tension normal to fibres are investigated numerically using a finite element model. Two initial scenarios are considered: Scenario 1 where a pre-existing single fibre/matrix debond crack kinks out into the matrix and induces fibre/matrix debonding at neighbouring fibres, and Scenario 2 where multiple pre-existing debond cracks link up by the debond growth and crack kink-out process. The 2-D finite element model consists of a circular region of matrix with a central fibre surrounded by six fibres in a hexagonal pattern. The region is embedded in a homogenized UD composite of rectangular outer boundary. Energy release rates (ERRs) of interface cracks and kinked-out cracks are calculated under applied tension normal to fibres. Results show that Scenario 2 is more likely to lead to formation of a transverse crack than Scenario 1. These results provide understanding of the roles of fibre clustering and fibre volume fraction on transverse crack formation in composites.