- 2018 Elsevier Ltd Transverse crack formation in epoxy based unidirectional composites is treated by means of a percolation concept. A previously proposed criterion for brittle cavitation in epoxies is used as the criterion for fibre/matrix debonding. Then, neglecting the details of debond linkups, a transverse crack is assumed to form when the intensity of the linkup process crosses a threshold analogous to the percolation threshold. The predicted applied strain to transverse crack formation based on this assumption agrees well with experimental data. A parametric study is conducted to reveal the effects of thermal cooldown temperature, fibre volume fraction and fibre elastic properties on the percolation threshold for transverse crack formation.