This thesis deals with the application of the coupled criterion to characterize the first steps of failure mechanisms in composites. The coupled criterion was developed in the context of the finite fracture mechanics and is based on assuming that a crack of a finite extension appears abruptly when a stress criterion is fulfilled and this crack onset is energetically admissible. Using this criterion, the critical load leading to a crack onset can be expressed as a function of well-known physical properties of the material as strength and fracture toughness. The thesis outline can be divided...