Abstract. The manufacturing of composite materials can be ensured by several industrial processes, like Liquid Composite Molding (LCM). This technology is used to produce composite parts with complex geometries because it provides a very good compromise in terms of repeatability, production rates and cost. During the forming phase of an LCM process, a fabric can be formed by highly double curved punch geometries where it could be submitted to several deformations and mechanical stresses that lead to the appearance of different types of defects: buckles, gapping, in-plane pull-out, etc. In order to understand their phenomenology, different types of defects were generated inside samples of glass and carbon fabrics so as to understand their mechanisms of appearance. This work focuses on the phenomenology of appearance of forming defects and the definition of experimental parameters allowing the generation of calibrated defects, such as buckles and gapping, inside samples of glass and carbon woven fabrics. The definition of these parameters allows the manufacturing of composite parts with calibrated defects, which in turn helps to define the influence of these defects on the mechanical behavior of composites materials.