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This paper describes a new approach of Nondestructive Evaluation (NDE) using fibrous sensors inserted inside composite reinforcements during their weaving. A flexible piezoresistive fibrous sensor has been developed and optimized for in situ structural deformation sensing in carbon composites. The sensor was inserted, in the weft direction, in three-dimensional warp interlock reinforcement during the weaving process on a special weaving loom. The reinforcement was then impregnated in epoxy resin and was later subjected to quasi-static tensile strain. It was found that the sensor was able to detect deformations in the composite structure until rupture since it was inserted together with reinforcing tows. The morphological and electromechanical properties of the fibrous sensors have been analyzed using scanning electron microscopy, tomography and yarn tensile strength tester. An appropriate data treatment and recording device has also been developed and utilized. The results obtained for carbon composite specimens under standard testing conditions (NF EN ISO 527-4 Plastiques, CEN, 1997) have validated in situ monitoring concept using our textile fibrous sensors.