A method for producing a closed-section portion of an aircraft provided with deformation sensors comprising the steps of: a) inserting inside a mould equipped with a first part and a second part: - at least a first sheet of composite material, - at least a second sheet of composite material facing the first sheet and spaced apart therefrom; and - interconnection elements in a composite sheet material that extend between the first and the second sheets (3 and 4) of composite material to delimit an elongated cavity; b) inserting a tool (plug) inside the elongated cavity, said tool comprising a rigid elongated body delimited by a number of flat faces and having a cross-section corresponding to that of the cavity - the entire length of said rigid body being covered by and enveloped in a tubular sack made of a deformable material; c) creating a vacuum in an autoclave inside which the mould is arranged to allow expansion of the sack, which presses against the walls of the cavity during a first heating cycle in which the sheets of composite material fuse with each other; d) extracting the tool from the cavity at the end of the first heating cycle after having re-established the original pressure - at the end of the first heating cycle said cavity is delimitated by rigid walls, the method further comprising the steps of: e) preparing a flat tape having dimensions (l 1 by l 2) that allow it to be deposited on a face of the tool covered by a tubular sack made of a deformable material; f) permanently fastening a strain gauge on said flat tape; g) arranging said tape on the tool with its first face carrying the strain gauge facing the tool and the sack, and a second face facing the outside; h) depositing an adhesive layer on the second face of said tape; i) inserting said tool supporting said tape in said cavity; j) creating a vacuum and performing a second heating cycle to obtain the expansion of said vacuum sack, which performs the pressure gluing of said tape onto a wall of said cavity; k) extracting said tool from said cavity - at the end of said second heating cycle, said tape being integrally fixed on a wall of said cavity such that any deformations applied to said wall are transmitted to said strain gauge. a) inserting inside a mould equipped with a first part and a second part: - at least a first sheet of composite material, - at least a second sheet of composite material facing the first sheet and spaced apart therefrom; and - interconnection elements in a composite sheet material that extend between the first and the second sheets (3 and 4) of composite material to delimit an elongated cavity; - at least a first sheet of composite material, - at least a second sheet of composite material facing the first sheet and spaced apart therefrom; and - interconnection elements in a composite sheet material that extend between the first and the second sheets (3 and 4) of composite material to delimit an elongated cavity; b) inserting a tool (plug) inside the elongated cavity, said tool comprising a rigid elongated body delimited by a number of flat faces and having a cross-section corresponding to that of the cavity - the entire length of said rigid body being covered by and enveloped in a tubular sack made of a deformable material; c) creating a vacuum in an autoclave inside which the mould is arranged to allow expansion of the sack, which presses against the walls of the cavity during a first heating cycle in which the sheets of composite material fuse with each other; d) extracting the tool from the cavity at the end of the first heating cycle after having re-established the original pressure - at the end of the first heating cycle said cavity is delimitated by rigid walls, e) preparing a flat tape having dimensions (l 1 by l 2) that allow it to be deposited on a face of the tool covered by a tubular sack made of a deformable material; f) permanently fastening a strain gauge on said flat tape; g) arranging said tape on the tool with its first face carrying the strain gauge facing the tool and the sack, and a second face facing the outside; h) depositing an adhesive layer on the second face of said tape; i) inserting said tool supporting said tape in said cavity; j) creating a vacuum and performing a second heating cycle to obtain the expansion of said vacuum sack, which performs the pressure gluing of said tape onto a wall of said cavity; k) extracting said tool from said cavity - at the end of said second heating cycle, said tape being integrally fixed on a wall of said cavity such that any deformations applied to said wall are transmitted to said strain gauge.