A computer-implemented method for designing a 3D modeled object that represents a mechanical part, wherein the mechanical part comprises a bent and/or stamped planar sheet, the method comprising: - providing (S10, S30), at a CAD system, a B-Rep skin that represents the mechanical part, the B-rep skin having a B-rep portion including: ▪ a boundary face bounded by one or more boundary edges of the β-rep skin, and ▪ one or more first faces each adjacent to the boundary face at a respective internal edge of the B-rep skin; - defining, by graphical user-interaction with the CAD system, a surface; - constructing, automatically by the CAD system, a B-rep replacement, the constructing including: o creating one or more second faces each adjacent to the boundary face and bounded by a respective boundary edge of the B-rep skin; ∘ generating a B-rep assembly which includes: ▪ one or more third faces each coincident with the surface, each third face having one or more edges, each edge of each third face being internal to the B-rep assembly, ▪ one or more fourth faces each coincident with a respective first face and adjacent to a respective third face, and ▪ one or more fifth faces each coincident with a respective second face and adjacent to a respective third face; and o removing from the B-rep assembly each fifth face; - replacing, in the B-Rep skin, the B-rep portion by the B-rep replacement; and - using the 3D modeled object for manufacturing a physical product corresponding in shape to the 3D modeled object. - providing (S10, S30), at a CAD system, a B-Rep skin that represents the mechanical part, the B-rep skin having a B-rep portion including: ▪ a boundary face bounded by one or more boundary edges of the β-rep skin, and ▪ one or more first faces each adjacent to the boundary face at a respective internal edge of the B-rep skin; ▪ a boundary face bounded by one or more boundary edges of the β-rep skin, and ▪ one or more first faces each adjacent to the boundary face at a respective internal edge of the B-rep skin; - defining, by graphical user-interaction with the CAD system, a surface; - constructing, automatically by the CAD system, a B-rep replacement, the constructing including: o creating one or more second faces each adjacent to the boundary face and bounded by a respective boundary edge of the B-rep skin; ∘ generating a B-rep assembly which includes: ▪ one or more third faces each coincident with the surface, each third face having one or more edges, each edge of each third face being internal to the B-rep assembly, ▪ one or more fourth faces each coincident with a respective first face and adjacent to a respective third face, and ▪ one or more fifth faces each coincident with a respective second face and adjacent to a respective third face; and o removing from the B-rep assembly each fifth face; o creating one or more second faces each adjacent to the boundary face and bounded by a respective boundary edge of the B-rep skin; ∘ generating a B-rep assembly which includes: ▪ one or more third faces each coincident with the surface, each third face having one or more edges, each edge of each third face being internal to the B-rep assembly, ▪ one or more fourth faces each coincident with a respective first face and adjacent to a respective third face, and ▪ one or more fifth faces each coincident with a respective second face and adjacent to a respective third face; and ▪ one or more third faces each coincident with the surface, each third face having one or more edges, each edge of each third face being internal to the B-rep assembly, ▪ one or more fourth faces each coincident with a respective first face and adjacent to a respective third face, and ▪ one or more fifth faces each coincident with a respective second face and adjacent to a respective third face; and o removing from the B-rep assembly each fifth face; - replacing, in the B-Rep skin, the B-rep portion by the B-rep replacement; and - using the 3D modeled object for manufacturing a physical product corresponding in shape to the 3D modeled object.