Abstract: The rigidity of a panel of honeycomb material formed by parallel bands of thin metal spot-welded together and spread apart so as to form regular cavities, is broken by passing this panel in a gripped state between a plurality of cylinders 3, 4, 5 having a hard surface and whose axes are parallel and disposed in such manner as to produce in the panel as many changes in direction at roughly a right angle so as to fold or corrugate the walls of the cavities in a direction roughly parallel to the edges of the apices formed by the sides of the apices. A sandwich is formed by adhering at least one panel 6 treated in this way on a sheet 9 of a thermoplastic material, the sandwich is heated to a temperature for softening the thermoplastic material and the sandwich is placed on a form having the desired curvature and allowed to cool.