Double-sided stiffened composite panel and method for producing such a panel

1. A method for producing a stiffened panel comprising a laminated skin extending in two directions, made of a resin and a continuous fiber reinforcement, a first stiffener and a second stiffener, the method comprising the steps of;

drape forming the laminated skin from a continuous fiber reinforcement impregnated with a first thermoplastic resin having a first melting temperature Tf 1 ;

integrating in the drape forming on outer sides of the laminated skin and at predetermined locations for the stiffeners, films of a second thermoplastic resin, the second thermoplastic resin being at least partly miscible in the first thermoplastic resin and having a second melting temperature Tf 2< Tf 1 and not being degraded when the second thermoplastic resin is taken to the first melting temperature Tf 1 ;

compacting and consolidating the drape formed skin comprising the films of the second thermoplastic resin at the first melting temperature Tf 1 so that the films of the second thermoplastic resin are melted during the compacting and the areas in the predetermined locations are enriched in the second thermoplastic;

making the stiffeners from the continuous fiber reinforcement impregnated with the first thermoplastic resin, each stiffener is in the form of a beam made of a resin with the continuous fiber reinforcement,

assembling the skin and the stiffeners to provide an assembly, both in a compacted and consolidated state, by applying a contact pressure on the stiffeners placed on the skin at the predetermined locations and by raising a temperature of the assembly to the second melting temperature Tf 2 wherein the first stiffener extends on a first side of the skin in a first direction, the second stiffener extends on a second side of the skin in a second direction, and wherein the resins which make up the skin and the stiffeners are thermoplastic in nature and the stiffeners are closely joined to the skin by welding.

2. The method according to claim 1 , further comprising the step of placing a strip made of the second thermoplastic resin at an interface between the skin and the stiffeners at the predetermined locations where the stiffeners are assembled with the skin, before the stiffeners are applied to the skin.

3. The method according to claim 1 , wherein the first thermoplastic resin is a polyetheretherketone and the second thermoplastic resin is a polyetherimide.

4. The method according to claim 1 , further comprising the step of depositing a sealing agent on edges of the stiffeners at their interface with the skin.

5. The method according to claim 1 , wherein the step of making the stiffeners comprises the step of stamping a pre-consolidated blank at the first melting temperature Tf 1 ; and further comprising the step of integrating a layer made of the second thermoplastic resin before stamping on the surfaces of the stiffener to be in contact with the skin at the assembly step, the layer made of the second thermoplastic is melted during the stamping step.

6. The method according to claim 1 , wherein at least one of the stiffeners is joined to the skin by two parallel surfaces; and further comprising the step of inserting an inflatable core inside said at least one of the stiffeners between the parallel surfaces of said at least one of the stiffeners welded to the skin.