1. Field
The presently disclosed embodiment relates to the general field of producing tanks made of high-performance composite materials, intended for storing pressurized fluids.
The presently disclosed embodiment applies in particular to tanks made of thermoplastic matrix composite materials, but may be applied to tanks made of composite materials comprising other types of matrices (radiation-polymerizable, thermosetting, etc. matrices).
More generally, the presently disclosed embodiment applies to the field of the transport and storage of fluids, optionally cryogenic storage, for example to the transport of liquefied gas by methane tankers, to the storage of liquid hydrogen for automotive applications, or to the liquid propellant tanks of launch vehicles such as the Ariane European launch vehicle.
2. Brief Description of Related Developments
High-performance tanks made of composite material are in general produced by the technique of filament winding (i.e. by winding) of (pre)impregnated fibers or else by a variant thereof such as fiber placement.
High-performance tanks are understood to mean tanks that are optimized in terms of weight/strength ratio, such as those used in particular in the fields of the space industry and the transport industry generally.
In the particular case of applications in the space industry, the use of high-performance tanks is induced by the need to store gases under pressure, optionally under cryogenic conditions, in other words at very low temperature.
The high-performance composite tanks intended in particular for storing pressurized fluids are generally designed by separating the functions of leaktightness and of mechanical resistance to the pressure. Such a tank thus comprises:                a shell made of metal or made of a polymer material, referred to as a “liner”, responsible for ensuring the containment of the fluid, that is to say the leaktightness of the tank and, optionally, the chemical protection of the wall made of composite material with respect to the fluid contained;        a composite coating that ensures the mechanical strength of the tank, resistance to the internal pressures in particular, which coating is formed of fibers deposited on the liner by filament winding, or by any equivalent method.        
Since the shell (liner) has no mechanical function, it is in principle thin, knowing that, in a high-performance tank, it is naturally sought to minimize the masses. Since this thinness gives it little mechanical strength, the liner is usually placed on a support mandrel.
However, for reasons of simplification of the manufacture, it is possible to give the liner other functions, in particular at the stage of producing the tank. Thus, in the case of small-sized tanks, the liner may be used as a winding mandrel during the deposition of the composite material fibers. It must therefore be able to withstand, without deforming, the forces induced by the deposition of the fibers, which requires the liner to have a sufficient thickness to withstand these forces
Similarly, the liner may be used as a reference surface of the composite structure deposited. It must then be able to exhibit a certain stiffness.
When such structures are produced, one problem to be taken into account is that of the behavior of the liner during the use of the tank, which is characterized by successive fillings and emptyings, therefore successive pressurizations, and also by thermal cycles in the case of cryogenic fluids. In particular, the emptying operations result in a compression of the liner by the composite wall.
Consequently, there are two cases:                either the tank is of small size (1 m3 typically). In such a case, the liner may have a sufficient thickness to withstand this compression without buckling, this thickness being, for example, imposed by the manufacturing principle of the liner or else by usage characteristics of the liner such as the direct use of the liner as a winding tool. There is thus no particular precaution to be taken regarding a bond between the liner and the composite wall.        or the tank is of large size or else the liner is very thin or has a very low stiffness. In such a case, it cannot withstand this compression. It is then necessary to connect the liner to the composite wall, in general by adhesive bonding.        
Consequently, there is therefore, in a good many production cases, a need to be able to connect the liner to the composite wall.
However, although the joining of two high-performance thermoplastic matrix composite materials by welding generally requires known joining processes (cf. the book by Michael J. Troughton entitled “Handbook of Plastics Joining” (Plastics Design Library) ISBN: 978-1-884207-17-4), no technique exists to date, among the known joining techniques, that makes it possible to join an element made of thermoplastic material to an element made of another material, especially a polymer or metal material, and that gives the joint produced the required qualities, in terms of mechanical strength in particular.
This is especially the case as regards the joining of the liner that forms a tank and of the protective wall that covers it.