EP0219421B1 - Sonar dome - Google Patents

Description

The invention relates to a dome for sonar, that is to say a dome for protecting the antennas of a hull sonar or of a towed sonar.

The antennas of -sonar systems must be mechanically protected against shock, flow noise, corrosion, etc. This protection is provided by a hollow or massive dome which must therefore be made of a material which is very mechanically resistant and acoustically transparent, at least in the frequency band used. Acoustically transparent materials are those whose density is close to that of water and where the speed of propagation of sound waves is substantially equal to that in water. The transmission coefficient is then close to 1.

It is known from US Pat. No. 4,307,457 to produce a sonar dome comprising a shell of glass fiber fabric and of resin, filled with thick foam.

It is known, in particular from French patent application No. 2 069 875, filed by the Applicant, to produce massive domes in thermoset polyurethane. As this material is soft, it makes it necessary to produce massive domes which are ill-suited when it comes to coating antennas of ellipsoidal or spherical shape because then these antennas are no longer accessible. In addition, near 0 _° Celsius, the sound transmission coefficient of this material becomes poor. This phenomenon is annoying because it sometimes takes several hours for the mass of the material to resume the water temperature. For the protection of antennas whose transducers are arranged on the surface of a ship's hull, it is known to produce a dome in the shape of a hull, in a composite material consisting of webs woven from glass fibers, impregnated with a resin polyester. This material has a modulus of elasticity (defined by the ratio between the applied stress and the resulting deformation) of a value such that it leads to relatively large shell thicknesses, resulting in poor transmission.

We also know a composite material consisting of carbon fibers impregnated with a resin, whose elastic modulus is higher, but we do not make domes entirely with this material because it is more expensive. US Pat. No. 3,858,165 describes domes made of glass fibers and polyester resin in which acoustic windows made of carbon fibers are provided to improve transmission. These windows are made of carbon fibers impregnated with polyester resin replacing part of the thickness of the hull in glass fibers and polyester resin, on the inner side of the hull. However, this structure retains the disadvantage of being heavy, and is difficult to build.

The object of the invention is to produce a dome having a low weight and good acoustic transmission, while being easy to construct. The object of the invention is a dome of composite material of the sandwich type comprising a layer of glass fibers and two layers of carbon fibers, impregnated with a resin, combined so as to avoid the drawbacks of domes of known types.

According to the invention, a sonar dome comprises a wall formed of several plies woven in fibers, impregnated with a resin and is characterized in that the wall comprises a central layer formed of several plies woven in glass fibers, impregnated with a resin, and two peripheral layers, located on either side of the central layer and formed of several layers woven of carbon fibers, impregnated with a resin.

• - la figure 1 montre en coupe un exemple de dôme de sonar selon l'invention;
• - la figure 2 montre, en l'agrandissant, une coupe de la paroi de cet exemple de réalisation;
• - la figure 3 montre le tissage d'une des nappes de fibres de carbone que comporte cet exemple de réalisation.

The invention will be better understood and other details will appear with the aid of the description below and the figures relating thereto:

• - Figure 1 shows in section an example of a sonar dome according to the invention;
• - Figure 2 shows, by enlarging, a section of the wall of this exemplary embodiment;
• - Figure 3 shows the weaving of one of the layers of carbon fibers that this exemplary embodiment comprises.

In Figure 1, the dome has a hull shape whose wall 3 is made in one piece, bolted on its periphery to the hull 1 of a ship. A sonar antenna, 2, fixed to the hull 1 of the ship, is located in the hollow of the dome and bathes in water 4, retained by the dome.

• - deux couches périphériques 6 et 8, constituées de trois nappes tissées, appelées "plis", formées de fibres de carbone;
• - une couche centrale 7 constituée de huit plis de fibres de verre.

Figure 2 shows, by enlarging it, a section of the wall 3. It is composed of a sandwich formed of three layers:

• - two peripheral layers 6 and 8, consisting of three woven layers, called "folds", formed of carbon fibers;
• - A central layer 7 consisting of eight plies of glass fibers.

The three layers 6, 7, and 8 are impregnated with a thermosetting epoxy resin. During the hardening of the resin, these three layers are strongly pressed together and polymerized, in an oven, with a rise in temperature and pressure adapted to the resin used and the choice of which is within the reach of the man of the art.

The central layer has a thickness of 5 mm. The two peripheral layers have an identical thickness: 1.5 mm, in order to obtain a stability of the material from a mechanical and thermal point of view, that is to say avoid deformations due to mechanical or thermal stresses.

The dome according to the invention has good acoustic transmission and great resistance to bending thanks to the carbon fibers which are located far from the neutral axis.

FIG. 3 represents an example of weaving one of the plies of carbon fibers constituting the layers 6 and 8. Wicks of carbon fibers are crossed with an identical pitch, for example 3, in the transverse direction and a pitch of 3 in the longitudinal direction due to the stiffness of the carbon fibers. The weaving of the glass fiber plies, which is not shown, is carried out with a ratio of 1/1.

The wall thus produced has a tensile elasticity module of 28 Gigapascals with an acoustic transmission coefficient of 98%. If the dome had been made entirely of glass fibers and polyester resin, the thickness necessary to obtain the same resistance would have been 16 mm and would have given a transmission coefficient close to 92%. In addition, the weight gain is in a ratio 2.

The invention is not limited to this exemplary embodiment. It is within the reach of those skilled in the art to use a type of resin other than epoxy resins, for example a thermoplastic resin.

It is also possible to use a different resin in each layer. For example, a resin particularly suitable for carbon fibers can be used in the two peripheral layers and another resin, particularly suitable for glass fibers, can be used for the central layer.

According to a variant, the wall of the dome according to the invention may comprise additional layers of materials having good acoustic transparency, for example an outer layer of aramid fibers impregnated with epoxy resin, in order to obtain a particularly high impact resistance.

Claims (4)

1. A sonar dome comprising a wall (3) constituted of several resin-impregnated woven fiber layers, characterized in that the wall comprises a central layer (7) formed of several resin-impregnated woven glass fiber layers and of two peripheral layers (6, 8) situated on either side of the central layer (7) and formed of several resin-impregnated woven carbon fibers layers.

2. A dome according to claim 1, characterized in that the two peripheral layers (6 and 8) have the same thickness.

3. A dome according to claim 1, characterized in that the three layers (6, 7, 8) are impregnated with the same resin of the epoxide type.

4. A dome according to claim 3, characterized in that the three layers (6, 7, 8) are submitted to a compression and a temperature rising during the hardening of the resin.

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