A multi-layered transparent unit and a process for manufacturing thereof

Abstract

A multi-layered transparent unit (1), comprising at least one pane (4) having a curved laminated composite structure, the at least one pane (4) comprising: − at least two curved transparent plies (5, 15) of which at least one is made of soda-lime or borosilicate or alumino-silicate glass or glass-ceramic, and − an interlayer (6) for coupling the two curved transparent plies (5, 15) stacked together, the interlayer (6) being interposed between the two curved transparent plies (5, 15) and being made of silicone-based or polyurethane-based material; − a contouring frame (8) having a groove configured to receive edges of the at least one pane (4).

Description

[0001] DESCRIPTION

[0002] A MULTI-LAYERED TRANSPARENT UNIT AND A PROCESS FOR MANUFACTURING THEREOF

[0003] Technical field

[0004] The present invention relates to a multi-layered transparent unit and to a process for manufacturing thereof.

[0005] The invention finds a direct application in aerospace engineering since it is specifically designed to be used in the outer space.

[0006] The present invention may also be used in terrestrial application. In particular, the invention is applicable in the following technical fields:

[0007] - automotive industry, especially for low-weight transparent surfaces to be used in electric vehicles, racing cars, supercars;

[0008] - shipbuilding, to produce large transparent windows in the ship hull, below the water line or submersibles;

[0009] - architectural applications, such as bulletproof, shockproof, explosionresistant glazing.

[0010] - aerospace and defense (canopies, protective shields).

[0011] Background art

[0012] The state-of-the-art in glazing for aerospace applications is represented by the observatory module presently installed on the International Space Station (ISS), usually referred to as the “Cupola”.

[0013] The Cupola was first conceived as a workstation for operating robotic arm, maneuvering vehicles outside the ISS, and observing and supporting spacewalks.

[0014] The Cupola is an 18 kN-weight aluminum structure, 2 meters in diameter and 1 .5 meters high.

[0015] The Cupola’s outer framework is made from a forged aluminum unit, with an inner skeleton made of aluminum plates and beams.

[0016] The “shirt sleeve” module is composed of six trapezoidal side windows, made of fused silica glass panes, plus a circular top window 80 cm in diameter. External shutters are used for protection against travelling debris and solar radiation, which also provide thermal insulation.

[0017] Each window has three subsections:

[0018] - two pressure panes to maintain cabin pressure and environment;

[0019] - an inner scratch pane to protect the pressure panes from accidental damage from inside;

[0020] - a debris pane on the outside to protect the pressure panes from space debris when the Cupola shutters are open.

[0021] The design lifetime is 10-year on-orbit. The entire window or the individual scratch and debris panes can be replaced in space. To replace an entire window, an astronaut should first fit an external pressure cover over the window during a spacewalk.

[0022] Concerning terrestrial application, glass and glass-based laminated structures are widely used in the architectural field. In recent decades, there has been a rapid worldwide development of structural elements and components based on load bearing structural glass.

[0023] The main glass-based structural components and systems may be classified into:

[0024] - laminated glass;

[0025] - insulating glass units;

[0026] - glass-based hybrid solutions.

[0027] Laminated glass is composed of two or more glass plies permanently bonded by one or more thermoplastic polymeric interlayers, with a treatment in autoclave at high pressure and temperature.

[0028] Three main commercial polymeric films are usually used as interlayers, i.e., Polyvinyl Butyral (PVB), Ethylene Vinyl Acetate (EVA), and lonoplasts. The permanent bonding of the glass plies with the polymeric films allows to reduce the risk of catastrophic collapse of the windows.

[0029] An insulating glass unit consists of two or more glass panes held together by structural edge seals, entrapping a gas in the gap between the panes for thermal and acoustic insulation. The panes, made of monolithic glass, laminated glass, or a combination of monolithic and laminated glass, are structurally fixed along their perimeter by various types of edge seal systems. The interaction between panes and gas is structurally beneficial because it permits the sharing among the different panes of the applied loads.

[0030] Glass-based hybrid solutions employ glass in combination with other materials, mainly steel, aluminum, timber, in the form of a reinforcing frame confining the borders of the glass pane. There are several examples, first proposed at the level of technological research, but now used in practice. The association with the contouring frame allows to mitigate the glass intrinsic brittleness and increases the post-breakage capacity.

[0031] It must be noticed that state-of-the-art solutions still show drawbacks.

[0032] In the Cupola, fused silica for the panes is chosen to reduce the thermal stress in the outer space, where no atmosphere can shield the solar radiation, thanks to the small coefficient of thermal expansion of silica. Nevertheless, fused silica does not allow to manufacture large size panes. Indeed, the diameter of 800 mm of the ISS windows can be considered an upper bound.

[0033] Furthermore, the production of curved plates is likewise very difficult because of the effect of gravity, which renders it difficult to pour a layer of constant thickness of fused material on a curved mold.

[0034] Another issue resides in the thermal mismatch between the silica and the aluminum frame, due to the very different coefficients of thermal expansion. This requires very thick gaskets to accommodate the relative displacement, which pose difficulties in manufacturing, considering that they shall be air-tight and made of a soft material (usually silicone), compatible with the outer space environment.

[0035] Last but not least, the mechanical properties of fused silica are quite poor: the critical stress intensity factor is of the order of 0.66 MPa m05, and the nominal bending strength approximately 60 MPa. Worst of all, on the contrary of glass, fused silica does not exhibit a static fatigue limit. Static fatigue is a subtle mechanism of damage, according to which the surface cracks (defects) can slowly grow in time even when the opening stress is below the critical limit. This implies that, whatever the level of the applied stress, there will be a time at which the pane breaks.

[0036] The panes of the Cupola cannot be manufactured using laminated glass as employed for terrestrial applications. As a matter of fact, the temperature in the pane is expected to vary from -100°C to +100°C during the time of revolution of the space station, which is approximately 90 minutes for low orbits. No commercial polymeric interlayer can withstand such a temperature range.

[0037] For this reason, the Cupola employs monolithic flat panels, which are nevertheless not robust since the onset of the first crack provokes catastrophic failure. In order to assure redundancy, two identical panes, each one able to withstand the same design pressure (interior approximately one atmosphere; vacuum at the exterior), have been installed in the ISS Cupola.

[0038] Apart from the increase in weight, a reason of concern is the maintenance of vacuum in the gap between the two panes. The inner pane shall carry the whole internal design pressure and, in case it breaks, the external pane has a fallback function.

[0039] However, due to the physiological leakage of the gaskets, the gap needs to be periodically re-evacuated, for example by means of a circuit of pipes and valves. This increases the structural complexity and is also prone to accidental damage.

[0040] Disclosure of the invention

[0041] In this context, the technical task at the basis of the present invention is to propose a multi-layered transparent unit and a process for manufacturing thereof, which overcome the drawbacks of the prior art cited above.

[0042] In particular, it is an object of the present invention to provide a multilayered transparent unit, which is lighter and achieves larger surface size than prior art solutions.

[0043] Another object of the present invention is to propose a multi-layered transparent unit having a higher fracture toughness and higher endurance limit under static and cyclic fatigue than prior art solutions.

[0044] Another object of the present invention is to propose a multi-layered transparent unit having better performance in terms of robustness and resilience (high post-breakage capacity) against accidental failure, or failure caused by the impact of small debris.

[0045] Another object of the present invention is to propose a multi-layered transparent unit which is resistant to cyclic thermal changes and to thermal shocks due to uneven temperature distribution in the pane.

[0046] Another object of the present invention is to propose a multi-layered transparent unit which is easier to maintain and fix than prior art solutions.

[0047] Another object of the present invention is to propose a process for manufacturing a multi-layered glass unit which shows larger size than prior art solutions.

[0048] The stated technical task and specified aims are substantially achieved by a multi-layered transparent unit according to one or more of the attached claims.

[0049] Brief description of drawings

[0050] Further characteristics and advantages of the present invention will more fully emerge from the non-limiting description of a preferred but not exclusive embodiment of a multi-layered transparent unit and a process for manufacturing thereof, as illustrated in the accompanying drawings in which:

[0051] - figure 1 shows the curves of subcritical-crack-propagation velocity as a function of the stress intensity factor measured for four different materials (silica, aluminosilicate, borosilicate glass, soda lime glass);

[0052] - figure 2 illustrates a multi-layered transparent unit, according to the present invention, in a perspective view;

[0053] - figure 3(a) illustrates the multi-layered transparent unit of figure 2, in a schematical cross-section view;

[0054] - figure 3(b) is an enlarged view of a portion close to the edge of the multi-layered transparent unit of figure 3(a);

[0055] - figures 4(a) and 4(b) show the results of the measured shear stiffness of silicone interlayers in a double lap shear test on specimens with the same composite package of figure 2, respectively at two different temperatures, i.e. at room temperature (20°C) and at high temperature (80°C);

[0056] - figure 5 is a schematic representation of an embodiment of space module employing a plurality of multi-layered transparent units of figure 2;

[0057] - figures 6(a) and 6(b) are schematic representations of two embodiments of the master frame of the space module;

[0058] - figures 7(a) and 7(b) illustrate the results of FEM analysis of the pressure pane of the multi-layered transparent unit composed of two plies and one interlayer, subjected to a pressure of 2 atmospheres, under various boundary conditions;

[0059] - figure 8 illustrates an embodiment of the contouring frame and gasket system forming the multi-layered transpared unit according to the present invention.

[0060] Detailed description of preferred embodiments of the invention

[0061] With reference to the drawings, number 1 denotes a multi-layered transparent unit or cell, also shortly referred to as “unit” or “cell” in the following description. As already stated, the multi-layered transparent unit 1 can be employed in aero spatial or terrestrial applications.

[0062] In this context, the term “transparent” means to have a light transmittance of at least 30%, calculated as indicated in EN 410:2011 and ISO 9050:2003.

[0063] The multi-layered transparent unit 1 comprises at least one pane 4 having a curved laminated composite structure which will be better described hereafter. According to one embodiment of the invention, shown in figures 2 and 3, the at least one pane 4 acts as an intermediate pane.

[0064] As a matter of fact, the multi-layered transparent unit 1 preferably comprises an internal pane 2 and an external pane 3, whereas the intermediate pane 4 is interposed between the internal pane 2 and the external pane 3.

[0065] In this context, the internal pane 2 is also referred to as “scratch pane”, the external pane 3 is also referred to as “debris pane” and the intermediate pane 4 is also referred to as “pressure plane”. These terms are commonly used in aerospace applications.

[0066] The intermediate pane 4 is distanced from the internal pane 2, therefore a first gap 10 is obtained between the intermediate pane 4 and the internal pane 2.

[0067] In aerospace applications, the pressure in the first gap 10 is the same of the interior of the space module, which will be disclosed later.

[0068] The intermediate pane 4 is also distanced from the external pane 3, therefore a second gap 11 is obtained between the intermediate pane 4 and the external pane 3.

[0069] In aerospace applications, the pressure in the second gap 11 is the same as the exterior (vacuum).

[0070] According to one aspect of the invention, both the internal pane 2 and the external pane 3 have a planar extension. In other words, the internal pane 2 and the external pane 3 can be substantially flat panes because they are not out-of-plane loaded. Such plates could also be curved to fit with the gabarit of the intermediate pane 4.

[0071] This is because they are not subjected to different pressures on the faces. Originally, the intermediate pane 4 has a curved laminated composite structure.

[0072] The intermediate pane 4 comprises:

[0073] - at least two curved transparent plies 5, 15 stacked on each other, and

[0074] - an interlayer 6 interposed between the two curved transparent plies 5, 15, for coupling them together.

[0075] This configuration of the intermediate pane 4, which has a curved laminated composite structure, is also referred to as a “shell” throughout the description.

[0076] In particular, the interlayer 6 is configured to permanently bond the two curved transparent plies 5, 15.

[0077] Preferably, the two curved transparent plies 5, 15 have substantially the same extension, i.e., the same size.

[0078] At least one of the two curved transparent plies 5, 15 is made of a transparent material chosen among: soda-lime glass, borosilicate glass, alumino-silicate glass, glass-ceramic.

[0079] The other of the two curved transparent plies 5, 15 can be made of sodalime or borosilicate or alumino-silicate glass or glass-ceramic, or polycarbonate or acrylic glass.

[0080] Therefore, in one embodiment, one of the two curved transparent plies 5, 15 can be made of a transparent material other than glass.

[0081] The interlayer 6 is made of a transparent temperature-resistant and radiation-resistant material. Silicone has proved to be effective. Alternatively, polyurethane-based adhesive materials could be used.

[0082] The multi-layered transparent unit 1 also comprises a contouring frame 8 made of a stiff material.

[0083] According to an embodiment of the invention, shown in figure 3b, the contouring frame 8 comprises a main frame (also indicated as 8) and an assembly of a plurality of parts made of stiff material, indicated as 8a, 8b, 8c, 8d, 8e.

[0084] As stiff materials, metals such as Titanium and Aluminum, as well as steelalloys, can be conveniently used. Carbon-based composites could also represent an alternative.

[0085] The contouring frame 8 has a groove configured to receive edges of the intermediate pane 4. In particular, the edges of the two curved transparent plies 5, 15 fit into the groove of the contouring frame 8. In particular, a first inner part 8a and a second inner part 8d of the frame 8 are shaped and arranged so as to define the groove configured to receive the edges of the two curved transparent plies 5, 15.

[0086] The role of the contouring frame 8 is that of hooping the curved pressure pane 4 at the border in order to balance the thrust of the resulting shell. In particular, thanks to the contouring frame 8 the state of stress in the pressure pane 4 approaches the optimal condition of equi-biaxial compression.

[0087] Although the bending stiffness of the contouring frame 8 is limited, its axial stiffness is noteworthy: consequently, the contouring frame 8 behaves as a truss, and provides an efficient constrains that limits the in-plane displacement only in proximity of the corners of the pressure pane 4. This may produce stress concentrations, which can be limited by rounding the corners of the pressure pane 4.

[0088] The role of the contouring frame 8 is particularly important in the post- glass-breakage phase. In fact, it avoids the catastrophic failure of the shell in case of glass breakage: the glass fragments can still support compression stresses along the meridians by direct contact of their surfaces of breakage, since they are kept together by the interlayer 6.

[0089] Indeed, it is the robustness and resilience of the element in the postbreakage phase, rather than its geometric thickness, that determines the actual level of safety.

[0090] According to one embodiment of the invention, the contouring frame 8 is made of Titanium, which has a coefficient of thermal expansion (of about 8.5 10’6K’1) very similar to that of glass. This is the preferable choice for minimizing the relative displacement consequent to temperature variations when the transparent plies 5, 15 are made of glass.

[0091] Alternatively, the contouring frame 8 frame can be made of Aluminum. In this case, it is expected that the differential thermal expansion might limit the efficiency of the contouring frame 8 in equilibrating the thrust from the pressure pane shell 4, especially in the pre-breakage phase, when the pressure pane 4 is stiff. However, the contouring frame 8 will still remain effective in the post-breakage phase, when the pressure pane 4 loses stiffness as it turns out to be an assembly of coherent glass fragments, which will deform until they establish the contact with the edge constraint. As already said, other materials for the contouring frame 8 can be Steelalloys or Carbon-based composites.

[0092] According to the embodiment disclosed and illustrated herewith, the contouring frame 8 has two further grooves configured to receive edges of the internal pane 2 and edges of the external pane 3.

[0093] The first gap 10 and the second gap 11 do not need to be closed respectively from the inner and outer environment because the pressure pane 4 is deputed to support the difference in pressure.

[0094] According to one embodiment of the invention, the multi-layered transparent unit 1 further comprises a gasket system located within the contouring frame 8.

[0095] In particular, the gasket system comprises a side gasket 9a and air-tigh sealing gaskets 9b.

[0096] The side gasket 9a is interposed between the borders of the pressure pane 4 and the internal walls delimiting the groove of the contouring frame 8.

[0097] The side gasket 9a is preferably made of a plastic material, or composites, and is arranged so as to avoid direct contact between the curved transparent plies 5, 15 and the stiff material of the contouring frame 8.

[0098] The main role of the side gasket 9a is to provide a structural fixing (by equilibrating the lateral thrust from the pressure pane 4) and thermal barrier.

[0099] The air-tight sealing gaskets 9b are arranged between the intermediate pane 4 and the contouring frame 8. In particular, the air-tight sealing gaskets 9b are arranged between the first inner part 8a and the intermediate pane 4, between the second inner part 8d and the intermediate pane 4, between the first inner part 8a and the main frame 8 and between the second inner part 8d and the inner part 8e.

[0100] In the illustrated embodiment, the gasket system comprises further sealing gaskets 9c arranged between the internal pane 2 and external parts 8c, 8e of the frame 8 and between the external pane 3 and another external part 8b or the main frame 8. These further sealing gaskets 9c are not necessarily air-tight.

[0101] According to a preferred embodiment of the invention, the multi-layered transparent unit 1 further comprises a peripheral layer 7, preferable in the shape of strips, arranged between the two curved transparent plies 5, 15 so as to contour, trap and hoop the interlayer 6. In addition, the peripheral layer 7 serves for withstanding peripheral pressure for sealing, transmitted through the gasket system 9b.

[0102] In particular, the peripheral layer 7 is made of one of the following materials: PTFE, EPDM, PIB, synthetic rubber.

[0103] The material of the peripheral layer 7 is stiffer than the material of the interlayer 6 for the reasons which will be explained below.

[0104] According to one embodiment of the invention, the peripheral layer 7 comprises strips made of PIB (or other materials previously listed), which are interposed between the two curved transparent plies 5, 15 so as to contour the interlayer 6.

[0105] The peripheral layer 7 performs at least three functions. First of all, during the lamination process, the peripheral layer 7 serves to contain the silicone-based material, usually in the liquid phase when applied (the silicone solidifies after the setting time). Secondly, it permits to press the borders of the curved laminated composite structure of the intermediate pane 4 against the air-tight sealing gaskets 9b, preventing the silicone- base material from squeezing out.

[0106] Thirdly, the peripheral layer 7 confines the interlayer 6 under operating condition. Therefore, when the pressure is on the convex surface of the laminated curved pane, the state of stress in the interlayer 6 becomes closed to be that of hydrostatic pressure, for which the interlayer material results very stiff (it is almost incompressible under hydrostatic pressure). Advantageously, the choice of soda-lime glass or borosilicate glass for the curved transparent plies 5, 15 have noteworthy mechanical properties, which surpass the property of fused silica.

[0107] First of all, soda-lime glass and borosilicate glass present a lower bound for the occurrence of phenomenon of static fatigue. As long as the stress intensity factor for the intrinsic cracks remains below the level Kio= 0.25- 0.30 MPa m1 / 2(soda-lime glass) or Kio= 0.30 MPa m1 / 2(borosilicate glass), static fatigue does not occur and the element can withstand the applied stress, at least in principle, forever.

[0108] This is shown in figure 1 , taken from Wiederhorn and Bolz, J. Am. Cer. Soc., 53, 1970, which shows the measured subcritical-crack-propagation velocity as a function of the stress intensity factor for silica and aluminosilicate, on the one hand, and soda-lime and borosilicate glasses, on the other hand.

[0109] It has to be noticed that the trend of the measured points indicates that for the last two materials the velocity tends to zero for finite values of the stress intensity factor.

[0110] It should be also mentioned that the critical value of the stress intensity factor (0.75 MPa m1 / 2) is higher than that of fused silica (0.66 MPa m1 / 2).

[0111] The average macroscopic bending strength of glass ranges from 60 MPa (for soda-lime glass) to 80 MPa (for borosilicate glass), but it can be noteworthy increased, up to 150-200 MPa, with a chemical toughening process, which can be used also for curved glass without producing an appreciable distortion of the geometric shape. Such a process induces a permanent surface compression, which prevents the opening of the surface cracks. The chemical toughening process also enhances the endurance limit with respect to the phenomenon of static fatigue because cracks can only open after that the permanent surface compression has been overcome.

[0112] It is true that the coefficient of thermal expansion of glass is higher than that of fused silica, being of the order of 3 - 6-1 O’6K’1for borosilicate glass and 9-1 O’6K-1for soda-lime glass, but the increase of the developed thermal stress can be counterbalanced by the higher mechanical strength. Another advantage of using soda-lime glass or borosilicate glass for the curved transparent plies 5, 15 is that they can be conveniently produced in large size since there is not a limitation due to the fusion process.

[0113] Last but not least, the mismatch due to the different thermal expansion with the contouring frame 8 is much less than for fused silica.

[0114] Therefore, the gasket system should accommodate a lower relative displacement.

[0115] As already stated, materials other than glass can be used to obtain the curved transparent plies 5, 15. For example, glass-ceramic is often used in aerospace applications, as well as polycarbonate, or acrylic glass. Plies made of different materials can be combined in one laminated package to enhance its overall capacity.

[0116] Regarding the material chosen for the interlayer 6, experiments have been conducted by the Applicant using a silicone-based material known on the market as DOWSIL™ 9955, produced by Dow Silicones Belgium SRL.

[0117] The experiments have demonstrated that said material is stable in the design temperature range. The shear stiffness does not exhibit appreciable variations, being the shear modulus of the order of 1 -2 MPa.

[0118] In particular, it was performed a double lap shear test on specimens comprising four soda-lime glass plies 5, 15 of 100 mm x 100 mm made of coupled by three interlayers of DOWSIL™ 9955.

[0119] The load was applied on the two inner plies using a spacer, and the relative displacement with respect to the two outer plies was measured by a displacement transducer.

[0120] Figure 4(a) reports, in MPa, the stiffness of the interlayers 6 for relaxation tests at room temperature (20°C) over a time interval comparable with 90 minutes, i.e., the time of revolution of the space station, whereas figure 4(b) reports the results of a similar test at a temperature of 80°C. As said, other materials, such as polyurethane-based resins, could be used to form the interlayer 6.

[0121] According to other embodiments of the invention, the intermediate pane 4 comprises further curved transparent plies which are stacked together and bonded by further interlayers.

[0122] The further curved transparent plies can be made of the same material mentioned above. In particular, they can be made of one of the following materials: soda-lime glass, borosilicate glass, alumino-silicate glass, glassceramic, polycarbonate, acrylic glass.

[0123] Coupling of the further curved transparent plies can be made by silicone- based or polyurethane-based materials.

[0124] In one embodiment of the invention, curved plies made of polycarbonate are alternated to curved plies made of glass.

[0125] The interlayers made of silicone-based material are effective in bonding acrylic glass and polycarbonate. The polycarbonate is certainly less brittle than glass, but it remains brittle at very low temperatures, such as those experienced in the outer space environment

[0126] For example, the embodiment of figures 3(a) and 3(b) shows a curved laminated structure for the intermediate pane 4 comprising three curved glass plies 5, 15, 25 and two interlayers 6, 16 which are surrounded by corresponding peripheral layers 7, 17.

[0127] The whole intermediate pane 4 is fit with its edges in the contouring frame 8.

[0128] A single side gasket 9a is interposed between the border of the intermediate pane 4 and the internal walls delimiting the groove of the contouring frame 8. In practice, the side gasket 9a is in contact with the edges of all the three curved glass plies 5, 15, 25.

[0129] In an alternative embodiment (not illustrated), the edges of the pressure pane 4 are obliquely cut and the groove of the contouring frame 8 is counter-shaped to them so that it is obtained a conical coupling between the pressure pane 4 and the contouring frame 8. According to one aspect of the invention, each of the curved transparent plies 5, 15 has a radius of curvature comprised between 2 m and 3 m. Smaller or larger curvatures can be used, according to the size of the windows, but in general the radius of curvature should be higher than 50% of the diameter of the window to be obtained.

[0130] The radius of curvature depends on the diameter of the window since what is important is the ratio diameter / sag.

[0131] According to one aspect of the invention, the transparent plies 5, 15 are curved with a process of hot bending.

[0132] For this purpose, the hot bending process comprises a step of heating the transparent plies 5, 15 until they become soft enough to adopt the desired form of a mold.

[0133] In case of plies made of glass, the temperature shall be around 580- 600°C.

[0134] The process of hot bending is known by itself, but it has been customized due to the use of silicone-based material for the interlayer 6. As a matter of fact, the silicone-based material is a liquid component which solidifies over time.

[0135] Therefore, the plies 5, 15 have to be hot bent in the required shape taking care of imposing a gap of prescribed thickness between the said plies 5, 15. In practice, the gap is destined to be filled by the interlayer 6.

[0136] The thickness of the interlayer 6 is obtained by interposing a foil, usually made of PTFE, between the stacked transparent plies 5, 15, which are consequently hot bent together.

[0137] The interposed foil is therefore used to ensure geometric congruence.

[0138] The interposed foil is then removed from inside the curved transparent plies 5, 15 thus leaving a gap therebetween.

[0139] Subsequently, the liquid silicone-based material is poured into the gap between the curved transparent plies 5, 15, taking care of having previously applied the strips 7 to the edge, of the same thickness as the gap, to confine the liquid silicone-based material. The strips 7 will be permanently bonded since they serve as a counter to the pressure exerted by the side gasket 9a.

[0140] The same process can be used when other initially liquid materials are used as interlayers.

[0141] The strips 7 along the borders of the gap between the curved transparent plies 5, 15 aim at controlling the design thickness when the gap is successively filled up with the silicone-based material.

[0142] The interlayer 6 originates from the solidification of the silicone-based material poured between the curved transparent plies 5, 15.

[0143] The assembled composite may be successively cured with a process at controlled temperature and pressure in autoclave.

[0144] Figure 8 discloses a possible embodiment of the contouring frame 8 and of the gasket system 9.

[0145] The contouring frame 8 is axially pre-stressed, to generate a hoop stress on the curved laminated composite structure.

[0146] To generate the prestress, the contouring frame 8 preferably comprises an internal ring 80 and an external ring 81 .

[0147] Both the internal ring 80 and the external ring 81 are made of metal.

[0148] The internal ring 80 is composed of one or more conical elements. In figure 8 the internal ring 80 comprises only one conical element. In particular, the internal ring 80 is cut in radial slices.

[0149] The external ring 81 is composed of one or more conical elements 81 a, 81 b. The contact surfaces L between the external ring 81 and the internal ring 80 may be conical or biconical.

[0150] The internal ring 80 is interposed between the external ring 81 and a gasket system 9 which has grooves configured to receive the edges of the intermediate pane 4.

[0151] The gasket system 9 comprises as many grooves as the number of curved transparent plies forming the intermediate pane 4.

[0152] As already said, the curved transparent plies are at least two, but they can be more. In particular, in figure 8 the gasket system 9 comprises three grooves, each groove being configured to receive one of the three curved transparent plies 5, 15, 25.

[0153] In particular, the gasket system 9 acts as a peripheral containment for

[0154] - spacing between plies 5, 15, 25;

[0155] - containing the interlayers 6;

[0156] - mating with outer hoop stress internal ring 80;

[0157] - pushing on outer truss composing the master frame (not shown in figure 8), with axial reaction on surface F or on spherical surface E.

[0158] In the embodiment shown in figure 8, the prestress is obtained by tightening screws along a transversal direction I, so as to reduce the distance between the two conical elements 81 a, 81 b of the external ring 81 and to obtain a pushing effect of said two conical elements 81 a, 81 b on the internal ring 80 through the contact surfaces L. This produces the radial shrinkage of the slices of the internal ring 80 and generates a radial compression of the curved laminated composite structure.

[0159] Other possible embodiments may be obtained, with the same concept, by modifying the geometry, for example by inverting the orientation of the conical surfaces L, or by using one biconical element for the external ring 81 and two conical elements for the internal ring 80.

[0160] As said, irrespectively of the specific embodiment, the structural role of the contouring frame is that of an encircling tie rod, to hoop the curved laminated composite structure and to absorb its horizontal thrust.

[0161] With reference to figures 5, 6(a) and 6(b), number 100 denotes an observatory or space module comprising a plurality of multi-layered transparent units 1 , in particular suitable to be installed in a space station operating at low orbit.

[0162] In the illustrated embodiment, the observatory 100 has a form of a truncated icosahedron, formed by a spatial master frame 101 , hosting the transparent units 1 occupying the pentagonal and hexagonal faces of the truncated icosahedron. The master frame 101 is made of metal, for example Aluminum, or composite.

[0163] In particular, figure 6(b) shows a master frame 101 with a top part removed for allowing to create an access to the inner part of the space station.

[0164] In practice, the units 1 are arranged in the master frame 101 to occupy the windows of the observatory 100.

[0165] The suggested shape is one example dictated by the load condition: the external environment is vacuum, whereas the internal space is under atmospheric pressure (nominally 1 atm). The shape appears optimal, since it is inspired by an inflated soccer ball: this shall allow to minimize the bending stress in the element of the master frame 101 .

[0166] In practice, this geometry reduces the bending of the constituent members, so to minimize the thickness and facilitate a 360° view.

[0167] Alternatively, the master frame 101 can have other shapes than the truncated icosahedron. In particular, other forms of Archimedean solids, cluster of solids, faceted solids, with faces in the shape of polygons or circular windows, can be used.

[0168] The master frame101 is designed in such a way to permit the installation of external pressure covers on the single window. These will permit the substitution of the units 1 from the interior of the space module 100.

[0169] The units 1 of the observatory 100 have the structure with three panes 2, 3, 4.

[0170] The multi-layered transparent units 1 are arranged to occupy the windows of the master frame 101 with the concavity facing outwards.

[0171] In particular, the pressure panes 4 have a curved laminated structure with center of curvature on the outside of the observatory 100.

[0172] The shape has been chosen in such a way that the internal pressure produces a state close to that of biaxial compression (membrane stress) in the transparent pressure pane 4.

[0173] In one example, the side of the hexagonal / pentagonal faces is approximately 600 mm in order to maintain the diameter of the larger pane of the order of 1000 mm, to permit the passage of the unit 1 through the docking ports of the space station.

[0174] In fact, the substitution of a unit 1 in case of accidental rupture shall always be made from the interior of the observatory 100, after an external pressure cover has been fixed on the master frame 101 .

[0175] In one example, the radius of curvature of the transparent spherical pane 4 is of the order of 2500 mm. The average radius of observatory 100, obtained by approximating the truncated icosahedron with the circumscribed spherical surface, is approximately 1350 mm.

[0176] It must be noticed that there is no direct contact between the panes 2, 3, 4 of the units 1 and the master frame 101 since the panes 2, 3, 4 are supported by the contouring frame 8.

[0177] As said, the multi-layered transparent unit 1 can have different shapes depending on the specific needs and applications.

[0178] Therefore, the panes 2, 3, 4 of the unit 1 can have different shape.

[0179] According to one embodiment, shown in figures 5, 6(a) and 6(b), the panes 2, 3 and 4 have a hexagonal and pentagonal shape, specifically conceived to form the truncated icosahedron observatory 100.

[0180] The intermediate pane 4 or pressure pane represents the main structural part of the whole unit 1.

[0181] As said, from a structural point of view the intermediate pane 4 is a shell with the concavity directed outwards in order to develop membrane compression stresses under internal pressure. It represents the barrier between the internal and external environmental pressures.

[0182] In submarine applications, where the external pressure is higher than internal pressure, the transparent units 1 are arranged to occupy the windows of the master frame 101 with the concavity directed inwards.

[0183] For the sake of illustration, figure 7 synthetically represents the result of a FEM simulation of a hexagonal pressure pane 4 under an applied (internal) pressure of 2 atm (twice the nominal value).

[0184] In this simulation, the pressure pane 4 is a laminated spherical shell with radius of curvature 2500 mm, consisting of only two glass plies 5, 15 of thickness 10 mm, sandwiching a 1 .52 mm thick silicone interlayer 6.

[0185] The Young’s modulus of glass and Poisson ratio have been respectively set equal to 70 GPa and 0.23, whereas the shear modulus of the silicone interlayer 6 is 1 MPa.

[0186] Two limit cases for the boundary conditions have been considered.

[0187] In figure 7(a) the displacement of the border of the shell is prevented in all directions; in figure 7(b) only the “out-of-plane” component of displacement is prevented, whereas the “in-plane” displacement is left free.

[0188] The first condition represents the ideal situation in which the shell is rigidly hooped by the contouring frame 8; the second condition simulates the case in which the shell is simply supported at the border.

[0189] The actual condition shall be intermediate between the two cases. The state of stress in figure 7(a), shown on both internal and external surfaces, tends to that of a uniform equi-biaxial compression, which is the most favorable for the shell. On the other hand, the boundary conditions of figure 7(b), also shown on the two surfaces, involves the onset of tensile stresses, which are in any case limited to 35 MPa at most, i.e., within the limit dictated by material strength.

[0190] It should be noticed that, thanks to the curvature of the surface, apart from the limitation of the stress level, there is another positive effect. In fact, the interlayer 6 is only mildly stressed in shear.

[0191] On the contrary, if the intermediate pane 4 was flat, the load bearing capacity of the laminate would be dictated by the coupling of the glass plies 5, 15 through the interlayer 6, which depends on the shear stiffness of the interlayer itself.

[0192] In conclusion, whereas in a flat laminated pane a soft interlayer of silicone (shear modulus of the order of 1 MPa) would have been insufficient to impart a response similar to that of a monolith, in a shell (curved structure) its role is principally that of assuring the sharing of the pressure load to the various glass plies 5, 15. For this, a high stiffness is not required to the interlayer 6.

[0193] The external pane 3 performs various functions. First of all, it represents a sacrificial element that should act as a primary shield for the inner panes in the event of an impact from small debris.

[0194] Preferably, the external pane 3 has a selective coating for reflecting part of the solar radiation.

[0195] Finally, the external pane 3 provides thermal insulation.

[0196] As shown in figure 3(b), there are no air-tight gaskets between the external pane 3 and the intermediate pane 4.

[0197] Since no pressure load acts on the faces of the external pane 3, this can be conveniently made flat, but it could be curved to comply with the gabarit of the pressure pane 4.

[0198] According to one embodiment, the external pane 3 is made of a chemically tempered glass in order to withstand the thermal stress consequent to the uneven temperature distribution.

[0199] Alternative, the external pane 3 can be a made of glass laminated with silicone or other temperature resistant materials.

[0200] Since the second gap 11 is under vacuum condition, there is no heat exchange under the form of convection between the external pane 3 and the intermediate pane 4. Conduction can be limited, similarly to what is done in insulating glass units by interposing insulated spacers. An excellent thermal insulation can thus be obtained.

[0201] Since the external pane 3 plays no structural role, it can be readily substituted, in case of rupture, with extravehicular activity (EVA).

[0202] With reference to the internal pane 2, it must be noticed that its role is simply to protect the intermediate pane 4 from accidental shocks from the inside, for example caused by floating objects and people under microgravity condition.

[0203] As a matter of fact, the first gap 10 between the intermediate pane 4 and the internal pane 2 is in direct contact with the internal environment. Consequently, the pressure is the same on the two surfaces of the internal pane 2.

[0204] The internal pane 2 can be easily substituted.

[0205] Since it is not in contact with the external environment, the internal pane 2 is preferably made of ordinary laminated glass (with commercial polymeric interlayers).

[0206] The first gap 10 between the intermediate pane 4 and the internal pane 2 provides additional insulations. Dehumidifier-filled spacers can be added, as done in traditional insulated glass units, in order to diminish the risk of condensation in the gap filled with air.

[0207] The internal pane 2 can be coated with an anti-reflection coating and can embody a thin chemically tempered glass layer (having a thickness lower than 1 mm), presenting increased scratch resistance and impact strength. The anti-reflection coating is advantageous when it is necessary to eliminate or reduce reflections in case of installation in the space module 100.

[0208] In alternative embodiment, the internal pane 2 and / or the external pane 3, as well as the plies of the pressure pane 4, can be made of other transparent materials such as glass-ceramic.

[0209] An example of transparent glass-ceramic is represented by aluminum oxynitride, composed of aluminum, oxygen, and nitrogen. The mechanical properties are much higher than that of glass: bending strength of the order of 380-700 MPa; critical stress intensity factor 2.0 MPa m1 / 2, Young’s modulus 334 GPa and Poisson ratio 0.24. Also, the coefficient of thermal expansion is lower than that of glass, of the order of 4.7 10’6K’1. On the other hand, this material is heavier than glass (density 3700 kg / mA3 rather than 2500 kg / mA3).

[0210] Glass-ceramic for aerospace applications is produced by a few companies. A further example is represented by ZERODUR® produced by Schott Inc.

[0211] It is worth mentioning that the optimization of composite transparent units 1 (in terms of size, shape, curvature, and thickness) is in strict relation with the optimization of the geometry of the master frame101 of the space module 100. In fact, the global optimization procedure needs to consider the shape and size of the entire structure, defined by the association of “transparent + frame”, which has to be designed for minimum weight. The geometry just outlined before represents only one of the possible choices, which does not consider other possible design constraints that could affect the final layout

[0212] The windows of the space module 100 shall be equipped with shutters (not illustrated) to protect them from damage from micrometeoroids and orbital debris, providing a radiation / thermal shield as well which protects the glass when the space module 100 is not used.

[0213] The characteristics of the multi-layered transparent unit and a process for manufacturing thereof, according to the present invention, are clear, as are the advantages.

[0214] The multi-layered transparent unit composed by curved laminated composite structure and contouring metallic frame represents a unitized hybrid cell, with enhanced mechanical properties, in which the glass and the metal play a synergetic role.

[0215] The possibility of obtaining curved laminated transparent shells has noteworthy advantages. In the pre-breakage phase, the membrane stress can equilibrate the high design pressure. In the post-breakage phase, the shell can maintain a residual load bearing capacity, because the fragments can still carry the membrane stress, via their direct contact.

[0216] In particular, thanks to the curved geometry which exploits the load bearing capacity of the whole thickness via the development of membrane stress, the multi-layered transparent unit shows a reduced thickness and weight with respect to prior art solutions.

[0217] The curved laminated composite structure of the proposed unit shows no gap between the curved transparent plies, and, obviously, there is no need of periodic re-evacuation.

[0218] In addition, all curved transparent plies contribute to support, in parallel, the applied pressure. Consequently, the stress state in each ply is reduced with respect to the condition in which the plies are monolithic and spaced by a gap.

[0219] Furthermore, rupture of one ply does not imply the failure of the whole laminated structure. In fact, the interlayer acts as a crack arrestor: the fracture does not propagate from one ply to the other. This increases the robustness and the resilience of the laminated structure.

[0220] In addition, the proposed unit has higher resistance against the impact of debris. Fracture will remain limited to the directly impacted plies and will not propagate to the successive plies because arrested by the presence of the interlayers.

[0221] The possibility offered by lamination also permits to associate layers of transparent materials with different properties, to synergistically increase the capacity of the panel.

[0222] The proposed unit finds application in manufacturing large windows, in particular for a space station or a space aircraft. In fact, glass is the main structural transparent material which can be produced in large panes. The curved geometry allows to withstand the high-pressure load with no intolerable increase in thickness.

[0223] Finally, the proposed manufacturing process takes into account the issues of inserting an initially liquid material between two glass surfaces, keeping the thickness constant, so adapting the hot bending to the specific case.

Claims

CLAIMS1. A multi-layered transparent unit (1 ), comprising at least one pane (4) having a curved laminated composite structure, said at least one pane (4) comprising:- at least two curved transparent plies (5, 15) of which at least one is made of soda-lime or borosilicate or alumino-silicate glass or glassceramic, and- an interlayer (6) for coupling the two curved transparent plies (5, 15) stacked together, said interlayer (6) being interposed between said two curved transparent plies (5, 15) and being made of silicone-based or polyurethane-based material;- a contouring frame (8) having a groove configured to receive edges of said at least one pane (4).

2. The multi-layered transparent unit (1 ) of claim 1 , wherein each of said at least two curved transparent plies (5, 15) has a radius of curvature not less than 50% of the diameter of a window to be obtained.

3. The multi-layered transparent unit (1 ) of claim 1 or 2, wherein the other of said at least two curved transparent plies (5, 15) is made of soda-lime or borosilicate or alumino-silicate glass or glass-ceramic, or polycarbonate, or acrylic glass.

4. The multi-layered transparent unit (1 ) of any one of the previous claims, further comprising a peripheral layer (7) arranged between the at least two curved transparent plies (5, 15) so as to contour, trap and hoop said interlayer (6) and to withstand peripheral pressure.

5. The multi-layered transparent unit (1 ) of claim 4, wherein said peripheral layer (7) is made of one of the following materials: PIB, PTFE, EPDM,synthetic rubber.

6. The multi-layered transparent unit (1 ) of any one of the previous claims, wherein the contouring frame (8) is made of Titanium or Aluminum or Steel-alloy or Carbon-based composite.

7. The multi-layered transparent unit (1 ) of any one of the previous claims, further comprising a gasket system located within the contouring frame (8) and comprising at least one side gasket (9a) interposed between the edges of said at least one pane (4) and internal walls delimiting the groove of the contouring frame (8), and air-tight sealing gaskets (9b) arranged between the intermediate pane (4) and the contouring frame (8).

8. The multi-layered transparent unit (1 ) of any one of the previous claims, wherein said at least one pane (4) comprises:- further curved transparent plies (25) made of a material chosen among the following ones: soda-lime glass, borosilicate glass, alumino-silicate glass, glass-ceramic, polycarbonate, acrylic glass;- further interlayers (16) made of silicone-based or polyurethane-based material for coupling together the further curved transparent plies (25) and for coupling them to said at least two curved transparent plies (5, 15);- further peripheral layers (17) arranged between pairs of said curved transparent plies (5, 15) and / or further curved transparent plies (25) so as to contour, trap and hoop the further interlayers (16) and to withstand peripheral pressure.

9. The multi-layered transparent unit (1 ) of any one of the previous claims, further comprising an internal pane (2) and an external pane (3), said at least one pane (4) being interposed between the internal pane (2) and the external pane (3) and being distanced from both the internal pane (2) andthe external pane (3) so as to obtain corresponding gaps (10, 11 ) therebetween, said contouring frame (8) having two further grooves configured to receive edges of the internal pane (2) and edges of the external pane (3).

10. The multi-layered transparent unit (1 ) of any one of the previous claims, wherein the contouring frame (8) comprises an internal ring (80) made of one or more conical elements and an external ring (81 ) made of one ore more conical elements (81a, 81 b), contact surfaces (L) between the external ring (81 ) and the internal ring (80) being conical or biconical.

11. The multi-layered transparent unit (1 ) according to claim 10, wherein the internal ring (80) is cut in radial slices.

12. A process for manufacturing a pane (4) for a multi-layered transparent unit (1 ) according to any one of the previous claims, said process comprising the steps of:- interposing a foil between two stacked transparent plies (5, 15) of which at least one is made of soda-lime or borosilicate or aluminosilicate glass or glass-ceramic;- hot bending the two stacked transparent plies (5, 15) so as to curve them;- removing the interposed foil from inside the two curved transparent plies (5, 15) thus leaving a gap therebetween;- pouring a liquid silicone-based or polyurethane-based material within the gap between the two curved transparent plies (5, 15) so as to obtain an interlayer (6) bonding the two curved transparent plies (5, 15).

13. The process of claim 12, further comprising a step of applying strips (7) between edges of the curved transparent plies (5, 15) after havingremoved the interposed foil and before pouring the liquid silicone-based or polyurethane-based material within the gap.

14. A module (100) for spatial or terrestrial or submarine applications, comprising:- a master frame (101 ) made of metal or composite and having the shape of an Archimedean solid with faces in the shape of regular polygons or faceted solids with polygonal-like and / or circular windows;- a plurality of multi-layered transparent units (1 ) according to any one of claims 1 to 11 , said multi-layered transparent units (1 ) occupying the windows.

15. The module (100) of claim 14, wherein the multi-layered transparent units (1 ) are arranged to occupy the windows with the concavity facing outwards.

16. The module (100) of claim 14, wherein the multi-layered transparent units (1 ) are arranged to occupy the windows with the concavity directed inwards.

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