Patent Publication Number: US-2011052910-A1

Title: High toughness fiber-metal laminate

Description:
PRIORITY CLAIM 
     The present application is a national phase application filed pursuant to 35 USC §371 of International Patent Application No. PCT/EP2009/050883, filed 27 Jan. 2009; which application claims the benefit of European Patent Application No. 08150772.5, filed 29 Jan. 2008; all of the foregoing applications are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a fiber-metal laminate comprising fiber-reinforced composite layers and thin metal sheets. More particularly, the invention relates to a fiber-metal laminate comprising fiber-reinforced composite layers and thin metal sheets having a high combined level of strength and toughness. 
     BACKGROUND OF THE INVENTION 
     Generally, composite panels are manufactured from tapes of prepregs (pre-impregnated fibers) consisting of various types of reinforcing fibers impregnated with a thermoplastic or thermosetting adhesive, such as an epoxy. These thin sheets of prepreg are then arranged in various configurations and built up to a desired thickness by “laying them out” on a flat or curved surface and stacking them to the desired thickness. To produce a finished product, the stack may then be sealed from the atmosphere and drawn under a vacuum to remove entrapped air. When using thermosetting adhesives, the stack is subsequently placed in an autoclave or a press where it is heated to the required curing temperature for the required curing time and the required pressure is applied. Upon cooling to room temperature, a solid composite product is obtained. 
     Composites offer considerable weight advantage over other preferred materials, such as metals, that are used to manufacture various industrial components. Generally, the weight savings are obtained at the sacrifice of other important material properties such as ductility, toughness, conductivity and cold forming capability. To overcome these deficiencies, new hybrid materials called fiber-metal laminates have been developed to combine the best attributes of metal and composites. These materials are for instance described in U.S. Pat. No. 4,500,589 and U.S. Pat. No. 5,039,571, and are obtained by stacking alternating thin layers of metal (most preferably aluminum) and fiber-reinforced prepregs with the outermost layers being generally metal and the inner layers alternating between the metal and prepreg. The stack is cured under heat and pressure to provide the fiber-metal laminate. Fiber-metal laminates were developed to provide for lighter weight components than if the component were made of just metal alone. While the laminate does provide for lighter weight, the laminate is still heavier than pure composite components however. 
     WO 2005/110736A2 describes a fiber-metal laminate comprising fiber-reinforced composite layers and thin metal sheets. The reinforcing fibers used in the fiber-reinforced composite layers are organic polymeric fibers having a modulus of elasticity greater than 270 GPa. These very high modulus fibers carry most of the stress applied to the laminate and therefore permit the use of a low amount of metal in the laminate. 
     U.S. Pat. No. 5,227,216 also describes a fiber-metal laminate comprising fiber-reinforced composite layers and thin metal sheets. The reinforcing fibers used in the fiber-reinforced composite layers are bi-directionally oriented. The laminate is post-stretched along an axis that bisects the two fiber orientations. 
     Fiber-metal laminates are increasingly used in industries such as the transportation industry, for example in cars, trains, aircraft and spacecraft. They can be used as a reinforcing element and/or as a stiffener for wings, fuselage and tail panels and/or other skin panels for aircraft. Fiber-metal laminates of the type described provide excellent resistance to fatigue crack propagation and generally provide an improved fatigue resistance over bare aluminum alloys. The strength of fiber-metal laminates represents a “volume averaged” strength, by which is meant that the strength typically corresponds to a weighted sum of the strength of each of the metal and composite components of the laminates, each component strength value being factorized by the volume fraction of the components. This so-called rule of mixtures prediction of strength has been confirmed many times. To obtain suitable strengths for most applications, the known fiber-metal laminates generally have an elevated metal volume fraction, typically larger than 47%. As a result of recent terrorist attacks in and around aircraft, protection of the aircraft, and therefore the people seated in the aircraft, has increasingly become an issue. In order to improve safe (air) transport of high-risk and/or suspect goods, explosion containment systems have been developed. Since fiber-metal laminates have the favorable property, among others, of having a relatively high resistance during a considerable impact or blast, such as for instance an explosion, the wall parts of the known explosion containment device are preferably manufactured from this material, and in particular from Glare®. Although the known explosion containment device shows a good energy-absorbing capacity per unit of mass, there is a strong need to further improve this capacity. Since the energy-absorbing capacity depends on the combination of strength and toughness, there is a need to further improve the combination of strength and toughness of fiber-metal laminates. 
     In accordance with the present invention, a fiber-metal laminate configuration is provided that is lighter weight than the known laminate configurations, as described in U.S. Pat. No. 4,500,589 and U.S. Pat. No. 5,039,571 for instance. 
     In addition a fiber-metal laminate is provided having a level of toughness and strength that is greater than that expected from the sum of the contributing volume fraction of the metal and composite components. 
     In another aspect of the invention, a fiber-metal laminate is provided having an improved energy-absorbing capacity per unit of mass that is greater than the known fiber-metal laminate. 
     In still another aspect of the invention, an improved explosion resistant device is provided, which device is produced with the fiber-metal laminate of the invention. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a fiber-metal laminate is provided comprising fiber-reinforced composite layers and thin metal sheets, wherein the total metal volume fraction of the laminate is between 0 vol. % and 47 vol. %. Such a configuration is readily obtained by arranging alternating layers of prepreg and metal in such a manner that the volume fraction of metal ranges from between 0% and 47%. This may be accomplished by building a stacking sequence with the outer layers being thin sheets of prepreg and thin metal layers are symmetrically spaced in the interior. With between 0 vol. % and 47 vol. % is meant in the context of the present application a range of metal volume fractions that does not include the extreme values of 0 vol. % and 47 vol. %. 
     In another preferred aspect of the invention, a fiber-metal laminate is provided comprising fiber-reinforced composite layers and thin metal sheets, wherein the total metal volume fraction of the laminate is between 5 vol. % and 41 vol. %, more preferably between 10 vol. % and 35 vol. %, and most preferably between 15 vol. % and 30 vol. %. Another aspect of the invention is to build a fiber-metal laminate having a metal volume fraction in the indicated range by having one outer layer consisting of a thin sheet of metal. Another aspect of the invention is to have both outer sheets consisting of thin metal sheets and the inner layers consisting of composite prepregs such that the metal volume fraction is in the indicated range. In another preferred aspect of the invention, the fiber-reinforced composite layers in the fiber-metal laminate comprise substantially continuous fibers that extend mainly in one direction. Another aspect of the invention is to have arrangements as specified above, but with unidirectional layers of prepregs arranged in cross ply patterns. In still another aspect of the invention the fiber-reinforced composite layers in the fiber-metal laminate comprise a woven structure of reinforcing fibers, preferably in the form of a woven fiber prepreg. 
     In addition to the advantage that the fiber-metal laminate according to the invention provides for a level of toughness and strength that is greater than that expected from the sum of the contributing volume fraction of the metal and composite components, a device manufactured from the fiber-metal laminate according to the invention shows improved explosion-resistance at reduced weight. Reducing weight by lowering the metal volume fraction in the laminate may be straightforward. What is highly unexpected however is that by reducing the metal volume fraction, toughness and/or strength increase. Decreasing total mass of airborne devices is usually very relevant, since the total cost of air transport, in particular the fuel costs, are closely related to this total mass. It is generally assumed that one pound (about 0.5 kg) of dead weight represents a cost of US $100 or more in fuel annually. The reduction of the total weight of airborne devices, and therefore saving of transport costs, is generally of great relevance for airlines. Application of the invented fiber-metal laminate in airborne devices, such as in explosion containment systems, may result in substantial savings in (transport) cost as well as in an improved protection against explosion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1-Illustrates  the effect of metal volume fraction on the stress-strain curve of fiber-metal laminates. 
         FIG. 2-Illustrates  the effect of metal volume fraction on area under the stress-strain curve (toughness). 
         FIG. 3-Illustrates  the relationship of tensile strength and toughness as affected by metal volume fraction in a fiber-metal laminate. 
         FIG. 4-Illustrates  a sheet of unidirectional composite prepreg used in the construction of a fiber-metal laminate according to the invention. 
         FIG. 5-Illustrates  a sheet of bi-directional or woven fabric prepreg used in the construction of a fiber-metal laminate according to the invention. 
         FIG. 6-Illustrates  an embodiment of the fiber-metal laminate according to the invention showing a single sheet of metal sandwiched between alternating layers of unidirectional prepreg stacked in a cross ply arrangement. 
         FIG. 7-Illustrates  a cross section of the fiber-metal laminate according to the invention of  FIG. 6 . 
         FIG. 8-Illustrates  another embodiment of the fiber-metal laminate according to the invention showing a single sheet of metal sandwiched between layers of unidirectional prepreg in a unidirectional arrangement. 
         FIG. 9-Illustrates  a cross section of the fiber-metal laminate according to the invention of  FIG. 8 . 
         FIG. 10-Illustrates  still another embodiment of the fiber-metal laminate according to the invention showing a single sheet of metal on the outer surface of a number of prepreg plies. 
         FIG. 11-Illustrates  still another embodiment of the fiber-metal laminate according to the invention showing two sheets of metal sandwiched between alternating layers of unidirectional prepreg stacked in a cross ply arrangement. 
         FIG. 12-Illustrates  still another embodiment of the fiber-metal laminate according to the invention showing a single sheet of metal sandwiched between layers of woven fabric prepreg. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, reference is made to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific embodiments or processes in which the invention may be practiced. The present invention, however, may be practiced without the specific details or with certain alternative equivalent methods to those described herein. 
     The basis of the present invention is a unique arrangement of fiber-reinforced composite layers and at least one thin metal sheet. In accordance with the invention a fiber-metal laminate is provided comprising fiber-reinforced composite layers and thin metal sheets, wherein the total metal volume fraction of the laminate is between 0 vol. % and 47 vol. %, preferably between 5 vol. % and 41 vol. %, more preferably between 10 vol. % and 35 vol. %, and most preferably between 15 vol. % and 30 vol. %. The fiber-reinforced composite layers preferably comprise fibers pre-impregnated with adhesive (prepreg). The system of prepreg layers and metal sheets can be, for instance, processed under heat and pressure (cured) to form a solid panel or component. 
     Generally, it is presumed that mechanical properties (and some physical properties) of fiber-metal laminates follow a rule of mixtures such that when fiber-reinforced composite layer prepreg and metal components are combined to form a single component, the resulting property is the sum of the products of the property of the components and the volume fraction of the component. Contrary to this behavior, it has now been discovered by the inventors that toughness of certain fiber-metal laminates made from aluminum and unidirectional, cross ply prepreg of S2-glass fibers was highest when metal volume fraction of the fiber-metal laminate was between 0% and 47% and more specifically 21%. This behavior is illustrated in  FIGS. 1 to 3 , and does not depend on the type of fiber used. With reference to  FIG. 1  the effect of the metal volume fraction on the stress-strain curves of fiber-metal laminates made with 2024 aluminum sheets and unidirectional, cross ply, S2-glass prepreg is shown. Curve  1  represents the stress-strain curve of a pure fiber-reinforced cross-plied composite (metal volume fraction=0, 12 plies of S2 glass prepreg with epoxy matrix). Curve  2  represents the stress-strain curve of a fiber-metal laminate consisting of 8 layers of the same prepreg as the one used for curve  1 , and a single sheet of 2024 aluminum. Curve  3  finally represents the stress-strain curve of a known Glare® fiber-metal laminate consisting of 3 sheets of 2024 aluminum and 2 fiber-reinforced composite layers of the same prepreg as the one used for curve  1 . Comparing curves  1 ,  2  and  3  shows that the laminate according to the invention (curve  2 ) is tougher than the pure composite (curve  1 ) and the known fiber-metal laminate (curve  3 ). Moreover the invented laminate has a substantially higher strain to break. Surprisingly, its strength is on the same level as the pure composite strength (about 1030 MPa or 150 ksi), whereas the rule of mixtures would dictate a lower strength. The area under a stress-strain curve represents the total specific energy that the laminate was able to store during deformation until fracture. This specific stored energy (expressed in MPa) is generally viewed as an adequate measure of toughness. The measured specific stored energy values for curves  1 ,  2  and  3  were 5.07 ksi (35 MPa), 5.82 ksi (40 MPa) and 7.15 ksi (50 MPa) respectively. The measured toughness of the fiber-metal laminate according to the invention is therefore at least 25% higher than that of the known fiber-metal laminate. 
       FIG. 2  shows the toughness (stored specific energy) for a number of fiber-metal laminates with varying metal volume fraction. The upper curve  4  (and data) represents the stored specific energy obtained at a strain rate of 8.67 per second, whereas the lower curve  5  (and data) represents the stored specific energy obtained at a strain rate of 0.005 per second. The difference is attributable to viscoelastic effects: at the lower strain rates a part of the stored elastic energy is dissipated. It is clear that in the range between 0% and 47% metal volume fraction, toughness is higher than expected. The average toughness achieved in this range is 30 about 6.3 ksi (43.4 MPa) for a strain rate of 8.67 per second, whereas the average toughness of the known fiber-metal laminate is about 5 ksi (34.5 MPa). 
       FIG. 3  shows the outstanding combination of toughness and tensile strength associated with fiber-metal laminates covered by this invention and made using 2024 aluminum and unidirectional, cross plied, S2 glass fiber prepreg. The toughness-tensile strength data combination for fiber-metal laminates according to the invention are clustered in the upper right corner of the graph. 
     The fiber-reinforced composite layers in the fiber-metal laminates according to the invention are light and strong and comprise reinforcing fibers embedded in a polymer. The polymer may also act as a bonding means between the various layers. Reinforcing fibers that are suitable for use in the fiber-reinforced composite layers include glass fibers, carbon fibers and metal fibers, and if required can also include drawn thermoplastic polymer fibers, such as aramid fibers, PBO fibers (Zylon®), M5® fibers, and ultrahigh molecular weight polyethylene or polypropylene fibers, as well as natural fibers such as flax, wood and hemp fibers, and/or combinations of the above fibers. It is also possible to use commingled and/or intermingled rovings. Such rovings comprise a reinforcing fiber and a thermoplastic polymer in fiber form. Preferred fibers include reinforcing fibers with a relatively high tensile strength and/or stiffness, of which class high strength glass fibers, such as S2-glass fibers, and HS2 and HS4 fibers (PPG) are particularly preferred. Preferred reinforcing fibers include glass fibers having a tensile strength of at least 3 GPa, more preferred of at least 3.5 GPa, even more preferred of at least 4 GPa, and most preferred of at least 4.5 GPa. Other preferred reinforcing fibers include glass fibers having a tensile modulus of at least 70 GPa, more preferred of at least 80 GPa, even more preferred of at least 85 GPa, and most preferred of at least 90 GPa. Another preferred range of glass fibers has a tensile modulus of at most 100 GPa. The most preferred reinforcing fibers include glass fibers having a tensile strength of at least 3 GPa, and a tensile modulus of at least 80 GPa, and/or a tensile modulus of at most 100 GPa. 
     Examples of suitable matrix materials for the reinforcing fibers are thermoplastic polymers such as polyamides, polyimides, polyethersulphones, polyetheretherketone, polyurethanes, polyethylene, polypropylene, polyphenylene sulphides (PPS), polyamide-imides, acrylonitrile butadiene styrene (ABS), styrene/maleic anhydride (SMA), polycarbonate, polyphenylene oxide blend (PPO), thermoplastic polyesters such as polyethylene terephthalate, polybutylene terephthalate, as well as mixtures and copolymers of one or more of the above polymers. The preferred thermoplastic polymers further comprise an almost amorphous thermoplastic polymer having a glass transition temperature Tg of greater than 140° C., preferably greater than 160° C., such as polyarylate (PAR), polysulphone (PSO), polyethersulphone (PES), polyetherimide (PE1) or polyphenylene ether (PPE), and in particular poly-2,6 dimethyl phenylene ether. According to the invention, it is also possible to apply a semicrystalline or paracrystalline thermoplastic polymer having a crystalline melting point T m  of greater than 170° C., preferably greater than 270° C., such as polyphenylene sulphide (PPS), polyetherketones, in particular polyetheretherketone (PEEK), polyetherketone (PEK) and polyetherketoneketone (PEKK), “liquid crystal polymers” such as XYDAR® by Dartco derived from monomers biphenol, terephthalic acid and hydrobenzoic acid. Suitable matrix materials also comprise thermosetting polymers such as epoxies, unsaturated polyester resins, melamine/formaldehyde resins, phenol/formaldehyde resins, polyurethanes, etcetera. 
     In the laminate according to the invention, the fiber-reinforced composite layer comprises if desired substantially continuous fibers that extend in two almost orthogonal directions (so called isotropic woven fabric). However it is preferable for the fiber-reinforced composite layer to comprise substantially continuous fibers that mainly extend in one direction (so called UD material). It is advantageous to use the fiber-reinforced composite layer in the form of a pre-impregnated semi-finished product. Such a “prepreg” shows generally good mechanical properties after curing thereof, among other reasons because the fibers have already been wetted in advance by the matrix polymer. 
     Fiber-metal laminates may be obtained by connecting a number of metal layers and intermediary fiber-reinforced composite layers to each other by means of heating under pressure and then cooling them. The fiber-metal laminates of the invention have good specific mechanical properties (properties per unit of density). Metals that are particularly appropriate to use include steel (alloys) and light metals, such as titanium and in particular aluminium alloys. Suitable aluminum alloys are based on alloying elements such as copper, zinc, magnesium, silicon, manganese, and lithium. Small quantities of chromium, titanium, scandium, zirconium, lead, bismuth and nickel may also be added, as well as iron. Preferred aluminum alloys include aluminum copper alloys (2xxx series), aluminum magnesium alloys (5xxx series), aluminum silicon magnesium alloys (6xxx series), aluminum zinc magnesium alloys (7xxx series), aluminum lithium alloys (8xxx series), as well as aluminum magnesium scandium alloys. Particularly preferred aluminum alloys include 2024, 7075, 7475 or 6013 alloys. Also, aluminum magnesium alloys with a magnesium content lower than 6% by weight are preferred, particularly in combination with aluminum zinc magnesium alloys of the 7xxx series. In other respects, the invention is not restricted to laminates using these metals, so that if desired other aluminum alloys and for example steel or another suitable structural metal can be used. The laminate of the invention may also comprise metal sheets of different alloys. 
     Although the thickness of the composite layers and the thin metal sheets may be varied within a large range, in a preferred embodiment the thickness of the metal sheets is between 0.2 mm and 1.5 mm, and more preferably between 0.3 and 0.8 mm. Although applying thinner metal sheets per se leads to higher costs and is therefore not naturally obvious, it turns out that applying them in the laminate leads to an improvement in the properties of the laminate. The laminate according to the invention is additionally advantageous in that only a few thin metal sheets have to be applied in the laminate to be sufficient to achieve these improved properties. The same advantages are achieved if the thickness of the fiber-reinforced composite layers in the laminate is less than 0.8 mm, and preferably inclusive between 0.2 and 0.6 mm. 
     A fiber-metal laminate according to the invention will generally be formed by a number of metal sheets and a number of fiber-reinforced composite layers, with the proviso that such laminate comprises less than 47% by volume of metal. The outer layers of the fiber-metal laminate may comprise metal sheets and/or fiber-reinforced composite layers. The number of metal layers may be varied over a large range and is at least one. In a particularly preferred fiber-metal laminate, the number of metal layers is two, three or four, between each of which fiber-reinforced composite layers have preferably been applied. Depending on the intended use and requirements set, the optimum number of metal sheets can easily be determined by the person skilled in the art. The total number of metal sheets will generally not exceed 20, although the invention is not restricted to laminates with a maximum number of metal layers such as this. According to the invention, the number of metal sheets is preferably between 1 and 10, and more preferably between 1 and 5, with the metal sheets preferably having a tensile strength of at least 0.20 GPa. 
     To prevent the laminate from warping as a result of internal tensions, the laminate according to the invention can be structured symmetrically with respect to a plane through the center of the thickness of the laminate. To restrict galvanic corrosion, a laminate according to the invention having aluminum alloy sheets can be designed such that if a layer of electrically conductive fibers, for example carbon fibers, is applied, it is covered on either side with a layer of electrically non-conductive fibers, for example glass fibers. 
     Fiber-metal laminate configurations according to the invention are readily obtained by arranging (alternating) layers of fiber-reinforced composite, preferably in the form of prepregs, and at least one thin metal sheet in such a manner that the volume fraction of metal ranges from between 0% and 470/0, preferably between 5 vol. % and 41 vol. %, more preferably between 10 vol. % and 35 vol. %, and most preferably between 15 vol. % and 30 vol. %. The fiber-metal laminates can be designed in many different arrangements.  FIGS. 4 and 5  schematically show single sheets of prepreg that can be used to construct the configurations covered by this invention.  FIG. 4  represents a unidirectional prepreg with fibers  100  running in one direction only (denoted X-direction in  FIGS. 4 and 5 ), whereas  FIG. 5  represents a woven fabric prepreg with fibers ( 100 ,  110 ) running in two perpendicular directions X and Y, corresponding to the warp and weft directions of the woven fabric. 
       FIG. 6  shows an embodiment (in exploded view) of the fiber-metal laminate  10  according to the invention in the form of a flat rectangular sheet.  FIG. 7  shows the same laminate in a cross-sectional view. In the exemplary embodiment shown, the laminate  10  comprises one metal sheet  11  having a thickness of about 0.2 mm, the sheet comprising an aluminum alloy, which in this case is 2024-T3. The metal sheet  11  is securely interconnected with four fiber-reinforced composite layers  12 - 15  on each side. Each fiber-reinforced composite layer  12 - 15  comprises and is formed of S2-glass fibers impregnated with an epoxy resin, having a fiber volume content of approximately 60 vol. %. The prepreg layers  12 - 15  with a thickness of approximately 0.25 mm are formed of (unidirectional) S2-glass fibers extending parallel to each other in the direction indicated. As shown in  FIG. 6 , layers  12  and  14  have fibers directed in the Y-direction, whereas the fibers in layers  13  and  15  extend in the X-direction of the laminate. Such a cross-ply arrangement yields the same properties in the X- and Y-direction. 
     Laminate  10  is produced by preparing a stack of a first set of layers  12 - 15 , metal sheet  11  and another set of layers  12 - 15  in the sequence shown in  FIG. 6 , for example on a flat mold. After lamination, the overall structure  10  is cured at a temperature suitable for the epoxy resin, for instance in an autoclave, and preferably under vacuum in order to expel entrapped air from the laminate. For most applications, an epoxy resin with a high glass transition temperature will be most suitable. Any epoxy resin may be used however. Epoxy resins are generally cured at or slightly above room temperature, at a temperature of approximately 125° C. or at a temperature of approximately 175° C. After curing under pressure a consolidated laminate is obtained. 
       FIG. 8  shows another embodiment (in exploded view) of a fiber-metal laminate  20  according to the invention in the form of a flat rectangular sheet.  FIG. 9  shows the same laminate in a cross-sectional view. In the exemplary embodiment shown, the laminate  20  comprises one metal sheet  21  having a thickness of about 0.2 mm, the sheet comprising an aluminum alloy, which in this case is 2024-T3. The metal sheet  21  is securely interconnected with four fiber-reinforced composite layers  22 - 25  on each side. Each fiber-reinforced composite layer  22 - 25  comprises and is formed of S2-glass fibers impregnated with an epoxy resin, having a fiber volume content of approximately 60 vol.-%. The prepreg layers  22 - 25  with a thickness of approximately 0.25 mm are formed of (unidirectional) S2-glass fibers extending parallel to each other in the direction indicated. As shown in  FIG. 8 , all layers  22 - 25  have fibers directed in the X-direction. Such a unidirectional arrangement yields different properties in the X- and Y-direction (anisotropy). Laminate  20  is produced in the same way as laminate  10 , as described above. 
       FIG. 10  shows still another embodiment (in exploded view) of a fiber-metal laminate according to the invention in the form of a flat rectangular sheet. In the exemplary embodiment shown, the laminate  30  comprises two metal sheets  31  positioned on the outer surfaces of the laminate, sheet  31  having a thickness of about 0.2 mm and comprising an aluminum alloy, which in this case is 2024-T3. The metal sheets  31  are securely interconnected with eight fiber-reinforced composite layers  32 - 39  on the back side. Each fiber-reinforced composite layer  32 - 39  comprises and is formed of S2-glass fibers impregnated with an epoxy resin, having a fiber volume content of approximately 60 vol.-%. The prepreg layers  32 - 39  with a thickness of approximately 0.25 mm are formed of (unidirectional) S2-glass fibers extending parallel to each other in the direction indicated. As shown in  FIG. 10  layers  32 ,  34 ,  36  and  38  have fibers directed in the Y-direction, whereas the fibers in layers  33 ,  35 ,  37  and  39  extend in the X-direction of the laminate. Such a cross-ply arrangement yields the same properties in the X- and Y-direction. Laminate  30  is produced in the same way as laminate  10 , as described above. 
       FIG. 11  shows still another embodiment (in exploded view) of a fiber-metal laminate according to the invention in the form of a flat rectangular sheet. In the exemplary embodiment shown, the laminate  40  comprises two metal sheets  41  and  42 , symmetrically positioned in the Z-direction with respect to seven fiber-reinforced composite layers  44 - 50 . Metal sheets ( 41 ,  42 ) have a thickness of about 0.2 mm and comprise a 2024-T3 aluminum alloy. The metal sheets ( 41 ,  42 ) are securely interconnected with seven fiber-reinforced composite layers  44 - 50  in de sequence indicated in  FIG. 11 . Each fiber-reinforced composite layer  44 - 50  comprises and is formed of S2-glass fibers impregnated with an epoxy resin, having a fiber volume content of approximately 60 vol.-%. The prepreg layers  44 - 50  with a thickness of approximately 0.25 mm are formed of (unidirectional) S2-glass fibers extending parallel to each other in the direction indicated. As shown in  FIG. 11  layers  45 ,  47  and  49  have fibers directed in the Y-direction, whereas the fibers in layers  44 ,  46 ,  48  and  50  extend in the X-direction of the laminate. Such a cross-ply arrangement yields the same properties in the X- and Y-direction. Laminate  40  is produced in the same way as laminate  10 , as described above. 
     Finally,  FIG. 12  shows still another embodiment (in exploded view) of a fiber-metal laminate  60  according to the invention in the form of a flat rectangular sheet. In the exemplary embodiment shown, the laminate  60  comprises one metal sheet  61 , centrally disposed between two sets of four fiber-reinforced composite layers  62 - 65 . Metal sheet  61  has a thickness of about 0.2 mm and comprise a 2024-T3 aluminum alloy. The metal sheet  61  is securely interconnected with the eight fiber-reinforced composite layers  62 - 65  in the sequence indicated in  FIG. 12 . Each fiber-reinforced composite layer  62 - 65  comprises and is formed of a square woven fabric of S2-glass fibers impregnated with an epoxy resin, having a fiber volume content of approximately 50 vol.-%. Such a woven fabric arrangement yields about the same properties in the X- and Y-direction. Laminate  60  is produced in the same way as laminate  10 , as described above. 
       FIGS. 4 through 12  show various possible fiber-metal laminate configuration covered by this invention. These figures are meant to show examples of configurations but do not limit the wide range of possible configuration covered by this invention. The laminate is advantageously used in constructing impact resistant objects, such as explosion resistant aircraft luggage containers and wing leading edges.