Laminate of a Metal Sheet and an Adhesive Layer Bonded Thereto

Abstract

Disclosed is a laminate including a first metal sheet and an adhesive layer bonded to the first metal sheet, in which the following relationship applies: 1(E.sub.metal*t.sub.metal)/(E.sub.adh*t.sub.adh)15 (1), where E.sub.metal=tensile Young's modulus of the first metal sheet, t.sub.metal=thickness of the first metal sheet, E.sub.adh=tensile Young's modulus of the adhesive layer, and t=thickness of the adhesive layer. The adhesive layer may include reinforcing fibers. The laminate may be used for providing a fatigue resistant structure, such as an aerospace structure, and shows a high crack growth resistance, in particular near edges of the structure.

Claims

1.-22. (canceled)

23. A laminate comprising a first metal sheet and an adhesive layer bonded to the first metal sheet, wherein the following relation applies:
3.5(E.sub.metal*t.sub.metal)/(E.sub.adh*t.sub.adh)15(1) wherein E.sub.metal=tensile Young's modulus of the first metal sheet, t.sub.metal=thickness of the first metal sheet, E.sub.adh=tensile Young's modulus of the adhesive layer, and t.sub.adh=thickness of the adhesive layer; wherein the laminate comprises a second metal sheet bonded to the adhesive layer and having a thickness t.sub.metal.

24. The laminate according to claim 23, wherein the first metal sheet has a thickness t.sub.metal of larger than 0.5 mm.

25. The laminate according to claim 23, wherein: 4.25(E.sub.metal*t.sub.metal)/(E.sub.adh*t.sub.adh)13.5.

26. The laminate according to claim 23, wherein the adhesive layer comprises reinforcing fibers to form a fiber-metal laminate, and E.sub.adh=tensile Young's modulus of the fiber reinforced adhesive layer in a direction of maximum stiffness, and t.sub.adh=thickness of the fiber reinforced adhesive layer.

27. The laminate according to claim 23, comprising N metal sheets having a thickness equal to t.sub.metal, and M metal sheets having a thickness t.sub.metal, wherein N 2 and M 1.

28. The laminate according to claim 23, comprising P second metal sheets directly bonded to a first metal sheet, wherein P 1.

29. The laminate according to claim 23, wherein the thickness of the second metal sheet is less than 0.8 mm.

30. The laminate according to claim 23, wherein the first and/or other metal sheets have a variable thickness, and the thickness t.sub.metal of the first metal sheet in relation corresponds to the largest thickness of the first metal sheet, whereby the area of largest thickness extends over more than 80% of the laminate's area.

31. The laminate according to claim 23, comprising metal sheets of different metal alloys.

32. The laminate according to claim 23, comprising metal sheets of an aluminum alloy.

33. The laminate according to claim 26, comprising a fiber-reinforced adhesive layer with at least two different fibers, and/or comprising fiber-reinforced adhesive layers that differ in fiber.

34. The laminate according to claim 26, wherein a fiber-reinforced adhesive layer comprises high strength glass fibers having a tensile Young's modulus of at least 80 GPa.

35. The laminate according to claim 23, comprising two first metal sheets that are connected to each other by a number of second metal sheets and intermittent adhesive layers.

36. A process for providing a fatigue resistant structure comprising providing a laminate according to claim 23.

37. An aerospace structure comprising a laminate according to claim 23.

38. The aerospace structure according to claim 37, comprising a fuselage structure, a tail plane structure, or a wing structure.

39. The laminate according to claim 32, comprising at least one aluminum lithium sheet.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0043] FIG. 1is a view in perspective of a fiber-metal laminate according to an embodiment of the present invention;

[0044] FIG. 2is a view in perspective of a fiber-metal laminate according to another embodiment of the present invention;

[0045] FIGS. 3-10are perspective views of other embodiments of a fiber-metal laminate according to the invention having a reduced laminate thickness in an edge area of the laminate; and

[0046] FIGS. 11-13are cross-sectional views of other embodiments of a fiber-metal laminate according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0047] 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 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.

[0048] The basis of the present invention is a unique arrangement of at least one metal sheet and an adhesive layer adhered thereto. The adhesive layer is preferred embodiments comprise reinforcement fibers. In accordance with certain embodiments, a fiber-metal laminate is provided comprising fiber-reinforced composite layers and metal sheets, wherein a fiber-reinforced composite layer and an adjacent first metal sheet have related properties in a specific manner, as given by relation (1). The fiber-reinforced composite layers preferably comprise fibers pre-impregnated with a composite matrix system, preferably a metal adhesive (prepreg). The system of composite layers and metal sheets is preferably processed under heat and pressure to cure the adhesive and form a solid panel or component.

[0049] It has been discovered by the inventor that laminates with metal sheet and adhesive layer properties according to equation (1) have better structural properties in fatigue, in particular a higher resistance against crack growth than fiber-metal laminates of which the relevant properties are not in accordance with relation (1). The parameters used in equation (1) are well known to the person skilled in the art and this person will have no difficulty in determining the properties mentioned. The invention is based on the insight that the extensional stiffness of a metal sheet and an adjacent adhesive layer (preferably fiber-reinforced composite layer) are related in view of obtaining a high crack growth resistance.

[0050] 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 typically acts as a bonding means between the various layers. Reinforcing fibers that are suitable for use in the fiber-reinforced composite layers depend on the choice of metal in the metal sheets (see equation (1)) but may include glass fibers, aramid fibers, PBO fibers, carbon fibers, copolymer fibres, boron fibres and metal fibers and/or combinations of the above fibers.

[0051] Examples of suitable matrix materials for the reinforcing fibers include but are not limited to thermoplastic polymers such as polyamides, polyimides, polyethersulphones, polyetheretherketone, polyurethanes, polyphenylene sulphides (PPS), polyamide-imides, polycarbonate, polyphenylene oxide blend (PPO), as well as mixtures and copolymers of one or more of the above polymers. Suitable matrix materials also comprise thermosetting polymers such as epoxies, unsaturated polyester resins, melamine/formaldehyde resins, phenol/formaldehyde resins, polyurethanes, of which thermosetting polymers epoxies are most preferred.

[0052] In the laminate according to the invention, the fiber-reinforced composite layer preferably comprises substantially continuous fibers that extend in multiple direction (like 0, 90 and angles with respect to 0) and more preferable in two almost orthogonal directions (for instance isotropic woven fabrics or cross plies). However it is even more 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.

[0053] In some embodiments of the invention, fiber-metal laminates may be obtained by connecting a number of metal sheets and fiber-reinforced composite layers to each other by means of heating under pressure and subsequent cooling. 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 aluminum alloys and in particular titanium 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. Suitable 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 (2xxx, 8xxx series), as well as aluminum magnesium scandium alloys. Suitable titanium alloys include but are not limited to alloys comprising Ti-15V-3Cr-3Al-3Sn, Ti-15Mo-3Al-3Nb, Ti-3Al-8V-6Cr-4Zr-4Mo, Ti-13V-11Cr-3Al, Ti-6Al-4V and Ti-6Al-4V-2Sn. In other respects, the invention is not restricted to laminates using these metals, so that if desired other metals, for example steel or another suitable structural metal can be used. The laminate of the invention may also comprise metal sheets of different alloys.

[0054] A fiber-metal laminate according to some embodiments of the invention may be formed by combining a number of metal sheets and a number of fiber-reinforced composite layers, with the proviso that the extensional stiffness of a metal sheet and an adjacent adhesive layer satisfies equation (1).

[0055] 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 50, 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 40, and more preferably between 1 and 25.

[0056] 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.

[0057] Fiber-metal laminate configurations according to some embodiments of the invention are readily obtained by arranging (alternating) layers of fiber-reinforced composite, preferably in the form of prepregs, and at least one metal sheet. The fiber-metal laminates can be designed in many different arrangements.

[0058] With reference to FIG. 1, a fiber-metal laminate according to one embodiment is shown, wherein the total number of layers is 3, and wherein layer 1 and layer 3 comprise a metal sheet and layer 2 a fibrous composite layer. Alternatively, layer 1 and layer 3 comprise a fibrous composite layer and layer 2 is a metal sheet. Layer 1 and layer 3 can comprise the same metal alloy or may be made of a different kind of metal alloy. The fibrous composite layer(s) may contain fibers in multiple directions as well as different kind of fibers. At least one of the combinations of layers 1 and 2, or 2 and 3, fulfills the requirement set in equation (1).

[0059] With reference to FIG. 2, a fiber-metal laminate according to another embodiment is shown, wherein the total number of layers is n, and wherein layer 1 is a metal sheet and layer 2 is a fibrous composite layer, which will be alternating until layer n-1 and layer n. Alternatively, layer 1 is a fibrous composite layer and layer 2 is a metal sheet, which will be alternating until layer n-1 and layer n. The alternating metal sheets can be made of the same metal alloy or be made from a different kind of metal alloy, and may have different thicknesses. Also, at least one of the alternating fibrous composite layers may contain fibers in multiple directions as well as different kind of fibers. According to the invention, at least one combination of a fiber-reinforced composite layer (for instance layer 2) and an adjacent metal sheet (for instance layer 1 or 3) needs to satisfy relation (1). In case metal sheets (1) and (3) differ in thickness, the thickest metal sheet is selected as first metal sheet in the combination. In case the outer layer of the laminate is a fibrous composite layer, this layer preferably needs to fulfill the requirements set in equation (1) with respect to its adjacent metal sheet, unless another metal sheet with its adjacent fiber composite layer already fulfills the requirements of equation (1). If the outer layer is a metal sheet, it preferably needs to fulfill the requirements set in equation (1) with respect to its adjacent fibrous composite layer, unless another metal sheet with its adjacent fibrous composite layer already fulfills the requirements of equation (1).

[0060] The laminates are produced by preparing a stack of fibrous composite and metal sheets in the sequence as exemplified in FIGS. 1 and 2, for example on a flat or single, double or multiple curved mold. After lamination, the overall structure is cured at a temperature suitable for the matrix resin, preferably an 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. As mentioned above, it is also possible to use a thermoplastic resin.

[0061] FIG. 3 shows another embodiment of a laminate in accordance with the invention. The laminate 10 comprises 5 layers in total. Laminate 10 in particular comprises an aluminum sheet 1 with a thickness t.sub.metal of 1.2 mm, a high strength glass fiber epoxy composite layer 2 bonded to the first aluminum sheet 1, a second aluminum sheet 3 with a thickness of 0.6 mm (smaller than t.sub.metal) and bonded to composite layer 2, another high strength glass fiber epoxy composite layer 4 bonded to the second aluminum sheet 3, and another aluminum sheet 5 bonded to composite layer 4 and having a thickness of 1.2 mm. The composite layer 2 has about 45 vol % of glass fibers running in a length direction 11 of the laminate. The fibers have a Young's modulus of about 85 GPa. The thickness of layer 2 is about 0.2 mm. The extensional stiffness E*t of layer 1 is about 72 GPa*1.2 mm, whereas the extensional stiffness of layer 2 is about 0.45*85 GPa*0.2 mm. Equation (1) then yields a value of about 11.3 which is within the claimed range.

[0062] Laminate 10 further has an edge 13 and the total thickness 14 of the laminate 10 is reduced in an edge area of laminate 10 towards the edge 13. The thickness reduction is achieved by ending the first aluminum sheet 1 at a first distance 15 from the laminate edge 13, optionally ending another aluminum sheet 3 at a second distance 16 from the laminate edge 13, and ending the adhesive layer 2 adjacent to the first metal sheet 1 at a third distance 17 from the edge 13. The distance 15 in the present embodiment corresponds to the distance over which the edge area extends from edge 13. Another adhesive layer 4 is ended at yet another distance 18 from the edge 13. The distances 15 to 18 all differ from each other, in fact these distances decrease from distance 15 to distance 18 to achieve a tapered laminate in the edge area.

[0063] FIG. 4 shows another embodiment of a laminate in accordance with the invention. The laminate 10 comprises the same 5 layers as those of the embodiment of FIG. 3. However, the first aluminum sheet 1 has a variable thickness, which in the embodiment shown varies from a constant thickness of 1.2 mm to a constant thickness of 0.6 mm in a stepwise fashion. The largest thickness of the first aluminum sheet (1.2 mm) is taken as t.sub.metal in relation (1).

[0064] Laminate 10 further has an edge 13 and the total thickness 14 of the laminate 10 is reduced in an edge area 15 of laminate 10 towards the edge 13. The thickness reduction is achieved by reducing the thickness of the first aluminum sheet 1 at a first distance 15 from the laminate edge 13 (which is the same as ending part of the first aluminum sheet 1), ending the first aluminum sheet 1 at a distance 15a from the laminate edge 13, optionally ending another aluminum sheet 3 at a second distance 16 from the laminate edge 13, and by ending the adhesive layer 2 adjacent the first metal sheet 1 at a third distance 17 from the edge 13. Another adhesive layer 4 is ended at another distance 18 from the edge 13. The distances 15, 15a to 18 all differ from each other, in fact these distances decrease from distance 15 to distance 18.

[0065] FIG. 5 shows yet another embodiment in accordance with the invention. The laminate 10 comprises the same 5 layers as those of the embodiment of FIGS. 3 and 4. Laminate 10 again has an edge 13 and the total thickness 14 of the laminate 10 is reduced in an edge area 15 of laminate 10 towards the edge 13. The thickness reduction is achieved by ending the first aluminum sheet 1 at a distance 15 from the laminate edge 13, optionally ending another aluminum sheet 3 at a second distance 16 from the laminate edge 13. The adhesive layer 2 adjacent the first metal sheet 1 is ended at a third distance 17 from the edge 13, which distance 17 in the present embodiment is equal to the first distance 15. Another adhesive layer 4 is ended at a distance 18 from the edge 13 which is equal to distance 16. Adhesive fiber-composite layers 2 and 4 have fibers running in the length direction 11 but are void of reinforcing fibers at extreme ends (2a, 4a).

[0066] FIG. 6 shows yet another embodiment of a laminate in accordance with the invention. The laminate 10 comprises 5 layers in total. Laminate 10 in particular comprises an aluminum sheet 1 with a thickness t.sub.metal of 1.0 mm, a high strength glass fiber epoxy composite layer 2 bonded to the first aluminum sheet 1, a second aluminum sheet 3 with a thickness of 0.5 mm (smaller than t.sub.metal) and bonded to composite layer 2, another high strength glass fiber epoxy composite layer 4 bonded to the second aluminum sheet 3, and another aluminum sheet 5 bonded to composite layer 4 and having a thickness of 1.5 mm. The composite layer 2 has about 55 vol % of glass fibers running in a length direction 11 of the laminate. The fibers have a Young's modulus of about 85 GPa. The thickness of layer 2 is about 0.25 mm. The extensional stiffness E*t of layer 1 is about 72 GPa*1.0 mm, whereas the extensional stiffness of layer 2 is about 0.55*85 GPa*0.25 mm. Equation (1) then yields a value of about 6 which is within the claimed range.

[0067] In FIG. 7 another embodiment of a laminate in accordance with the invention is shown. The laminate of FIG. 7 differs from the laminate of FIG. 6 in that the outer aluminum sheet 1 has a reduced thickness in an edge area towards the edge 13 of aluminum sheet 1 over a distance 15. The thickness reduction is achieved by reducing the thickness of the first aluminum sheet 1 at a first distance 15 from the laminate edge 13 (which is the same as ending part of the first aluminum sheet 1), and ending the first aluminum sheet 1 at a distance 15a from the laminate edge 13, the distance 15a being smaller than the thickness 15.

[0068] FIG. 8 shows another embodiment of a laminate in accordance with the invention. The laminate of FIG. 8 is largely the same as that of FIG. 7 with the exception that the thickness reduction of aluminum sheet 1 is gradual (or tapered) from a distance 15 of the edge 13 to a distance 15a from the edge 13.

[0069] FIG. 9 shows yet another embodiment of a laminate in accordance with the invention. The laminate 10 comprises 9 layers in total. Laminate 10 in particular comprises an aluminum sheet 9 with a thickness t.sub.metal of 3.0 mm, a high strength glass fiber epoxy composite layer 8 bonded to the first aluminum sheet 9, a second aluminum sheet 7 with a thickness of 0.4 mm (smaller than t.sub.metal) and bonded to composite layer 8, another high strength glass fiber epoxy composite layer 6 bonded to the aluminum sheet 7, and another aluminum sheet 5 bonded to composite layer 6 and having a thickness of 0.4 mm, another high strength glass fiber epoxy composite layer 4 bonded to the aluminum sheet 5, and another aluminum sheet 3 bonded to composite layer 4 and having a thickness of 0.4 mm, another high strength glass fiber epoxy composite layer 2 bonded to the aluminum sheet 3, and another aluminum sheet 1 bonded to composite layer 2 and having a thickness of 0.4 mm. The outer aluminum sheet 9 can have a constant thickness, a tapered thickness or, as shown in FIG. 9 a thickness reduction. The thickness reduction is achieved by reducing the thickness of metal sheet 9 at a distance 95 from the edge 13. Composite layer 8 ends at a distance 85 which is larger than distance 95. The composite layer 8 has about 55 vol % of glass fibers running in a length direction 11 of the laminate. The fibers have a Young's modulus of about 90 GPa. The thickness of layer 8 is about 0.4 mm. The extensional stiffness E*t of layer 9 is about 72 GPa*3.0 mm, whereas the extensional stiffness of layer 8 is about 0.55*90 GPa*0.40 mm. Equation (1) then yields a value of about 11 which is within the claimed range.

[0070] FIG. 10 shows another embodiment of a laminate in accordance with the invention. The laminate 10 comprises 9 layers in total. It is largely equivalent to the laminate of FIG. 9 with the exception that aluminum sheet 1 has a thickness of 2.0 mm instead of 0.4 mm and that sheet 1 has a reduced thickness towards the edge 16 of aluminum sheet 1 over a distance 17.

[0071] FIGS. 11-13 finally represent cross-sections of three other embodiments of a laminate in accordance with the invention. The laminate 10 of FIG. 11 comprises an alternating stack of relatively thick metal sheets (1, 5, 9, 23) and relatively thin metal sheets (3, 7, 21). The metal sheets (1, 3, 5, 7, 9, 21, 23) are mutually bonded by intermittent fiber composite layers (2, 4, 6, 8, 20, 22). At an edge area of the laminate 10, the metal sheets and fiber composite layers end at different distances from the edge 13, so as to produce a tapered part of the laminate 10 at the edge area.

[0072] The laminate 10 of FIG. 12 has two relatively thick metal sheets (1, 9) as outer layers in the stack, and a number of 3 relatively thin metal sheets (3, 5, 7) in between the outer metal sheets (1, 9). The metal sheets (1, 3, 5, 7, 9) are mutually bonded by intermittent fiber composite layers (2, 4, 6, 8), of which layers (4, 6) have a smaller thickness than layers (2, 8). At an edge area of the laminate 10, the metal sheets and fiber composite layers end at different distances from the edge 13, so as to produce a tapered part of the laminate 10 at the edge area. Further, metal sheets 1 and 9 have a reduced thickness towards their edge.

[0073] The laminate 10 of FIG. 13 finally combines two laminates 10 according to FIG. 12. The laminate has 9 metal sheets and 8 fiber composite layers (2, 4, 6, 8, 20, 22, 24, 26) in total. Metal sheets (1, 9 and 27) are thicker than metal sheets (3, 5, 7, 21, 23, and 25) in accordance with the invention. The thickness of the relatively thick metal sheets (1, 9, 27) is again reduced at their respective edges.

Experiments

[0074] Four different laminate configurations were tested in fatigue. In particular, fatigue crack growth was measured at a maximum stress level of 120 MPa and at a ratio R=0.1; whereby R is the ratio between the minimum stress level and the maximum stress level.

[0075] All four tested configurations are so-called GLARE 2 laminates in a 3/2 lay-up. GLARE 2 uses fiber reinforced adhesive layers in the form of prepregs having all fibers extending in one direction parallel to each other. The direction of the fibers is parallel to a rolling direction of the metal sheets used in the laminates and also parallel to the loading direction in the fatigue tests. The metal applied in the metal sheets comprises aluminum alloy 2024-T3 with a tensile modulus E=72.4 GPa. The prepregs applied comprise S2-glass fibers embedded in an epoxy matrix system. The nominal fiber volume content of the prepreg is 19.8% in configurations 1 and 2, and 35.0% in configurations 3 and 4. The respective thickness after curing is 0.38 mm (configurations 1 and 2) and 0.65 mm (configurations 3 and 4)).

[0076] Configuration 1 is a laminate which consists of three metal layers of a thickness of 2.0 mm and one prepreg layer, placed in between each metal layer. This laminate is referred to as GLARE 2-3/2-2.0-1 pp. Configuration 2 is a laminate which consists of three metal layers of a thickness of 2.0 mm and three prepreg layers, placed in between each metal layer. This laminate is called GLARE 2-3/2-2.0-3 pp. Configuration 3 is a laminate which consists of three metal layers of a thickness of 1.3 mm and one prepreg layer, placed in between each metal layer. This laminate is called GLARE 2-3/2-1.3-1 pp. Configuration 4 finally is a laminate which consists of three metal layers of a thickness of 1.3 mm and three prepreg layers, placed in between each metal layer. This laminate is called GLARE 2-3/2-1.3-3 pp.

[0077] The S2-glass fiber applied in the prepregs has an E-modulus of 88 GPa and the applied epoxy system has an E-modulus of 2.2 GPa. The stiffness ratio according to equation (1) of claim 1 can be determined for the different configurations as shown in Table A.

TABLE-US-00001 TABLE A Fibre (E.sub.metal*t.sub.metal)/ Configuration Volume (E.sub.adh*t.sub.adh) 1 GLARE 2-3/2-2.0-1 pp 19.8% 19.7 2 GLARE 2-3/2-2.0-3 pp 35.0% 6.9 3 GLARE 2-3/2-1.3-1 pp 19.8% 12.8 4 GLARE 2-3/2-1.3-3 pp 35.0% 4.5

[0078] Table A clearly shows that configuration 1 is outside the range of eq. (1). Configuration 3 on the other hand has a stiffness ratio of 12.8 which is relatively close to the upper border value of eq. (1).

[0079] Figure B shows the obtained results in terms of crack growth data da/dn, where n denotes the number of fatigue cycles, versus the half crack length a. Results are shown for configurations 1-4 and for a sheet of monolithic aluminum alloy 2024-T3. It may be inferred from Figure B that the crack growth rate of the monolithic aluminum alloy is highest and shows failure of the specimen at a half crack length a=21 mm. The specimen outside the range of eq. (1) with configuration 1 has failed like the aluminum specimen at a half crack length a=26 mm and further appears to have a slope of the crack growth rate in the same range as the slope of the crack growth rate of the aluminum alloy.

[0080] While the aluminum alloy and laminate according to configuration 1 failed at a relatively small crack length, the other configurations 2-4 which are according to the invention could be loaded without failure to much higher half crack lengths.

[0081] The configurations 2-4 further all show significantly smaller crack growth rates with configuration 4 showing the best performance. This configuration has the lowest stiffness ratio (Eq. (1)).