Laminate of Mutually Bonded Adhesive Layers and Metal Sheets, and Method to Obtain Such Laminate

Abstract

Described is a laminate, including a stack of mutually bonded adhesive layers and metal sheets. The laminate includes abutting and/or overlapping metal sheet edges that extend along a length direction within a splicing region. A splice strap is connected to the laminate at an outer surface of the laminate across said splicing region. The splice strap includes one layer of fiber-reinforced adhesive or of metal sheet, or stacked layers of fiber-reinforced adhesive and/or metal sheets. A widest splice strap layer is connected to the laminate over a transverse distance of at least 5 times the widest strap layer thickness, and the widest strap layer has a lower bending stiffness than the bending stiffness of one of the spliced metal sheets.

Claims

1.-25. (canceled)

26. A laminate comprising a stack of mutually bonded layers of adhesive and metal sheets, the laminate comprising spliced metal sheets with abutting and/or overlapping metal sheet edges that extend along a length direction within a splicing region, wherein a splice strap is connected to the laminate at and over an outer surface of the laminate and extending in the length direction across said splicing region over a certain width in a transverse direction perpendicular to the length direction, the splice strap comprising a layer of fiber-reinforced adhesive and/or a metal sheet layer, or stacked layers of fiber-reinforced adhesive and/or metal sheets, wherein a widest splice strap layer is connected to the laminate over a transverse distance of at least 5 times the widest strap layer thickness, and the widest strap layer has a lower bending stiffness (E*t.sup.3).sub.strap layer than the bending stiffness (E*t.sup.3).sub.spliced layer of one of the spliced metal sheets
(E*t.sup.3).sub.strap layer<(E*t.sup.3).sub.spliced layer and wherein further E.sub.strap layer>10 GPa.

27. Laminate according to claim 26, wherein the bending stiffness of the splice strap layer and the spliced metal sheet is the bending stiffness in the transverse direction.

28. Laminate according to claim 26, wherein (E*t.sup.3).sub.strap layer<0.9(E*t.sup.3).sub.spliced layer, more preferably (E*t.sup.3).sub.strap layer<0.75(E*t.sup.3).sub.spliced layer, and most preferably (E*t.sup.3).sub.strap layer<0.50(E*t.sup.3).sub.spliced layer.

29. Laminate according to claim 26, wherein the tensile strength P.sub.strap of the total strap layer is larger than 0.6 times the tensile strength of one of the spliced metal sheets P.sub.spliced layer.

30. Laminate according to claim 29, wherein the laminate comprises spliced metal sheets with overlapping metal sheet edges.

31. Laminate according to claim 26, wherein the bending stiffness (E*t.sup.3).sub.spliced layer of one of the spliced metal sheets is lower than 90 GPa mm.sup.3, more preferably lower than 65 GPa mm.sup.3.

32. Laminate according to claim 29, wherein the tensile strength P.sub.strap of the total strap layer is larger than the tensile strength of one of the spliced metal sheets P.sub.spliced layer, and more preferably larger than 1.2 times P.sub.spliced layer.

33. Laminate according to claim 31, wherein the laminate comprises spliced metal sheets with abutting metal sheet edges.

34. Laminate according to claim 26, wherein the splice strap comprises a metal sheet layer that is connected to the laminate with a layer of fiber-reinforced adhesive.

35. Laminate according to claim 26, wherein an outer surface of the splice strap is flush with the outer surface of the laminate.

36. Laminate according to claim 26, wherein splice strap layers each have a width in the transverse direction across the splicing region, and the width of the layers decreases over the splice strap thickness towards the laminate to form staggered layers.

37. Laminate according to claim 26, wherein the splice strap comprises a tapered edge over a tapered transverse distance, and the splice strap has a lower bending stiffness than the bending stiffness of one of the spliced metal sheets, whereby the bending stiffness of the splice strap is evaluated by taking the thickness equal to the mean thickness across the tapered transverse distance.

38. Laminate according to claim 26, wherein the modulus of elasticity of the widest splice strap layer E.sub.strap layer>15 GPa, more preferably >20 GPa, and most preferably >25 GPa.

39. Laminate according to claim 26, wherein splice strap layers each have a width in the transverse direction across the splicing region, and the width of the layers is equal over the splice strap thickness, wherein the splice strap has a lower bending stiffness than the bending stiffness of one of the spliced metal sheets.

40. Laminate according to claim 26, further comprising a bonded second splice strap extending in the length direction across said splicing region and positioned within the laminate stack, or being positioned adjacent to the spliced metal sheets and at a side of the spliced metal sheets that is opposite to the outer surface of the laminate.

41. Laminate according to claim 26, wherein the splicing region comprises deformed metal sheets.

42. Laminate according to claim 41, wherein the deformed metal sheets are bend along a line parallel to the length direction.

43. Laminate according to claim 26, wherein the outer surface of the laminate is substantially smooth and a second outer surface opposite said outer surface is curved.

44. Laminate according to claim 26, wherein the adhesive layers comprise reinforcing fibers to form a fiber-metal laminate.

45. Structural component for a vehicle, spacecraft, or aircraft, comprising a laminate according to claim 26.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0059] The invention will now be further elucidated on the basis of the exemplary embodiments shown in the figures, without however being limited thereto. The same or similar elements in the figures may be denoted by the same or similar reference signs. In the figures:

[0060] FIG. 1is a view in perspective of a fiber-metal laminate according to the state of the art;

[0061] FIG. 2is a view in perspective of a fiber-metal laminate according to the state of the art;

[0062] FIG. 3is a cross-sectional view in a transverse direction of a fiber-metal laminate according to an embodiment of the present invention;

[0063] FIG. 4is a cross-sectional view in a transverse direction of a fiber-metal laminate according to another embodiment of the present invention;

[0064] FIG. 5is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0065] FIG. 6is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0066] FIG. 7is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0067] FIG. 8is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0068] FIG. 9is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0069] FIG. 10is a cross-sectional view in a transverse direction of a fiber-metal laminate according to yet another embodiment of the present invention;

[0070] FIG. 11is a cross-sectional view in a transverse direction of an assembly of a forming substrate and a fiber-metal laminate, illustrating an embodiment of a method for manufacturing the laminate;

[0071] FIG. 12is a cross-sectional view in a transverse direction of an assembly of a forming substrate and a fiber-metal laminate, illustrating an embodiment of a method for manufacturing another laminate according to the invention;

[0072] FIG. 13is a cross-sectional view in a transverse direction of a fiber-metal laminate according to the state of the art having a butted splice in the uppermost metal layer;

[0073] FIG. 14is a cross-sectional view in a transverse direction of a fiber-metal laminate according to the state of the art having an overlap splice in the uppermost metal layer; and

[0074] FIG. 15is a cross-sectional view in a transverse direction of a fiber-metal laminate according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0075] With reference to FIG. 1, a fiber-metal laminate according to the state of the art is shown. The laminate has a total number of three layers, of which layers 1 and 3 comprise a metal layer and layer 2 comprises a fiber-reinforced adhesive layer. Alternatively, layer 1 and 3 may comprise a fiber-reinforced adhesive layer and layer 2 a metal layer. Layers 1 and 3 may comprise the same metal alloy or may be built from a different kind of metal alloy. The fiber-reinforced adhesive layers may contain fibers in multiple directions as well as different fiber types. The laminate is typically built by providing a forming substrate, providing a first layer 3 on the forming substrate and stacking layers 2 and 1 on top of layer 3 to produce a stack of layers 1-3, which stack is then consolidated under the application of heat and pressure into a cured laminate.

[0076] As shown in FIG. 2, a fiber-metal laminate may comprise more layers up to a layer n, where n may range from 4 to more than 30 for instance. The outer layers 1 and n may be metal layers and/or fiber-reinforced adhesive layers. In the laminate, metal layers generally alternate with fiber-reinforced adhesive layers. Metal layers may be built from one metal sheet having a width in a transverse direction 25 that is sufficiently large to cover the entire width 6 of the laminate. As shown in FIG. 2, metal sheets may not be available in widths covering the entire width 6 of the laminate, and metal layers may have to be built up of at least two metal sheets with abutting metal sheet edges that form a splice 7, extending along a length direction 24 of the laminate within a splicing region 8 of the laminate (an extension of one splice line only is shown in FIG. 2 for clarity reasons and as minimum coverage area of the strap). As shown in FIG. 3, the at least two metal sheets may also comprise overlapping edge parts within a splicing region 8.

[0077] Referring now to FIGS. 3-7, several embodiments of the invented laminate are shown.

[0078] The fiber-metal laminate of FIG. 3 comprises a stack of 2 fiber-reinforced adhesive layers (2, 4) and three metal sheets (1, 3, 5). The metal sheets (1, 3, 5) are bonded to the adhesive layers (2, 4) by the adhesive present in the adhesive layers (2, 4). Outer metal sheet layer 1 is composed of two metal sheets (1a, 1b), edge parts whereof mutually overlap over a distance 9. The overlapping edges extend along the length direction 24 within a splicing region 8. According to the invention, a splice strap 12 is bonded to the laminate 10 at an outer surface 10a of the laminate 10 and extends in the length direction 24 within or across said splicing region 8. The splice strap 12 is made of metal, in casu an aluminum alloy, and has a lower bending stiffness than that of the spliced metal sheets (1a, 1b). It is connected over a distance 12c that is larger than 5 times its thickness. Splice strap 12 could also be made of a fiber reinforced adhesive layer, having also a lower cured bending stiffness than that of one of the spliced metal sheets (1a, 1b).

[0079] As shown in FIG. 3, an outer surface 12a of the splice strap 12 protrudes from the outer surface 10a of the laminate 10 by an off-set thickness which is about half the thickness of the splice strap 12. The outer surface 10a of the laminate is substantially smoothapart from the slightly protruded splice strap 12and a second outer surface 10b opposite said outer surface 10a is curved. A substantially smooth outer surface 10a is preferred for aircraft components from an aerodynamics point of view. To accommodate the overlapping edge parts of sheets (1a, 1b) as well as the splice strap 12, and still produce a substantially smooth or flat outer surface 10a, may in some embodiments require that metal sheets are deformed in the splicing region 8. In particular, as shown, metal sheets are bent along a line parallel to the length direction 24 towards the splice strap 12 and/or overlapping edge parts. The splice strap 12 extends over a width 12c which encompasses the splice region. At the left hand of the splice strap 12, the splice strap 12 extends further than the end of layer 1b, whereas at the right hand of the splice strap 12, the splice strap 12 extends equally far as the layer 1a. In other preferred embodiments however, the right side of the splice strap 12 may extend further than the layer 1a.

[0080] Another useful embodiment of a fiber-metal laminate 10 is shown in FIG. 4 and comprises a bonded second splice strap 13 positioned within a stack of 2 fiber-reinforced adhesive layers (2, 4) and three metal sheets (1, 3, 5). The second splice strap extends in the length direction 24 across said splicing region 8, just as a splice strap 12 provided at the outer surface 10 a of the laminate 10 extends in the length direction 24 across said splicing region 8. The respective widths (12c (see FIG. 3), 13c) of both splice straps (12, 13) need not be the same, as shown. Outer metal layer 1 is composed of two spliced metal sheets (1a, 1b), edge parts whereof abut to form a splice line 7, extending along the length direction 24 within a splicing region 8. The aluminum splice strap 12 has a lower bending stiffness than the bending stiffness of the spliced metal sheets (1a, 1b). The second aluminum splice strap 13 is positioned adjacent to the spliced metal sheets (1a, 1b) but at a side of the spliced metal sheets (1a, 1b) that is opposite to the outer surface 10a of the laminate 10. The second splice strap 13 in other words is positioned directly below the abutting end parts of metal sheets (1a, 1b). Its bending stiffness is lower than the stiffness of the spliced metal sheets (1a, 1b). Instead of aluminum sheets also fiber reinforced layers or combinations can be applied as splice strap having the same requirements.

[0081] Yet another useful embodiment of a fiber-metal laminate 10 is shown in FIG. 5. The laminate 10 comprises a splice strap 22 bonded to an outer surface 10a of the laminate, which comprises a stack of 2 fiber-reinforced adhesive layers (2, 4) and three metal sheets (1, 3, 5). The strap 22 extends in the length direction 24 across a splicing region 8a. Outer metal layer 1 comprises two metal sheets (1a, 1b), an edge part of sheet 1a overlapping with an edge part of sheet 1b. The splice strap 22 comprises two layers (22-1, 22-2) of fiber-reinforced adhesive (prepreg), whereby the layer 22-2 in contact with the outer surface 10a extends over a larger width than the outermost layer 22-1 of the splice strap 22. According to the invention, the bending stiffness of the widest layer 22-2 (after cure) is lower than the bending stiffness of the metal sheets (1a, 1b). Further, it is connected to the outer surface 10a of the laminate 10 over a distance that is larger than 5 times the thickness of layer 22-2. In this embodiment, an outer surface the splice strap 22 protrudes from the outer surface 10a with an off-set thickness. However, the splice strap 22 may also be flush with the outer surface 10a of the laminate 10. Layer 22-1 can also be a metal sheet.

[0082] Yet another useful embodiment is shown in FIG. 6. The splice strap 22 in this embodiment comprises stacked layers (22-1, 22-2, 22-3), bonded to the laminate 1 by an additional adhesive layer 30. Fiber-reinforced adhesive layers 22-1, 22-2 are bonded to a metal sheet layer 22-3 by co-curing the layers. The adhesive layer 30 has a modulus of elasticity of lower than 10 GPa. According to the invention, the bending stiffness of layer 22-3 is lower than the bending stiffness of the metal sheets (1a, 1b). Further, it is connected to the outer surface of the laminate 10 over a distance that is larger than 5 times its thickness. The layers (22-1, 22-2, 22-3) of the splice strap 22 each have a width across the splicing region and the width of the layers (22-1, 22-2, 22-3) is seen to increase towards the laminate. Although the splice strap 22 is seen to protrude from the outer surface 10a with an off-set thickness, the splice strap 22 may also be flush with the outer surface 10a of the laminate 10.

[0083] Another embodiment of a laminate according to the invention is shown in FIG. 7. The splice strap 22 in this embodiment comprises stacked layers (22-1, 22-2, 22-3) of fiber-reinforced adhesive, bonded to another metal sheet layer 22-4. The layers (22-1, 22-2, 22-3, 22-4) of the splice strap 22 each have a width across the splicing region and the width of the layers (22-1, 22-2, 22-3, 22-4) is seen to decrease towards the laminate. The splice strap 22 is bonded to the laminate 10 by an additional adhesive layer 30, and two adhesive layers (31, a, 31b,) for bonding the largest width layer 22-1 to the laminate 10. According to an embodiment of the invention, the bending stiffness of layers 22-4 and in particular widest layer 22-1 is lower than the bending stiffness of the metal sheets (1a, 1b). Widest layer 22-1 is further connected over a distance that is larger than 5 times its thickness. This distance is equal to the difference in width of layers 22-1 and 22-2. Although the splice strap 22 is seen to protrude from the outer surface 10a with an off-set thickness, the splice strap 22 may also be flush with the outer surface 10a of the laminate 10. Also 22-1 can be a metal, 22-2 can be fibre reinforced metal layer; 22-3 can be a metal layer and layer 22-4 can be a fibre reinforced adhesive layer.

[0084] Another embodiment of a laminate according to the invention is shown in FIG. 8. The splice strap 22 in this embodiment comprises two stacked layers (22-1, 22-2) of fiber-reinforced adhesive in the form of prepregs. The splice strap 22 is bonded to the laminate 10 by the adhesive that is part of the prepreg of layer 22-2 and that partly flows out of the prepreg during cure. This embodiment does not require an additional adhesive layer for bonding to the laminate. The layers (22-1, 22-2) of the splice strap 22 in the embodiment shown have an equal width 22c across the splicing region. According to such embodiment of the invention, the bending stiffness of the combined layers 22-1 and 22-2 (after cure) is lower than the bending stiffness of the metal sheets (1a, 1b). Although the splice strap 22 is seen to protrude from the outer surface 10a with an off-set thickness, the splice strap 22 may also be flush with the outer surface 10a of the laminate 10.

[0085] FIG. 9 shows a basic spliced laminate with 3 metal layers (b.sub.1, b.sub.2, b.sub.3, b.sub.4; where b.sub.1 and b.sub.4 are spliced) of an aluminum alloy 2024-T3 (E.sub.metal=72.4 GPa) and b.sub.1=b.sub.2=b.sub.3=0.3 mm), and each metal layer of the basic laminate is bonded with an adhesive layer (a.sub.1 and a.sub.2). This adhesive layer may contain reinforcing fibers. On top of the splice region is positioned a splice strap with 3 fiber reinforced layers (Layer 1, Layer 2, Layer 3) of the same length l.sub.s. All 3 layers of the splice strap are the same and comprise unidirectional (UD) glass fibers (E.sub.fiber=88 GPa) having a fiber volume content of 57%. The glass fibers are embedded in a matrix resin (E.sub.matrix=5000 MPa). The resulting elongational stiffness of each of the layers therefore is E.sub.layer=52.3 GPa). Each layer of the splice strap has the same cured thickness of t.sub.layer=0.13 mm. Since all 3 layers of the splice have the same width l.sub.s the total thickness of the splice is preferably taken into account to meet the requirements of the invention (E.sub.metal*t.sub.metal.sup.3)>(E.sub.splice*t.sub.splice.sup.3). Therefore E.sub.splice=E.sub.layer and t.sub.splice=3*t.sub.layer=3*0.13 mm=0.39 mm. Consequently (E.sub.metal*t.sub.metal.sup.3)=1954.8 MPa*mm.sup.3 and (E.sub.splice*t.sub.splice.sup.3)=3103 MPa*mm.sup.3, which shows that this configuration is not meeting the requirements of the invention and therefore is part of the prior art.

[0086] FIG. 10 shows a laminate in accordance with an embodiment of the invention. Having the same basic laminate, as well as the same splice strap as in FIG. 9, the width of the 3 layers of the splice strap differ in length. Layer 3 of the splice strap has a width of l.sub.s, Layer 2 has a width of (l.sub.s2*l.sub.b) and Layer 1 has a width of (l.sub.s2*l.sub.c). The length of l.sub.b=10 mm and of l.sub.c=20 mm. In order to meet the requirements of the invention, the overlaps of the outer layer (Layer 3) are connected to one of the outer metal sheets (b1, b4) of the basic spliced laminate over a distance l.sub.b of at least 5 times the thickness of Layer 3, i.e. at least 0.65 mm. Furthermore Layer 3 needs to fulfill the stiffness requirements of the invention; i.e. (E.sub.metal*t.sub.metal.sup.3)>(E.sub.Layer 3*t.sub.Layer 3.sup.3). This requirement is fulfilled since (E.sub.Layer 3*t.sub.Layer 3.sup.3)=115 mPa*mm.sup.3 and (E.sub.metal*t.sub.metal.sup.3)=1954.8 MPa*mm.sup.3. Layer 1 is also meeting the stiffness requirements of the invention.

[0087] A method for making a laminate 10 in accordance with the present invention is illustrated in FIGS. 11 and 12. The method comprises providing a forming substrate 30 extending in a transverse direction 35, a thickness direction 36 and a length direction 34, and provided with a shape defining upper surface 31. The upper surface 31 of the forming substrate 30 comprises a recess 31a which extends in the length direction 34 of the forming substrate 30 across a splicing region for accommodating a splice strap (12, 18). In FIG. 11, the recess gradually builds up from an upper surface 31 outside the splicing region to achieve a final recess depth 31a at a discontinuous end line. The shape of the recess 31 mirrors the shape of the protruded part 18 of the metal sheet 1b of the laminate 10 of FIG. 6. In FIG. 12, the recess is provided in the upper surface 31 as a constant thickness trough 31a, which of course mirrors the shape of the splice strap 12 of the laminate of FIG. 3 or 4.

[0088] In the embodiment of FIG. 11, a first metal sheet 1b is then provided onto the tapered upper surface 31 of the forming substrate 30 such that an end part 18 thereof abuts against the upstanding end wall of the recess 31a. In the embodiment of FIG. 12, a metal or fiber-reinforced adhesive splice strap 12 is provided on the upper surface of the recess 31a within the confines of the recess 31a, the first metal sheet 1b and the splice strap 12 extending over part of the forming substrate 30 in the length direction 34 across a splicing region. A stack of three metal sheets (1, 3, 5) and two adhesive layers (2, 4) is then applied on top of the first metal sheet 1b (FIG. 11) or the splice strap 12 (FIG. 12). Edges of the metal sheets (1, 3, 5) extend along the length direction 34 and abut and/or overlap within the splicing region, and the stack (1-5) extends beyond the boundaries of the splice strap 12 or tapered metals sheet section 18. Heat and pressure are then applied to the thus obtained stack (1-5), in which process metal sheets (1a, 1b, 3, 5) deform across the splicing region. The deformed shape is then consolidated by curing a thermosetting adhesive in the fiber-reinforced adhesive layers (2, 4), or by cooling down a thermoplastic adhesive in the fiber-reinforced adhesive layers (2, 4). As shown, the metal sheets (1, 3, 5) are elastically bent over the splice strap 12 (FIG. 12) or first metals sheet portion 18, since metal sheets (1, 3, 5) are forced to take on the shape of the splice strap 12 or first metal sheet portion 18, provided in the recess 31a of forming substrate 30.

[0089] Heating and applying pressure may be achieved in a press or alternatively using an autoclave. Conventional pressure and heat levels may be used, for instance 4-10 bar at 120-175 C. The splice straps 12 and metal sheets (1a, 1b) may if desired be subjected to a degreasing treatment followed by etching or anodizing, and a primer may be applied onto the surface of the forming substrate. Although the forming substrate in the examples has a substantially flat upper surface, it does not need to be flat, and may for instance be shaped as the mirror image of a single- or double-curved body panel for an aircraft, or may have other shapes. The laminate is in particular applied in structural components for a vehicle spacecraft, or aircraft.

EXAMPLES

Calculation of Parameters

[0090] Calculation of the claimed parameters is illustrated by reference to the laminate of FIG. 15. The basic laminate is a laminate with 3 layers of titanium, whereby each layer has a thickness t.sub.ti=0.8 mm. The outside layer is butt spliced at critical location 7. The titanium layers are bonded together by a metal adhesive. The applied titanium Ti-6Al-4V has a TUS=923 MPa. The strength of the spliced metal layer therefore is given by:

P.sub.spliced layer=923*0.8=738.4 MPa mm

[0091] The strap over the critical location 7 consists of 4 layers. These layers are positioned symmetrically over the critical location 7 with increasing width towards the outside of the structure. The overlap is at each side minimally 5*t.sub.layer i. Therefore:

0.5(ba)5*t.sub.layer 2, 0.5(cb)5*t.sub.layer 3 and 0.5(dc)5*t.sub.layer 4

[0092] Layer 1 and layer 3 are UD CFRP layers with a fibre strength of 4,000 MPa and a fiber volume fraction FVF of 50% and a composite layer with a of thickness t.sub.c=0.15 mm and layers 2 and 4 are aluminium 2024-T3 layers with a thickness of t.sub.alu=0.4 mm. The TSC of the CFRP layers will be determined hereunder and the TUS of aluminium 2024-T3=440 MPa.

[0093] To determine the strength of the strap at critical location 7 it will be essential to determine the strength of the metal parts as well as the strength of the composite parts.

[0094] The strength of the metal parts is:

(TUS*t.sub.m).sub.metal layers=440*0.4*2=352 MPa

[0095] The strength of the composite parts is determined as follows:

[0096] Each composite layer has a FVF=60%, so the strength of the composite layer is 2,400 MPa. Consequently the TSC=2,400 MPa. This means that:

(TSC*t.sub.c).sub.composite layers=2,400*0.15*2=720 MPa mm.

Consequently:

[0097]
P.sub.strap=(TUS*t.sub.m).sub.metal layers+(TSC*t.sub.c).sub.composite layers=352+720=1072 MPa mm.

[0098] This results in that the strap has adequate strength:

P.sub.strap>1.2 P.sub.spliced layer

[0099] Applying this requirement to the example according to FIG. 15 gives for the bending stiffness of the spliced layer:

(E*t.sup.3).sub.spliced layer=110*0.8.sup.3=56.32 GPa mm.sup.3

[0100] Whereby the E.sub.titanium=110 GPa

[0101] The bending stiffness of the strap will be the bending stiffness of layer 4, since it is the widest strap over the splice, i.e.

(E*t.sup.3).sub.strap=72.4*0.4.sup.3=4.64 GPa mm.sup.3

[0102] Whereby the E.sub.aluminum=72.4 GPa.

[0103] So this strap configuration fulfils the requirements required by this invention.

[0104] For purpose of understanding it is now assumed that all layers (1-4) in FIG. 15 have the same width.

[0105] In this case the bending stiffness will be determined by first determining the average E-modulus E.sub.strap=(E.sub.i*t.sub.i)/t.sub.i

[0106] The stiffness of the applied Carbon fibre is 230 GPa, with the applied FVF=60% the stiffness of the UD layer will be E.sub.c=138 GPa. So:

E.sub.strap=(138*0.15*2+72.4*0.4*2)/(0.15*2+0.4*2)=90.29 GPa

Consequently:

[0107]
(E*t.sup.3).sub.strap=90.29*1.1.sup.3=120.2 GPa mm.sup.3

[0108] So the bending stiffness of this strap is more than twice the bending stiffness of the splice layer and therefore this strap does not fulfil the requirement according to the invention.

Specimen Configurations

[0109] Two basic series of spliced laminates were tested. A first series comprises aluminum sheets with t=0.5 mm and a second series has aluminum sheets with t=1.3 mm. In both series, the applied aluminum is a 2024-T3 alloy (TUS=440 MPa and E=72.4 GPa). In applicable exemplary laminates, the applied composite layers in the strap are UD-glass prepreg with t=0.13 mm, E.sub.glass fibre=88 GPa, the strength of the glass fibre is 4,890 MPa and the prepreg layer has a fiber volume fraction FVF=57%. All specimens are flush at the strap side of the laminate structure, unless otherwise indicated. In exemplary laminates wherein a staggered strap was applied, the stagger is inside-out, meaning that the widest strap layer is at an innermost position, relative to the laminate.

Spliced Laminates with Applied Metal Thickness of t=0.5 mm

Overlap Splice

[0110] Specimen O-0.5-1: this specimen is a basic laminate having three aluminium layers and two fiber reinforced layers, as shown in FIG. 14.

[0111] Specimen O-0.5-2: this specimen has a strap of a 2024-T3 aluminum alloy with a thickness of t=0.5 mm bonded over the spliced area of the laminate of FIG. 14.

[0112] Specimen O-0.5-3: this specimen has a strap of a 2024-T3 aluminum alloy with a thickness of t=0.3 mm bonded over the spliced area of the laminate of FIG. 14.

[0113] Specimen O-0.5-4: this specimen has a strap consisting of an UD glass prepreg layer adjacent to the spliced metal sheets and attached to it (at the outside) an aluminium 2024-T3 layer with t=0.3 mm having the same width as the glass prepreg layer.

[0114] Specimen O-0.5-5: this specimen has a strap equal to specimen 0.0.5-4, but with a glass prepreg layer that is wider than the aluminum layer of the strap. At each side, the extension of the glass prepreg layer is 20 mm, which is more than the required 5*t.sub.c.

[0115] Specimen O-0.5-6: this specimen has two glass prepreg layers with different width. The widest strap layer is adjacent to the spliced laminate and has the same extension as for specimen O-0.5-5. Furthermore, this specimen is not flush at the strap side, but is flush on the opposite side.

[0116] Specimen O-0.5-7: this specimen has a strap with a glass prepreg adjacent to the spliced laminate and on the outside an aluminum 2024-T3 layer of t=0.3 mm attached to it. The aluminium layer is wider than the glass prepreg layer whereby the extension of the aluminium layer on each side is 15 mm, which is significantly more than the required 5*t.sub.alu. Furthermore a small layer of adhesive is applied to fill the gap of 15 mm on each side of the glass prepreg layer.

Butt Splice

[0117] Specimen B-0.5-1: this specimen is the basic laminate as is shown in FIG. 13.

[0118] Specimen B-0.5-2: this specimen has a strap of a 2024-T3 aluminum alloy with a thickness of t=0.5 mm bonded over the spliced area of the laminate.

[0119] Specimen B-0.5-3: this specimen has a strap consisting of an UD glass prepreg layer adjacent to the spliced metal sheets and attached to it (at the outside) an aluminium 2024-T3 layer with t=0.3 mm having the same width as the glass prepreg layer.

[0120] Specimen B-0.5-4: this specimen has a strap of t=0.3 mm aluminum 2024-T3 bonded over the spliced area.

[0121] Specimen B-0.5-5: this specimen a strap with a glass prepreg adjacent to the spliced laminate and on the outside an aluminum 2024-T3 layer of t=0.3 mm attached to it. The aluminum layer is smaller than the glass prepreg layer whereby the extension of the glass prepreg layer on each side is 20 mm, which is significantly more than the required 5*t.sub.c.

[0122] Specimen B-0.5-6*): this specimen has two glass prepreg layers with different width. The widest strap layer is adjacent to the spliced laminate and has the same extension as for specimen B.0.5-3. Furthermore, this specimen is not flush at the strap side, but is flush on the opposite side.

[0123] Specimen B-0.5-7: this specimen has a strap with a glass prepreg adjacent to the spliced laminate and on the outside an aluminium 2024-T3 layer of t=0.3 mm attached to it the aluminium layer is wider than the glass prepreg layer whereby the extension of the aluminium layer on each side is 15 mm, which is significantly more than the required 5*t.sub.alu. Furthermore a small layer of adhesive is applied to fill the gap of 15 mm on each side of the glass prepreg layer.

[0124] Table 1 summarizes the relevant parameters of the tested spliced laminate configurations.

[0125] Table 2 summarizes spliced laminate configurations according to embodiments of the invention (Overall Yes) and those that are part of the state of the art (Overall No). Specimens for which the last column indicates Overall NA are the basic splice laminates.

TABLE-US-00001 TABLE 2 0.5 mm basic laminate parameter values for spliced laminates having 0.5 mm thick metal sheets. strap meeting requirements specimen strap configuration P.sub.strap > P.sub.sl [00001]$t_{\mathrm{sl}}<c.Math.\sqrt[3]{\frac{1}{E_{\mathrm{sl}}}}$ (E.sup.*t.sup.3)strap < (E.sup.*t.sup.3)sl Overall O-0.5-1 Basic laminate NA NA NA NA O-0.5-2 0.5 mm bonded NA Yes No No O-0.5-3 0.3 mm bonded NA Yes Yes Yes O-0.5-4 0.3 mm + pp same length NA Yes Yes Yes O-0.5-5 0.3 mm + pp extended NA Yes Yes Yes O-0.5-6 *) 2x prepreg different length NA Yes Yes Yes O-0.5-7 0.3 mm + pp shorter NA Yes Yes Yes extended w adhesive B-0.5-1 Basic laminate NA NA NA NA B-0.5-2 0.5 mm bonded No NA No No B-0.5-3 0.3 mm + pp same length Yes NA Yes Yes B-0.5-4 0.3 mm bonded No NA Yes No B-0.5-5 0.3 mm + pp extended Yes NA Yes Yes B-0.5-6 *) 2x prepreg different length Yes NA Yes Yes B-0.5-7 0.3 mm + pp shorter Yes NA Yes Yes extended w adhesive Remarks index sl means spliced layer; example P.sub.sl means P.sub.spliced layer C.sub.s = 0.6 for overlap splice C.sub.s = 1.2 for butt splice

[0126] Fatigue tests were performed on the laminates with R=0.1 (R=maximum load divided by minimum load) at a maximum tensile load of 120 MPa for about 60,000 cycles, whereafter the load was increased to 180 MPa and held at this level until failure. The tests have been continued up to first cracking or delamination or stopped after a high number of cycles (above 500,000 cycles).The fatigue test results of the above configurations are shown in Graph 1.

[0127] The overlap splice configurations with aluminum layer thicknesses of t=0.5 mm fulfil the requirement with respect to the stiffness of the splice metal sheets since this requires t.sub.spliced layer<1.08 mm. The specimens O-0.5-2 and B-0.5-2 do not meet the claimed requirements and therefore represent state of the art laminates. Specimen B-0.5-4 is less preferred since it does not meet the strength requirement. The specimens that do not meet the claimed bending stiffness ratio requirement show hardly to no fatigue improvement at all. The fatigue results of B-0.5-4 and B-0.5-3 show that the most preferred bending stiffness ratio of <0.50 shows the best fatigue performances.

Spliced Laminates with Applied Metal Thickness of t=1.3 mm

Overlap Splice

[0128] Specimen O-1.3-1: this specimen is the basic overlap laminate as is shown in FIG. 14.

[0129] Specimen O-1.3-2: this specimen has a strap consisting of an UD glass prepreg layer adjacent to the spliced metal sheets of the laminate and on the outside an aluminium 2024-T3 layer with t=1.0 mm having the same width of the glass prepreg.

Butt Splice

[0130] Specimen B-1.3-1: this specimen is the basic butt splice laminate as is shown in FIG. 13

[0131] Specimen B-1.3-2: this specimen has a strap of aluminum 2024-T3 with t=1.3 mm bonded to the spliced aluminium layers

[0132] Specimen B-1.3-3: this specimen has a strap consisting of an aluminum 2024-T3 layer t=0.3 mm bonded to the spliced metal layers. On top of this aluminum layer an UD glass prepreg layer with a smaller width than this aluminium layer is placed. The UD prepreg layer is on both sides 20 mm smaller than the underlying aluminum layer (thereby fulfilling easily the requirement of 5*t.sub.tot; t.sub.tot=0.3+0.13=0.43 mm)

[0133] Specimen B-1.3-4: this specimen has a strap consisting of 2 layers of of aluminium 2024T3 with t =0.3 mm. Between these layers is placed an UD glass prepreg and this total package is connected to the splice metal sheets of the laminate by another UD glass prepreg layer of the same width as the adjacent aluminium sheet of the strap. The two aluminium layers of the strap and the glass prepreg layer in between these aluminium layers are staggered. So the UD glass prepreg layer adjacent to the spliced metal sheets and the aluminium on top of it have the same width the UD glass prepreg layer on top of the aluminium layer has a smaller width than this aluminium layer the width reduction is on both sides 15 mm. On top of this prepreg layer is placed the aluminium layer with a reduced width compared to the adjacent UD glass prepreg. The reduction is at both sides also 15 mm. In both cases the requirement of 5*t is met.

[0134] Specimen B-1.3-5: this specimen has a strap of equal shape as specimen B-1.3-4. The main difference is that the prepreg layer adjacent to the spliced metal sheets is extended to the aluminium sheet op top of it. The extension is at both end of the prepreg 20 mm.

[0135] Specimen B-1.3-6*): this specimen is in configuration the same as the previous specimen B-1.3-5, with one exception. This specimen is non flush on the strap side, but flush on the opposite side.

[0136] Table 1 summarizes the relevant parameters of the tested spliced laminate configurations.

TABLE-US-00002 TABLE 1 stiffness and strength parameters of the tested spliced laminate configurations strap end of strap strength Et.sup.3.sub.str/ P.sub.strap/ specimen strap configuration Et.sup.3.sub.spl P.sub.spliced layer 0.5 mm basic laminate O-0.5-1 Basic laminate O-0.5-2 0.5 mm bonded 1.000 1.00 O-0.5-3 0.3 mm bonded 0.216 0.60 O-0.5-4 0.3 mm + pp same length 0.577 2.25 O-0.5-5 0.3 mm + pp extended 0.012 2.25 O-0.5-6 *) 2 prepreg different length 0.012 3.29 O-0.5-7 0.3 mm + pp shorter extended 0.216 2.25 w adhesive B-0.5-1 Basic laminate B-0.5-2 0.5 mm bonded 1.000 1.00 B-0.5-3 0.3 mm + pp same length 0.577 2.25 B-0.5-4 0.3 mm bonded 0.216 0.60 B-0.5-5 0.3 mm + pp extended 0.024 2.81 B-0.5-6 *) 2 prepreg different length 0.012 3.29 B-0.5-7 0.3 mm + pp shorter extended 0.216 2.25 w adhesive 1.3 mm basic laminate O-1.3-1 Basic laminate O-1.3-2 1.0 mm + pp 0.634 1.40 B-1.3-1 Basic laminate B-1.3-2 1.3 mm bonded 1.000 1.00 B-1.3-3 2 0.3 mm + pp, bonded 0.012 1.10 B-1.3-4 2 0.3 mm + 1 pp ext 0.033 1.73 B-1.3-5 2 0.3 mm + 2*pp ext 0.001 1.73 B-1.3-6 *) 2 0.3 mm + 2*pp ext 0.001 1.73 Remarks: all specimens are flush on the strap side, except specimens marked *) which are flush on the opposite side O = Overlap splice B = Butted splice

[0137] Table 3 summarizes spliced laminate configurations according to embodiments of the invention (Overall Yes) and those that are part of the state of the art (Overall No). Specimens for which the last column indicates Overall NA are the basic splice laminates.

TABLE-US-00003 TABLE 3 1.3 mm basic laminate parameter values for spliced laminates having 1.3 mm thick metal sheets. strap meeting requirements specimen strap configuration P.sub.strap > P.sub.sl [00002]$t_{\mathrm{sl}}<c.Math..Math.\sqrt[3]{\frac{1}{E_{\mathrm{sl}}}}$ (E*t.sup.3)strap < (E*t.sup.3)sl Overall O-1.3-1 Basic laminate NA NA NA NA O-1.3-2 1.0 mm + pp NA No Yes No B-1.3-1 Basic laminate NA NA NA NA B-1.3-2 1.3 mm bonded No NA No No B-1.3-3 2 0.3 mm + pp, bonded No NA Yes No B-1.3-4 2 0.3 mm + 1 pp ext Yes NA Yes Yes B-1.3-5 2 0.3 mm + 2*pp ext Yes NA Yes Yes B-1.3-6 *) 2 0.3 mm + 2*pp ext Yes NA Yes Yes Remarks index sl means spliced layer; example P.sub.sl means P.sub.spliced layer C.sub.s = 0.6 for overlap splice C.sub.s = 1.2 for butt splice

[0138] Graph 2 illustrates the fatigue results obtained on the different laminate configurations having spliced aluminium layers of 1.3 mm thick sheets. The fatigue results show that the fatigue performance of the basic overlap splice is about equal to the performance of the basic butt splice configurations.

[0139] Overlap spliced laminates with a metal sheet thickness of t=1.3 mm do not meet the bending stiffness requirement of the metal sheets, since this would require that t.sub.spliced layer<1.08 mm. The spliced layer has a thickness of t=1.3 mm and such laminates are less preferred. The specimens B-1.3-2 and B-1.3-3 do not fulfil all requirements. Specimen B-1.3-2 corresponds to a state of the art laminate and specimen B-1.3-3 is less preferred since the strength requirement is not met. The remaining specimens B-1.3-4 to B-1.3-6*) meet all requirements and show consequently very good fatigue results. These specimens are preferred.