NL2017849B1

NL2017849B1 - Laminate of mutually bonded adhesive layers and spliced metal sheets

Info

Publication number: NL2017849B1
Application number: NL2017849A
Authority: NL
Inventors: Willem Gunnink Jan
Original assignee: Gtm Advanced Products B V
Priority date: 2016-11-23
Filing date: 2016-11-23
Publication date: 2018-05-28
Also published as: WO2018097720A1; EP3544801A1

Abstract

Described is a laminate, comprising a stack of mutually bonded adhesive layers and metal sheets. The laminate comprises abutting metal sheet edges that extend along a length direction within a splicing region. A splice strap is connected to the stack at an outer surface of the stack across said splicing region, the splice strap comprising at least one layer of fiber-reinforced adhesive or of metal sheet, or stacked layers of fiber-reinforced adhesive and/or metal sheets. The laminate comprising the splice strap has specific shear properties.

Description

FIELD OF THE INVENTION

The present invention relates to a laminate comprising a stack of mutually bonded layers of adhesive and metal sheets. The invention further relates to a structural component for a vehicle, spacecraft, or aircraft, comprising a laminate according to the present invention. The invention further relates to an aircraft comprising a laminate according to the present invention.

BACKGROUND ART

Laminates of mutually bonded adhesive layers and metal sheets are used for structural purposes, for instance in the aircraft industry. In order to obtain large panels of such laminates, and because metal sheets are available in limited widths only, typical laminates comprise abutting and/or overlapping metal sheet edges, extending along a length direction within a splicing region of the laminate. This may for instance occur in wings of aircraft, where the longitudinal (spanwise) direction of the wing corresponds to the length direction.

A laminate comprising a splicing region is for instance known from US 5,429,326, which discloses a laminated body panel for aircraft applications. The panel comprises at least two metal layers with a typical thickness of 0.3 mm, and an adhesive layer provided in between the metal layers. Some metal layers are composed of two or more metal sheets which are generally disposed coplanar in a layer and separated by a splice or splice line extending in a length direction of the laminate. Splices in a metal layer are typically staggered with respect to splices provided in other metal layers in order to prevent the laminate from weakening too much. Using splices in a laminate no longer restricts the maximum width of a laminate to a metal sheet width that is limited by present day metal sheet manufacturing technology.

In some laminates, the splice region of the laminate is covered with a splice strap or doubler to prevent exposure of the splices to environmental conditions, and to strengthen the laminate in a direction transverse to the length direction of the laminate.

The known laminate may suffer from internal stresses, for instance induced by their manufacturing process. The internal stresses may negatively affect strength and fatigue life of the laminate, which strength and fatigue life are an important design parameter, in particular for aircraft structures. The negative effects on strength and fatigue life may be worsened in laminates having relatively thick and/or stiff metal layers, in particular exceeding 0.3 mm for aluminum layers, and /or at relatively low temperatures, for instance below 0°C.

It is an object of the present invention to provide a laminate with an adequate strength and / or adequate stiffness and improved fatigue behavior.

SUMMARY OF THE INVENTION

This and other objects are achieved by providing a laminate in accordance with claim 1.

The laminate comprises a stack of mutually bonded layers of adhesive and metal sheets, wherein an outer metal sheet defining an outer surface of the stack is a spliced metal sheet with abutting metal sheet edges extending along a length direction and forming a splice within a splicing region; and a splice strap connected to the outer surface of the stack and extending in the length direction and in a transverse direction perpendicular to the length direction across said splicing region over a splicing region width, the splice strap comprising mutually bonded layers of at least one adhesive layer and at least one metal sheet, wherein a splice strap adhesive layer is provided for connection to the outer surface;

wherein the following relation is satisfied within the splicing region:

0*5 Gstack no splice — Gl_am — 1*5 Gstack no splice wherein Gi_am is defined as the shear stiffness of the laminate, i.e. including the splice strap. G_stackno _Spii_Ce is defined as the shear stiffness of the stack of mutually bonded layers of adhesive and metal sheets, wherein the metal sheets do not have a splice in the metal sheets. G_stackno splice does not include the splice strap. As such, G_stack _no splice is a shear stiffness of the stack of mutually bonded layers of adhesive and metal sheets having no splices in the metal sheets and having no splice strap attached to the stack. Please note that the shear stiffness has the dimensions of N/m, whereas a shear modulus has the dimensions of N/m².

The shear stiffness of the laminate Gi_am is defined by:

Glam — Gsieck I Y (Gj tj)strap ( for j — 1 to m);

wherein m = the number of layers in the splice strap; tj is the thickness of the j^th layer in the splice strap; and Gj is the shear modulus of the j* layer in the splice strap.

The shear stiffness of the stack G_stack is defined by:

Gstack = Σ (Gi*ti)stack( for i = 1 to n);

wherein n = the number of layers in the stack; f is the thickness of the i^th layer in the stack, and G is the shear modulus of the 1^th layer in the stack; and wherein G, = 0 for the outer spliced metal sheet in the stack and wherein G = 0 for any two other spliced metal sheets in the stack within the splicing region that are adjacent to each other and have splices that are within a distance in the transverse direction of 5 times the larger thickness f of the two spliced metal sheets. By using these equations, the shear stiffness of the laminate can easily be determined.

Furthermore, the shear stiffness of the stack of mutually bonded layers of adhesive and metal sheets, wherein the metal sheets do not have a splice in the metal sheets, is defined by:

Gstack no splice ⁼ Σ (Gi ti)stack (for 1 — 1 to n);

wherein n = the number of layers in the stack; f is the thickness of the i^th layer in the stack, and G the shear modulus of the i^th layer in the stack. As the metal sheets do not have a splice, all metal layers are taken into account in the calculation.

As defined herein, the shear modulus of each layer, G or Gj, is defined in the same direction as the shear stiffness of the whole stack Gstack no splice or G_stack and of the whole laminate Gi_am. For instance, when the shear stiffness Gi_am is to be calculated between the length and the transverse directions, the appropriate shear moduli are those acting between the length and the transverse direction of the laminate.

The shear modulus of each layer, G or Gj, may be determined experimentally, such as by using ASTM E143 - 13. Alternatively, and preferably, the shear modulus of each layer, G or Gj, is calculated from tensile elastic moduli of the layer. For example, when calculating the shear modulus of an isotropic layer, such as a metal sheet, the shear modulus is defined by:

G = E/(2(l+v));

wherein G is the shear modulus of the isotropic layer; E is the tensile elastic modulus of the isotropic layer and V is the Poisson’s ratio.

When calculating the shear modulus of a non-isotropic layer, such as an adhesive layer containing reinforcing fibers under some angle(s), the shear modulus of the layer may be determined by the appropriate formulae for fiber composite materials. These formulae are well known to one skilled in the art and may be found in handbooks of composite materials, such as for instance “Mechanics of composite materials” by Robert M. Jones, ISBN 0-07-032790-4.

The laminate in accordance with the invention has inter alia an improved fatigue life. An improved fatigue life, in the context of the present application means that the laminate may have a lower crack growth rate and/or higher number of load cycles to failure at a certain load. The splicing region in the laminate is defined as that region of the laminate wherein splice lines between abutting metal sheets and/or overlapping edge parts occur in at least one of the outer metal sheet layers of the laminate. The splicing region in a transverse direction (perpendicular to the length direction) of the laminate extends across at least one abutting edges of metal sheets or across at least one edge of a metal sheet that overlaps with another metal sheet. The adhesive layer between metal sheets is preferably continuous through the splicing region and therefore bridges splice lines and the like. A spliced layer in the laminate comprises two abutting metal sheets and/or two metal sheets with overlapping edge parts.

The shear stiffness Gi_am of the laminate including the splice strap is at least 0.5 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice. In this way, the shear stiffness of the spliced laminate at the splice strap supports a torsional stiffness of the laminate. In an example, such a laminate according to the present invention, when used in a wing of an aircraft, provides improved fatigue life. The shear stiffness Gi_am of the laminate including the splice strap is at most 1.5 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice.

The adhesive layers in the laminate and/or the splice strap for some embodiments may be used as such. Preferred embodiments of the invention however provide a laminate and/or splice strap wherein the adhesive layers comprise reinforcing fibers to form a fiber-metal laminate and/or splice strap.

The splice strap extends across the splicing region, by which is meant that the width of the splice strap covers at least the width of the splicing region or a part of the width of the splicing region. The wording ’ substantially’ in the context of the present inventions means at least 90% of the indicated variable or subject.

Connecting the splice strap to the laminate may be achieved by any means such as by mechanical means or by an adhesive. Any adhesive may be used, including the same adhesive as that used in the adhesive layers of the laminate. Such adhesive may be applied as a separate layer. The strap bonding adhesive layer may also be provided with reinforcing fibers, if desired. It is also possible that the strap layer comprises a fiber reinforced adhesive layer, for instance in the form of a prepreg. Such splice strap may be bonded to the laminate as such, and the adhesive within the splice strap will partly form the adhesive layer connecting the splice strap to the laminate.

The splice strap in useful embodiments comprises a metal strip, for instance made from the same metal as the laminate metal sheets. In accordance with another embodiment of the invention, a laminate is provided wherein the splice strap comprises stacked splice strap layers, preferably of fiber-reinforced adhesive, in another embodiment of metal sheets, and in yet another embodiment of a combination of mutually bonded metal sheets and fiber-reinforced adhesive layers. The stacking sequence of the splice strap can be provided outside-in or, preferably, inside-out; meaning respectively that the smallest layer is adjacent to the laminate, or the widest strap layer is adjacent to the laminate

In an embodiment of the invention, the shear stiffness Gi_am of the laminate including the splice strap is in the range at least 0.75 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice.

In a further embodiment of the invention, the shear stiffness Gi_am of the laminate including the splice strap is in the range at most 1.25 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice.

In yet another embodiment of the invention, the shear stiffness Gi_am of the laminate including the splice strap is in the range at least 0.85 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice.

In a further preferred embodiment of the invention, the shear stiffness Gi_am of the laminate including the splice strap is in the range at most 1.10 times the shear stiffness Gstack no splice of the stack of mutually bonded layers of adhesive and metal sheets without splice.

Although the invented laminate may be used in conjunction with any type of discontinuity in the laminate stack, an embodiment of the invention relates to a laminate comprising spliced metal sheets with abutting metal sheet edges. The advantages of the invention are particularly apparent for these types of splices.

In an embodiment of the invention, at least one of the adhesive stack layers of the stack comprises reinforcing fibers forming a fiber-reinforced adhesive stack layer. The fibers reinforce the stack thereby improving the mechanical strength and/or the crack growth performance.

In an embodiment of the invention, at least one of the adhesive splice strap layers comprises reinforcing fibers forming a fiber reinforced adhesive splice strap layer.

In an embodiment of the invention, at least 30% of the reinforcing fibers in the stack are oriented at an acute angle with the length direction within the splice strap region, the acute angle being within the range between - 45° and 45° with respect to the length direction, more preferably within the range between -30° and 30° with respect to the length direction, and most preferably within the range between -15° and 15° with respect to the length direction.

In a preferred embodiment, at least 50% of the reinforcing fibers in the stack are oriented at an acute angle with the length direction within the splice strap region, the acute angle being within the range between - 45° and 45° with respect to the length direction, more preferably within the range between -30° and 30° with respect to the length direction, and most preferably within the range between -15° and 15° with respect to the length direction.

In an embodiment, at least 65% of the reinforcing fibers in the stack are oriented along said acute angle with respect to the length direction.

In an embodiment, at least 75%, and more preferably at least 90% of the reinforcing fibers in the stack are oriented along said acute angle with respect to the length direction.

In an embodiment, at least 50% of the reinforcing fibers in the splice strap are oriented at an acute angle with the transverse direction within the splice strap region, the acute angle being within the range between - 45° and 45° with respect to the transverse direction, more preferably within the range between -30° and 30° with respect to the transverse direction, and most preferably within the range between -15° and 15° with respect to the transverse direction.

In a preferred embodiment, at least 50% of the reinforcing fibers in the stack are oriented at an acute angle with the length direction within the splice strap region, the acute angle being within the range between -15° and 15° with respect to the length direction, and at least 50% of the reinforcing fibers in the splice strap are oriented at an acute angle with the transverse direction within the splice strap region, the acute angle being within the range between -15° and 15° with respect to the transverse direction. In this way, the fibers in the stack provide strength along the length direction while the fibers in the splice stack enhance the shear stiffness of the laminate. This combination is especially beneficial for providing adequate strength and improved fatigue behavior.

In an embodiment, the thickness of the spliced metal layer of the stack is at least 0.3 mm, more preferably between 0.3 mm and 1.3 mm, even more preferably between 0.4 mm and 1.2 mm, and most preferably between 0.6 mm and 1.0 mm. The thickness of the metal sheets in the laminate is typically limited to 5-10 mm, in accordance with generally accepted definitions of what constitutes a foil, a sheet and a plate.

In an embodiment, the spliced metal sheet comprises the metal sheet in the stack that is in contact with the splice strap.

In an embodiment, the spliced metal sheet comprises the thickest metal sheet in the stack.

In an embodiment, the blunt notch strength of the laminate at a location of the splicing region P_bnlam is in the range of about 0.75 * P_bnstack to 1.0 * P_bnstack wherein P_bnstack is the tensile strength of the stack of the laminate without the splice in the metal sheet, preferably P_{bn [am} is equal to or more than 0.85 * P_bn stack, more preferably P_bni_am is equal to or more than 0.95 * P_bn stack·

In an embodiment, a splice strap layer most closely arranged to the outer surface of the stack extends over a total transverse distance of at least 25 times the thickest metal sheet in the stack, more preferably over a total transverse distance of at least 50 times the thickest metal sheet in the stack.

In an embodiment, the splice strap comprises a metal sheet layer that is connected to the laminate with an adhesive layer and/or a layer of fiber-reinforced adhesive.

In an embodiment, an outer surface of the splice strap is flush with an outer surface of the stack.

The strap may comprise a number of strap layers, of which one is the widest splice strap layer. In such an embodiment, less wide strap layers may be closer to the laminate’s outer surface than the widest strap layer. The widest strap layer may thus be connected to the laminate at its sides only, for instance symmetrically with respect to its central extension. The widest splice strap layer is then connected to the laminate over a transverse distance of at least 5 times the widest strap layer thickness at both sides of the splice strap layer.

In case the splice strap comprises one layer only, the widest splice strap corresponds to this one strap layer. The strap may also comprise a number of strap layers of equal width. In this case, all the strap layers can be considered as the widest strap layer.

According to the invention, the splice strap or a widest splice strap layer is connected to the laminate over a transverse distance of at least 10 times the widest strap layer thickness. In more preferred embodiments, a widest splice strap layer is connected to the laminate over a transverse distance of at least 25 times the widest strap layer thickness, even more preferred over at least 50 times the widest strap layer thickness, even more preferred over at least 80 times the widest strap layer thickness, even more preferred over at least 100 times the widest strap layer thickness, and most preferred over at least 200 times the widest strap layer thickness. Other preferred embodiments relate to a laminate, wherein the widest splice strap layer is connected to the laminate over a transverse distance of at most 500 times the widest strap layer thickness, more preferably over at most 400 times the widest strap layer thickness, and most preferred over at most 300 times the widest strap layer thickness.

A particularly useful embodiment offers a laminate wherein the 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. In another embodiment, the splice strap layers each have a width in the transverse direction across the splicing region and the width of the layers increases over the splice strap thickness towards the laminate to form staggered layers.

The splice strap layers may be staggered on one or both sides of the splice strap to provide a splice strap with staggered edges. In an embodiment of the invention, the laminate is characterized in that the splice strap layers are staggered on each side of the splice strap by a length of at least 5 times the widest strap layer thickness, and more preferably by a length of at least 10 times the widest strap layer thickness.

In another embodiment, a laminate is provided wherein the splice strap comprises a staircase edge over a stair cased transverse distance, and a virtual splice strap with a thickness equal to the thickness of the widest stair has a lower bending stiffness than the bending stiffness of one of the spliced metal sheets.

A splice strap with tapered edges may have a continuously tapered edge, for instance linear, and/or a continuous variable edge, for instance parabolic. It is also possible to provide staircased edges, the staggered edges then showing discontinuities.

In a further embodiment, the 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 (the assembly of all bonded splice strap layers) has a lower bending stiffness than the bending stiffness of one of the spliced metal sheets.

The splice strap extends in the transverse direction of the laminate across at least a part of the splicing region. However, in some embodiments, the splice strap may extend across the splicing region or even beyond the splicing region. In further embodiments, the splice strap may even extend in the transverse direction of the laminate over substantially the complete laminate width.

According to the invention, a laminate may be provided wherein an outer surface of the splice strap protrudes from the outer surface of the laminate by an off-set thickness, for instance ranging from 0% to more than 100% of the splice strap thickness. In a preferred embodiment, the off-set thickness is 0 (zero), and the outer surface of the splice strap is flush with the outer surface of the laminate. In such embodiment, the splice strap is embedded in the laminate and a substantially smooth outer surface of the laminate ensues. In embodiments having a non-zero off-set thickness, the splice strap protrudes from an outer surface of the laminate in the splicing region and a discontinuous outer surface of the laminate ensues in the splicing region. This will in particular embodiments provide a ridge that extends in the length direction of the laminate.

The laminate according to the invention in some embodiments needs to accommodate a splice strap and/or overlapping metal sheet edges in the thickness direction. In order to provide a smooth continuous outer surface of the laminate, some metal sheets then are provided with a lower thickness or need to be deformed. A useful embodiment of the invention therefore provides a laminate wherein the splicing region comprises deformed metal sheets.

In embodiments wherein the splice strap extends substantially parallel to the length direction of the laminate, the deformed metal sheets are preferably bend along a line parallel to the length direction.

Deforming metal sheets in the laminate may produce a laminate wherein, in an embodiment, the outer surface of the laminate is substantially smooth and a second outer surface opposite said outer surface is curved. The outer surface is then typically used as outbound surface of an aircraft component for instance, whereas the curved second outer surface is used as inbound surface of the aircraft component. The inbound surface may typically be covered with interior cladding and the like.

The laminates according to the present invention preferably comprise from 0 to 50 metal layers and about 1 to 49 adhesive layers. The metal layers may have any thickness such as the relatively thin metal layers of the prior art spliced laminates. Metal sheet thicknesses of between 0.1 and 2 mm may be used. The metal sheets in the present invention preferably have a thickness of more than 0.2 mm, more preferably more than 0.3 mm, and most preferably more than 0.6 mm.

The splice strap according to the invention preferably comprises from 0 to 8 metal layers and/or from 0 to 8 fiber-reinforced adhesive layers. The layers may have any thickness as long as the requirements of claim 1 are satisfied.

The metal sheets are preferably made from a metal having a tensile strength of more than 200 MPa. Examples of suitable metals are aluminum alloys, steel alloys, titanium alloys, copper alloys, magnesium alloys, and aluminum matrix composites. Aluminum-copper alloys of the AA2000 series, aluminum manganese alloys of the AA3000 series, aluminum-magnesium alloys of the AA5000 series, aluminum-zinc alloys of the AA7000 series, and aluminum-magnesium-silicon alloys of the AA6000 series are preferred. Some particularly preferred alloys are AA2024 aluminum-copper, AA5182 aluminum alloy, AA7075 aluminum-zinc, and AA6013 aluminummagnesium-silicon. When improved corrosion resistance is desired, a sheet of AA5052 alloy or AA5024, AA5083 or AA5182 alloy may be included in the laminate. The laminates may also comprise metal sheets of a different alloy. Other useful alloys comprise aluminum-lithium alloys, such as AA2090, AA2098, and AA2198 alloys.

The adhesive layers of the laminate and/or splice strap are in preferred embodiments provided with reinforcing fibers, which fibers preferably bridge the splice lines and metal sheet edge overlaps and therefore are continuous across the splicing region. The reinforcing fibers may be oriented in one direction or in several different directions, depending on the loading conditions of the laminate structure. Preferred reinforcing fibers comprise continuous fibers made of glass, aromatic polyamides (aramids), carbon, basalt, and/or polymeric fibers such as PBO for instance.

Preferred glass fibers include S-2, S-3 and/or R-glass fibers, as well as carbonized silicate glass fibers, although E-glass fibers are also suitable. Preferred fibers have a modulus of elasticity of between 60 and 650 GPa, and an elongation at break of between 0.1 and 8%, preferably above 1.6%, more preferably above 2.0%, and most preferably above 3.0%

The adhesive layers preferably comprise synthetic polymers. Suitable examples of thermosetting polymers include epoxy resins, unsaturated polyester resins, vinyl ester resins, and phenolic resins. Suitable thermoplastic polymers include polyarylates (PAR), polysulphones (PSO), polyether sulphones (PES), polyether imides (PEI), polyphenylene ethers (PEE), polyphenylene sulphide (PPS), polyamide-4,6, polyketone sulphide (PKS), polyether ketones (PEK), polyether ether ketone (PEEK), polyether ketoneketone (PEKK), and others. The laminate and/or splice strap may be provided with additional adhesive in certain areas, apart from the adhesive present in the adhesive layers. The thickness of the adhesive layers may be similar to that of the metal sheets but adhesive layers in the laminate and/or splice strap are preferably thinner.

The reinforcing fibers in the laminate and/or splice strap layers may be provided in the form of prepregs, an intermediate product of reinforcing fibers embedded in a partly cured thermosetting resin or in a thermoplastic polymer. Typically fiber volume fractions range from 15 to 75%, and more preferably from 20 to 65% of the total volume of adhesive and reinforcing fiber in the adhesive layers. The effective fiber volume fraction in an adhesive layer may be lowered by adding plain adhesive layers to reinforced adhesive layers.

The laminate in accordance with the invention may be manufactured by a method that comprises the steps of providing a forming substrate with an upper surface; providing a splice strap on the upper surface of the forming substrate, the splice strap extending over part of the forming substrate in a length direction across a splicing region; providing a stack of at least one adhesive layer and metal sheets, of which edges extend along the length direction and abut and/or overlap within the splicing region, the stack extending beyond the boundaries of the splice strap; the splice strap having a smaller thickness than a thickness of a metal sheet, positioned adjacent to the splice strap in the stack; and applying heat and pressure to the thus obtained stack.

Metal sheets may deform across the splicing region during the application of heat and pressure, and the deformed shape may be consolidated. The shape may be consolidated by curing the thermosetting resin in the adhesive layers, or by lowering the temperature below the melt temperature of a thermoplastic polymer in case such polymer is used in the adhesive layers. The metal sheets will bend towards the splice strap. The metal sheets may be deformed elastically (below the elastic limit) and/or may be deformed plastically (beyond the plastic limit). Which type of deformation prevails depends on the type of metal used, on shape and dimensions, on manufacturing conditions, and more.

In useful embodiments of the invention, a splice strap comprises stacked layers of fiber-reinforced adhesive. Several of such layers are preferably applied to the forming substrate on top of each other to build up thickness.

Another aspect of the invention finally relates to a structural component for a vehicle, spacecraft, or aircraft, comprising a laminate according to one of the described embodiments, and in particular to an aircraft comprising such a laminate.

BRIEF DESCRIPTION OF THE FIGURES

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:

Figure 1 - is a view in perspective of a fiber-metal laminate according to the state of the art;

Figure 2 - is a view in perspective of a fiber-metal laminate according to the state of the art;

Figure 3 - is a cross-sectional view in a transverse direction of a stack of the laminate used as a reference for the laminate of an embodiment of the present invention, which is shown in Figure 4; Figure 4 - is a cross-sectional view in a transverse direction of the laminate according to the embodiment of the present invention;

Figure 5 - is a plane view of the fatigue loading direction of the stack of the laminate shown in Figure 3;

Figure 6 - is a plane view of the fatigue loading direction of the laminate shown in Figure 3;

Figure 7 - is a schematic illustration of the fatigue results of the stack of the laminate of Figure 3 and the embodiment of the laminate of figure 4.

DETAIFED DESCRIPTION OF EXEMPFARY EMBODIMENTS

With reference to figure 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. Fayers 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.

As shown in figure 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 fiberreinforced 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 figure 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 figure 2 for clarity reasons and as minimum coverage area of the strap). The at least two metal sheets may also comprise overlapping edge parts within a splicing region 8.

Experiments

Fatigue tests were performed on a fiber/metal laminate as shown in figure 3, as well as on a spliced laminate as shown in figure 4. The laminate of figure 4 comprises a spliced aluminum sheet having a constant thickness of 1.3 mm. A splice strap is attached to an outer surface of the spliced sheet and comprises 3 layers of S2-glass prepreg and an outer aluminum sheet having a constant thickness of 0.3 mm.

The laminate of figure 3 is used as a reference and comprises a stack of layers 11-19. Layer 11 is a sheet of aluminum 2024-T3 with a nominal thickness of t = 1.3 mm. The layers 13, 15, 17, 19 comprise sheets of aluminum 2024-T3 with a nominal thickness of t = 0.4 mm. The layers 12, 14, 16, 18 are S2-glass prepreg layers with a nominal thickness of t = 0.26 mm and a fiber direction along a length direction 24, which is perpendular to the plane of viewing. The laminate of figure 3 does not have a splice in any of the aluminum layers (11, 13, 15,17, 19).

An embodiment of a laminate according to the present invention is shown in figure 4. The laminate comprises a stack of mutually bonded layers of adhesive and metal sheets 11-19, wherein a metal sheet 11 is a spliced metal sheet with abutting metal sheet edges extending along a length direction within a splicing region 7. Sheet 11 is split in two parts (11a, lib) that extend at both sides from a splice 28 running along the length direction 24. Furthermore the laminate comprises a splice strap connected to an outer surface of the stack and extending across said splicing region 7 over a certain width in a transverse direction 25 perpendicular to the length direction 24. The splice strap comprises a layer 21 of aluminum 2024-T3 having a nominal thickness of t = 0.4 mm and a width of 140 m in the transverse direction 25. The splice strap further comprises layers 22, 23, 26, which are S2-glass prepreg layers with a nominal thickness of t = 0.13 mm. Each of the prepreg layers 22, 23, 26 comprises fibers, which are running substantially along the transverse direction 25 as indicated in figure 4. Layer 27 is an adhesive layer, which is provided to connect the splice strap 20 to the stack of the laminate. The width of the layer 22 is 110 mm, the width of the layer 23 is 80 mm; the width of the layer 26 is 50 mm; and the width of the layer 26 is 30 mm, each along the transverse direction 25.

The layers 22, 23, 26, 27 are arranged as staggered layers, wherein the widest layer 22 is arranged away from the stack of the laminate. The structure is flush at the strap side.

Shear stiffness

Aluminum 2024-T4 has a tensile ultimate strength of TUS = 465 MPa and an elastic (Young’s) modulus E = 72.4 GPa. Consequently the shear modulus of the aluminum sheets is G = 0.38 * 72.4 = 27.51 GPa. Indeed, the Poisson’s ratio v = 0.32.

The shear modulus of each of the fiber reinforced composite layers (12, 14, 16 and 18) can be calculated according to the Halpin-Tsai equations, known in the art, to be G = 5 GPa. The fibers in the composite layers (12, 14, 16, 18) extend in the transverse direction 25 and therefore do not substantially contribute to the strength of the laminate in the length direction 24 perpendicular to the transverse direction 25. The composite layers (between two successive metal layers) each have a thickness of t = 0.266 mm. The tensile ultimate strength of the S2 glass fibers TUS = 4,890 MPa.

The prepregs applied in each of the composite layers have a fiber volume fraction FVF =57%

The above result in a calculated shear modulus G _stack = 27.51 * (1.32 + 4 * 0.38) + 5 * 4 * 0.266 =

83.5 GPa*mm.

In the laminate shown in figure 4, the splice area 7 and splice 28 are covered by 3 layers of S2 glass prepreg 22, 23, 26 with reinforcing fibers running perpendicular to the transverse direction 25, each with a thickness of t = 0.133 mm and over it an aluminium 2024-T3 sheet 21 with a thickness of t = 0.3 mm.

The shear modulus of this combination is G _[am = 27.51 * (0.27 + 4 * 0.38) + 5 * 4 * 0.266 + 5 * 3 * 0.133 = 56.6 GPa*mm. So the ratio between G i_am and G _stack is 0.68 and the laminate of figure 4 therefore fulfils the requirements of the claimed invention.

Fatigue results

The laminates according to figures 4 and 5 were subject to fatigue tests. Figure 5 shows a plane view of a fatigue specimen comprising a laminate according to figure 3. The fatigue loading direction of the laminate is parallel to the transverse direction 25 shown in figures 3 and 5.

Figure 6 shows a plane view of a fatigue specimen comprising a laminate according to figure 4.

The fatigue loading direction of the laminate again is parallel to the transverse direction 25 shown in figures 3 and 5.

The specimens as shown in figures 5 and 6 have a central crack 34 (saw cut) of about 4 mm extension. The crack in the laminate with splice 28 and strap 20 is applied at the splice location of this specimen. The fatigue loading direction is along the transverse direction 25 and therefore perpendicular to the extension of the saw cut 34.

A constant amplitude cycling test was applied at a maximum stress of 100 MPa and a minimum stress of 5MPa. The stress level of the spliced laminate with strap was determined by the cross section outside the strap location.

Figure 7 shows the fatigue test results of the two configurations of figure 3 and 4. The fatigue results for the laminate of figure 3 are annotated with V, while the fatigue crack growth results for the laminate with splice and strap of figure 4 are shown with the annotations Si (for crack growth at the underside of the stack) and with Ss (for the crack growth at the side of the strap).

The spliced laminate with strap of figure 4 has a significant higher fatigue life compared to the stack of figure 3. The fatigue life of the spliced laminate with strap is at least 50% longer than the fatigue life of the laminate of figure 3.

Secondly, a significant difference in fatigue crack growth could be observed for the spliced laminate with the crack at the strap side compared to the opposite side of this laminate.

Thirdly, the stack of Figure 3 test panel showed complete failure at about 45,000 cycles, while the spliced laminate with strap at 70,000 cycles was still in tact.

Blunt Notch Strength

The blunt notch strength of the laminate including the splice strap can be compared to the blunt notch strength of the stack of the laminate without a splice in any of the metal sheets and without the splice strap. The blunt notch strength can be determined experimentally, as described in relation to Figures 8 and 9.

For test specimens see Figures 8 and 9 showing a stack of the laminate specimen and a laminate with strap, respectively. In Figure 8 is shown in a plane view a test specimen of the laminate of figure 3. In Figure 9 is shown in a plane view a test specimen of the laminate of figure 5. A circular hole 44 is provided in each of the laminates having a diameter of 6.35 mm. for the laminate shown in Figure 9, the circular hole 44 is arranged centrally within the splicing region 7 in both the length direction 24 and the transverse direction 25. The circular hole 44 is arranged at the splice 28 of the outer metal sheet of the stack of the laminate.

The loading direction in the blunt notch strength test is along the transverse direction 25.

The test results of the blunt notch strength test show that:

Pbn stack = 1306 MPa*mm.

Pbn laminate = 1150 MPa*mm.

According to the experimentally determined blunt notch strength, the ratio P_bn laminate / Pbn stack between the blunt notch strength of the laminate including the strap P_bn laminate and the blunt notch strength of the stack P_bn stack is 0.88.

Claims

CONCLUSIESCONCLUSIONS

1. Een laminaat omvattende een stack van onderling verbonden lijmlagen en metaalplaten, waarin een buitenste metaalplaat een buitenste oppervlak van de stack definieert en een zich in een lengterichting uitstrekkende gesplitste metaalplaat met aanliggende metaalplaatranden omvat die een splice in een splitsingsgebied vormt; en een splice strap verbonden met het buitenste oppervlak van de stack en zich uitstrekkend in de lengterichting en in een dwarsrichting loodrecht op de lengterichting in genoemd splitsingsgebied over een splitsingsgebiedbreedte, waarbij de splice strap onderling verbonden lagen van tenminste één lijmlaag en tenminste één metaalplaat omvat, waarin een splice strap lijmlaag is verschaft voor verbinding met het buitenste oppervlak;A laminate comprising a stack of interconnected adhesive layers and metal sheets, wherein an outer metal sheet defines an outer surface of the stack and comprises a longitudinally extending split metal sheet with adjacent metal sheet edges forming a splice in a split region; and a splice strap connected to the outer surface of the stack and extending in the longitudinal direction and in a transverse direction perpendicular to the longitudinal direction in said splitting region over a splitting region width, wherein the splice strap comprises mutually connected layers of at least one adhesive layer and at least one metal plate, wherein a splice strap adhesive layer is provided for connection to the outer surface;

waarin de volgende relatie is voldaan in het splitsingsgebied:in which the following relationship is satisfied in the division:

0.5 Gstack no splice — Glam — E5 Gstack no splice waarin n = het aantal lagen in de stack;0.5 Gstack no splice - Glam - E5 Gstack no splice where n = the number of layers in the stack;

m = het aantal lagen in de splice strap;m = the number of layers in the splice strap;

Glam — G_stack + Σ (Gj*tj)strap ( voor j — 1 tot m);Glam - _Gstac k + Σ (Gj * tj) strap (for j - 1 to m);

Gstack = Σ (Gi*ti)stack( voor i = 1 tot n) waarin G = 0 voor de buitenste gesplitste metaalplaat in de stack en waarin G_; = 0 voor elke twee andere gesplitste metaalplaten in de stack in het splitsingsgebied die naast elkaar liggen en die splices hebben die binnen een afstand in de dwarsrichting liggen van 5 keer de grootste dikte f van de twee gesplitste metaalplaten;Gstack = Σ (Gi * ti) stack (for i = 1 to n) where G = 0 for the outer split metal sheet in the stack and where G _; = 0 for every two other split metal plates in the stack in the splitting area that are adjacent to each other and that have splices that are within a transverse distance of 5 times the greatest thickness f of the two split metal plates;

Gstack no splice ⁼ Σ (Gi ti)stack (νΟΟΓ i ⁼ 1 tot n);Gstack no splice ⁼ Σ (Gi ti) stack (νΟΟΓ i ⁼ 1 to n);

t, is de dikte, en Gj de afschuifmodulus van de i'^il laag in de stack;t, the thickness, and Gj is the shear modulus of the i ^'il layer in the stack;

tj is de dikte en Gj de afschuifmodulus van de j^th laag in de splice strap.tj is the thickness and Gj is the shear modulus of the j ^th layer in the splice strap.

2. Het laminaat volgens conclusie 1 waarin 0.75 Gsocknospike< G_iam < 1.25 G_stack,_rospli_ce The laminate of claim 1 wherein 0.75 Gsocknospike <G _iam <1.25 G _stack , _rospl i _ce

3. Het laminaat volgens conclusie 1 of 2 waarin 0.85 Gstackno spiiee < G_]am < 1.10 G_staCk no spikeThe laminate according to claim 1 or 2, wherein 0.85 Gstackno spiiee <G _{] am} <1.10 G _staC k no spike

4. Het laminaat volgens één der vorige conclusies, waarin tenminste één van de lijmlagen van de stack versterkingsvezels omvat die een vezel versterkte lijmlaag van de stack vormen.The laminate according to any of the preceding claims, wherein at least one of the adhesive layers of the stack comprises reinforcing fibers that form a fiber-reinforced adhesive layer of the stack.

5. Het laminaat volgens één der vorige conclusies, waarin tenminste één van de lijmlagen van de splice strap versterkingsvezels omvat die een vezel versterkte splice strap lijmlaag vormen.The laminate according to any of the preceding claims, wherein at least one of the adhesive layers of the splice strap comprises reinforcing fibers that form a fiber-reinforced splice strap adhesive layer.

6. Het laminaat volgens conclusie 5 of 6, waarin ten minste 30% van de versterkingsvezels in de stack onder een scherpe hoek met de lengterichting zijn georiënteerd in het splice strap gebied, waarbij de scherpe hoek in het gebied ligt tussen - 45° en 45° ten opzichte van de lengterichting, meer bij voorkeur in het gebied tussen -30° en 30° ten opzichte van de lengterichting, en met de meeste voorkeur in het gebied tussen -15° en 15° ten opzichte van de lengterichting.The laminate according to claim 5 or 6, wherein at least 30% of the reinforcement fibers in the stack are oriented at an acute angle with the longitudinal direction in the splice strap area, wherein the acute angle is in the range between - 45 ° and 45 ° relative to the longitudinal direction, more preferably in the range between -30 ° and 30 ° relative to the longitudinal direction, and most preferably in the range between -15 ° and 15 ° relative to the longitudinal direction.

7. Het laminaat volgens conclusie 6, waarin ten minste 50% van de versterkingsvezels in de stack zijn georiënteerd volgens genoemde scherpe hoek ten opzichte van de lengterichting.The laminate of claim 6, wherein at least 50% of the reinforcement fibers in the stack are oriented according to said acute angle with respect to the longitudinal direction.

8. Het laminaat volgens conclusie 6, waarin ten minste 65% van de versterkingsvezels in de stack zijn georiënteerd volgens genoemde scherpe hoek ten opzichte van de lengterichting.The laminate of claim 6, wherein at least 65% of the reinforcement fibers in the stack are oriented according to said acute angle with respect to the longitudinal direction.

9. Het laminaat volgens conclusie 6, waarin ten minste 75%, en meer bij voorkeur ten minste 90% van de versterkingsvezels in de stack zijn georiënteerd volgens genoemde scherpe hoek ten opzichte van de lengterichting.The laminate of claim 6, wherein at least 75%, and more preferably at least 90%, of the reinforcing fibers in the stack are oriented according to said acute angle with respect to the longitudinal direction.

10. Het laminaat volgens één der conclusies 6-9, waarin ten minste 50% van de versterkingsvezels in de splice strap zijn georiënteerd onder een scherpe hoek met de dwarsrichting in het splice strap gebied, waarbij de scherpe hoek in het gebied ligt tussen - 45° en 45° ten opzichte van de dwarsrichting, meer bij voorkeur in het gebied tussen -30° en 30° ten opzichte van de dwarsrichting, en met de meeste voorkeur in hel gebied tussen -15° en 15° ten opzichte van de dwarsrichting.The laminate according to any of claims 6-9, wherein at least 50% of the reinforcement fibers in the splice strap are oriented at an acute angle with the transverse direction in the splice strap area, the acute angle being in the range between - 45 ° and 45 ° with respect to the transverse direction, more preferably in the range between -30 ° and 30 ° with respect to the transverse direction, and most preferably in the range between -15 ° and 15 ° with respect to the transverse direction.

11. Het laminaat volgens één der vorige conclusies, waarin de dikte van de gesplitste metaal laag van de stack ten minste 0.3 mm is, meer bij voorkeur tussen 0.3 mm en 1.3 mm, nog meer bij voorkeur tussen 0.4 mm en 1.2 mm, en met de meeste voorkeur tussen 0.6 mm en 1.0 mm.The laminate according to any of the preceding claims, wherein the thickness of the split metal layer of the stack is at least 0.3 mm, more preferably between 0.3 mm and 1.3 mm, even more preferably between 0.4 mm and 1.2 mm, and with most preferred between 0.6 mm and 1.0 mm.

12. Het laminaat volgens één der voorgaande conclusies, waarin de gesplitste metaalplaat de metaalplaat in de stack omvat die in contact is met de splice strap.The laminate according to any of the preceding claims, wherein the split metal plate comprises the metal plate in the stack that is in contact with the splice strap.

13. Het laminaat volgens één der voorgaande conclusies, waarin de gesplitste metaalplaat de dikste metaalplaat in de stack omvat.The laminate according to any of the preceding claims, wherein the split metal plate comprises the thickest metal plate in the stack.

14. Het laminaat volgens één der vorige conclusies, waarin een splice strap laag die het dichtste bij het buitenste oppervlak van de slack is gelegen zich uitstrekt over een totale dwarse afstand van ten minste 10 keer de dikste metaalplaat in de stack, meer bij voorkeur over een totale dwarse afstand van ten minste 25 keer de dikste metaalplaat in de stack.The laminate of any one of the preceding claims, wherein a splice strap layer that is closest to the outer surface of the slack extends over a total transverse distance of at least 10 times the thickest metal sheet in the stack, more preferably over a total transverse distance of at least 25 times the thickest metal plate in the stack.

15. Het laminaat volgens één der vorige conclusies, waarin de splice strap een metaalplaatlaagThe laminate of any one of the preceding claims, wherein the splice strap is a metal sheet layer

5 omvat die is verbonden met het laminaat met een laag van vezelversterkte lijm.5 which is connected to the laminate with a layer of fiber-reinforced glue.

16. Het laminaat volgens één der vorige conclusies, waarin een buitenste oppervlak van de splice strap is uitgelijnd met een buitenste oppervlak van de stack.The laminate of any one of the preceding claims, wherein an outer surface of the splice strap is aligned with an outer surface of the stack.

1010

17. Het laminaat volgens één der voorgaande conclusies, waarvan de blunt notch strength in een locatie van het splitsingsgebied P_bnlam van 0.75 * P_bn stack tot PO * Pbnstack bedraagt, waarin P_b„ stack de treksterkte van de stack van het laminaat is zonder de splice in de metaalplaat, bij voorkeur is P_{bn Um} gelijk aan of meer dan 0.85 * P_{bn stack}, meer bij voorkeur is P_{bn tam} gelijk aan of meer dan 0.95 * P_bn «bThe laminate according to any of the preceding claims, wherein the blunt notch strength in a location of the splitting area P _{bnlam is} from 0.75 * P _bn stack to PO * Pbn stack, wherein P _b 'stack is the tensile strength of the stack of the laminate without the splice in the metal sheet, preferably P _{bn Um is} equal to or more than 0.85 * P _{bn stack} , more preferably P _{bn tam is} equal to or more than 0.95 * P _bn «b

18. Structuurdeel voor een voertuig, ruimtetuig of vliegtuig, omvattende een laminaat volgens één der voorgaande conclusies.A structural part for a vehicle, spacecraft or aircraft, comprising a laminate according to any one of the preceding claims.

19. Vleugel van een vliegtuig, omvattende een laminaat volgens één van de conclusies 1-16.The wing of an aircraft comprising a laminate according to any one of claims 1-16.

2/42/4

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I 17 ¹ Saw cut/crack locationI 17 ¹ Saw cut / crack location