WO2018100350A1

WO2018100350A1 - Composite structural laminate

Info

Publication number: WO2018100350A1
Application number: PCT/GB2017/053572
Authority: WO
Inventors: Stephen John Kennedy
Original assignee: Intelligent Engineering (Bahamas) Limited; Leeming, John Gerard
Priority date: 2016-11-30
Filing date: 2017-11-28
Publication date: 2018-06-07
Also published as: GB201620318D0; GB2557214A

Abstract

A composite structural laminate comprise two outer metal, e.g. steel, layers and an intermediate elastomer core, e.g. of unfoamed polyurethane. The elastomer has a modulus of elasticity of at least 4 MPa at a sustained temperature of 60 ºC and a modulus of elasticity of at least 1000 MPa at a sustained temperature of -80 ºC, and a ductility exceeding that of the metal layers. The composite structural laminate member acts a single member under load, buckling globally, not asymmetrically.

Description

Composite Structural Laminate

[0001] The present invention relates to structural members having a sandwich structure, and particularly to members that can be used in loadbearing applications in cold temperature

environments.

[0002] In applications such as ship hulls or bridge decks it has been known to increase the stiffness of steel plates by providing elongate stiffeners that comprise further steel girders welded perpendicularly to the main plate. The stiffeners may run in one direction or two orthogonal directions, depending on the forces to be borne by the plate. The use of stiffeners complicates the manufacturing process, adds significant weight and makes corrosion prevention and maintenance of the complete structure more difficult.

[0003] WO/1999/058333 and WO/1998/021029 disclose a structural laminate member comprising an intermediate elastomer layer sandwiched between two metal layers. The elastomer has a modulus of elasticity greater than or equal to about 250 MPa and a ductility exceeding that of the metal layers. The use of such structural laminate members in complex structures, e.g. ships, allows reductions in complexity, weight and cost by eliminating the need for some or all stiffeners, eliminating or increasing the spacing of longitudinal and transverse girders, reducing surface areas requiring coating and reducing locations susceptible to corrosion.

[0004] It was previously thought that the modulus of elasticity for the elastomer had to be at least 250 MPa to be capable of transferring transverse forces that are encountered or expected in use between the two metal layers, such that the laminate behaves under load as a single member. However, it has since been found that such structural laminate members are not optimal for some applications, such as for structures operating in cold climates and for structures in which acoustic attenuation is required.

[0005] The present invention provides a structural laminate member comprising:

a first metal layer having a first inner surface and a first outer surface,

a second metal layer having a second inner surface, and a second outer surface, the second metal layer being spaced apart from said first metal layer; and

an intermediate layer comprising an elastomer located between and adhered to said first and second inner surfaces, said elastomer having a modulus of elasticity, E, of at least 140 MPa at a sustained temperature of 60 °C and a modulus of elasticity of at least 1000 MPa at a sustained temperature of -80 °C, and a ductility exceeding that of the metal layers.

[0006] The essential requirement of the invention is that the laminate behaves under load as a single member rather than three individual components in cold temperature environments, e.g. between -80 °C and 30 °C, and the mechanical properties of the intermediate layer and its bond to the outer layers must be selected to effect that. The intermediate layer must therefore have sufficient modulus of elasticity and ductility to be capable of transferring transverse forces that are encountered or expected in use between the two metal layers in a cold temperature environment. Sufficient bond strength to transfer shear forces is also desirable.

[0007] In applications where the ability to withstand impacts is important, e.g. ship building, the intermediate layer must additionally have sufficient yield strength not to rupture under designed impact loads. Under extreme loads, the member will absorb greater energy than comparable single sheet metal members through strain dissipation, increased puncture resistance and inelastic membrane action of the member as a whole.

[0008] Preferably, the relative strengths and proportions of the two metal layers and the intermediate layer, particularly the stiffness of the intermediate layer, are selected such that the member, when subjected to extreme hogging and sagging loads, will buckle globally (as a whole) rather than anti- symmetrically or locally.

[0009] Also preferably, the intermediate layer should have ductility and a modulus of elasticity that is sufficient to spread a stress concentration at the tip of a crack in one metal layer in transferring it to the other so that the crack is prevented from propagating between the layers. The intermediate layer will also have a retarding effect on propagation of the crack in the layer in which it started.

[0010] The metal layers are preferably made of steel and each of thickness in the range of 3.5 to 25 mm. The minimum thickness is the thinnest sheet that can be effectively butt- welded, which is necessary for strength. At the upper limit, the advantages of the invention decrease. It is not necessary that the two metal layers are of the same thickness. In particular it is possible to provide a sacrificial excess on the side that, in use, will face a corrosive environment.

[0011] The plastic material preferably behaves as an elastomer at the loads expected in use and has a thickness in the range of from 15 to 100 mm, optionally 20 to 100 mm. The thickness of the intermediate layer may vary across a member in some applications. The material is preferably compact, i.e. unfoamed, though some void spaces may be present, either intentionally or as a side- effect of the manufacturing method used, provided the desired properties of the composite are not reduced. It is believed that maximum acceptable void space in the intermediate layer is up to 60%.

[0012] The use of the invention in complex structures operating in cold climates, e.g. ships, bridges, pipelines and mining equipment working in the Arctic, achieves optimal performance and ductility, while still providing the structure with adequate strength at sustained maximum temperatures of 60 °C.

[0013] Use of the invention in indoor structures provides improved acoustic attenuation (noise damping) for airborne and structural borne noise and flanking noise, while still providing the structure with adequate strength. In particular, the softer (lower modulus of elasticity) continuous core material provides improved absorption of sound waves within the intermediate layer. Acoustic attenuation is also improved due to scattering of sound waves by internal reflection off the metal plates.

[0014] Exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which:

[0015] Fig. 1 is a cross-sectional view of a laminate member according to the invention.

[0016] Fig. 2 is a graph showing the variation of the modulus of elasticity with temperature of an elastomer that can be used in the intermediate layer of the present invention.

[0017] Fig. 3 is a partly cut-away perspective view of a laminate member according to the present invention including spacers.

[0018] In the figures, like parts are identified by like numerals.

[0019] The expression "cold temperature" used throughout the description is defined as being between -80 °C and 30 °C.

[0020] The expression "at a sustained temperature of ... " means the state when the material is thermally stabilised at the specified temperature, i.e. there is a uniform temperature throughout the material. For example, "at a sustained temperature of 60 °C" means that the elastomer has a uniform temperature of 60 °C throughout the elastomer. This state can be obtained by subjecting the material to the specified temperature over a time period in the order of hours.

[0021] Fig. 1 is a cross-sectional view of a laminate member 10 according to the present invention. The laminate member 10 comprises a first outer layer 1, a intermediate, or core, layer 2 and a second outer layer 3. The intermediate layer 2 is bonded to each of the first and second outer layers 1, 3 with sufficient strength to transfer shear loads between the outer layers so as to form a composite structural member capable of bearing loads significantly greater than self-weight.

[0022] The precise load to be borne by the laminate member will depend on the application to which it is to be put. For example, if the laminate member is to be used as a ship's hull plate in a 40,000 DWT oil tanker, it should be capable of withstanding an in-plane load of at least 10-12,000 kN in a 2 m width without buckling or a transverse load of at least 100 kPa, preferably 1,000 kPa or greater, without rupturing. For smaller vessels, especially yachts, the laminate member need not be so strong.

[0023] The first and second layers 1, 3 are made of metal and the intermediate layer 2 is made of a plastic or elastomeric material. The absolute and relative dimensions of the member and the precise materials employed will depend on the application to which the member is to be put. At a minimum the first and second outer layers will have a thickness of 3 mm and the intermediate layer 15 mm, optionally 20 mm.

[0024] The elastomer of the intermediate layer has a modulus of elasticity (Y oung's modulus), E, of at least 104 MPa at a sustained temperature of 60 °C and a modulus of elasticity of at least 1000 MPa at a sustained temperature of -80 °C.

[0025] Preferably, the elastomer of the intermediate layer has a modulus of elasticity between 104

MPa and 200MPa, optionally between 104 MPa and 175 MPa, or between 104 MPa and 150 MPa at a sustained temperature of 60 °C.

[0026] Preferably, the elastomer of the intermediate layer has a modulus of elasticity of at least 1200 MPa, optionally at least 1400 MPa at a sustained temperature of -80 °C.

[0027] Preferably, the elastomer of the intermediate layer has a modulus of elasticity between 1000 MPa and 3000 MPa, optionally between 1000 MPa and 2500 MPa, between 1000 MPa and 2000 MPa, between 1200 MPa and 2500MPa, between 1200 MPa and 2000 MPa, between 1400 MPa and 2500 MPa, or between 1400 MPa and 2000 MPa at a sustained temperature of -80 °C.

[0028] The elastomer can have any combination of the above values for the modulus of elasticity at a sustained temperature of 60 °C and the above values for the modulus of elasticity at a sustained temperature of -80 °C in order to provide an elastomer with optimal properties in cold temperature applications. For example, the elastomer for the intermediate layer may have modulus of elasticity of at least 104 MPa at a sustained temperature of 60 °C and a modulus of elasticity of at least 1200 MPa at a sustained temperature of -80 °C. In another example, the elastomer for the intermediate layer may have a modulus of elasticity between 104 MPa and 200MPa at a sustained temperature of 60 °C and a modulus of elasticity between 1000 MPa and 3000 MPa at a sustained temperature of -80 °C.

[0029] The elastomer preferably has a modulus of elasticity between 300 and 600 MPa, optionally between 400 and 500 MPa at a sustained temperature of 20 °C.

[0030] The elastomer preferably has a glass transition temperature of about 60 °C, as measured using ISO 527.

[0031] Figure 2 shows how the modulus of elasticity varies with sustained temperature for an example elastomer according to the present invention. In this example, the elastomer has a modulus of elasticity of 150 MPa at a sustained temperature of 60 °C and a modulus of elasticity of 1400 MPa at a sustained temperature of -80 °C.

[0032] The tear, compression and tensile strengths as well as the elongation should be maximised to enable the composite laminate to absorb energy in unusual load events, such as impacts. In particular, the compressive and tensile strengths of the elastomer should be at least 10, preferably 20, 30 or 40, MPa. The compressive and tensile strengths can, of course, be considerably greater than these minima.

[0033] The metal layers are preferably structural steel though may also be aluminium stainless steel or other structural alloys in speciality applications where lightness, corrosion resistance or other specific properties are essential. The metal should preferably have a minimum yield strength of 240 MPa and an elongation of at least 20%. For many applications, especially ship building, it is essential that the metal is weldable.

[0034] The ductility of the elastomer at the lowest operating temperature (e.g. -80 °C) must be greater than that of the metal layers, which is about 10%. A preferred value for the ductility of the elastomer at lowest operating temperature is 20%, optionally 30%, 40% or 50%. The thermal coefficient of the elastomer must also be sufficiently close to that of the steel so that temperature variation across the expected operating range, and during welding, does not cause delamination. The extent by which the thermal coefficients of the two materials can differ will depend in part on the elasticity of the elastomer but it is believed that the thermal expansion coefficient of the elastomer may be about 10 times that of the metal layers. The coefficient of thermal expansion may be controlled by the addition of fillers to the elastomer.

[0035] The bond strength between the elastomer and metal layers is at least 2, preferably 4, MPa over the entire operating range. This is preferably achieved by the inherent adhesiveness of the elastomer to steel but additional adhesives may be provided.

[0036] Additional requirements if the member is to be used in a ship building application, include that the tensile strength across the interface must be sufficient to withstand expected negative hydrostatic pressure and delaminating forces from steel connections. The elastomer must be hydrolytically stable to both sea and fresh water and if the member is to be used in an oil tanker must have chemical resistance to oils.

[0037] The elastomer therefore essentially comprises a polyol (e.g. polyester or polyether) together with an isocyanate or a di-isocyanate, a chain extender and a filler. The filler is provided, as necessary, to reduce the thermal coefficient of the intermediate layer, reduce its cost and otherwise control the physical properties of the elastomer. Further additives, e.g. to control hydrophobicity or adhesion, and fire retardants may also be included.

[0038] The ratio of the total thickness of the outer layers to the thickness of the elastomer, (Tl + T3) / T2, is in the range of from 0.05 to 3.5, preferably 0.1 to 2.5.

[0039] Coatings, e.g. for cosmetic or corrosion resistance reasons, may be applied to the outer surfaces of the metal layers either before or after fabrication of the laminate.

[0040] The member of the present invention is substantially stronger and stiffer than a member of the same thickness of metal but no intermediate layer. This is because the member acts in an analogous manner to a box girder or I-beam with the intermediate layer performing the function of the web(s). To so function the intermediate layer itself and the bonds to the outer layers must be sufficiently strong to transfer the forces that will arise in use of the member.

[0041] A further advantage of the present invention, of particular benefit in ship building, is that the intermediate layer acts to prevent crack propagation between the inner and outer layer. The elasticity of the intermediate layer prevents the stress concentration at the tip of a crack in one outer layer being transmitted to the other as a rigid connection would, instead the load is spread out.

[0042] For indoor structures (in environments between 15 and 25 °C, for example), the present invention is particularly useful for improving acoustic attenuation. The relatively low modulus of elasticity of the elastomer provides a soft core material that results in effective absorption of sound waves as they pass through the structure. Sound waves may also be scattered by internal reflection off a metal plate of the member, thereby further reducing transmission of sound through the structure.

[0043] The preferred method of fabricating a laminate member according to the invention is to cast, or inject, the elastomer directly into a cavity formed by the two metal layers. If this is done horizontally, the metal plates are preferably held apart by spacers, which may be metal or elastomeric. If the spacers are elastomeric then they must be compatible with the material forming the bulk of the intermediate layer and slightly taller than the desired spacing so that they compress to the correct distance under the weight of the upper plate. The spacers may be elongate to divide the cavity into spaces that may be filled separately or simply plugs around which the elastomer flows. If elongate, the spacers may be rectangular or trapezoidal in cross-section and may vary in height along their length to provide members with varying elastomer thickness. The spacers must be bonded to the steel plates with bonding agents or elastomer compatible compounds with sufficient strength to hold the plates in place during the injection process until the elastomer is sufficiently cured.

[0044] Figure 3 shows, for illustrative purposes, three different types of spacer that may be used in constructing laminate members according to the invention. A cylindrical elastomer plug 4A is used to support the upper plate without dividing the cavity to be filled. If the cavity needs to be bounded or divided an elongate metal spacer 4B or an elongate elastomer spacer 4C may be used. The metal spacer 4B may be fillet welded to the lower plate and support a butt weld between two sections of the upper plate, or act as backing bar for that weld. The elastomer plug 4A and the elongate elastomer spacer 4C may be adhered to the metal plates before casting and may be made of substantially the same elastomer as will be injected or a different elastomer compatible with the elastomer to be injected. An actual laminate member may not require all of these different types of spacer.

[0045] During casting the plates may be held at an incline to assist elastomer flow, or even vertical, though the hydrostatic head of the elastomer during casting should not be excessive and the flow of the displaced air should be optimised. The plates may also be fixed in place in the structure and filled with elastomer in situ.

[0046] To enable welding of the member to other members or an existing structure, it is necessary to leave a sufficient weld margin around the edges to ensure that the elastomer and its bond to the steel plate are not damaged by the heat of welding. The width of the weld margin will depend on the heat resistance of the elastomer and the welding technique to be used but may be about 25 mm. If the elastomer is cast between the plates, the welding margin will need to be defined by elongate spacers.

[0047] The number of injection ports required will depend on the available equipment for pumping the components of the elastomer and to provide minimum splash (ideally splash free) and air entrainment (to minimise void spaces) as well as the gel time of the elastomer. The ports should be situated in appropriate places for the use to which the member is to be put. If the member is to be used as a hull plate in a double-hulled ship, the injection ports are ideally situated so as to face the inter hull gap rather than the sea or cargo space. The injection ports are ideally quick disconnect ports, possibly with one way valves, that can be ground off after casting. They may also be sealed with plugs that are ground smooth after casting.

[0048] Air vents are placed in each cavity to allow escape of all air in the cavity and to ensure no void space is left. The air vents may be threaded to allow insertion of plugs after filling or include valves or other mechanical devices which close after filling. The air vents and any plug or valve may be ground smooth after the elastomer has set.

[0049] Plugs inserted in injection ports or air vents should be made of a material that has galvanic characteristics compatible with the metal layers. If the metal layers are steel, the plugs may be of nylon.

Claims

[0050] The injection process must be monitored to ensure even filling of the cavity without any back pressure which might cause swelling and uneven plate thickness. The injection can also be carried out using tubes that are withdrawn progressively as the cavity fills. [0051] After manufacture and during the life of the laminate, it may be necessary to verify that the elastomer has correctly adhered to the metal layers. This can be done using sound. [0052] To repair damaged members, or if the elastomer has not properly adhered, the damaged region of the steel plate is sawn (cold cut) or flame cut and the elastomer is cut or gouged out, e.g. using a router or pressurised water (hydro blasting) until good elastomer is exposed and a weld margin is created. The exposed surface of the remaining elastomer must be sufficiently clean for new elastomer, cast in situ, to adhere. A new backing strip or plate is welded along this leading edge. An alternate method of fabrication is to glue preformed slabs of elastomer to metal plates. [0053] The present invention has been described above largely in relation to ship building applications. However, the invention is also useful in other applications, especially those where high in-plane and transverse loads are expected, where high rupture strength, high fatigue strength or high resistance to crack propagation is desirable. [0054] Those skilled in the art will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Modifications and variations of the present invention are possible in light of the above teaching without departing from the spirit and scope of the invention as defined in the appended claims. [0055] The present invention claims priority from British Patent Application no 1620318.4 filed 30 November 2016, which document is hereby incorporated by reference in its entirety. CLAIMS

1. A structural laminate member comprising:

a first metal layer having a first inner surface and a first outer surface;

an intermediate layer comprising an elastomer located between and adhered to said first and second inner surfaces, said elastomer having a modulus of elasticity, E, of at least 104 MPa at a sustained temperature of 60 °C and a modulus of elasticity of at least 1000 MPa at a sustained temperature of -80 °C, and a ductility exceeding that of the metal layers.

2. A structural laminate member according to claim 1 wherein said elastomer has a modulus of elasticity between 104 MPa and 200 MPa at a sustained temperature of 60 °C.

3. A structural laminate member according to claim 1 or claim 2 wherein said elastomer has a

modulus of elasticity between 1000 MPa and 3000 MPa at a sustained temperature of -80 °C.

4. A structural laminate member according to any one of the preceding claims wherein said

elastomer has tensile and compressive strengths of at least 10 MPa.

5. A structural laminate member according to any one of the preceding claims wherein said

elastomer is compact.

6. A structural laminate member according to claim 5 wherein the total void space in the

intermediate layer is less than about 60% of the total volume of the intermediate layer.

7. A structural laminate member according to any one of the preceding claims wherein said

elastomer is polyurethane.

8. A structural laminate member according to any one of the preceding claims wherein said

intermediate layer has a thickness in the range of from about 15 to about 100 mm.

9. A structural laminate member according to any one of the preceding claims wherein at least one of said first and second metal layers is formed of steel.

10. A structural laminate member according to any one of the preceding claims wherein said first and second metal layers each has a thickness in the range of from about 3.5 to about 25 mm.

11. A structural laminate member according to any one of the preceding claims wherein the ratio of the total thickness of the first and second metal layers to the thickness of the intermediate layer is in the range of from 0.05 to 3.5.

12. A method of making a structural laminate member comprising the steps of:

providing first and second metal layers in a spaced apart relationship so that a core cavity is defined therebetween;

filling said core cavity with an uncured elastomer that, when cured, will have a modulus of elasticity, E, of at least 104 MPa at a sustained temperature of 60 °C and a modulus of elasticity of at least 1000 MPa at a sustained temperature of -80 °C and a ductility exceeding that of the metal layers; and

curing said elastomer so that it adheres to said metal layers.

13. A method according to claim 12 wherein said filling step is carried out to minimise air

entrainment such that the proportion of void space after curing is less than 60%.

14. A method according to claim 12 or 13 further comprising the step, prior to said filling step, of providing at least one vent aperture in said cavity.

15. A method according to claim 14 further comprising the step, after said curing step, of sealing said vent aperture.

16. A method according to any one of claims 12 to 15 further comprising the step of providing spacers to maintain the separation of said first and second metal layers during said filling and curing steps.

17. A method according to claim 16 wherein said spacers are provided to define side edges of the cavity such that said intermediate elastomer layer is recessed from the first and second metal layers at least one edge to provide a welding margin.