Modern Windships; Phase 2

Appendix 3. Material choice, background

Steel
Aluminium
Space-frame/Fabric
Composites
Aluminium
Fibre Composites, Fibre Material

In the following the material advantages and disadvantages are described briefly.

Steel

Steel is a well-known construction material, and the most commonly used in shipbuilding. It exists an many different qualities with distinct properties, such as stainless, high tensile strength etc. The qualities fit for the current application can be categorized as having the following properties.

Advantages

	Cheap
	Well-known
	Easy to repair - weldable
	Stiff

Disadvantages

	Heavy
	Not corrosion resistant
	High surface finish requires post-treatment
	Complicated thin shapes can be difficult to build

Material Properties

In Table 29 below the allowed stress values for mild steel and high tensile steel according to DNV are listed.

Aluminium

Aluminium is a construction material widely used in the aeronautical industry. It is then riveted or glued. In the shipbuilding industry it has lately gained some success in high speed applications and as superstructures. In these applications the aluminium is often welded, which adversly affects the material properties.

For yacht masts aluminium is the dominating material. These masts are mainly extruded profiles, which can only be produced up to certain sizes. Larger aluminium ships are often rationally built by welding together extruded profiles, already incorporating stiffeners.

Advantages

	Light.
	Weldable.
	Relatively cheap.

Disadvantages

	Corrosion resistance generally good, but in combination with steel it can corrode badly, see for example Ref. 1
	Welding degrades the physical properties, like fatigue properties.
	Surface finish of welded parts requires post-treatment to achieve smooth surfaces.
	Thin sheet-metals necessary for optimum design will have to be riveted or glued.

Space-frame/Fabric

An alternative way to build wing-profiles is to use fabric covered space frames. This was the way the first aircraft wings were built, and can properly engineered result in extremely light structures. They can be differentiated from other wings as their skins are not stressed, they are not load carrying in any sense other than to provide the profile shape and resist the wind pressure. The principle is still used in light aircrafts, wingliders, etc. where the span/corda ratio is not too high.

Advantages

	Light weight
	Can be built of completely corrosive - free materials

Disadvantages

	Structures will often have a higher maintenance requirement than stressed skin structures
	Sun exposed fabric will need to be changed regularly
	Not very damage tolerant, rips and tears in the fabric will quickly propagate

Composites

Composite materials can be made of a large group of different constituents with very different properties and an almost endless range of combinations. Before going through the advantages / disadvantages of the material, the definitions should be clear.

Composite material - a conscious combination of two or more distinct material phases into one engineering material where the phases are still discernible

This commonly used definition of a composite material in this context we further reduce into that at least one of the pertaining materials should consist of fibres, another of a man made polymer. This way we are not including ceramic-, metallic-, paper and all naturally occurring composites like wood, bone, mollusk shell, insect exosceleton etc.

Sandwich constriction is often mentioned in conjunction with composite materials. This has had the side-effect of mixing up the terminology somewhat. ASTM for example states that:

A structural sandwich is a special form of a laminated composite comprising of a combination of different materials that are bonded to each other so as to utilise the properties of each separate component to the structural advantage of the whole assembly.

In this context the term "laminated composite" refers to that the material combination forming a sandwich often consists of different materials, stiff face sheets, a lighter core material and some adhesive to transfer loads between the face sheets and the core material. However this definition is somewhat flawed, as a sandwich can easily be manufactured using only one material, such as aluminium foam covered with aluminium face sheets, or poly-carbonates foamed with lower densities in the middle of the foam sheets.

It is therefore better to concentrate on the function. A sandwich structure consists of two or more relatively thin, stiff and strong faces separated by a relatively thick, lightweight and weaker core material. The face sheets are connected through the core material, often using some adhesive. The mode of operation of a sandwich is much the same as an I-beam. As much as possible of the load carrying material is placed far away from the bending neutral axis, leaving only enough connecting the flanges as to make them work together and resist shear and buckling. See Figure 85 below.

Figure 85. A typical structural sandwich material.

Using the definitions above we see that, for example, a aluminium honeycomb core material clad with bonded aluminium face sheets is not a composite material, but definitely a structural sandwich. The aluminium honeycomb is often anisotropic, and the resulting sandwich will most likely be treated as a ortothropic construction component. Changing the face sheets from aluminium to polyester impregnated fibreglass we now have a composite sandwich material. By tailoring the fibre directions of the glass fibres in the face sheets we can further direct the stiffness and strength of structural sandwich component in the preferred direction.

Structural Considerations

When building something from composites there is a question that needs attention from start; single skin or sandwich construction. A single skin structure is much like a normal stiffened shell, composite panels are stiffened with internal stringers to provide the necessary strength and stiffness. In a sandwich structure the distance between the internal load distributors can be increased, allowing for the sandwich to work as described above.

Typically a single skin composite is favourable when dealing with small panels having small building depths. Load introduction in sandwich structures can also be complicated, so panels having many load introduction points, fasteners etc. can often benefit by being single skin. Handling requirements can also dictate a minimum laminate thickness which is so large that sandwich is not necessary to fulfil strength requirements.

As a sandwich is more complicated, and thereby more expensive, to manufacture than a single laminate it is important not to overlook this option. However, for the panels considered here there are relatively few load introductions and the areas / volumes are large. Being of the same shape only one mould is required. It is therefore recommended that the panels should be of sandwich construction in order to save weight.

Manufacturing

A big difference from "normal" steel construction using composites is that the structural material is actually fabricated in the same step as the construction of the component itself. As will be evident from below the combination possibilities are almost endless. For stress calculation purposes a designer must start with finding a manufacturer of the component, in order to find out which material properties can be achieved. These will vary depending on the raw material supplier and the production method used by the manufacturer.

Many different composite manufacturing processes exist. Basically fibres are laid up in a form and resin (matrix) is applied. After the matrix has hardened the component is ready for further processing. The main difference between the different production processes is the way the matrix is distributed over the fibres. This will, amongst other things, depend on which matrix is chosen.

Naturally different matrixes will yield different properties of the finished composite material. Using the same matrix and fibre the properties will also vary depending on the relative content of the different constituents of the composite. Typically a good laminate produced using pressure assisted devices can contain up to 60-70 weight % fibres, while a less good hand made laminate in a yacht can contain 20-30 weight % fibres. Obviously the properties of the two laminates will vary greatly since the fibres are the main load carriers in the composite.

Face Sheets

As explained above the face sheets of a sandwich can consist of both metal and composites. The only relevant metal here is aluminium. The aluminium is then glued to the core material to form the structural sandwich.

Composite face sheets can consist of a multitude of materials in different combinations. Choosing the correct combination is the designers task, often guided by what is locally available as much as what is structurally optimal.

Aluminium

Aluminium face sheet sandwich materials are quite common. They can for example be found in internal structures of high-speed crafts, aircraft and trains were non-combustibility and low weight is crucial.

Advantages

	Smooth finish
	Low weight
	Good mechanical properties

Disadvantages

	Doubly curved surfaces complicated to manufacture, best for flat or slightly curved panels.
	Gluing to core often requires large and expensive press equipment
	Availability in Denmark of production facilities low?

Fibre Composites, Fibre Material

As fibre material in the composite there are basically three options:

1. Glass
2. Carbon
3. Aramid/Kevlar™

From these, glass is the cheapest being the less stiff and heaviest fibre. It is also the most widely used.

Carbon fibres are stiff, having good tensile properties. Compression strength is less good. Gaining ever increased use, specially in sports/leisure and military applications, the world production is constantly increasing and prices thereby gradually sinking. The price is otherwise a hindrance to wider use of the carbon fibre.

Aramid fibres are more known under the trade name Kevlar. They have good tearing and shearing resistance, making them the preferred fibre for bullet-proof vests etc. As with carbon fibres the compressive properties are not very good. Price and property-wise they are located somewhere in-between glass and carbon, but with the recent price drop in carbon fibre price their justification as a structural fibre is diminishing. In building of yacht hulls they have had some success based on the assumption that they perform better in grounding situations. A disadvantage with aramid fibres is that they are hygroscopic, absorbing water in humid environment. They are often used for sail making, as they are not as brittle as glass or carbon fibres when folded.

All fibres above exist in a wide variety of qualities and grades. Structural manufacturers will normally stock a few standard grades from a few manufacturers, thereby reducing the number of choices.

More exotic fibres exist, having better properties, but they are not relevant in this context due to their high price.

Fibre Directions

Fibres can be applied in many ways to build the structural component. Most common are weaves, unidirectional tapes and non-crimp fabrics. The number of options available when it comes to different weaves, fabrics and tapes available using the same fibre is staggering. When ordering larger quantities the designer can sometimes specify the amount of fibre in each direction to be used in each weave or fabric. Again structural manufacturers store a few standard weaves, fabrics and tapes in order to reduce the number of choices. The laminates are then built up as required by adjusting the number of layers and direction of each individual weave/fabric/tape layer.

Matrix Material

Matrix material are commonly divided into thermosets and thermoplastics. Here we will concentrate on thermosets. These are two-component mixtures, after the two components are mised an irreversible exotherm process is started, and there is a finite time before the matrix has hardened. The most often used thermosets are:

1. Polyester
2. Epoxy
3. Vinylester

Polyester is the cheapest and most commonly used, but having the lowest properties. Styrene emissions during curing can also be high. Epoxies generally have the best properties, but is the most expensive, is more difficult to process and is allergenic when not hardened. Vinylester is the compromise, with properties and price in-between epoxy and polyester.

The choice is often most depending on which manufacturer is designated to build the structure, since the different materials require slightly different equipment and routines manufacturers tend to stick to one instead of varying in the production.

Core Material

As with fibres a bewildering amount of options are available to the designer. Basically they can be divided into:

1. Foam
2. Honeycomb
3. Balsa wood

Foam materials are made by blowing a gas through a base material which then stiffens in a "bubbly" state. By varying the base material and blowing agent open or closed cell materials can be made and foam densities varied. The foam density is directly proportional to the strength properties of the foam. Common densities vary from 40 to 300 kg/m³.

The temperature resistance of the foam will normally be directly dependent on the base material, something to remember if the structure is to survive in a hot and sunny climate near the equator. The foam materials are much used in yacht and shipbuilding. Their price and properties are on the low end of the core material scale.

The material exhibiting the best properties are honeycomb materials. Their name stems from their hexagonal cell shape, copied from the bees honeycombs. The honeycomb itself are often of aluminium or Nomex, a "paper" containing aramid fibres. A drawback is that cracks in the face sheets will let water in, filling the cells and gradually destroying the core material. Another is the price of the material.

Balsa wood is quite common in decks of yachts. The balsa is cut in smaller slabs and positioned on a mat in an upright position, with the fibres in vertical between the face sheets. It is then called end grain balsa, EGB. The properties are quite good for its weight, and temperature resistance is good. For natural reasons the density variations can be quite significant within the same quality, and the balsa is best used in relatively thin sheets. Water absorption can be a real problem, completely destroying the balsa by rotting. EGB is however better than normal balsa as the water is not transported sideways by the fibres.

Ultimatively the core material choice is best solved co-operating with the manufacturer, depending which material he has experience with. In the current application a closed cell foam core material is recommended to increase damage tolerance.