Materials

Mark Chapman

Design would be a whole lot easier if the stores had inexhaustible supplies of unobtainium and nomassium, those most valuable of  materials, solving all the troublesome problems - when you need something as stiff as carbon, as strong as titanium, but as cheap as steel. Unfortunately there is no universal material, each has its strengths and weaknesses (pardon the pun), so for the most part material selection will always be a compromise, balancing factors such as cost, weight, strength and stiffness.

The two that best describe the engineering properties of a material (as much as two numbers ever can) are the strength of a material ie. How much force is needed to break it, and its stiffness (the Young’s Modulus, E).  Young’s Modulus is best thought of as a measure of whether a material is springy, floppy, flexible or stiff (high E).  These two figures give a pretty good idea of what you’re looking at, a biscuit is stiff but weak, steel is stiff and strong, a plastic such as nylon is strong and flexible and jelly is wobbly and weak.
  

As a quick guide, these tables compares some typical values for the tensile strength and Young’s Modulus for a range of generic engineering materials

From the table above you can see that, for the metals, the specific stiffness is pretty much identical, meaning that for a frame with a given structural stiffness we could make it equally well from titanium, aluminium or steel and come out at roughly the same weight. 
 

The main differences will come when we start to need to consider strength

From this you can see that though we can build a structure of equal stiffness for a given weight, when high strength becomes a requirement then alloy steels and titanium come into their own, with titanium head and shoulders above.  However, the choice between them has to consider other factors such as cost, availability and the ease of manufacture and fabrication.  Once you start to consider these, then for the most part the weight penalty of steel is more than offset by its relative low cost and ease of working.

The one wonder material I haven’t mentioned from the above table is the carbon filament, which, on the face of it seems the best of the lot.  However, it is what it is, a filament, or single fibre, and its outstanding material properties are when it is in tension.  In order for it to be a useful engineering material it must be woven into a fabric then held in place with a matrix or resin.  The way in which it’s woven and laid in the structure has a massive effect upon the strength of the resulting component.  For example if all the fibres are laid in one direction, then the panel will be as strong as the carbon along its length, but, pull it across the plies, then the panel is only as strong as the resin.  This is known as anisotropy, where the properties of the material are highly dependent on direction.  This property can be used to the designers’ advantage by allowing the lay-up to be tuned to the stresses in the part, ensuring the maximum stress is in the direction of the maximum strength.  This is one of the principal advantages of composite components, being able to control its anisotropy by design and fabrication.  However, to fully exploit this you need one vital piece of information, you need to know how the stresses flow through the component.  Without this knowledge, it’s almost better to produce a structure with quasi-isotropic properties, where its properties are pretty much independent of direction.  At it’s most basic this means using a chopped strand mat, with fibres randomly orientated in all directions, to be a bit more sophisticated you’d make the panel with sheets of woven fabric, each ply set at 45o to the last giving approximately the same properties irrespective of how the panel is stressed.

Composites have two other advantages, and one significant drawback.  The advantages are to do with their fabrication, in that with the correct choice of fabric weave, highly complex surfaces can be produced with relative ease, especially when compared with the efforts that would be needed to produce those shapes in metal, making composites ideal for external bodywork.  The other is that with the addition of a lightweight core material it’s possible to produce an extremely stiff panel at very low weight, the core acting much like the web in an I section steel beam.  The drawback is that the composite is only as strong as its weakest point, and is highly susceptible to voids, inclusions, delamination and pockets of rich resin, these have to be eliminated through a careful manufacturing process, this is doubly so as composites do not lend themselves well to the thorough Non-Destructive Testing that is commonplace with metals.

There is one area of BLOODHOUND SSC where there is an additional, unusual and quite important material requirement.  The ton or so of High Test Peroxide (HTP) that we’re carrying is a fantastically good oxidiser to such an extent that there aren’t many things that won’t make it go fizz (by which I mean go bang).  So, everything it comes into contact with before it reaches the catalyst pack, must either be non-reactive or passivated.  The special stainless steels that make up the pump impellor and pipework don’t react to the peroxide, and neither do the Teflon liners in the flexible pipe joints.  However, the aluminium that forms the pump body and the HTP tank itself need to be introduced gently to peroxide at ever increasing concentrations to allow a non-reactive layer to build up and stop fizzing, before it’s passivated and non-reactive to the full strength HTP.

The challenge with BLOODHOUND SSC will be to come up with the right material choices for each job, without the lengthy analysis, testing and trial. Hopefully we’ll manage to avoid picking the jelly and biscuits.