Rolls Royce exam

Metal crystal lattice structure there are three basic structures, Body Centred Cubic (BCC), Face Centred Cubic (FCC) and Closely Packed Hexagonal (CPH)

The ‘cell unit’ of a BCC is based on a simple cube.

A metal atom occupies each corner of the cube and another metal atom sits in the centre of the cube (hence the name ‘body centred cube’). Metals with this structure are characterised as being relatively strong. They have 9 atoms in the structure (one at each corner [8] + 1 in the centre = 9)

Face centred cubic metals have a cell unit that is based on a simple cube. They have an atom at each corner of the cube but, unlike the body centred cube they do not have an atom in the middle of the cube, they have an atom in the centre of each face, therefore they have 14 atoms in each structure. (One at each corner [8] + one on each face [6] = 14). Metals with this structure tend to be tough and ductile but relatively weak, such as copper and aluminium.

Closed packed hexagonal structure is more complex. It is based in a hexagon rather than a cube. There is an atom on each corner of the hexagon (12) and three more forming an equilateral triangle sitting between the two hexagons, holding the structure together and one on each top/bottom face giving a total of 17 atoms. Metals with this structure tend to be relatively weak and brittle, such as zinc and pure titanium.

So, the structure of a material governs the materials behaviour.  But why does the crystal lattice structure govern the mechanical properties?

When a piece of metal is formed, bent, rolled or squeezed during manufacture,  vast numbers of atoms in every crystal must be moved. A metals mechanical properties are directly related to the ease (or difficulty) of this atomic movement. In FCC metals, this movement is quite easy, as a result the metal tend to be relatively weaker but more ductile.

Metals have a crystalline structure (see above), when a metal solidifies from the molten state, millions of tiny crystals start to grow. The longer the metal takes to cool the larger the crystals grow. These crystals form the grains in the solid metal. Each grain is a distinct crystal.

The areas between the grains are known as grain boundaries. Within each grain, the individual atoms form a crystalline lattice. Each atom will have a certain number of close neighbours with which it shares loose bonds. (The number of neighbouring atoms depends upon the structure of the lattice.)

When stress is applied to the metal, the atoms will start to spread apart. The atomic bonds stretch, and the attractive forces between the atoms will oppose the applied stress, like millions of tiny springs.

Metals solidify at different temperatures, however they all go through the same process.

Aluminium freezes at 660 degrees C, at which point it solidifies. This occurs in a series of steps.

The first stage is called nucleation, where atoms (in the liquid state) start to bond into tiny masses of solid metal.

These small clusters of atoms begin to grow larger, this is stage two, but they do not develop into spheres but form tree like structures as more and more tiny masses of atoms clump together, this is called dendritic growth.

The clusters grow and eventually become dendrites (Dendrite comes from the Greek ‘dendros’ meaning ‘tree’), this is stage three.

The dendrites grow larger, developing more complex branched structures. This is because of the way that the atoms in the liquid metal bond with the atoms of solid metal. The dendrites become so large that they make contact with each other, this is stage four. When they do this they DO NOT become attached, what they do at this stage is stop growing any further immediately. The metal now consists of ‘large’ dendrite structures (in this case of solidified aluminium) with liquid metal (again in this case aluminium) in between the dendrite arms.

Stage five sees the liquid metal between the dendrites solidifying, the complex dendritic structure becomes one consisting of solid grains of crystals of pure metal (in this case aluminium). The structure is called polycrystalline, this is the structure of all metals. (That is, it comprises of many crystals).  

A monomer is a molecule of low molecular weight capable of reacting with identical or different molecules of low molecular weight to form a polymer (a chain of monomers), sometimes in the presence of a catalyst. For example, ethylene is the monomer that is used to produce the polymer, polyethylene.  

The word polymer comes from the Greek poly (many) and mer (parts). A polymer is composed of many repeated subunits (monomers). An example of a polymer is polyethylene (polythene). This is composed of many monomers (in this case ethylene), which react together forming a long chain molecule, (as explained below).  In order to create a polymer two or more monomers must react together, for example monomers of ethylene.

To look at how polymerisation occurs think of a polymer we all know, polythene. Ethylene is a gas, with the chemical formula C2 H4.  It consists of two carbon atoms sharing a double bond, and four hydrogen atoms, two attached to each carbon atom, this is explained below .

Polymerization is where a monomer reacts to create a long chain polymer.

Ethylene is a gas, with the chemical formula C2 H4.  It consists of two carbon atoms sharing a double bond, and four hydrogen atoms, two attached to each carbon atom.

The double bond between the two carbon atoms is a point of weakness, so with a little persuasion (in the form of heat energy and pressure) along with the presence of a catalyst, (titanium III chloride) we can break that bond. This then leaves a carbon atoms valence electron looking for something to covalently bond with (remember covalent bonding, two non-metals sharing valence electrons?).

So, the carbon atom shares its spare valence electron with another carbon atom in the same situation who shares with the next carbon atom, and so on.....

Particulate composites consist of a matrix phase (for example a resin), with a reinforcement phase in the form of dispersed particles.

The effect of the dispersed particles on the composite properties depends on the particles dimensions.

Very small particles (less than 0.25 micron in diameter) finely distributed in the matrix impede movement within the material, creating a strengthening effect.

Large dispersed phase particles have low strengthening effect but they share any load applied to the material, resulting in increase of stiffness and decrease of ductility.

Laminate composites consist of a matrix phase, reinforced with a dispersed phase in the form of sheets.

When a fiber reinforced composite consists of several layers with different fiber orientations, it is called a multilayer (angle-ply) composite. Laminate composites provide increased mechanical strength.

Dispersed phase in form of fibers (Fibrous Composites) improves strength, stiffness and Fracture Toughness of the material, impeding crack growth in the directions normal to the fiber.

Effect of the strength increase becomes much more significant when the fibers are arranged in a particular direction (fiber alignment) and a stress is applied along the same direction.

The strengthening effect is higher in long-fiber (continuous-fiber) reinforced composites than in short-fiber (discontinuous-fiber) reinforced composites.

Short-fiber reinforced composites, consisting of a matrix reinforced with a dispersed phase in form discontinuous fibers (length < 100*diameter), has a limited ability to share load.

Load, applied to a long-fiber reinforced composite, is carried mostly by the dispersed phase - fibers. The matrix phase in such materials serves only as a binder of the fibers keeping them in a desired shape and protecting them from mechanical or chemical damages.