The challenge: build a device to crush a car into as small an object as possible.

Heavyweight crusher Pros Faster than the guillotine. This is the hare in this race. In theory, a mechanical method is more reliable than a hydraulic one – a simpler build has less that can go wrong. Cons Can’t smash as large a surface area as the hydraulic machine. Could find it more difficult to make sure that every inch is squashed, which will add to the volume. Won't produce nearly as much power as that exerted by hydraulic rams.

Pros

• Hydraulics are supremely powerful. Strength and power are needed when it comes to flattening cars.
• Systematic method of flattening: each part of the car can be crushed equally.

Cons

• Slow and depends on the power of the pump. This is certainly the tortoise in this race.
• Hydraulics are tricky. If there are leaks, they don’t work.
• Quite a hefty build.

Metal facts

Fact 1 To be considered a metal, a substance must:

• conduct electricity.
• conduct heat.
• make a ringing sound when hit.
• reflect light.
• be malleable – able to be moulded into different shapes, especially by hammering into thin sheets.
• be ductile – able to be stretched, bent, twisted and rolled without breaking.
• have a dense molecular structure.
• react with oxygen (except gold, platinum and silver).
• react with acids to form salts.

Fact 2 Metals have a fine crystalline structure. When metal is strained and distorted mechanically - as happens during the crushing of a car – the crystals become stressed and form into 'tetrahedrals'. These are crystals made from four atoms with very strong electrical bonds where the atoms touch. This makes the metal very hard and brittle.

Fact 3 To relieve the stresses and relax the crystals caused by distortion, metal must be heated to a temperature just short of its melting point – a process known as annealing. This makes the metal soft and easier to work.

Fact 4 Ductility is the ability of a metal to be deformed without breaking or fracturing. It is particularly important in the making of wire and in stamping metal sheets. Platinum, steel, copper and tungsten all have high ductility. The science of rheology, which focuses on ductility, is the study of how materials deform and flow in response to force.

Fact 5 Alloys are mixtures of metals. When one of those metals is copper, the alloy will be prone to 'age hardening'. This is why antique silver tends to crack easily.

Fact 6 Platinum is a particularly interesting metal. It has an extremely high melting point: 1769ºC (3216ºF) and is very wear resistant. It doesn't react with oxygen at normal temperatures nor with common acids - so it doesn't corrode.

Fact 7 Steel is an alloy of at least 98.5% iron and less than 1.5% carbon. The carbon gives the metal strength but reduces its malleability and ductility. When steel consists of 11–14% chromium, it becomes 'stainless steel' with corrosion resistance.

Fact 8 Rust is a brown solid formed when iron and the oxygen in water and air react together – a process called oxidation. Iron objects are given a protective coating to prevent rust. For example, cars are painted and grease is applied to engine parts. Iron and steel can also be protected from rust by galvanising – coating the objects with a layer of zinc. The zinc on the surface oxidises in air, preventing the zinc and iron below from becoming oxidised. Galvanised cars remain rust-free far longer than those that have not undergone this process.

Fact 9 Metalworkers hammer sheet metals to harden them without making them brittle. The hammering moves dislocations along intersecting slip planes within the metal until they meet and stop. These meeting places act as barriers to the movement of any other dislocations, making the metal stronger.

Fact 10 Engineers use high-grade steel girders in their work because good steel is usually highly resistant to cracking. If a crack does appear, the metal is generally ductile enough for the edges of the crack to flow together, which will diminish the danger of the crack extending very far. In poor steels, cracks may develop rapidly. This is a particular danger in bridges where the steel is exposed to changes in temperature.

Fact 11 Hydraulic presses can make shaped body panels from single flat sheets of metal in a single operation. A hydraulic ram presses a piston carrying a 'former' of the shape required against the metal sheet with tremendous force. The high pressures generally needed call for costly presses, but their speed usually justifies the expense. For more information about hydraulics, see the Scrap science section of the Demolition challenge.

Fact 12 Metal shapes can also be made by the process of extrusion. Cold metal is forced through a hole - a 'die' – of the required shape and size. The great pressures exerted on the metal cause it to assume a 'plastic' condition so that it is able to 'flow' smoothly through the extrusion die.

Find out more from the Scrapheap Challenge advisers about possible solutions for building a heavyweight crusher and a hydraulic guillotine.

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