Lockdown Challenge: ‘Pykrete’ – make some ice that’s stronger than concrete

Each year in winter, we get to know how pathetically weak ice is. Drop a slab of ice from even from a few inches, and it shatters into dozens of pieces. But ice doesn’t have to be like that…

In the Second World War, Project Habbakuk was a plan hatched to make ice stronger. Much, much stronger. Ice was made that was so strong that it could be used to make giant ice aircraft carriers in Canada. Famously, when the generals were told about the remarkable properties of Geoffrey Pyke’s invention they went ahead and tested it straight away: they blasted it with bullets. Bullets broke the plain ice easily – but bounced off the ‘Pykrete’, so much so that they narrowly avoided injury from the ricochets! Huge pieces of Pykrete were made – up to hundreds of tons. One took three years to melt, and they were immensely strong. Here’s how to make your own version of this magic.

To get started, use something like a sandwich container with a snap-on lid as the mould. Pack it with around two per cent by weight cushion fibres (they are a kind of polyester), then fill up with water, trying to get all the bubbles out before putting it outside or in the kitchen freezer to freeze. Maybe then make up a matching piece with just the water and no fibre to compare.

Now take your hammer, don your safety glasses, and tap your test sample with the impact of a small hammer. Did it break? If not, hit it harder. Still not? Try a bigger hammer. If you made an ice sample, test that too.

Try different ice/fibre proportions and different fibres. Maybe more linear fibres like wool or string? The original military Pykrete used wood pulp fibres, at a high level, 15 per cent. Try long thin shapes, flat shapes. But remember when choosing a mould that you have to get your test piece out of it after freezing.

You can make accurate hammer tests by arranging a hammer (of known hammerhead weight) to swing through an arc from vertically upwards to vertically downwards. A simple wood stand will do this, using a long nail as the pivot near the end of the handle. The sample can be held in position loosely at the bottom end of the arc in a thin plastic bag using drawing pins. Alternatively, you can put your test piece in front of a couple of heavy blocks – wood maybe, with the sample spanning between the blocks like a bridge sideways. This is something like the IZOD or Charpy Impact Tester seen in engineering labs.

The size of the impact energy you get is simply proportional to the height of the hammer before you let it swing times the hammerhead weight times gravity. Try small samples of concrete or roof tiles to compare the fibre-reinforced ice. Is your Pykrete actually stronger than concrete?

How can the fibres weave this magic? The sample is under compression where the hammer hits, while the side away from the hammer is under tension. Many brittle materials – including ice – are easily broken because of the tensile force on the bottom. The fibres help provide tensile strength where it is needed. The fibres also help prevent crack propagation. Once something has a crack in it, as you stress it more, the crack speeds along, until the sample is completely broken. But the crack can be stopped when it hits something different – like a fibre.

Now, what can you make with your super strong ice? Strengthen those trucking roads across the frozen lakes of the Canadian outback? A bullet-proof sleigh? Or maybe just a more sophisticated attempt on the world’s tallest snowman record (currently 38m).

If you liked this, you will find lots more fun science stuff in my books like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org