The Iceberg Ship: A Cautionary Tale

This story takes place during the dark days of World War II, when Nazi forces occupied much of central and western Europe. The island of Great Britain held out against nightly aerial bombing raids. US merchant mariners braved the North Atlantic to keep the besieged British people supplied with food, fuel, ammunition, and medical supplies. However, land-based aircraft could protect ships only within a few hundred miles of the coast. Beyond the reach of the planes, deadly "wolf packs" of German submarines operated freely -- sending thousands of ships, along with their crews and life-sustaining cargoes, to the bottom of the ocean.

Neither the British nor their US allies had enough steel available to construct the aircraft carriers that would be needed to extend air cover over the entire North Atlantic. But a British inventor named Geoffery Pyke came upon an alternate material that was cheap, plentiful, and strong: ice.

Pyke's idea got the attention of Prime Minister Winston Churchill, who became enthusiastic about the possibility of towing icebergs from the North Sea and converting them into giant aircraft carriers. He directed the British military establishment to provide all resources needed to turn this idea into reality.

However, engineers quickly realized that natural icebergs would not be suitable for this purpose. First of all, though icebergs are big, most of their volume is underwater. The part of the iceberg that is above water, even if flattened out, wouldn't be big enough to allow aircraft to take off and land. Also, though natural ice is fairly strong relative to its weight, it is also brittle, especially under rapidly applied loads. Finally, ice has an unfortunate tendency to melt.

Fortunately, a pair of researchers in New York discovered that it was possible to combine ice with wood pulp to create a composite material. The addition of 14% wood pulp increased the tensile strength by a factor of four and also reduced the melting rate by decreasing thermal conductivity. As the ice melted, the wood pulp would create an insulating barrier around it, dramatically slowing any further melting. This new material was named "pykrete," in honor of the project's originator, Geoffery Pyke.

To demonstrate the new material's properties, a demonstration was arranged in Canada (outside the reach of Nazi intelligence) for top US and British military brass. A bullet was fired at a block of ordinary ice, which shattered. A bullet was then fired at a block of pykrete. The bullet ricocheted off the pykrete and struck a high-ranking officer, who fortunately survived. Despite its near-disastrous outcome, this dramatic test convinced many in the military leadership of the new material's potential.

In spite of pykrete's ballistic properties, it was susceptible to another failure mode: creep. This is a material's tendency to deform permanently over time, at loads less than the material's yield strength. For example, rubber o-rings sometimes take a permanent set under compression. Some plastics are susceptible to creep at room temperature. Metals will also creep, especially at temperatures greater than 50% of their melting points. Creep is a common failure mode for high-temperature components, such as gas turbine blades.