Cryo FaultCryogenic Fracture

Cryo Fault does not bend. That was the problem with the ships.

Most metal parts fail politely: overload a steel bar at room temperature and it stretches and tears, soaking up energy. Cool some steels far enough and that changes. Below a transition temperature the same alloy can crack straight across with almost no warning, like glass. Face-centred cubic metals such as copper and aluminium have no such transition. Ordinary structural steels, whose iron sits in a body-centred cubic lattice, do.

In 1921 Alan Arnold Griffith explained why brittle materials break at a small fraction of their theoretical strength: they contain cracks. A crack grows when the elastic energy it releases by lengthening exceeds the energy needed to create the two new surfaces. The first formula on this page is his result: the stress needed to break falls as one over the square root of the crack length. The second is the engineering form still used today. Each material has a fracture toughness, and when the stress concentrated at a crack tip exceeds it, the crack runs.

In the Second World War the United States built thousands of welded ships, Liberty ships among them, because welding was fast. More than a thousand cases of brittle fracture were reported, mostly in cold weather. On 16 January 1943 the new tanker Schenectady, moored in calm water at its Portland, Oregon, fitting-out dock, cracked almost in half, with a report heard a mile away. Later analysis blamed mainly steel that lost its toughness in the cold. Welding rarely started cracks, but it made each hull one continuous piece, so a crack that began at a sharp hatch corner had no seam to stop it. The investigations helped found fracture mechanics.

Cryo Fault stands still. Each step tests a number they'd rather not know.