Stories from Physics for 11-14 14-16
Richard Feynman famously demonstrated the role of brittleness in the failure of a rubber O-ring in the Challenger Space Shuttle disaster. The properties of materials have contributed to a number of accidents.
During the Second World War, the demand for new ships was high. US manufacturers switched to using welded joints rather than the traditional, but slower, riveted joints. Ships produced this way were referred to as Liberty Ships and, of the 2,700 vessels produced, 400 suffered from fractures, with the hulls of 20 ships breaking in two. In one dramatic case, in 1943, the tanker SS Schenectady had just completed sea trials and was in a fitting-out dock in Portland, Oregon. Though the conditions were calm, it was cold — the air temperature was -3°C and the sea a chilly 4°C. The Schenectady fractured suddenly, with such force that the sound of rupture was heard a mile away. A large fracture ran through the deck and the sides of the hull, causing the ship to jack-knife, lifting the central part of the vessel out of the water and forcing the bow and stern down to the floor of the dock. The cause of the failure was the brittle fracture of the low-grade steel used in the hull, made more brittle by the low temperatures. The Schenectady was repaired and able to re-enter service only four months after the fracture.
In January 1919, at the Purity Distilling Company in Boston, a tank filled with 8.7 million litres of molasses burst, killing 21 people and injuring 150. Eyewitnesses reported hearing a thunderclap, then a noise like a machine-gun as the tank’s rivets were forced out and they felt the ground shake. The accident produced a wave of molasses 8 m high at its peak which travelled at 56 km/h. The relatively high density of molasses meant the wave had considerable momentum, destroying over 150 buildings and buckling overhead railway tracks. Cold temperatures preceding the disaster had caused the ductile rivets to become brittle and investigators cited brittle failure of the tank’s rivets as a cause of the disaster.
Modern metallurgic analysis of steel taken from the wreck of the Titanic suggests that brittleness may have contributed to the disaster. The chemical composition of a steel plate recovered from the wreck showed it to be plain carbon steel with above-average levels of phosphorous and sulphur but much lower levels of manganese than modern steel, reducing its toughness. Researchers concluded that the steel used in the Titanic would have been prone to brittle fracture, behaviour that would have been exacerbated by the sea temperature of -2°C at the time of the collision.
How pigeons broke the Brooklyn Bridge
In 1981, two steel cables supporting the deck of the Brooklyn Bridge snapped, injuring a man. One of the 180 m long stays hit Akira Armi, fracturing his arm and skull. The other cable damaged the wooden planks of a footpath. The cables had been supporting the bridge since 1883 and their failure is attributed to the acidic wear caused by pigeon droppings.
The saggy suspension bridge
Samuel Brown began his career in 1795 as a naval captain and developed an interest in the construction of wrought iron chains. On retiring from the navy, he founded a chain and anchor works. This commercial concern with chains led Brown to develop designs for suspension bridges and he bid, but lost out to Brunel, to build the Clifton Suspension Bridge. He was, however, successful in winning the contract for the Stockton Railway Bridge over the Tees which was to be the first rail suspension bridge. Once complete, a test steam locomotive pulling a train of carriages loaded with coal was driven over the bridge. As the train passed over the cable-supported deck, it began to sag significantly.
The trial crossing was witnessed by Robert Stephenson, the son of George Stephenson, inventor of the first steam locomotive. He later reported that “when the engine and train went over for the first time there was a wave before the engine of something like two feet, just like a carpet”. To rectify this dramatic behaviour, a pier was built under the bridge to support it and only horse-drawn carriages were allowed to use the crossing. In 1842, Stephenson replaced the suspension bridge with a cast-iron span.
A train accident in Versailles in 1842 prompted the start of systematic research interest into the phenomenon of metal fatigue. Almost 800 travellers returning from a celebration in honour of King Louis Phillipe had boarded a train to return to Paris. During the journey, a locomotive axle snapped, causing one of the engines pulling the carriages to leave the track, spraying hot coals and igniting the wooden carriages. As was customary at the time, the doors of the carriages had been locked, and many people (estimates vary from 60-100) died in the accident. Following the disaster, August Wöhler developed the stress-life method for predicting metal fatigue.
R. P. Feynman, ‘What Do You Care What Other People Think?’: Further Adventures of a Curious Character. London, Penguin Books, 1998.