Monday, December 22, 2008

A Riveting Story: The Titanic's 94-Year-Old Mystery Solved

The sinking of the White Star Line's luxury passenger liner Titanic on April 14, 1912 has got to be one of the most famous engineering failures in history. Everybody knows the story: how the ship ran full speed into an iceberg despite warnings relayed by the then-new wireless, and sank less than three hours later with the loss of over 1500 lives. Since the discovery of the wreck in 1985, researchers have been able to recover hundreds of artifacts and subject them to modern forensic analyses. Two of these researchers—metallurgical experts Jennifer Hooper McCarty and Tim Foecke—have written a book about their discoveries. What Really Sank the Titanic clears up a long-standing mystery about the tragedy and points the finger of blame in a surprising direction.

Boards of inquiry held immediately after the disaster obtained enough information from survivors to piece together the following story. At about 11:40 PM, as the Titanic moved at about 22 knots through a near-freezing sea "as smooth as glass," lookouts spotted an iceberg in the path of the ship. The steersman had just begun to turn the bow to port when the berg scraped along the starboard side of the ship, making a long-lasting noise that was described variously as tearing, jarring, or ripping. Although the ship's six watertight compartments were immediately sealed when it was discovered that water was coming in, there were enough holes in different parts of the hull that eventually all six compartments filled up, and the ship sank. The ship's designer, Edward Wilding, said at one inquiry that a long, narrow series of slit-like openings about two hundred feet long and only an inch or so wide would have accounted for the fashion and speed with which the ship foundered. But since steel is much harder than iceberg ice, he could not explain how such openings could have occurred. There the mystery lay at the bottom of the North Atlantic for over eighty years, until salvage expeditions began to bring pieces to the surface.

In a decade-long investigation, McCarty and Foecke, respectively graduate student at Johns Hopkins University and staff member at the Gaithersburg, Maryland office of the National Institute of Standards and Technology, obtained samples of the Titanic's hull, which consisted of large steel plates held together by rivets (electric-arc welding was not to become the standard steel-fabrication method until World War II). McCarty's archival research in England revealed that Harland & Wolff, the ship's Belfast builders, used two kinds of rivets: the more modern machine-formed steel rivets for the central part of the ship, and the old-fashioned hand-formed wrought-iron type for the stern and bow sections, where much of the collision impact probably occurred.

The making and installing of wrought-iron rivets was largely a manual operation. The Titanic needed over three million rivets in all, and this huge demand led flocks of entrepreneurial ironmakers to enter the field. The hand-stirred "puddling" process then used to make wrought iron from ore required strong and highly experienced workers, of which there were not enough in 1912. So it turned out that Harland & Wolff bought wrought iron from a wide variety of suppliers, some of whom were much less experienced than others. McCarty and Foecke have proof of this in the form of long, stringy slag inclusions they found in some of the recovered rivets. These inclusions tended to make wrought iron, an already a less satisfactory material than steel, even weaker.

Why weren't steel rivets used throughout? Besides reasons of cost, steel rivets had to be formed with hydraulic riveters—large U-shaped steel machines upwards of six feet high that had to be laboriously positioned on either side of the plate to be riveted. Then the rivet, shaped much like a blunt round-headed nail, would be heated, inserted into its hole through the two overlapping hull plates to be joined, and squeezed between the jaws of the riveter. This squeezing formed heads on both ends, and as the rivet cooled, the resulting shrinkage provided tension that held the two steel hull plates together in a watertight joint.

At least, that was how it was supposed to work. The problem was there was not enough room in the bow and stern areas to maneuver the hydraulic riveter. So the builders resorted in those areas to the older hand-forming way of riveting, which couldn't use steel rivets because of reasons to do with the different ways steel and iron cool. Wrought iron was more forgiving to the delays and variations involved when a boy tossed a red-hot rivet from a portable stove to the rivet gang, which placed it in its hole and pounded it in by hand.

When the Titanic embarked on her maiden voyage on April 10, 1912, her bow hull plates were held together by wrought-iron rivets. The iron itself had probably never undergone any systematic quality testing, and the only quality tests done on the finished riveting job was a hurried hammer tap by an inspector, who listened to the sound it made. All this inspection could detect was loose rivets, not those made from defective wrought iron.

Then came the iceberg. While ice itself will crumble if forced against solid steel, the typical iceberg was a lot heavier than the Titanic. So in a glancing collision, the iceberg exerted tremendous localized force against only a few hull plates at a time. While even poorly-made rivets can withstand the mainly sideways stress that uniform pressure causes (e. g. hydraulic pressure on a water tank or a ship's hull), some of the forces that the iceberg caused tended to pull the hull plates apart, causing tensile stress. And the researchers found that wrought-iron rivets made of bad iron with lots of slag inclusions pop their heads off much more easily than either steel rivets or wrought-iron rivets made with better material. Significantly, many of the steel plates recovered from the wreckage were missing their rivets altogether. And riveting is not a gracefully-degrading fastening method. Once one rivet in a row pops, the ones next to it get much higher stress and are likely to fail as well, leading to a kind of chain-reaction zipper effect.

That is exactly what McCarty and Foecke say must have happened as the iceberg bounced repeatedly along the side of the ship, popping rivets and opening up long, narrow slots between hull plates—exactly what designer Wilding said in 1912 must have happened, though he couldn't explain exactly how. The researchers also show in detail how a rival theory—one that says the cold Atlantic waters made the plates themselves brittle enough to shatter like glass—is full of holes, so to speak.

So the roots of the Titanic disaster prove to go in several directions: to the heedlessness of the captain who failed to slow down in a known field of icebergs, to the rulemakers who didn't require lifeboats for everybody, and, surprisingly, to little mom-and-pop wrought-iron puddling operations that sprang up all over the United Kingdom in response to increased demand for wrought iron. McCarty and Foecke conclude that if all the ship's rivets had been steel, the ship still might have sustained serious damage, but not so much as to sink it in less than three hours. Even a few hours longer afloat could have given time for nearby ships to arrive and save most or all the passengers. But that was not the way it happened.

Sources: What Really Sank the Titanic was published in 2008 by Citadel Press. I also thank my wife Pamela for her thoughtfulness in this birthday-gift selection.

1 comment:

  1. Strikingly similar to another great Engineering disaster - The West Gate Bridge collapse in Melbourne where a serious of errors accumulated to cause a catastrophe. In that case the major cause was a loosening of bolts to enable two spans to come together during construction. However, just like the rivets on the Titanic, that alone would not have caused the accident had it not not been for other numerous errors - mainly a lack of communication between the parties involved in the design and construction.