Titanic was designed to be ‘virtually unsinkable’ but a collision with an iceberg opened six of her compartments to the sea
An engineering tragedy consists of a sequence of events, some normal and some unusual, which must occur in a certain order and with a certain timing, for the accident to take place. A forensic investigation of such an accident must consider all possible factors, and also step though the timeline to identify the critical issues.
What information about Titanic and her sinking is available for such an investigation?
Since the discovery of the wreckage site in 1985, over a dozen expeditions to the ship have allowed a limited amount of forensic study to take place, including visual and sonar imaging, and the recovery of samples of hull steel and rivets. Since 1996, we have been involved in analyzing the properties of the metallic samples, recovered to determine if they met specifications set at the time of her construction. In addition, eyewitness testimony is available in the form of two enquiries conducted shortly after the sinking, one in the US and the other in the UK. Going farther back in the timeline, we also have the plans of the ship, documentation of orders for the materials used, and minutes of the meetings held by the board of directors of Harland and Wolff, in which they discuss the day-to-day issues encountered during the ship's construction.
Titanic was built between 1911 and 1912. She was constructed of thousands of one inch-thick mild steel plates and two million steel and wrought iron rivets and equipped with the latest technology. She was designed to be ‘virtually unsinkable’, designed to stay afloat with four of 16 watertight compartments open to the sea.
The first phase of the wreck's analysis involved the scientific research performed in association with the expeditions to the wreck. As she sank, Titanic broke in half and the two pieces settled in 12 600 feet of water, approximately half a mile apart. The bow section is buried in the seabed mud, hiding the damage the ship sustained. Using sub-surface sonar, the iceberg damage has been mapped as a series of thin slits spanning the first six compartments. In addition, an array of steel samples from the hull and 48 rivets and fragments have been recovered for study.
The data collected point to two possible failure routes - cracking of the hull plates or failure of the riveted seams. However, the fact that the hull was not severely deformed, as evidenced by the sonar images and reported by the survivor Fireman Barrett, implies a fairly low energy failure. A suggestion of brittle fracture of the steel plates at ice brine temperatures was made by two groups in Canada in 1991 based on Charpy tests of a hull plate fragment. However, slow bend testing, a more likely applicable strain rate, of four hull plate samples showed average toughness of 55 MPa-m1/2 at 0°C, quite reasonable for this application. If it was not brittle steel, were the riveted seams strong enough?
Titanic's hull was triple riveted within the central 3/5ths length using mild steel rivets, and double riveted using wrought iron in the bow and stern. This was done to assure strength in the center, where the maximum wave flex stresses were assumed to be located. Analysis of the steel rivets has shown good strength, but the wrought iron rivets contained an average of three times more slag than optimal levels. In addition, the slag was in large pieces. Both of these facts point to fabrication by inexperienced tradesman, as wrought iron was made by hand at the time. Finite element models of rivets made from sub-standard materials show that they were already loaded near their ultimate strength when installed. The source of this poor quality material became clearer when the Harland and Wolff meeting minutes were examined, and it was seen that pressure to finish Titanic caused the company to order wrought iron that was one level below that generally specified for rivets and they had to use suppliers previously uncertified for this application.
Titanic experienced a glancing impact with an iceberg roughly ten times her size along her starboard bow, described by survivors as ‘slight’ and ‘a rumble’; a fairly minor impact. The collision opened six compartments to the sea, and she sank in two and a half hours. In the area of the hull where most of the damage is contained, the seams consist of double rows of wrought iron rivets. Our supposition is that if the iron rivets had been of high average quality, or if the designers had opted for triple rows of rivets or to use steel instead of iron, then fewer compartments would have flooded. If it had been five compartments, with the Carpathia only six hours away, she would have stayed afloat long enough for most people to have been rescued. If four compartments had flooded, she might have even limped into Halifax. We do not suggest that the ship would not have sustained significant damage in the collision if she had been built differently, but rather she would have sunk more slowly. And with the shortage of lifeboats, the time she spent afloat made all the difference in the tragedy.
Full details of this analysis, as well as other myth debunking, and a description of the ultimate fate of the wreck site can be found in What Really Sank The Titanic - New Forensic Discoveries (Citadel 2008).
The opinions expressed herein are not necessarily those of NIST or the US Government