Cost Of Compromise Or Why Titanic sank? - Mahendra Patel, GNFC Ltd., Bharuch
Do not compromise when Quality is concerned, you never know the final cost of a small compromise. Being part of industrial community we can hardly underestimate the impact quality standards may have on the shape of end results. Sinking of Titanic is one such case in point here. Who could believe that compromise on quality of as small and trivial an item as a rivet was the root cause of the entire catastrophe? But, as the investigations have revealed, this is the reality that surfaced out from the wreckage of this vessel, once thought "Unsinkable" and now resting on the sea bed in two pieces 12,600 feet apart. An engineering tragedy consists of a series of events, some normal and some unusual, occurring in a sequence with a certain timing, for the accident to take place. What are those events responsible for sinking of Titanic? How they occurred? Research investigations have been piecing together the details of what really occurred on April 14-15, 1912, after Titanic struck an iceberg, broke in half and caused deaths of more than 1,500 people. Why did the 46,000-ton ship sink in just two and a half hours? Well, the answer to this most elusive question comes in a report from NIST (National Institute of Standards and Technology, Maryland). As NIST metallurgist, Timothy Foecke says in the report, the culprit is most probably one of Titanic's smallest components -- the 3 million wrought iron rivets used to hold the hull plates together. Foecke performed metallurgical and mechanical analyses on steel and 48 rivet samples recovered from the Titanic's hull. His examinations revealed that the wrought iron in the rivets contained high slag (the glassy residue left behind after the smelting of ore), more than three times the allowable amount, thus making it less ductile and more brittle hence prone to fracture. This finding provides strong evidence that Titanic's collision with the iceberg caused the rivet heads to break off, popped the fasteners from their holes and allowed water to rush in between the separated hull plates, as published in various reports1, 2 from NIST. Earlier in 1991 two groups in Canada suggested possibility of brittle fracture of the steel plates at ice brine temperatures, which was later on ruled out based on more relevant slow bend testing of four hull plate samples, which showed average toughness of 55 MPa-m1/2 at 0° C, quite good for this application. Again, this confirmed that it was not brittle steel, but the riveted seams that gave way under impact. Titanic was built between 1911 and 1912 by well known shipbuilder M/s. Harland & Wolff, and she was designed to be ‘virtually unsinkable’, to stay afloat even if four of her 16 watertight compartments would get flooded with water. Titanic’s hull was triple riveted within the central 3/5th 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 which gets the maximum wave flex stresses. Analysis of the steel rivets showed good strength, but the wrought iron rivets contained three times more slag than optimal levels. In addition, the slag was in large clusters. Both of these facts indicate fabrication by inexperienced tradesman, as wrought iron was made by hand at the time.
Finite element models of rivets made from substandard materials show that they were already loaded near their ultimate strength when installed. The source of this poor quality material became clearer during researching in the Harland & Wolff archives, they discovered that the shipbuilder's ambitious plans to build three large ships at the same time had put a huge strain on its shipyard. "Not because of cost, but because of time pressures, they started using lower-quality material to fill the gaps," says Foecke. It also became very clear from their minutes of meeting that the pressure to finish Titanic in hurry, 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. The substandard rivets were pounded by hand into the ship's bow and stern, where the large machines required to pound in steel rivets didn't fit. Steel rivets, meanwhile, which are much stronger than iron, were put in the more-accessible middle of the ship. So after collision with iceberg, the seams with wrought iron rivets opened up, and as Foecke says, it's no accident, that the flooding stopped at the point in the hull where the steel rivets began. 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. NIST believes 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, thus avoiding the casualties. It is not to 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. So, at the end of the day, it is Quality that matters - even of a negligible component like a rivet.
References : 1. Materials Today, Elsevier, Oct-2008, Vol. 11, No. 10, p. 48 2. "Titanic Metallurgy" , in the Biweekly newsletter of NIST, Feb. 17, 1998. 3. Metallurgy of RMS Titanic, Report No: NISTIR 6118, Inquiries: mail to inquiries@nist.gov. 4. What Really Sank The Titanic - New Forensic Discoveries (Citadel 2008).