This Sunday will be 100th anniversary of the sinking of the Royal Mail Ship (RMS) Titanic. Over the years this tragic accident has been the topic of numerous books, films, exhibits—even a memorial cruise. Not only has this shipwreck captured the attention of the public, but the scientific community as well.
Ever since the wreckage of the Titanic was first discovered in 1985, researchers have tried to answer how and why did the Titanic—deemed ‘unsinkable’ by the press—sink in less than three hours time. Most attribute the sinking to design flaws, misjudgements in seamanship and sailing conditions. However, there has been some debate on the roles of metallurgy, the physical and chemical behaviours of metals, and the icy waters of the North Atlantic on the destruction of the Titanic.
Researchers examining the role of the ship’s construction in the sinking have focused on two areas: the quality of the ship’s steel hull, and the quality of the wrought iron rivets used to hold the steel panels together.
In 1995, Robert Gannon, a writer for Popular Science magazine, had a firsthand look at a piece of the Titanic’s hull that was salvaged. He described the edges of the hull to resemble broken china and did not show any signs of bending. In comparison, high-quality ship steel typically has more give (ductility).
To understand how the steel hull would have reacted when the ship collided with the iceberg, researchers have tested the recovered pieces of steel using an impact testing method known as a Charpy test. With this test, researchers were able determined the strength of the steel hull at the time of the accident (at the same temperature of the ocean waters) relative to the impact it would have had to sustain during the accident.
Researchers Felkins et al., 1998 and Leighly Jr. et al., 2001 found the ductile-brittle-transition temperature— when materials change from being ductile to brittle—of Titanic’s steel hull to be well above room temperature, compared to a sub-zero transition temperature for modern steel. Since the Titanic was sailing in -2˚C water this would mean the ship’s hull was extremely brittle at this temperature. Both believe the cold waters contributed to brittle fracture of Titanic after it hit the iceberg. More importantly both research teams have concluded the brittle fracture of steel hull plates contributed to its sinking.
What brittle fracture means is that when the Titanic collided with the iceberg, it pretty much just broke—rather than deforming and absorbing some of the energy from that impact. Brittle fracture is like cracking an egg open. Brittle materials require little energy to break them but ductile materials require more energy to break them.
The image on the right shows a piece of the Titanic’s hull that has been subjected to the Charpy test. The piece one of the right comes from modern high-quality steel, also subjected to the Charpy test. The hull sample is extremely brittle—looks like it just snapped in half. The modern steel sample is ductile and bends without breaking into pieces.
Image from Gannon, 1995
What’s different about the steel used in the Titanic and modern steel is the chemical composition. Felkin et al., 1998 reported that modern steel (ASTM A36 steel) has a higher manganese content and a lower sulfur content (i.e. a higher Mn:S ratio), which reduces the ductile-brittle transition temperature substantially. Thus modern steel would behave in a ductile manner at the temperature of the ocean waters.
Although the Titanic was constructed with steel that’s inferior in mechanical properties to the commercial steel available today, Felkin et al., 1998 concluded that the steel used was probably the best that was available at the time. But clearly it would not be acceptable for ship constructions today.
Aside from the steel hull Foeke, 1998 also examined the quality of the wrought iron rivets—a mixture of iron and iron silicate extruded into a layered structure. It’s interesting that in this 1998 report, Foeke concluded that given the knowledge available to engineers at the time of the ships construction, no apparent metallurgical mistakes were made in the construction of the Titanic. However he published in 2009 that poor manufacture methods were used because the Titanic rivets contained almost three times more slag particles (dendrites of iron oxide) than expected for rivet-quality wrought iron, and also contained large slag particle sizes (100 to 1000+ microns). In fact, he reported his analysis of construction records revealed the company had purchased sub-standard wrought iron rivets. Foeke believes decohesion along the iron-slag interface compromised the strength of the rivets, and was a critical factor in the Titanic’s sinking.
Recent publications may have focused on rivets as the sole cause of Titanic’s sinking. But as Leighly et al. pointed out, if sinking of the Titanic was just caused by fractured rivets which held the hull plates together, then the plates would not fracture when the iceberg gauged the hull. More importantly, the hull of the ship showed signs of brittle fracture. So with all the evidence available it is likely that a combination of metallurgical issues—in the hull and rivets— influenced how the ship was damaged by the iceberg.
Felkins, K., Leigh, H., & Jankovic, A. (1998). The royal mail ship Titanic: Did a metallurgical failure cause a night to remember? JOM, 50 (1), 12-17 DOI: 10.1007/s11837-998-0062-7
Foecke, T.J. (1998). Metallurgy of the RMS Titanic (NISTIR– 6118). NIST Interagency/Internal Report Retrieved from http://www.nist.gov/manuscript-publication-search.cfm?pub_id=852863
Foecke, T., & Hooper-McCarty, J. (2009). Quantitative Metallography And Microanalytical Analysis Of Particles In Iron Rivets Recovered From The Wreck Of The RMS Titanic Microscopy and Microanalysis, 15 (S2) DOI: 10.1017/S1431927609099127
Gannon, R. (1995, February). What really sank the Titanic: why did the ‘unsinkable’ ship go down only three hours after hitting an iceberg? A new scientific investigation answers an 80-year-old mystery. Popular Science. Retrieved from http://www.popsci.com/archive-viewer?id=c9F4ebT55DYC&pg=49&query=what+really+sank+the+titanic
Leighly Jr, H., Bramfitt, B., & Lawrence, S. (2001). RMS Titanic: A Metallurgical Problem Practical Failure Analysis, 1 (2), 10-37 DOI: 10.1361/152981501770352978