Titanic material failure
1/28/2019 11:49:40 AM
The sinking of the Titanic is a tragedy that captivated this nation for over a century. And please don’t tell me that this is a movie spoiler, we’ve just rolled past the 21st anniversary of the release of James Cameron’s Titanic, and the 106th anniversary of the actual tragedy itself. The stories of many characters, both fictional and real, have been told. But one of the main culprits of the disaster was not highlighted on the silver screen, and that is the catastrophic material failure on the ship.
Before we dive in, here’s some background on the material concepts we’ll need to understand what happened. Some materials have ductile properties, and this means that they will absorb more energy prior to breaking than materials with brittle properties, and will undergo plastic deformation before finally breaking. Plastic deformation is the idea that the material will change permanently under load before actually failing. Brittle materials will not experience plastic deformation, and absorb a smaller amount of energy before fracture.
The Titanic sailed through very cold water; on the night it sank the temperature was just below freezing. The makers of the Titanic may not have considered the effects of temperature change in material behavior, but the steel underwent a ductile-to-brittle transition at this lower temperature in the water. This is the idea that metals of a particular crystal structure that were ductile at a higher temperature will become brittle at lower temperatures and fail under lesser loads as a result.
We know this because samples were retrieved from the wreck of the Titanic, and a Charpy test was performed on them. In our lab section of ME 330, which is Engineering Materials, we conducted a Charpy impact test on 1045 Steel, 2024 Aluminum, HDPE, which is a plastic often used in grocery bags, and PMMA, which is a clear thermoplastic that can be used as a glass alternative. It’s quite a fun test, we get to press a big button and a heavy pendulum comes down and impacts the specimen. It felt a little bit like this to me:
We tested all of the materials at different temperatures, one was ice water, 0° C, another at room temperature, and one more in boiling 100° C water. You can see our steel specimen to the left, and notice the shear lips on the boiling water specimen. It had a much more ductile response. In contrast, you can see the flat surface of brittle failure clearly on the freezing water specimen.
The specimen recovered from the Titanic exhibited the same type of behavior that we saw in our ME 330 lab. On the left of this image you can see modern steel that was subjected to the Charpy test alongside the Titanic steel. You can see the indication of ductile failure with surfaces that looked like they fought to stay together. On the right, you see the specimen taken from the Titanic, which has the sharp and clean failure surface which indicates brittle failure.
I won’t be painting the full picture of the failure if I don’t mention the rivets and the “watertight” compartments. The Titanic scraped along the side of the iceberg, and the rivets holding the sides together sheared off. The force from the collision with the iceberg also caused rivets to simply pop off. Like the steel of the hull, they too failed in a brittle mode for the same ductile-to-brittle transition temperature reason.
The Titanic was divided into sixteen main compartments that were separated by bulkheads, which are water-tight walls. These walls only extended a few feet above the waterline, and when water began to flood into the first few compartments near the front of the ship, the water could continue to spill over into neighboring compartments, effectively weighing the ship down. If the water had been able to free flow in the base of the Titanic, the ship would have still sunk, but at a slower rate that could have allowed for more passengers to be rescued.
We can use this tragedy to remind us of the tremendous impact our work can have. But, on a positive note, modifications were made to other ships after the Titanic disaster. The height of the bulkheads on other ships were extended to make the base compartments completely watertight. Additionally, many ships that were already fitted with double bottoms were given an extension of that extra protection through a double layer all along the hull. As engineers and designers, we are always learning and always improving, and this case is no exception.