Rip, Tear, Crack, Split, Fracture

I ask a simple question and down the rathole we go . . .

SS Schenectady

Why is it so hard to start a tear in plastic film?
Once you have a tear started, it is easy to continue tearing.

I think it is derived from their long-chain molecular structure. Most plastics used in films will tend to deform when placed under tension, spreading the stress over a wider area. Once a tear is started, the stress becomes localised, and will exceed the limit quite easily. AndyTheGrump

Starting a tear, er, fracture. Figure by N. Bernstein & D. Hess, NRL

It's sort of the same reason that it's easier to pull apart a zipper [starting at one end but near impossible if you try and separate the whole thing at once]. Basically, at the onset, the force you apply is distributed throughout a line or plane. Once the tear is initiated, all of that force gets focused directly into the weak spot. Tearing is basically an example of fracture mechanics, and the details can get very complicated! A decent overview can be found here. Note that the ductile case of plastics works a bit differently than the brittle case. SemanticMantis
Part of the reason is already in the name: See Plasticity (physics). Just as when you're tearing on the cheese on your pizza, it just gets thinner and thinner, instead of breaking. — Sebastian

Then in the PDF from The University of Virginia, I found this:

Low temperatures can severely embrittle steels. The Liberty ships, produced in great numbers during the WWII were the first all-welded ships. A significant number of ships failed by catastrophic fracture. 

Ships failed? You're kidding, right? No, we're not. See the picture at the top.

Low Temperature Properties

Charpy Impact Resistance tells you how tough a material is. Nickel (the top line in the graph) doesn't much care about the temperature, but steel gets brittle when it gets cold. Fancy steel (the middle line) does better at all temperatures likely to be found on the surface of the Earth, but peasant steel (the line in the lower right quadrant) can become brittle in a mild West Coast winter, or more significantly, in the Middle of the North Atlantic.

Liberty ships were welded together, not riveted, as had been the standard practice up till that time. When they started breaking, it was assumed that the problem arose from the welding. The Brits called in Constance Tipper to look into the matter, and she found the problem was with the steel they were using. NewStatesman has a short story about her.

The SS Schenectady (shown at top) was built at the Swan Island Shipyard here in Portland, Oregon. Wikipedia has the story:

On 16 January 1943, she was moored at the fitting dock at Swan Island, in calm weather, shortly after returning from her sea trials. Without warning, and with a noise audible for at least a mile, the hull cracked almost in half, just aft of the superstructure. The cracks reached down the port and starboard sides almost to the keel, which itself fractured, jackknifing upwards out of the water as the bow and stern sagged to the bottom of the river. Only the bottom plates of the ship held. This was not the first of the war-built merchant fleet to fracture in this way – there had been ten other major incidents, and several more would follow – but it was perhaps the most prominent; it occurred in full view of the city of Portland, and was widely reported in the newspapers even under wartime conditions.