What I've Learned

The Kit Kat Bar jingle, as you may recall, says "Give me a break, give me a break, break me off a piece of that Kit Kat Bar."

The score lines in Kit Kat Bars are stress concentrations. That is to say, places in a material where stress is concentrated and cracks or breaks are more likely to occur.

In candy, stress concentrations are a good thing. In flying machines, not so good. A design flaw in the first commercial jetliner, the de Havilland DH 106 Comet, caused stress concentrations in the frame that in 1954, resulted in two Comets disintegrating in mid-flight. More than 50 people died.

The design flaw now seems laughable, a mistake no one would make. But at the time, it took a bit to figure out what was wrong. I'll sum it up in four words: windows with square corners.

A window with square corners has four points of stress concentration, points where cracks are likely to occur. You often see this in older brick buildings that have cracks in the brick work at the corners of window and doorways. In the de Havilland Comet, the repeated pressurization and depressurization of the cabin eventually caused cracks in the fuselage, beginning at the windows.

The solution was simple. Make the corners of the windows round, not square. By rounding the corners, less stress is concentrated there and fractures are less likely to occur. That's why airplane windows – and car windshields, for that matter – don't have square corners.

Boeing and Douglas, competitors to de Havilland, admitted that their engineers hadn't caught the design flaw either, and if the Comet hadn't been the first commercial jetliner, it might have been a Boeing or a Douglas aircraft that crashed because of square windows.

Stress concentration can not only down airplanes, it can sink ships.

Between 1941 and 1945, American shipyards built 2,751 cargo vessels called Liberty ships. They carried supplies, and sometimes troops, for the war effort. To speed the building process, shipyards welded the ships together instead of riveting them.

Some of the early Liberty ships developed cracks in the hulls and several of them sank. One suddenly broke completely in two.

At first it was thought that inexperienced welders were doing a shoddy job, but that turned out not to be the case. The problem was two-fold. One was the quality of the steel. Prolonged contact with the cold waters of the North Atlantic caused the steel to become brittle. The other was our old friend, stress concentration. Cracks were found, for example, where welded seams met the corners of rectangular hatchways.

When it comes to cracks, the difference between a riveted and a welded ship is profound. A hull made of steel plates riveted together tends to be less rigid than a welded ship and so can take more abuse. A crack in the hull of a riveted ship will run the length of the plate and stop. A crack in the hull of a welded ship will move through the welds and keep going. When cracks hit areas of steel made brittle by cold water, the results were disastrous.

The formula of the steel used in Liberty ships was changed, and designs were improved to reduce stress concentrations, both resulting in safer vessels.

I don't mean to imply that square corners are the only stress concentrations. There are many. The ends of a crack, itself, are powerful stress concentrations. That's why cracks tend to travel. Ding your windshield, a crack starts, and soon it's moved the length of the glass.

A happy use of stress concentration is lines of lesser thickness in chocolate bars that make them easier to break. Combine that with the lowering of emotional stress that dark chocolate is supposed to foster and you have an interesting situation: stress and anti-stress in the same substance. Break me off a piece of that.