On Christmas Eve, kid number 1 spotted a cracked in a window. It’s a double-glazed window, and inspection showed that the small, horizontal crack remained in the outermost pane. It to be perpendicular to the frame, about three-quarters that the way up one side. The beginnings are a mystery. It can have something to execute with recurring encounters with footballs, tennis balls (usually flying off the face of a cricket bat), rugby balls, frisbees and the like. Or it could be a thermal stress – this home window gets complete sun in the early morning.

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Over the next few days, this cracked grew, in rather an interesting way. It constantly grew in ~ night. We never saw any kind of discernable growth in it during daytime, but each morning the the next few days the crack was longer than it had actually been the night before. And also it do an astonishingly neat ‘wave’ pattern, as you’ll check out in the photo.


What is keep going here?

First, it’s worth noting that modern double-glazed windows are incredibly hard come break. The glass is toughened and the two panes together room able come absorb significant energy. Because that example, don’t count on being able come smash your way through one in the event of a fire, because that example. The video here will show you just what an effort it is! If you need to break v one, fight it tough near a bottom edge with something heavy and also pointed.

Second, crack act to concentration stresses at the crack’s point (where it end from gift a crack) which is why cracks, it is in they in pottery or rocks or file (think ‘tearing’ a paper of paper) have tendency to grow very easily. Once you gain them started, they have the right to often continue easily. Therefore there’s no surprise that the initial crack, a couple of centimetres in length, has actually now expanded itself under to the lower edge the the window.

But why the lovely tide pattern? here I’m speculating a bit. Oscillatory behaviour suggests a restoring pressure of some kind towards a main point. That is to say, when you room one side of the centre, you are pushed back towards the centre. In the special instance where we have a restoring force that is proportional come the street from the equilibrium point, we get simple harmonic movement – examples include an item bouncing up and also down ~ above the end of a spring, or a swinging pendulum. So perhaps when the crack moves towards an leaf of the window, the stresses brought about by the frame somehow result in a ‘force’ advertise the crack away from the edge and ago towards the centre. It then heads the way, dead on past the centre, and towards the other edge whereupon a similar thing wake up – it it s okay ‘pushed’ earlier to the center again. Whatever the reason, the result was pretty impressive.

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And why did that only thrive at night? more speculation here, however this can be down to thermal stress. The outer pane that glass would certainly drop in temperature during the night, yet then the early on morning sun would struggle it and also start increasing its temperature quickly. Yet the sun wouldn’t hit every the window at once. The shape of the house means that one next (the left together you look at the photo) would acquire the sun prior to the other side, i beg your pardon would continue to be shaded. For this reason we’d have at about 6 to be (before anyone it s okay up) half the home window facing the increasing sun and also heating rapidly, and fifty percent the window still in the shade of night. The thermal stress collection up can be significant here.