Only yesterday afternoon, I had to figure out a way of detecting a crack
in a steel tube that is welded to a base plate, and is constantly being
stressed in one second cycles. Eventually the tube cracks at the welded
joint, and then finally bends and breaks. The trick was to detect when
the crack appeared, which could take days.

The tube was filled with water above the level of the joint and I made a
moisture sensor out of placing two thin wires spaced about 1mm apart on
the sticky side of some cello-tape. I then stuck some paper towell on
the top of this and trimmed it so that I could wrap it around the welded
joint. The paper acts as an insulator until water seeps out of the
crack. The paper towell is very absorbent and as it quickly soaks up the
minute amount of escaping water, the resistance between the wires
changes and raises an alarm.

The things you have to come up with :-)

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Best regards

Tony

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