Here is a clever device, at my work in oil exploration and naval related equipment manufacturing, some time ago these washers became mandatory to meet production and safety regulations.
After having used them for a while I understand why, any bolt or nut they are used on will, simply said, never fall off after a correct torque tightening. As bolt or nut may try to unscrew itself, the surfaces will move against each other and expand in a way to lock everything even tighter and stop the process.
Of course, they allow unscrewing at any time by an operator and will 'click' off at a certain torque. They are use-once and replace.
Quite amazingly effective. Something to think about for special applications. 8mm bolt on the pictures (btw, a used washer, showing wear and rounding of locking surfaces after bolt being unscrewed).
