Bag and box handles that change colour when they are being overloaded
There is this tape / plastic that changes colour under stress. It should just be graded so the right grade is used on bags that can take different weights.
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Handy for bags but this stuff could also save lives if they get it right -https://www.sciencedirect.com/science/article/abs/pii/S0928493114005128
Found it, this is the stuff.