Hard facts
Theory of hardness testing
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Since the Knoop indentation is rather long and narrow and less deep compared with broader and "rounder" Vickers indentations, the Knoop method is primarily suitable for testing very thin layers (e.g. aluminium foil) as well as hard and brittle materials, such as glass and ceramics. The deeper Vickers indentation can cause cracks around the edges of the test indent in these materials, which can be avoided with the shallower Knoop indentation.
From a geographical perspective, the Knoop method is used above all in the USA, while it is less common in Europe.
