The longer diagonal of the Knoop indent is approximately seven times longer than the short one. Consequently, the longer diagonal can be measured comparatively accurately and the indent is more elongated than wide. The Knoop indenter only enters the material with approx. 1/30 of its longer diagonal (d), i.e. the indentation depth is minimal.
Therefore, the Knoop method is particularly suitable for thin and narrow specimens and for testing brittle materials (to prevent crack formation at the indent). According to the standard, the maximum permissible application time for the test force is 2 to 8 seconds (nominal time duration 7 s). The dwell time for the test force is 10 to 15 seconds (nominal time duration 14 s). If the dwell time is any longer, the duration in seconds must also be specified in the hardness value, e.g.: 870 HK 1/30 (dwell time of 30 s).
The test forces used in the Knoop method are very small. Consequently, the indents are also very small. Therefore, particular attention must be paid with this method to a test free from shock and vibration that could distort the measurement result.
In the Knoop test method, the indents must be positioned such that there is sufficient clearance from the specimen edge and between the individual indents. The minimum values to be observed in accordance with the standard can be found here.