Knoop testing procedure

General Information

In the Knoop hardness test, an optical method, the size of indentation left by the indenter is measured.

The larger the indent left by the indenter at a defined test force in the surface of a workpiece (specimen), the softer the tested material.

In order to determine the Knoop hardness, HK, (acc. ISO 4545) the pyramid-shaped rhombic indenter (longitudinal edge angle 172.5°, transverse edge angle 130°) is pressed into a specimen with a defined test load (between 1 gf and 2 kgf).

Calculation of the knoop hardness

The Knoop hardness (HK) results from the quotient of the test force (F in newtons N) and the projection surface (surface area) of the indent, calculated using the longer diagonal (d in millimetres) of the rhombic pyramidal indentation (see illustration below).

In contrast, the average of the two indentation diagonals of the square pyramidal indent is used for determining the hardness value in the Vickers method.

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.

Application of the test force

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.

Minimum distance of the test points

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.