Optical evaluation

In the Brinell, Vickers and Knoop hardness testing methods, the hardness value must be determined by optical evaluation of the diagonals of the hardness test indent.

There are various options for optical evaluation of the indentation:

1. Evaluation by measuring magnifier

The simplest way to measure and subsequently evaluate a hardness test indent is to use a measuring magnifier (loupe). Measuring magnifiers are available with different magnification strengths, scales and accuracies.

Three steps are required to evaluate a hardness test indent and obtain a measurement result:

First, the two diagonals of an indentation must be measured using the measuring magnifier.
Then the average diameter of the two diagonals must be calculated.
Finally, the respective hardness value can either be calculated using a formula or taken directly from the relevant standard hardness table for the respective test method.

Example values from a Brinell standards table (DIN EN ISO 6506):

Test procedure used: HBW 10/3000
Relevant ball diameter: D = 10 mm
Relevant force-diameter index: 0.102 x F/D² = 30
Calculated average indentation diameter of the indent: 2.95 mm

Resulting hardness value: 429 HBW 10/3000

2. Evaluation by focussing screen and calliper gauge

With older hardness testing machines, the hardness value can be evaluated directly on the machine using a focussing screen and a calliper gauge. This saves the operator from having to take the roundabout approach of determining the measured value from a table or calculation.

Admittedly, the operator must still always intervene in the measurement in this case.

Determination by means of focussing screen and calliper gauge is conducted as follows:

Firstly, the first diagonal of the hardness test indent is measured with the calliper gauge by positioning the rulers.
Then the entire calliper gauge can be rotated by 90° to measure the second diagonal length.
Once the two diagonals of an indent have been measured, the hardness testing machine automatically calculates and displays the correct hardness value.

3. Evaluation by camera system – state of the art

In the case of evaluation by means of camera system, as with focussing-screen devices, the operator positions measuring lines instead of rulers on the display at the indent and then the hardness value is determined by the hardness testing machine.

Using a camera system, however, it is also possible to conduct the evaluation fully automatically, with the camera system positioning the measuring lines itself and the operator no longer needing to do this. This facilitates a further reduction in operator influence.