The standard “EN ISO 2039 Plastics - Determination of hardness”, which is based on a European standard and later published by the International Standards Organization (ISO), consists of subsections published under the same title. Namely,
For the purposes of this standard, the impression hardness is the load on the impression maker divided by the surface area of the impression caused by the impression maker after a specified time of application of the load. It is expressed in Newtons per square millimetre.
This method consists of forcing a ball into the surface of a test specimen under a specified load. The depth of the impression is measured under load. The surface area of the impression is calculated from the depth. The ball indentation hardness is then calculated using the equation given in the standard.
The hardness tester used in the tests consists mainly of a frame with an adjustable platform equipped with a plate to support the test specimen, an indenter with associated fittings, and an equipment for applying the load without impact. The tester shall also be equipped with a device for measuring the depth of penetration of the indenter within 0,4 mm with an accuracy of plus/minus 0,005 mm. The frame shall not deform more than 0,05 mm under maximum load, the deformation shall be measured along the main axis of the applied force. The indenter shall be a hardened and polished steel ball. The ball shall not show any deformation or damage after the test. The diameter of the ball shall be 5,0 mm plus/minus 0,05 mm. A timing device with an accuracy of plus/minus 0,1 s is also used in these tests.
Each test specimen shall be a smooth, flat sheet or block of sufficient size to minimise edge effects during testing (e.g. 20 mm x 20 mm). The surfaces of the test specimen shall be parallel. A thickness of 4 mm is recommended. The supported surface of the test specimen shall not show any deformation after testing. If test specimens with a thickness of less than 4 mm are to be tested, it is possible to stack several test specimens on top of each other. However, the hardness values obtained on stacked test specimens and on single test specimens of the same thickness may differ.
Although every effort is made to ensure parallelism of the specimen, there are some cases where it is difficult to obtain perfectly flat test specimens, especially in injection-molded specimens of semi-crystalline thermoplastics. If such slightly curved test specimens are used, part of the measured impression depth actually corresponds to the distance traveled by the indenter as it presses the test specimen against the backing plate. This difficulty can be overcome by using a circular backing plate with a diameter of 10 mm plus7minus 1 mm. This diameter is also large enough for perfectly flat test specimens. It is also recommended that the test specimen be placed with the flatter side facing the backing plate.
Test samples are conditioned in one of the standard atmospheres specified in the ISO 291 standard prior to testing.
For materials with high creep and recovery, the time factors involved in applying major and minor loads have a significant effect on the measurement results. An alternative method of using the apparatus to give the hardness on the Rockwell hardness scale is included in the standard appendix.
This method is a method of determining hardness within specified limits for loading times by applying a constant minor load to a steel ball resting on the material to be tested, followed by a major load, and then returning to the same minor load. The actual measurement is obtained by subtracting the elastic recovery after a fixed time after removal of the major load from the total depth of penetration, minus the penetration due to the minor load. The Rockwell hardness number is derived from the net increase in depth of impression when the load in an indenter is increased from a constant minor load to a major load and then returned to the same minor load.
The standard test specimen shall be a flat sheet of at least 6 mm thickness. It shall have a suitable area to meet the requirements of this standard. The test specimen need not necessarily be square. It shall not show any traces of the indenter on the supported surface after a test.
When specimens of the minimum thickness specified in this standard are not available, the test specimen shall be constructed from a stack of thinner test specimens of the same thickness and the same material. However, the surfaces of the individual pieces shall be in full contact and shall not be separated by any surface defects (e.g. depression marks or burrs from saw cuts).
All indentations shall be made on one surface of the test specimen only. One test specimen is sufficient for determining the Rockwell hardness and for isotropic materials at least five measurements shall be made on each test specimen.
If the material being tested is anisotropic, the indentation direction shall be specified relative to the anisotropy axes. If results in more than one direction are required, test specimens shall be prepared so that at least five Rockwell hardness measurements can be determined in each direction.
Test samples are conditioned prior to testing in the atmosphere specified in the appropriate international standard for the material being tested or in one of those specified in the ISO 291 standard.
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