The verification of the geometry of Rockwell indenters has been widely studied in the past and, at the present, it is one of the most important tasks (in the uncertainty budget of hardness measurements) in discussion in many international organizations involved in the hardness field. The new measuring system designed in IMGC and developed in cooperation with AFFRI has been characterized and the results of the calibration are presented. The uncertainty evaluation has been calculated following the ISO guide on uncertainty evaluation. At the end, the results of the intercomparison between the new instrument and the instrument used up to now in IMGC laboratory, completely different from the point of view of measurement methodology, are presented.

As a matter of fact, global standards for hardness testing are set by International Organization for Standardization. The ISO standard for hardness testing consists of three parts: Part 1 is for test methods, Part 2 is for testing machines, and Part 3 is for hardness standard blocks. JIS provides three independent standards, but they have perfect consistency with the three parts of the ISO hardness-testing standard. These standards prescribe that hardness testing machines first be subject to direct verification of their test force, indenter, testing cycle, and hardness indicator, and then that hardness must actually be measured using standardized blocks for indirect verification. This is a globally agreed fact. In this connection, the traceability or the uncertainty of Rockwell hardness has been a topic of international debate. This paper discusses our position on this issue as a manufacturer of hardness standardized blocks.

In a previous report, the authors investigated changes in hardness measurements when load rise time (LRT) was changed over a broad range in Vickers harness tests with loads of between 15 kgf and 150 kgf. As a result, we reported that hardness measurements varied according to the LRT value for every test load, while they remained almost constant between 15 kgf to 150 kgf if LRT was the same, although the indentation velocity of the indenter differed by more than three times between the two loads. It is very interesting that the considerable difference in indentation velocity due to varied test loads was not reflected much in hardness measurements. We, therefore, verified the effects of loading speed on hardness measurements theoretically from the strain rates of indentation deformation under the indenter and obtained some findings as follows. (1) It is necessary to define loading conditions to ensure the reliability of hardness blocks (2) The loading condition, namely the strain rate under indentation could be determined by the equation which was newly introduced by the authors. (3) According to this equation, loading conditions for general harness tests could be defined by loading time (LRT) regardless of load values. (4) Also considering that hardness measurements can vary according to load holding time, we may say that testing conditions can be "defined by time,". (5) Consequently, from the viewpoint of industrial practice, it would be reasonable to have rough criteria, that is, for how many seconds the test should be conducted, rather than to have scrutinized discussions on indentation velocity.

Based on the estimation of uncertainty the present requirements of the standard are discussed. Additional requirements needed for improving the reliability and the reproducibility are proposed. Especially the compliance of the machine affects strongly the parameters EIT and HMs in the macro range.

Methods for hardness measurement on elastomers are characterized by a set-up where indenters with various defined geometries are pressed under defined test forces into the surface of testing material. The hardness value represents the resistance of the material to the rigid indenter and is calculated from the indentation depth of the indenter in the to be tested material. The test methods IRHD and Shore are the most common methods used in the field of rubber hardness measurements. Appropriate hardness measuring instruments are spread world wide. A periodic verification has to be carried out to assure the accuracy of these measuring instruments. Direct and indirect verification methods are recommended to be used. For the indirect method rubber hardness reference blocks are used. This indirect verification method is especially appropriate for the daily check of hand held rubber hardness testers. On the other side the direct verification of test force, depth measuring system and indenter geometry is suited for the periodic calibration of rubber hardness testers. For the rubber hardness reference blocks their accurate hardness reference value and the long-term stability are the most important characteristics. In this paper the long-term behaviour of hardness of rubber blocks which at present are available on the market and are made by different producers is presented. For the measurements the Shore and IRHD hardness standard devices in the PTB have been used. In order to determine the relationship between hardness and time under the specified conditions a regression analysis of the obtained measurement results was carried out. The results of this analysis can be used for the improvement of quality assurance systems in the field of rubber hardness measurements.