Hardness is an important material property describing a material's resistance to localized deformation from an indenter pressing or scraping against its surface. It is determined by measuring the indentation size realised on the tested material surface. This can be done by different hardness testing methods, like the Brinell, Vickers and Knoop scales. This project will investigate the indentation measurement phenomena for the above-mentioned scales to provide a better defined, more consistent, unified, and reliable measurement and traceability methodology to overcome the inconsistency between national metrology institutes (NMIs) and lower levels of indentation (hardness) measurements. The project outcomes will be used in the next generation of hardness definitions, instrumentation and standardisation to improve the accuracy of material testing in all engineering fields, including aerospace, automotive, health, industry and research and development.
In a comparison test the determinations of the modulus of elasticity were performed on samples from steel materials, brass, and aluminum with different tests of measurement (tensile test, bending test, laser induced surface acoustics wave, instrumented indentation test, impulse excitation of vibration, resonance method). Due to the unequal number of participants in the experimental groups, the statistical analysis was carried out of all values and the representative values of the groups (all groups with equal numbers of values). As a result, significant deviation across test groups and within the various test groups across.
The hardness test according Leeb is standardizing in DIN 50156 and ASTM A965. In 2013 will be published the ISO DIS 16859 standard for the Leeb hardness test. The Leeb hardness testing devices is built of guide tube with spring, impact device with impact body and magnet, support ring and the electronic device with digital display. During a hardness test, impacts the spherically intender (tungsten carbide or diamond tip) of the impact body the test surface from which it rebounds. In a definition distance from the test surface the impact and rebound velocities are measured. In the most devices is accomplished of a permanent magnet mounted in the impact body which, during the test, moves through a coil. These induced voltages are proportional to the respective impact and rebound velocities. The hardness value is definite as the quotient between rebound velocity and impact velocity multiplied by the factor 1000. The specifications of standard Leeb hardness testing devices in the standards are the mass of the impact body, radius, material an hardness of the spherical indenter, impact velocity, geometry of the support ring (only DIN 50156 and ISO DIS 16559) and the free fight length (only ASTM A 956) The direct calibration of the mass of the impact body, of the indenter (mass, material, hardness), geometry of the support ring are no problem from the measurement. So the direct calibration of the velocity was not possibly until now, in the MPA NRW constructed a built a special test equipment for the measurement of the impact velocity and the free fight length. The measuring of the velocity will be done with a laser vibrometer. In the report will be refer the conception and the technical realisation of the test equipment. The program for analyse of the measuring value with the uncertainty of measurement will explained. Also will be reported measurement results from the direct calibration of the velocity of Leeb hardness tester.
The specific hardness deviations of a Rockwell diamond indenter can be determined by a group standard, consisting of several indenters, using correction functions. In this paper the establishment of an HRC group standard in the PTB and comparative investigations of the group standards in the PTB and the calibration laboratory MPA NRW are reported. Above all the group standard is advantageous for maintaining the stability of the Rockwell hardness scales.
