Abstract:The industrial Brinell hardness test has been in common use for over 100 years. The test is defined by standardized procedures stating that the Brinell hardness number is proportional to the test force divided by the surface area of the indentation. The test procedures require that the surface area be determined by measuring the indentation diameter after removing the test force. This measurement is usually made using an optical microscope, but without having a physical definition of the indentation edge. This paper proposes a physical definition of the indentation edge such that the Brinell indentation diameter can be unambiguously measured.

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The measurement of hardness has been and continues to be of significant importance to many of the world's manufacturing industries. Conventional hardness testing is the most commonly used method for acceptance testing and production quality control of metals and metallic products. Instrumented indentation is one of the few techniques available for obtaining various property values for coatings and electronic products in the micrometre and nanometre dimensional scales. For these industries to be successful, it is critical that measurements made by suppliers and customers agree within some practical limits.To help assure this measurement agreement, a traceability chain for hardness measurement traceability from the hardness definition to industry has developed and evolved over the past 100 years, but its development has been complicated. A hardness measurement value not only requires traceability of force, length and time measurements but also requires traceability of the hardness values measured by the hardness machine. These multiple traceability paths are needed because a hardness measurement is affected by other influence parameters that are often difficult to identify, quantify and correct. This paper describes the current situation of hardness measurement traceability that exists for the conventional hardness methods (i.e. Rockwell, Brinell, Vickers and Knoop hardness) and for special-application hardness and indentation methods (i.e. elastomer, dynamic, portables and instrumented indentation).

Significant differences in Brinell hardness results have been observed worldwide, largely due to the curved surface at the indentation edge making it difficult to measure its diameter. The indenter/material contact boundary under the test force should be the basis for the Brinell indentation diameter; however, the contact boundary cannot be observed using an optical microscope after the indenter is removed as is required by the test methods. Finite element analysis (FEA) models were used to develop a method to effectively determine the location of the indentation contact boundary after unloading, allowing the indentation diameter to be physically defined and measured.