How to… perform different indendation hardness tests on one machine
A Buehler EMEA Technical and Laboratory Manager explains how to use a universal tester for indentation hardness testing of materials.
Testing is routinely carried out to provide insights into a material’s mechanical properties and ascertain their suitability for the intended applications. Tests can include tensile and compression testing, impact resistance, fracture toughness, fatigue and hardness. These techniques involve the application of a load under dynamic or static conditions, with resultant behaviour noted and used to evaluate material property behaviour. Hardness or indentation testing evaluates how well materials resist deformation when a force is applied. It is an empirical test that has excellent correlation to direct measurement of mechanical properties such as ultimate tensile strength.
The hardness testing involves application of a load through a mechanical indenter onto a sample, which on retraction of the load from the testing surface, leaves an impression of the indenter. The impression/indent is then measured optically, from which a hardness value is determined as a function of indenter geometry and the applied load. These include Brinell, Vickers and Knoop hardness testing scales. Non-optical techniques are based on the penetration depth of the indenter after main load application compared to the pre-load application. This is commonly referred to as Rockwell testing. These techniques are generally carried out on dedicated machines for each test method, such that one would either have a Vickers or a Knoop tester, a Brinell or a Rockwell testing machine.
But it is possible for one machine to perform all testing methods, without needing to mechanically change indenters or load configurations. Machines with these functions are known as universal hardness testers, such as Buehler’s UH4000 series.
Machine design considerations
The UH4000 series offers testing capability ranging from 0.5kg force (kgf) to 750kgf. The machine has a patented eight-position turret designed
to hold needed combinations of indenters and objectives, allowing for faster test cycles without manual indenter/objective changes. For easy set
up, the machine is equipped with a laser alignment tool, that allows precise indent placement for larger components as well as work place illumination for correct position for Rockwell testing of a cylindrical heat-treated drive shaft. To test such as sample, a V-shaped anvil has been added on the stage for better support and further clamping. Rockwell testing is then carried out using DiaMet software.
To perform a Brinell test, one has to first take into account sample geometry. For flat
samples, these are placed on top of the testing table and arranged with the aid of the laser alignment tool. Using the overview objective to check the surface to be tested, a Brinell indentation cycle is carried out. After retraction of the indenter, the use of ring light system on the overview objective allows easy and accurate measurement of the ball impression.
Surface finish is an area more often overlooked but has a direct effect on the measurement accuracy. For example, in a Vickers test, a rough surface finish results in a higher hardness reading compared with a metallographically prepared surface. Standards by the International Organisation for Standardisation and American Society for Testing and Materials, and American Society for Testing and Materials, provide a guideline for sample thickness considerations and how close one can space the indentations on a test sample.
The reasoning behind indent spacings and sample thickness is to eliminate indentation material deformation effect around one indent. When an impression is applied on a material, dislocation build-up and resultant deformation causes it to be highly stressed or work hardened. If another impression is placed close to the deformation zone of the first indent, the material resistance to this deformation is affected, resulting in a higher reading compared to that of undisturbed material.
Weld testing based on ISO 9015 requirements have been incorporated within the device’s DiaMet software for easy set-up and evaluation of weld hardness. This is based on specifications relating to number of indentations on base metal, heat-affected zones and weld metals, geometrical requirements for fillet, and butt welds for either single or multi-pass welds. In one example, Vickers indentation testing was used together with automated X-Y stage for the UH4250. The testing involved scanning a contour of the sample geometry followed by placing a grid pattern around the fillet weld and extending into both the base metal plates. The contour scan together with grid pattern allowing indentations to be carried out at desired positions. Defects presented in the sample, such as blowholes, can be edited out so that indentations will not be placed around those regions. The resolution of different zones within the weld depends on the load selected and the indentation spacings. A lower load selection allows closer indent spacing, which results in better hardness maps.
Indentation fracture toughness
The fracture toughness measurement is an important property of hard or brittle materials, such as nitrides, carbides, oxides and other ceramics. A tool on the UH4000 series allows fracture toughness measurements of materials to be carried out using indentation methodology. This is based on Vickers impressions and applicable when specimens cannot meet the required geometry as per ASTM C1421, but also accounts for notched specimens propensity to give erroneously high values. The software also includes IFT equations such as Niihara, Anstis, Laugier, Miyoshi and Shetty for accurate calculation of fracture toughness.
During K1C test set-up, a load – typically above 1 kilogram-force – is selected that would cause cracks to form on a test piece and in a regular manner. The software allows method election based on median cracks including Niihara, Anstis, Laugier or Miyoshi equations, or Palmqvist such as Shetty and Niihara cracks. The elastic modulus of the material being tested must be entered in the software before testing is carried out.