Mobile Hardness Measurement
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Mobile hardness measurement
General information
For conventional or recording hardness testing, laboratories typically use stationary table-top testing systems or machines. Whilst in macro-hardness testing of plastics – unlike with metallic materials – the effort required for specimen preparation is comparatively low, the characterisation of hardness at the micro- or nano-scale sometimes necessitates complex preparation of the specimens. These test specimens, which are often very small, must be embedded, grinded and polished in order to obtain meaningful hardness values or characteristic functions.
To assess the mechanical properties of components or plastics components, test specimens (see: test pieces) can be extracted from locations relevant to damage as part of damage analyses and subsequently examined using hardness testing methods. For components in practical use, this procedure is not permitted, as it would permanently impair their functionality or operational safety. On the other hand, information on the material condition and the predicted service life is urgently required, as the properties of plastic components can change significantly as a result of ageing or weathering processes. Hardness testing, utilising the TABOR relationship, represents a simple test method for monitoring and assessing the material condition of pipes, containers, wind turbine rotors or car tyres, and, for certain materials, allows conclusions to be drawn regarding the modulus of elasticity or the strength.
Mobile hardness testing
For this purpose, a portable or ambulatory hardness measurement technique is required which does not require any pre-treatment of the surface of the component, is robust, is not dependent on surface roughness, and exerts only a minimal subjective influence on the characteristic values determined. For metallic materials, there are various portable hardness measurement techniques involving static (quasi-static) or dynamic (impact) loading, based on a wide variety of operating and evaluation principles; however, hardness conversion is usually required to ensure comparability. Due to the specific characteristics of plastics, there is no single measurement technique that can be used for all plastics; consequently, a wide variety of testing instruments, based on different measurement principles, are employed.
The mobile dynamic hardness measuring devices are based on an impact loading with a specified kinetic energy content and a defined distance from the test specimen surface. In dynamic-plastic testing methods, the deformation (indentation diameter) is measured after the impact, whilst in dynamic-elastic methods, the measure of the loss of kinetic energy upon rebound of the indenter is carried out.
The oldest mobile dynamic hardness measuring devices capable of producing a plastic, measurable indentation in the test piece are the Poldi hammer and the Baumann hammer, both developed around 1900. In the Poldi hammer, a striking pin fitted with a hardened steel ball indenter 10 mm in diameter is driven simultaneously into both the test material and the reference material via a guide sleeve (Fig. 1a). By comparing the two indentation diameters, the hardness (converted to Brinell hardness) of the cast or forged test piece can be determined using the known hardness of the reference material. During the test with the Baumann hammer, the measuring device—which features an integrated tension spring—is pressed against the surface of the test specimen; this tensions the spring and, at a defined contact force, triggers the striking pin with a constant impact energy of 4.9 J. The indentation made by the indenter, which has a diameter of 10 mm, is measured using a magnifying glass and converted into Brinell hardness values (Fig. 1b) [1].
| Fig. 1: | Poldi hammer manufactured by BAQ GmbH, Braunschweig (a) and Baumann hammer manufactured by pce Deutschland GmbH, Meschede (b) |
Dynamic hardness testing techniques also include what is known as rebound hardness, in which the hardness value is determined from the rebound height of the indenter. The drop weight (Shore scleroscope) or the pendulum hammer (durometer), fitted with a rounded diamond tip or a steel ball, falls from a defined height onto the surface of the test specimen, rebounds, and the hardness can be read off a scale. The harder the test specimen, the greater the rebound height; however, as the results depend on the specific instrument used, the hardness values are only comparable within the respective material group. The hardness values correspond to a relative scale with a maximum value of 100 or 140, representing the maximum hardness value.
| Fig. 2: | Equotip 550 portable Leeb hardness tester manufactured by Proseq SA, Schwerzenbach, Switzerland |
The standardised LEEB rebound hardness test is a mobile dynamic hardness testing method in which a falling body with a hard metal tip and a defined energy content is dropped onto the surface of the test specimen. The elastic deformation resulting from the impact causes a loss of kinetic energy, which is determined non-contact by measuring the velocity before and after the impact. The measured Equotip hardness values can be converted into other hardness scales (Rockwell, Brinell or Vickers) (Fig. 2). The hardness values obtained depend on the impact energy, the modulus of elasticity of the material and the shape of the indenter.
Mobile static or quasi-static hardness testing techniques utilise the measurement principles according to Vickers, Brinell or Rockwell, involving measurements of the permanent plastic indentation, as well as methods under test load with evaluation of the total deformation, e.g. MARTENS hardness (see also: Martens) or UCI hardness. In these measurement methods, the test force is applied by applying compressive load to the measuring head or, for higher test loads, using clamps (Fig. 3a).
| Fig. 3: | Webster pliers WB20 (a) and UCI hardness tester AlphaDUR (b) from BAQ GmbH, Braunschweig |
The ultrasonic contact impedance (UCI) hardness method uses a Vickers diamond as the indenter, which is attached to the tip of an oscillating metal bar. As the penetration depth or the size of the indentation increases, the resonance frequency of this bar changes, with the change in frequency Δf representing the area of the indentation (Fig. 3b). By converting Δf, the UCI hardness can be determined; however, this depends on the measuring probe used, as well as the modulus of elasticity and the Poisson’s ratio of the material being tested.
The TIV (Through Indenter Viewing) method enables mobile Vickers hardness measurements to be carried out using a CCD camera, whereby the hardness indentation is measured fully automatically under load, providing visual information on the geometry of the indentation diagonals and thus on the quality of the hardness test under on-site conditions [2].
Applications for plastics (Barcol and BUCHHOLZ hardness testing)
Mobile hardness testing can also be carried out as recording measurements with simultaneous recording of the test load and the penetration depth in accordance with ISO 14577-1. By determining the MARTENS hardness (MH), such hardness testers can be used for both plastics and metallic test pieces [3].
The mobile hardness testers for plastics are adapted for use with different material groups. The most common SHORE hand-held hardness testers (see: SHORE Hardness) differ in terms of test load and the shape of the indenters, and are available for methods A, C and D. They are suitable for hard and soft elastomers as well as for hard and soft thermoplastic materials (Fig. 4) [4, 5].
| Fig. 4: | SHORE-C hardness testers manufactured by Zorn, Stendal (a), digital SHORE testers manufactured by Landtek, Padborg, Denmark (b) and by ZwickRoell GmbH & Co. KG, Ulm (c) |
The portable Barcol hardness tester in accordance with DIN EN 59 [6] is designed for testing glass-fibre-reinforced or hard plastics (e.g. wind turbine blades) and duromers. To determine the Barcol hardness, a steel cone indenter (26°, 0.157 mm diameter) is pressed into the surface of the test specimen via a spring system.
The penetration depth is measured directly by a load cell and the hardness is displayed as a dimensionless value on a scale from 0 to 100 (Fig. 5). A scale reading of 0 corresponds to the maximum penetration depth and thus to very low hardness, whilst a reading of 100 represents the maximum hardness, at which virtually no indentation is measured.
| Fig. 5: | Diagram and operating principle of the Barcol hardness tester (a) and the Barcol tester from Braive Instruments SA, Liège, Belgium (b) |
The Buchholz mobile testing method [7, 8] (see: BUCHHOLZ hardness) is used to determine the hardness of hard plastics or plastic coatings (see: plastic films & varnishes – surface technology) made from duromers. In this test, a pointed hard metal test wheel is pressed into the surface under test with a test load of approximately 4.9 N for 30 seconds. After the load is removed, the length l of the permanent indentation is measured using a measuring microscope (Fig. 6) and the BUCHHOLZ hardness is expressed as a relative value of 100 mm/l. This test method is also suitable for very thin layers in the range of 10 to 35 µm.
| Fig. 6: | Diagram and operating principle of the Buchholz hardness tester (a) and the Buchholz tester manufactured by Erichsen GmbH & Co. KG, Hemer (b) |
See also
- Conventional hardness testing
- Instrumented hardness testing
- Nanoindentation testing
- Plastic films & varnishes – Surface technology
References
| [1] | Michalzik, G.: Mobile Härteprüfung mit dem Kraft-Eindringtiefe-Verfahren, Quality Engineering 9 (1998) 60–62 |
| [2] | Frank, S., Lammerich, W.: TIV (Through Indenter Viewing) – Neue Möglichkeiten der mobilen Härteprüfung. DGZfP-Jahrestagung 2002, Weimar, Berichtsband 80-CD |
| [3] | Michalzik, G., Schneider, P.: Mobile Härteprüfgeräte einsetzen. Materialprüfung 39 (1997) 9, pp. 350–356 |
| [4] | ISO 868 (2003-03): Plastics and Ebonite – Determination of Indentation Hardness by Means of a Durometer mit einem Durometer (Shore hardness) |
| [5] | ISO 7619-1 (2010-10): Rubber, Vulcanized or Thermoplastic – Determination of Indentation Hardness – Part 1: Durometer Method (Shore Hardness) (withdrawn, replaced by ISO 48-4: 2018-08) |
| [6] | DIN EN 59 (2016-06): Glass Reinforced Plastics – Determination of Indentation Hardness by Means of a Barcol Hardness Tester |
| [7] | DIN 53 153 (1977-11): Testing of Paints, Varnishes and Similar Coating Materials – Buchholz Indentation Test on Paint Coatings and Similar Coatings (withdrawn) |
| [8] | DIN EN ISO 2815 (2003-10): Paints and varnishes - Buchholz indentation test |