Instrumented Hardness Measurement – Indentation Depth Measurement with Modified Contact Foot
Instrumentierte Härtemessung, Eindringtiefenmessung mit modifiziertem Tastfuß | A service provided by |
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Instrumented hardness measurement – Indentation depth measurement with modified contact (touch) foot
Factors influencing indentation depth measurement
Indentation depth measurement methods, such as recording macro hardness measurement, require the simultaneous measurement of the load F and the indentation depth h in the material. While the load measurement is influenced in particular by compliance, e.g. the expansion of the test frame (see also: tensile test compliance), various influencing factors come into play in indentation depth measurement. These are based on the type of path measurement, which can be based on the determination of the absolute or relative path. Absolute path measurement involves determining the indentation depth relative to a fixed, unchanging position of the measuring system.
In relative distance measurement, a contact foot establishes contact (force-controlled) with the surface of the test specimen, which is assumed to be sufficiently hard or rigid and therefore does not exhibit any deformation. Once contact has been established by the contact foot, the indenter positioned inside the contact foot also moves onto the test specimen surface in a force-controlled manner. Upon contact (Fig. 1a), the relative displacement is ‘zeroed’ and the test procedure begins by simultaneously recording the path h between the foot and the indenter at equal time intervals (Fig. 1b).
This type of indentation depth measurement causes problems when testing plastics at higher temperatures or relatively soft elastomers. In these cases, the contact foot can penetrate the comparatively soft test specimen surface and distort the path measurement by changing the reference point (Fig. 1c). The measured path signal h therefore becomes smaller and influences the material values of the hardness as well as the creep and relaxation.
| Fig. 1: | Indentation depth measurement: starting position a), measurement against hard surface b) and measurement against soft surface c) |
An alternative would be to equip the measuring system with an additional independent path measurement channel. The additional path sensor records the movement of the contact foot hex (Fig. 2a) relative to any fixed point, enabling a correction of the indentation depth as shown in Fig. 2. For this purpose, the internal path in the measuring head hin is added to the path measurement signal from the external sensor, allowing the indentation of the contact foot to be corrected for measurement purposes. However, since other effects, such as the bulging of the surface next to the indenter, can occur that cannot be compensated for metrologically, additional design changes must be made to the contact foot. This is necessary when testing soft plastics at elevated temperatures or elastomeric materials.
| Fig. 2: | Incorrect indentation depth measurement due to penetration of the contact foot a) and correction of the path signal using an external sensor b) |
Development of a modified contact (touch) foot
By modifying the design of the original contact foot with two set-on cutting edges (Fig. 3a) to create a flat surface (Fig. 3b), it was possible to significantly reduce the surface pressure (see Hertzian pressure), thereby minimising indentation and bulging next to the impression in soft plastics, especially at elevated temperatures. However, these measures are not sufficient for elastomers, so that the local pressure had to be further reduced with an additional adapter plate (Fig. 3c).
| Fig. 3: | Original contact foot from ZwickRoell GmbH & Co. KG] a), modified contact foot b) and use of the adapter plate for elastomers c) |
Creep tests on filled and glass fiber-reinforced polypropylene materials
The success of these design and measurement changes to the temperature-controlled, recording macro hardness measurement is demonstrated, for example, in creep experiments on filled and glass fibre-reinforced polypropylene (PP) materials (Fig. 4).
| Fig. 4: | Creep tests on polypropylene with 42 wt.-% chalk and PP materials reinforced with 20, 30 and 40 wt.-% short glass fibres (dashed lines correspond to the path measurement with modified contact foot) |
Within the temperature range, the curves obtained reflect the relevant experience that the greatest creep tendency occurs in chalk-filled PP materials and that the creep tendency of the materials decreases with increasing glass fibre content. If the original contact foot is used and no external sensor is employed, the incorrect distance measurement leads to a decrease in creep for all materials examined above 40 °C, which contradicts reality. When using the modified contact foot and the penetration depth correction via the external sensor, a further increase in creep tendency is recorded with increasing temperature (dashed line in Fig. 4), which corresponds to engineering experience with these materials.
See also
- Instrumented hardness test – method & material parameters
- Instrumented hardness measurement – creep
- Instrumented hardness measurement with tempering
- Instrumented hardness measurement – relaxation
- Hardness
References
Introduction to the method:
- Bierögel, C., Schöne, J., Lach, R., Lach, R., Grellmann, W.: Methodische und messtechnische Aspekte der instrumentierten Härteprüfung von Kunststoffen. In: Christ, H.-J. (Ed.): Fortschritte in der Werkstoffprüfung für Forschung und Praxis. 31. Vortrags- und Diskussionstagung Werkstoffprüfung 2013, 28.–29. November 2013, Neu-Ulm, Proceedings pp. 265–270 (ISBN 978-3-514-00806-9; see AMK-Library under M 26)
- Bierögel, C., Schöne, J., Lach, R., Grellmann, W.: Bewertung des temperatur- und zeitabhängigen Verhaltens von Thermoplasten und Elastomeren mittels der instrumentierten Makroeindringprüfung. In: Grellmann, W. (Ed.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderung an die Kennwertermittlung. "Werkstoffprüfung 2011", December 1 and 2, 2011, Berlin, Proceedings pp. 285–292 (ISSN 1861–8154; ISBN 978-3-9814516-1-0; see AMK-Library under A 13)
- Koch, T., Seidler, S.: Instrumented Hardness Values. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein. Volume VIII/6A3, Springer, Berlin (2014) 379–385, (ISBN 978-3-642-55165-9; see AMK-Library under A 16)
- Schöne, J.: Mehrparametrige Bewertung des temperaturabhängigen Eindruckverhaltens an Kunststofen mittels eines messtechnisch erweiterten, registrierenden Makrohärteprüfsystems. Martin-Luther-Universität Halle-Wittenberg, Promotion 2019, Shaker Publishing 2020 (ISBN 978-3-8440-7183-2, see AMK-Library under B 1-30)