Instrumented Hardness Measurement – Creep
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Instrumented hardness measurement – creep
General information
Unlike metallic materials, plastics exhibit a time dependence of their mechanical properties even at room temperature, which is referred to as viscoelasticity. Depending on the absolute level of stress or deformation, a distinction is made between linear viscoelastic and non-linear viscoelastic deformation (see: elasticity). In the case of long-term static stress, these time-dependent deformations are of considerable importance for practical applications, depending on the temperature and degree of stress. These plastic-specific deformation phenomena are referred to as creep (retardation) or stress relaxation.
Fundamentals of creep
Under constant static loading or stress, a spontaneous linear-elastic deformation is followed by a time-dependent increase in deformation, known as creep, which depends on the temperature and the level of stress (Fig. 1). If the stress conditions are limited to the linear viscoelastic range, the linear-elastic deformation will initially return without any time delay when the load is removed, and then the creep deformation will completely creep back (recreep) in a time-dependent manner. This reversibility does not occur in non-linear viscoelastic deformation due to the occurrence of initial microscopic damage processes.
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| Fig. 1: | Schematic representation of creep in plastics |
Depending on the load level and the test temperature, irreversible creep processes are observed which, under long-term stress, lead to creep failure or impermissible deformations, thereby causing dimensional deviations and loss of component functionality, with the absolute magnitude of these processes being largely determined by the type of plastics used.
To investigate such creep processes, tensile creep, flexural creep and creep compression tests are normally used, which are based on the generation of uniaxial stress states in the test specimen. The aim is to record the multi-parameter relationship between stress, strain and time as well as temperature, which is documented in the stress–strain–time diagram and should include test times > 104 h for dimensioning purposes.
Instrumented macrohardness measurement
If no test specimens are available for the plastic to be characterised (materials used in electronics, microsystems and medical technology) and results on the creep tendency of a material are required quickly, instrumented macro hardness measurement can also be used for this application (Fig. 2).
| Fig. 2: | Schematic arrangement of the instrumented macrohardness for creep experiments |
For this purpose, the universal testing machine applies a constant load F0 with a ramp function and then maintains it at a constant level for a specified time using force control (see tensile test control). Load control is necessary because relaxation occurs at the same time, which would reduce the test load acting on the indenter. Depending on the indenter used, the amount of force and the stress state caused, a linear elastic deformation or indentation depth initially occurs. Under the effect of the constant test load, an increase in the indentation depth is then recorded, which documents the creep behaviour of the material. After the holding time has ended, the load is reduced to zero in a load controlled manner, causing the elastic indentation portion to spontaneously return and subsequently a time-dependent creep of the indentation depth to occur.
With the connected temperature control chamber in the range from -100 °C to +100 °C, the test temperature can also be varied, ensuring a wide range of applications for instrumented macrohardness testing.
See also
- Instrumented hardness test – method & material parameters
- Instrumented hardness measurement – relaxation
- Instrumented hardness measurement – Indentation depth measurement with modified contact foot
- Hardness
References
- 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)
- Lach, R., Schöne, J., Bierögel, C., Grellmann, W.: Instrumented Macroindentation Techniques for Polymers and Composites – Mechanical Properties, Fracture Toughness and Time-Dependent Behaviour as a Function of the Temperature. Macromolecular Symposia 31 (2012) 125–131; https://doi.org/10.1016/j.polymertesting.2013.09.016