Tensile Test Event-related Interpretation
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Tensile test event-related interpretation
Informative character of the parameters
The tensile test is primarily used to determine the stress–strain behaviour under uniaxial tensile stress. The parameters derived from the tensile test represent distinctive, visually recognizable points on the diagram and can be used for simple design applications, material selection and development, and quality assurance. These parameters are not suitable for more exacting dimensioning tasks or the design of complex plastic components, e.g., using continuum mechanics methods or the finite element method (FEM). The tensile test can be used to classify the macroscopic deformation behaviour of plastics and, for example, divide them into brittle, ductile, and highly ductile behaviour, whereby special characteristics such as the occurrence of yield stress can be highlighted.
Information about microscopic deformation processes and damage mechanisms, which are of interest to material developers and designers for statements on the damage mechanics of plastics and composites, cannot be derived directly from the tensile test and the associated stress–strain diagram.
For such statements, the tensile test must generally be combined with non-destructive testing methods, also known as hybrid methods. These simultaneously applied testing methods can then be used to detect microstructural deformation and damage mechanisms in the polymer matrix, the interface between the inclusion and the matrix, and the filler or reinforcing material (see also: fibre-reinforced plastics) and to assign them to a specific point in time or to a specific load or deformation. As a rule, these methods work selectively, so that not all defect mechanisms can be detected at the same time. If the detected damage or deformation mechanisms have been verified without error, e.g., by microstructural investigations, then they can be assigned to the deformation phases in the tensile test, and an event-related interpretation of the tensile test is available, which also allows more in-depth statements to be made about the damage kinetics.
Coupling of the static tensile test with acoustic emission analysis
Figure 1 shows the simultaneous recording of sound emission on polyamide 6 materials with different short glass fiber contents under identical test conditions in a tensile test.
| Fig. 1: | Acoustic emission in tensile testing of polyamide 6 with 10 wt.-% GF a) and polyamide 6 with 30 m.-% GF b) (abbreviation: PA 6) [1] |
It is evident that the damage kinetics for both materials are completely different and that the relative onset of damage, in relation to the strain, occurs at different points in time. Despite the lower fiber content, the energies emitted are significantly higher for the polyamide with 10 m.-% glass fibers, which allows conclusions to be drawn about the degree of damage but also about the coupling of the fibres to the matrix (see also: fibre–matrix adhesion). However, this hybrid test method does not provide any information about the damage or deformation behaviour of the plastic material.
Volume dilatometry in tensile testing
Volume dilatometry (Fig. 2) is a hybrid method that can be used to obtain information on damage and deformation behaviour even in ductile materials such as unreinforced polyamide 6 (abbreviation: PA 6) [2].
| Fig. 2: | Schematic representation of volume dilatometry in tensile testing |
The test specimen is placed in a glass box and fixed with clamps. The box can be opened to change the test specimen. On the top, the linkage is connected to the load cell, and on the bottom, the test specimen is loaded via the movable traverse. The elongation or strain is measured with an optical sensor via measuring marks on the surface of the test specimen. A fine capillary tube is attached to the side of the container as a connected vessel, which is used to record changes in volume during the tensile test. These changes in volume are caused by the formation of cavitations, microcracks, or an increase in density as a result of increasing orientations.
For the interpretation of the deformation phases of the tensile test, the defect density QD is therefore used as the reciprocal mean value of the density ρ of the test specimen (Fig. 3).
| Fig. 3: | Event-related interpretation with volumetric dilatometry |
Figure 3 shows that there is no change in defect density in the linear-elastic and linear-viscoelastic deformation range due to the constant volume. In the non-linear viscoelastic range (see: elasticity), irreversible deformations occur, resulting in microcavitations and an increase in defect density until the yield stress is reached. In the necking range and in the range of steady-state flow, QD decreases again due to the increase in molecular orientation. This process continues with a further increase in the orientation state, even in the hardening range, and ends with an increase in defect density at ultimate fracture.
See also
- Tensile test true stress–strain diagram
- Micro-damage limits
- Bend test and sound emission analysis
- Laser longitudinal–transverse scanner
- Polymer diagnostics
References
| [1] | Bierögel, C., Fahnert, T., Grellmann, W.: Deformation Behaviour of Reinforced Polyamide Materials Evaluated by Laser Extensometry and Acoustic Emission Analysis. Strain Measurement in the 21st Century, Lancaster (UK) 5.–6. September 2001, Proceedings (2001) pp. 56–59 Download as pdf] |
| [2] | Bierögel, C.: Hybrid Methods of Polymer Diagnostics. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 497–513 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |