Bend Test and Light Microscopy

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Bend test and light microscopy

Methods for determining damage limits at deformation

The further development of hybrid methods for polymer diagnostics always pursues the goal of increasing the informative value of individual classic testing methods. Numerous examples from our own research work [1‒3] demonstrate the possibilities of quantifying micro-damage limits and the associated description of local microdeformation processes during the stressing of plastic components. The in-situ R-curve method under quasi-static loading was developed to elucidate the relationships between mechanical and fracture mechanical behaviour and microstructure on the basis of quantitative structure (morphology)–property correlations.

In-situ technique for recording crack resistance (R) curves under static bend loading

In order to determine fracture mechanics values taking into account physical crack initiation, in-situ observation of the deformation phenomena at the crack tip is necessary. One experimental option is to combine the quasi-static fracture mechanics test with classical light microscopy (Fig. 1). With the aid of this in-situ technique, crack resistance (R) curves can be recorded using single-sample measurement technology, as the respective applied force and the corresponding crack opening and crack growth values can be assigned to each load condition.

Physical crack initiation and the stretch zone

The evaluation of the microscopic images obtained in-situ makes it possible to determine qualitative and quantitative information about the respective shape of the crack tip in the propagation and blunting state and to specify a physical crack initiation value. In addition, statements about the deformation processes taking place are also possible. A particular advantage of the in-situ technique is the direct measurement of the extent of the stretch zone as a result of plastic deformation on the fracture surface. A disadvantage of the usually subsequent recording of the stretch zone height, e.g. with the aid of scanning electron microscopy, is the strong underestimation of the expansion due to the non-consideration of the elastic and viscoelastic deformation parts.

Test setup for recording in-situ R curves

For this hybrid method, a special bending test arrangement known as the ‘inverse’ bending test was developed at the Chair of Non-Metallic Materials (https://www.tuwien.at/mwbw/wwwt ) at the Vienna University of Technology [4]( https://www.tuwien.at/). The test arrangement ensures that the area of interest at the crack tip does not move out of the field of view of the light microscope, with the supports moving in the direction of the fixed bending punch (Fig. 1a). The test arrangement enables the recording of force–time and deflection–time signals and the in-situ video recording of the crack opening displacement δ and the stable crack growth Δa (Fig. 1b). The direct assignment of the recorded measured values enables the construction of crack resistance (R) curves in the form of J–Δa and δ–Δa curves, whereby the δ–Δa curves are preferable due to their direct assignment to the crack tip deformation processes (Fig. 1c).

Fig. 1:

Schematic test setup for recording in-situ R curves under quasi-static loading (a); crack tip of isotactic polypropylene (abbreviation: iPP) with determination of the direct measured variables δ and Δa (b) and example of an in-situ δ–Δa crack resistance curve (c)

Development trends in the evaluation of damage mechanisms at the crack tip

In addition to light microscopy, electron microscopy (SEM, ESEM) can also be used due to the geometric expansion of the stretch zone. The use of light microscopes for in-situ observation of crack initiation and crack propagation processes under quasi-static stress is relatively widespread due to its comparatively simple and cost-effective feasibility. There are two variants: either a light microscope is attached to commercial material testing machines (see Fig. 1a), or special in-situ testing devices are installed in a horizontal arrangement in a microscope, whereby a stereomicroscope is generally used in both cases. Such investigations into crack initiation and crack propagation behaviour allow the observation of micromechanical processes (see: micromechanics & nanomechanics) at the crack tip and conclusions to be drawn about the material-specific microdeformation mechanisms. The disadvantage of in-situ testing is that, due to the low thickness of the test specimens, it is limited to the area of plane stress state and comparatively low deformation rates. To evaluate the damage kinetics, quasi-static in-situ tensile tests can be performed on notched test specimens in an environmental scanning electron microscope (ESEM), whereby a schematic representation of a clamped test specimen and additionally applied acoustic emission sensors (see: in-situ tensile test in ESEM with SEA) are described in the literature by Zankel [5, 6] and Schoßig [7, 8].

References

[1]	Grellmann, W.: New Developments in Toughness Evaluation of Plymers and Composites by Fracture Mechanics. In: Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin Heidelberg (2001) pp. 3–26; ISBN 3-540-41247-6; see AMK-Library under A 7)
[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 978-1-56990-8066-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
[3]	Grellmann, W., Langer, B.: Methods for Polymer Diagnostics for the Automotive Industry. Materialprüfung 55 (2013) pp. 17–22 Download as pdf
[4]	Seidler, S., Koch, T., Kotter, I., Grellmann, W.: Crack Tip Deformation of PP-materials. In: Miannay D.; Cost, P.; Francois, D.; Pineau, A. (Eds.): Advances in Mechanical Behaviour, Plasticity and Damage. Volume 1. Elsevier Science Ltd, Oxford (2000) pp. 255–260; E-Book: ISBN 978-0-0805-5275-0
[5]	Zankel, A., Pölt, P., Ingolic, E., Gahleitner, M., Grein, C.: The Fracture Behaviour of Polymers – in situ Investigations in the ESEM. Imaging & Microscopy 7 (2005) 16–18; https://analyticalscience.wiley.com/content/article-do/fracture-behaviour-polymers---situ-investigations-esem (Access: 17.04.2026)
[6]	Zankel, A., Pölt, P., Gahleitner, M., Ingolic, E., Grein, C.: Tensile Tests of Polymers at Low Temperatures in the Environmental Scanning Electron Microscope: An Improved Cooling Platform. Scanning 29 (2007) 261–269; https://doi.org/10.1002/sca.20075
[7]	Schoßig, M.: Schädigungsmechanismen in faserverstärkten Kunststoffen – Quasistatische und dynamische Untersuchungen. Teubner Poublishing House, Wiesbaden (2010), (see AMK-Library under B 1–21) Content as pdf
[8]	Schoßig, M., Zankel, A., Bierögel, C., Pölt, P., Grellmann, W.: Acoustic Emission Analysis for Assessment of Damage Kinetics of Short-glass Fibre-reinforced Thermoplastics – ESEM Investigations and Instrumented Charpy Impact Test. In: Grellmann, W., Langer, B. (Eds.): Deformation and Fracture Behaviour of Polymer Materials. Springer, Berlin (2017) pp. 126‒149 (ISBN 978-3-319-41877-3; see AMK-Library under A 19)

Additional literature

Bierögel, C.: Bend Test on Polymers. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Munich (2022), 3rd Edition, pp. 133–143 (ISBN 978-1-56990-806-8; see AMK-Library under A 22)