Hybrid Methods, Examples
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Hybrid methods of plastic diagnostics, examples
The following section presents a number of examples illustrating the application of this testing methodology. Examples (2) to (4) are based on measurements carried out by Polymer Service GmbH Merseburg using equipment available within the company.
(1) Coupling of vibration analysis with dielectric measurement
In this example, tests on components (bushings) for the purpose of defect detection using
- vibration analysis to measure the natural frequency (eigenfrequency) and
- dielectrometry to measure the dielectric loss factor
were carried out (Fig. 1).
| Fig. 1: | The use of hybrid testing methods in the defect detection of plastics (Institut für Kunststoffkunde und -prüfung, Universität Stuttgart, Prof. Dr. G. Busse) |
Presenting the results of the vibration analysis and dielectric measurement separately did not yield any useful conclusions. By plotting the functional dependencies simultaneously, it was possible to distinguish between defect-free and defective bushings for quality control purposes.
(2) Coupling of the tensile test under static loading with the laser extensometry
Figure 2 shows an example of the evaluation of the local deformation behaviour of short-fibre reinforced polyamide materials under tensile loading using laser extensometry. The top left sub-image shows the laser extensometer used, manufactured by Fiedler Optoelektronik (Lützen), which has been adapted for use on a Z020 universal testing machine manufactured by ZwickRoell (Ulm/Einsingen). The manufacturer of the laser measurement system specifies a resolution of better than 1 μm for longitudinal strain and 0.15 μm for transverse strain.
The main component groups are the semiconductor diode laser with a wavelength of 670 nm, the rotating mirror scanner, the lens system, the photodiode (which acts as the receiver), and the data processing and evaluation unit.
| Fig. 2: | Application of laser extensometry to characterise local deformations |
The lower left sub-image shows that, in this case, 26 stripes (targets) have been applied to the test specimen using a film mask via screen or pad printing, with a freely selectable spacing of at least 1 mm. Depending on the number of stripes selected, n-1 stress (σ)–strain (ε) diagrams are then possible, or the measurement zones can be assigned freely across the surface.
The top-right sub-figure shows selected σ–ε diagrams. These are the integral σ–ε diagram for strips 1–26, the σ–ε diagram for strips 10–26, and the local stress-strain diagram for strips 12–13. These diagrams show very significant differences in strain behaviour, as the orientations (see: tensile test residual stresses orientations) of the test specimens affect the local strains.
The lower right sub-figure shows the integral strain and the maximum and minimum strains as a function of the glass fibre content (see: ashing method) for the polyamides studied. It can be seen that as the fibre content increases, the relative differences between the maximum and minimum strain increase, thereby increasing the heterogeneity of the materials. This naturally has consequences for the design of components.
(3) Coupling of sound emission and thermography with the tensile test on a statically loaded CT-specimen or multipurpose test specimen
The use of
- sound emission measurement techniques and
- video thermography
is also expected to broaden the scope of analysis regarding the assessment of local deformation behaviour and the energy-dissipating mechanisms at work.
| Fig. 3: | Application of acoustic emission (AE) analysis and videothermography to assess damage kinetics |
Figure 3 shows the setup of an acoustic emission (AE) and thermography measurement station for CT-specimens or multipurpose test specimens; however, specific experimental details cannot be discussed here.
In sound emission analysis, a sensor (narrow-band for high sensitivity, broadband for frequency analysis) is attached to the test specimen. This sensor can detect volume and surface waves generated under load, which are then amplified and filtered. The signals are then evaluated in terms of their energy content.
In video thermography, the surface of the test specimen is usually coloured black to achieve high emissivity. A thermal image of the test specimen’s surface is captured and stored by the sensor (infrared camera), which is cooled with liquid nitrogen. This allows for the measurement of heat generated by deformation or cooling effects.
Due to the need for a sufficiently high energy release rate, EA is only suitable to a limited extent for unreinforced thermoplastics, for example.
Consequently, coupled in situ techniques such as
- quasi-static tensile testing with CT-specimens and EA/thermography or
- quasi-static tensile testing with a multipurpose specimen and EA/thermography
are increasingly being used; these techniques selectively capture matrix processes on the one hand, whilst on the other hand allowing the characterisation of the composite component and its interaction with the matrix.
The right-hand sub-figure shows a description of the deformation and fracture behaviour, using PA6 reinforced with 5 % glass fibre as an example.
The upper sub-figure shows the σ–ϵ curve. The thermal emission is recorded later than the acoustic emission. The acoustic or thermal onset times, as well as the strain-related onset times shown here, can be used as material parameters.
If the experimental results of thermal and acoustic emission are related to local strain, this leads to the conclusion that, for a structure-related interpretation of the measurement results, it is necessary to carry out locally strain-controlled tensile tests.
The lower right sub-image shows the thermographic images taken in front of the crack tip in a notched multipurpose test specimen under various degrees of deformation. It can be seen that temperature differences of up to 8 °C can occur on the surface, even at low test speed.
(4) Other hybrid methods
- ICIT with AE
- In-situ Tensile Test in ESEM with AE
- In-situ tensile test in NMR
- In-situ ultramicrotomy
- Bend test and sound emission analysis
- Tensile test and sound emission analysis
- Tensile test, heat toning
See also
References
- 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)
- Grellmann, W., Langer, B.: Methods for polymer diagnostics for the automotive industry. Materialprüfung 55 (2013) pp. 17–22 (Download als pdf)
- Grellmann, W.: New developments in toughness evaluation of polymers and compounds 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 6)
- Osswald, T. A., Menges, G.: Materials Science of Polymers for Engineers. Carl Hanser, Mnich Vienna 3rd Edition (2012) (ISBN 978-1-56990-514-2; see AMK-Library under G 55)
- Roberts, J.: A critical strain design limit for thermoplastics. Materials & Design 4 (1983) pp. 791–793; https://doi.org/10.1016/0261-3069(83)90204-2
- Menges, G., Wiegand, E., Pütz, D., Maurer, F.: Ermittlung der kritischen Dehnung teilkristalliner Thermoplaste. Kunststoffe 65 (1975) pp. 368–371
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- Bierögel, C., Grellmann, W.: Evaluation of Thermal and Acoustic Emission of Composites by Means of Local Strain Measurements. ECF 9, European Confrence on Fracture, Varna 21.–25. September 1992, Proceedings Vol. 1 (1992) pp. 242–247
- Cowley, K. D., Beaumont, P. W. R.: Modeling problems of damage at nothes and the fracture stress of carbon-fiber/polymer composites: Matrix, temperature and residual stress effects. Composites Science and Technology 57 (1997) pp. 1309–1329; https://doi.org/10.1016/S0266-3538(97)00046-8
- Bartnig, K., Bierögel, C., Grellmann, W., Rufke, B.: Anwendung der Schallemission, Thermografie und Dielektrometrie zur Bewertung des Deformationsverhaltens von Polyamiden. Plaste und Kautschuk 39 (1992) pp. 1–8
- Bierögel, C., Grellmann, W.: Determination of local deformation behaviour of polymers by means of laser extensometry. In: Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin Heidelberg (2001) pp. 365–384, (ISBN 978-3-540-41247-2; see AMK-Library under A 7)
- Busse, G.: Hybride Verfahren in der zerstörungsfreien Prüfung (ZfP): Prinzip und Anwendungsbeispiele. In: Buchholz, O. W., Geisler, S. (Eds.): Herausforderung durch den industriellen Fortschritt. Verlag Stahleisen GmbH, Düsseldorf (2003) pp. 18–25, (ISBN 3-514-00703-9; see AMK-Library under M 11)
- Grellmann, W., Bierögel, C.: Laserextensometrie anwenden. Materialprüfung 40 (1998) pp. 452–459; https://doi.org/10.1515/mt-1998-4011-1206
- Markowski, W.: Ein neues Prinzip der Werkstoffprüfmaschine. Materialprüfung 32 (1990) pp. 144–148 https://doi.org/10.1515/mt-1990-320513
- 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) 56–59 Download as pdf