Hybrid Methods
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Hybrid methods of plastic diagnostics
Limitations of conventional polymer testing
In addition to suitable material properties that can be used in design, knowledge of stress-induced material damage is an essential prerequisite for the dimensioning of plastic components and the practical use of plastics. In-depth information about damage processes and deformation mechanisms is required, especially from the perspective of consistent, optimal use of material resources. The damage-specific parameters determined under mechanical, media and thermal stress provide material developers and designers with information about relevant stress limits and users with information about the remaining service life or functionality of a component. On the other hand, damage and accidents attributable to the failure of plastic components prove that material characterisation is often too one-sided and that the safety and quality characteristics used to date are not yet sufficient.
From the perspective of modern materials development, there is therefore a need for material-descriptive, structurally or morphologically based parameters that provide information about stress limits depending on complex load conditions and, in conjunction with suitable material laws, enable the selection and dimensioning of plastics in a manner appropriate to the material. Conventional test methods such as tensile testing or bending testing cannot meet these requirements, as the parameters determined are not always structurally or physically justifiable. One example of this is microdamage (see micro-damage limit), which occurs in the non-linear viscoelastic deformation range (see: elasticity) and cannot be derived from the stress–strain diagrams (see: tensile test) determined.
Development trends in the application of mechanical methods
The development of innovative new plastics and composite materials tailored to specific requirements is currently giving rise to the following trends in the application of conventional mechanical testing methods:
- Qualification of basic mechanical tests for polymer testing to demonstrate stress-induced property changes that can lead to loss of ductility or a decrease in strength,
- Determination of material damage as a precursor to the ultimate failure of plastic components (see: plastic component and component testing) and
- Representation of damage kinetics and dominant structurally influenceable damage mechanisms to describe material limit states or diagnostic functions for damage mechanics.
Hybrid methods and Instrumentation of testing methods
Methodologically, there are two main approaches, which are sometimes used in combination:
- Application of hybrid experimental methods, i.e. the in-situ coupling of basic mechanical and fracture mechanics tests with non-destructive testing methods to increase the informative value of material properties and to formulate damage functions or limit values, e.g. by means of mechanodielectrometry, acoustic emission analysis, thermography, computed tomography (radiographic testing) or ultrasonic testing, and
- Qualification of basic mechanical experiments through instrumentation and the application of improved measurement and evaluation techniques such as video extensometry, laser extensometry or field measurement techniques in conjunction with event- and structure-related interpretation of the deformation phases of plastics, which simultaneously results in increased requirements for the experimental control of these tests.
The overview in the Figure shows that, regardless of the selected stress conditions for such hybrid experimental investigations, continuous recording of the stress parameters is necessary. The test methods shown in the sensor block as examples must meet the following requirements:
- sufficient sensitivity and applicability of the test method for the plastic to be investigated,
- sufficient structural sensitivity or selectivity for the dominant damage mechanisms, and
- the deformation behaviour of the plastic should not be influenced by the sensors used as far as possible.
| Figure: | Hybrid methods for polymer testing and polymer diagnostics |
Requirements for test specimens
In polymer testing, experimental investigations are carried out in the field of conventional mechanical testing on multipurpose test specimens in accordance with ISO 3167 and in the field of fracture mechanics testing (see: fracture mechanical testing) on CT-specimens (see: compact tension (CT) specimens).
Measuring marks (targets) are attached to the tensile test specimen (multipurpose test specimen), for example, to enable the use of the laser extensometry method. Up to 63 targets can be applied to the test specimen to determine the local deformation behaviour. The minimum target spacing is 1 mm. When 10 targets are applied, the integral stress–strain diagram and 9 local stress–strain diagrams for each measurement zone or a selected number of zones are obtained. The CT test specimen can be prepared in a similar manner, but a different laser extensometer (laser double-scanner or laser multi-scanner) is used.
Increasing the informativeness of hybrid methods
Parameter block
The parameter block contains, for example, the applied force or the cross-sectional stress. To determine the characteristic values on CT test specimens, the load-line displacement and crack expansion are required as direct measured variables. In addition to time and speed, the stress variables listed include temperature and air humidity (see: test climate), which have a decisive influence on the characteristic value.
Sensor block
Due to the limited energy release rate in plastics, the use of various non-destructive testing (NDT) methods is appropriate.
The sensor block lists the following examples:
- Acoustic emission
- Laser extensometry
- Ultrasound method
- Video thermography
- Dielectrometry and
- Volume dilatometry
The aim of this approach is to meet the increasing demands on the informative value of mechanical material properties from quasi-static test methods. A further aim is to enable an event-related interpretation of deformation and fracture behaviour, regardless of the approach and load conditions.
Although many non-destructive testing methods meet these requirements in principle, contactless and inertial sensor technologies should be preferred whenever possible. In hybrid methods, examples, the advantages and informative potential of such hybrid testing methods for polymer testing and diagnostics are illustrated using various examples.
See also
- Hybrid methods, examples
- Polymer diagnostic
- Non-destructive polymer testing
- Non-destructive testing (NDT)
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 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)
- Grellmann, W., Langer, B.: Methods for Polymer Diagnostics for the Automotive Industry. Materialprüfung 55 (2013) pp. 17–22 Download as 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
- Menges, G., Wiegand, E., Pütz, D., Maurer, F.: Ermittlung der kritischen Dehnung teilkristalliner Thermoplaste. Kunststoffe 65 (1975) pp. 368–371
- Schreyer, G. W., Bartnig, K., Sander, M.: Bewertung von Schädigungseffekten in Thermoplasten durch simultane Messung der Spannungs-Dehnungs-Charakteristik und der dielektrischen Eigenschaften. Teil 1: Schädigungseffekte während der mechanischen Belastung und Möglichkeiten der experimentellen Bewertung. Materialw. und Werkstofftechnik 27 (1996) pp. 90–95
- 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
- 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; siehe 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
- Markowski, W.: Ein neues Prinzip der Werkstoffprüfmaschine. Materialprüfung 32 (1990) pp. 144–148
- 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