Bend Test and Sound Emission Analysis
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Bend test and sound emission analysis
Introduction
In the quasi-static bending test, the sound emissions occurring during loading on test specimens notched on one side are used to evaluate the damage kinetics. The use of notched test specimens makes it possible to specify a defined distance between the acoustic emission source and the sensor position, thereby deriving reproducible conditions. However, a disadvantage is that, due to the increased notch stress and local deformation, it is not possible to provide information on stress and strain. Due to the notch effect and the influence of the sensor position on the recording of sound emissions, different experimental conditions apply and the test was carried out in accordance with ISO 178 [1].
Experimental
The investigations focused on polypropylene (PP) materials reinforced with short glass fibres in the range of 0 to 50 per cent by mass. Due to the non-polar nature of polypropylene, maleic anhydride (MSA) was used as a coupling agent to optimise the bonding of the fibres to the matrix (see: fibre–matrix adhesion). Kardelky and Schröder demonstrated in [2] and [3] that Echtblau achieves the best mechanical properties compared to other nucleating agents for PP/GF composites at a content of 0.01 mass percent. For this reason, Echtblau was used as the nucleating agent.
The notch radius of the metal blades and the notch depth were 0.3 µm and 2 mm. Additional changes to the standard resulted from the application of the acoustic sensor to the test specimen. Due to the specified support width of 62 mm, the notch--sensor distance was 30 mm. The Zwick Z020 universal testing machine (ZwickRoell GmbH & Co. KG, Ulm) was used for the tests at a crosshead speed of 10 mm/min at room temperature.
The 3-channel AMSY-4 measurement system ([VALLEN SYSTEME GMBH, Wolfrathshausen, Germany https://www.vallen.de/de/kontakt/]) with an AEP-3 preamplifier and an AE204A broadband sensor was used to perform the sound emission measurements. The bandwidths of the preamplifier and the sensor were 95–1000 kHz and 150–650 kHz, respectively. Impedance matching was achieved when applying the sensor to the test specimen surface by using beeswax as an adhesive agent, and a constant contact pressure was ensured by using a clamp.
Figure 1 shows the test setup for a multipurpose test specimen under bend loading with the sensor applied.
| File:Biegev SEA HybMeth Bild1a.jpg |
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| Fig. 1: | Test setup for acoustic emission measurements in the bending arrangement |
Due to the increasing deflection of the test specimens, the test had to be stopped when the test specimen halves came into contact with the bending stamp. An additional difficulty arose when applying the sensor to the test specimen. It was not possible to use gel for impedance coupling, as increasing deflection of the test specimen caused both movement of the sensor and movement on the abutment, resulting in friction effects. The problem of securely attaching the sensor to the test specimen was solved by using beeswax.
Example
The results for the peak amplitude values Ap and the event duration tED are shown as examples for the PP/20 in Figs. 2a–b. As explained above, once the maximum load has been reached and due to the stable crack propagation, the test specimen moves on the supports, resulting in the generation of unwanted sound emissions. For this reason, the systematic evaluation was only carried out up to the maximum load. The results that were not taken into account are highlighted in grey in Figs. 2a–b.
Similar to the tensile test with coupled acoustic emission analysis, the bending test could also be divided into areas of different acoustic activity. This is illustrated by the dotted line in Figs. 2a–b. Area I is characterised by negligible activity, whereas in area II amplitude values between 40–61 dB and events with a duration of up to 380 µs occur. The dependencies of the maximum peak amplitude values Ap,max and the hits on the glass fibre volume content φ'v are shown in Figs. 2c–d.
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| Fig. 2: | Functional relationship between load F, amplitude values Ap (a), event duration tED (b) and deflection f for PP/20, as well as maximum peak amplitude values Ap,max (c) and hits (d) |
The Ap,max values determined up to the maximum load and thus up to the point where stable crack propagation begins are only slightly influenced by the glass fibre volume content, which is due to the consistent experimental conditions, in particular the sensor–notch distance and the amplification of the acoustic signal. A somewhat different result was obtained for the number of acoustic emissions (hits). The damage induced during crack propagation (see: crack opening) and recorded with SEA therefore decreases at higher fibre contents. On the one hand, it can be assumed that the higher glass fibre volume content leads to the superimposition of the stress fields formed around the fibres and thus to a reduction in local stress peaks [4], resulting in fewer acoustic emissions being detected. On the other hand, multiple damage to the glass fibres is possible at lower fibre contents [5].
See also
- Tensile test and sound emission analysis
- Tensile test event-related interpretation
- ICIT with AE
- Push-out test
- In-situ tensile test in ESEM with AE
- Sound emission analysis
References
| [1] | ISO 178 (2019-04): Plastics – Determination of Flexural Properties |
| [2] | Kardelky, S.: Einfluss der Nukleierungsmittelart auf die Deformations- und Bruchmechanismen von medial beanspruchten PP/GF-Verbunden. Diplomarbeit. Martin-Luther-Universität Halle-Wittenberg (2002); (see AMK-Library under B 3-101) |
| [3] | Schröder, D.: Kombinierte Wirkung des Faservolumen- und Nukleierungsmittelgehaltes auf das mechanische Eigenschaftsniveau von PP/GF-Verbunden. Diplomarbeit. Martin-Luther-Universität Halle-Wittenberg (2003); (see AMK-Library under B 3-102) |
| [4] | Bierögel, C.: Zur Problematik der Schallemissionsanalyse an verstärkten Thermo- und Duroplasten. Dissertation. Technische Hochschule "Carl Schorlemmer" Leuna-Merseburg (1983) |
| [5] | Ehrenstein, G. W., Wurmb, R.: Verstärkte Thermoplaste – Theorie und Praxis. Angewandte Makromolekulare Chemie, 60/61 (1977) 157–214; https://doi.org/10.1002/apmc.1977.050600108 |