ICIT with AE

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ICIT with AE

Coupling of the instrumented Charpy impact test with damage-sensitive sound emission analysis

Introduction

A fundamental prerequisite for the targeted development of short-fibre-reinforced composites is an understanding of the strength- and strain-dependent deformation and failure mechanisms. From a materials science perspective, the following aspects are crucial for the properties of the composites:

• the influence of matrix properties (e.g. molecular weight, degree of crystallinity),
• the influence of the fibres (e.g. fibre content, orientation, distribution and geometry) and
• the effect of modifiers (e.g. stabilisers, impact modifiers and fibre–matrix adhesion promoters).

A range of additives is available to optimise these complex influencing factors, but there is a lack of precise knowledge regarding their effect on the interaction between the matrix and the fibres. This directly implies the need to evaluate glass-fibre-reinforced composites using modern diagnostic methods with a view to fully exploiting the material properties in terms of their application limits, as these methods provide more detailed material information than conventional testing procedures. By combining mechanical and fracture mechanics methods with non-destructive testing methods, it is possible to gain a deeper understanding of material behaviour. For example, changes in the interface between fibre and matrix, which can lead to variations in the damage mechanisms under mechanical stress, can be indirectly detected by combining the tensile test with sound emission analysis as a hybrid method of plastics diagnostics. Sound emission analysis, as a quasi-non-destructive testing method, in principle enables the evaluation of the damage kinetics of fibre-reinforced plastics (see also: fracture behaviour). Furthermore, with the aid of frequency analysis of the recorded sound emissions, it is possible to correlate the damage mechanisms occurring with characteristic frequency ranges [1–4].

A prerequisite for the reliable application of these methods is prior validation carried out on model materials or using in-situ testing methods (see: hybrid methods in polymer diagnostic). Sound emission analysis exploits the fact that the acoustic emissions generated by the sudden release of elastic energy stored in the material are directly related to the underlying causes, thereby enabling them to be attributed to the damage mechanisms.

Experimental method

A polypropylene reinforced with 20 % by mass of short glass fibres (abbreviation: PP/20) was investigated. Due to the non-polar nature of polypropylene, maleic anhydride (MA) was used as a coupling agent to optimise the bonding of the fibres to the matrix. Kardelky and Schröder demonstrated in [5] and [6] that, at a content of 0.01 % by mass, Echtblau achieves the best mechanical properties compared to other nucleating agents for PP/GF composites. For this reason, Echtblau was used as the nucleating agent.

The evaluation of the predominantly unstable crack propagation under impact loading was carried out at the ICIT at room temperature. The test was conducted and the recorded load–time diagrams (F–t diagrams) were evaluated in accordance with the accredited test procedure MPK-ICIT “Testing of Plastics – Instrumented Charpy Impact Test – Procedure for Determining the Crack Resistance Behaviour using the Instrumented Impact Test” [7]. For the test in accordance with the standard, test specimens with dimensions of 80 x 10 x 4 mm³ (L x W x H) are used. The introduction of the notches was carried out using a manual notching device, whereby a metal blade (razor blade) is pressed into the test specimen with a constant feed rate. The metal blades used produced a notch radius of 0.3 µm and the notch depth a was 2 mm, which corresponds to a ligament length (W–a) of 8 mm and a notch depth-to-specimen width ratio (a/W ratio) of 0.2 (see: notch geometry).

The test was conducted under defined environmental conditions (23 °C and 50 % relative humidity), which were maintained by storing the test specimens for 16 hours and using room air conditioning. The test was carried out using an instrumented pendulum impact tester (see: impact loading pendulum impact tester) with a work capacity of 4 J at maximum drop height. Force measurement is achieved using semiconductor strain gauges attached to the hammer head, which are arranged in a Wheatstone bridge circuit. The path was measured via double integration in accordance with Newton’s second law, whereby the velocity of the pendulum hammer is first obtained as a function of time, and following a further integration, the deflection of the test specimen is obtained as a function of time. The force F was recorded using the Yokogawa DL 1620 digital oscilloscope (YOKOGAWA DEUTSCHLAND GMBH) and the deflection f was obtained by integrating with respect to time t. The in-house WinICIT-software programme [8] was used to record and evaluate the F-t diagrams.

In accordance with the test standard used, the span s = 40 mm and the test speed v of 1.0 m/s was achieved via a pendulum hammer deflection of 40°, corresponding to a pendulum hammer length of 220 mm.

Due to the experimental requirements, the notch–sensor distance is 30 mm with a span of 40 mm. The acoustic sensor was applied directly to the test specimen and, to avoid one-sided clamping of the test specimen against the support, no clamp was used. For this reason, beeswax had to be used as a coupling agent, as this was the only way to ensure a secure hold for the acoustic sensor. Figure 1 shows the test setup of a test specimen positioned on the support with the sensor applied.

A – instrumented pendulum hammer with a 4 J impact energy at maximum drop height B – specimen C – attached acoustic sensor using beeswax as the coupling medium

Fig. 1:

Experimental setup for coupling the ICIT with the AE

As a result of investigations carried out to optimise the frequency analysis, the inertial load was successfully filtered out of the measurement signal.

To carry out the sound emission testing, the AE 204A broadband sensor was connected directly to the Yokogawa DL 1620 digital oscilloscope, and the force triggers enabled time-synchronous recording of the damage-sensitive sound emissions. The sensor’s bandwidth was 150–650 kHz.

Example from polymer diagnostics

Assessment of the damage kinetics for PP/20

Figure 2a–b shows the electrical output voltage U of the acoustic sensor and the result of the wavelet transform, together with the load–time diagram (F–t diagram) for PP/20.

Fig. 2:

Recorded electrical output voltage U (a) and plot of the frequency characteristics (b) with the load–time diagram for PP/20 [9]

For PP/20, an elastic-plastic material behaviour with unstable crack propagation and low crack propagation energy was observed. This can be seen in Figure 2a in the linear increase in load F up to F_gy and the sharp drop once the maximum load F_max is reached. The acoustic emissions are recorded at different times depending on the glass fibre content. At the force maximum, i.e. the onset of unstable crack propagation, increased acoustic activity is recorded, some of which can also be attributed to the crack propagation range. The elastic energy stored in the test specimen is thus converted into mechanical and acoustic energy during crack propagation. The onset of sound emissions is recorded well before F_gy. The wavelet transform (see: frequency analysis) of the sound emissions recorded during the test reveals different frequency ranges at various points in time. This is illustrated in Figure 3a–b for the time point at 0.26 ms by the representation of the frequency characteristics within a narrower time window, from which three frequency ranges (Δf₁ to Δf₃) could be derived. The frequency range Δf₁ also occurs during the fracture of the test specimen. A specific assignment of the deformation mechanisms occurring to the frequency ranges requires validation, such as that which can be carried out, for example, by coupling the in-situ tensile test on notched test specimens in the environmental-SEM (ESEM).

Fig. 3:

Result of the wavelet transform (a) and a detailed view at the point of damage initiation to facilitate a better analysis of the frequency ranges (b) for PP/20 [9]

In order to define the exact moment at which sound emissions first occur and to demonstrate micro-damage limit, the stop-block method was also employed. In this method, the pendulum hammer is stopped by a metal block once the test specimen has deflected by a defined amount [7]. The blunting of the original crack tip as a result of plastic deformation is to be understood as the onset of material damage. The plastic deformation is characterised on the fracture surface as a stretch zone with the stretch zone height (SZH) and stretch zone width (SZW). Due to the fracture surface dominated by the glass fibres and the resulting difficulty in identifying the SZH and SZW, the recording of the acoustic emission characteristics served as indirect evidence that acoustic emissions are induced by the blunting of the crack tip (see also: crack opening, stretch zone and in-situ tensile test in ESEM with SEA) and the onset of stable crack propagation. The F–t and U–t diagrams of a completely fractured specimen served as a reference for F_max and f_max and for assessing the behaviour of the diagrams. Subsequently, defined specimen deflections were set using the stop block and the sound emissions were recorded simultaneously. The results are shown in Figure 4a–d. Examination of the functional relationships confirms the results previously discussed for PP/20. Thus, damage is detected by SEA both before the transition from elastic to elastic–plastic material behaviour and during the fracture of the test specimen, as can be seen in Figures 4a–b. By limiting the deflection, it was demonstrated that the acoustic emissions are caused by damage processes during the blunting of the crack tip.

In the case of PP/20, excellent fibre bonding was observed, resulting in effective load transfer from the fibres to the PP matrix (see: fibre–matrix adhesion). Due to stress concentration at the notch root and facilitated by microstructural damage caused by the introduction of the notches using a metal blade, acoustic emissions can be induced before F_gy is reached, i.e. before the transition from elastic to elastic–plastic material behaviour. As the damage progresses further, sound emissions are only recorded at the point of unstable crack propagation, i.e. due to material separation. The energy supplied during the phase of stable crack propagation is evidently consumed, and it is only during unstable crack propagation that the elastic energy stored in the test specimen is released.

Fig. 4:

U–t and F–t diagrams of a completely fractured test specimen and the result of the wavelet transformation (a, b), as well as the result for limiting deflection using the stop block (c, d) for PP/20 [9]

Further experimental results from Schoßig’s doctoral thesis [9] are presented in [10–12] for PP materials containing 10 m.-% GF.

See also

• Deformation mechanisms
• Frequency analysis
• Hybrid methods
• Hybrid methods, examples
• Instrumented Charpy impact test
• Polymer diagnostic
• Sound emission analysis

References

[1]	Bierögel, C.: Zur Problematik der Schallemissionsanalyse an verstärkten Thermo- und Duroplasten. PhD Thesis, Technischen Hochschule „Carl Schorlemmer“ Leuna-Merseburg (1983)
[2]	Bierögel, C.: Hybrid methods of polymer diagnostics. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2025) 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)
[3]	Krietsch, T.: Schallemissionsanalyse struktureller Versagensprozesse in faserverstärkten Polymeren. PhD Thesis, Technische Universität Berlin (1999)
[4]	Block, J.: Detektion von Schädigungsgrenzen in kohlenstoffaserverstärkten Kunststoffen mittels Schallemissionsanalyse. PhD Thesis, Universität Kassel (1988)
[5]	Kardelky, S.: Einfluss der Nukleierungsmittelart auf die Deformations- und Bruchmechanismen von medial beanspruchten PP/GF-Verbunden. Master-Thesis, Martin-Luther-Universität Halle-Wittenberg (2002) (see AMK-Library under B 3-101)
[6]	Schröder, D.: Kombinierte Wirkung des Faservolumen- und Nukleierungsmittelgehaltes auf das mechanische Eigenschaftsniveau von PP/GF-Verbunden. Master-Thesis, Martin-Luther-Universität Halle-Wittenberg (2003) (see AMK-Library under B 3-102)
[7]	MPK-Procedure MPK-ICIT (2016-08): Testing of Plastics – Instrumented Charpy Impact Test (ICIT): Procedure for Determining the Crack Resistance Behaviour Using the Instrumented Impact Test
[8]	Oluschinski, A., Schoßig, M., Bierögel, C., Grellmann, W.: winIKBV, Polymer Service GmbH Merseburg (2009-14)
[9]	Schoßig, M.: Schädigungsmechanismen in faserverstärkten Kunststoffen – Quasistatische und dynamische Untersuchungen. Vieweg+Teubner / GWV Fachverlage GmbH, Wiesbaden (2010), (ISBN 978-3-8348-1483-8; see AMK-Library under B 1-21)
[10]	Schoßig, M., Bierögel, C., Grellmann, W.: Simultane Aufzeichnung der schädigungssensitiven Schallemissionen im IKBV zur Bewertung der Risszähigkeit von kurzglasfaserverstärkten Kunststoffen. In: Grellmann, W. (Ed.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderung an die Kennwertermittlung. Tagung "Werkstoffprüfung 2011", December 1 and 2, 2011. Berlin, Proceedings pp. 201–206 (ISBN 978-3-9814516-1-0; see AMK-Library under A 13)
[11]	Schoßig, M., Bierögel, C., Grellmann, W.: Assessment of fracture behavior under impact loading with simultaneous recording of acoustic emission. Materialprüfung 55 (2013) 2, 84–91; https://doi.org/10.3139/120.110410
[12]	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) 126–149 (ISBN 978-3-319-41877-3; e-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19)