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{{PSM_Infobox}}
ICIT with AE
__FORCETOC__

==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|short-fibre-reinforced composites]] is an understanding of the [[Strength|strength]]- and strain-dependent [[Deformation Mechanisms|deformation]] and [[Fibre-reinforced Plastics Fracture Model|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|crystallinity]]),
* the influence of the fibres (e.g. [[Ashing Method|fibre content]], [[Fibre Orientation|orientation]], distribution and geometry) and
* the effect of modifiers (e.g. stabilisers, impact modifiers and [[Fibre–Matrix Adhesion|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 [[Polymer Diagnostic|diagnostic methods]] with a view to fully exploiting the [[Material & Werkstoff|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|fracture mechanics]] methods with [[Non-destructive Testing (NDT)|non-destructive testing methods]], it is possible to gain a deeper understanding of material behaviour. For example, changes in the [[Phase Boundary Surface|interface]] between [[Fibre–Matrix Adhesion|fibre and matrix]], which can lead to variations in the [[Deformation Mechanisms|damage mechanisms]] under mechanical [[Stress|stress]], can be indirectly detected by combining the [[Tensile Test|tensile test]] with [[Sound Emission Analysis|sound emission analysis]] as a [[Hybrid Methods|hybrid method of plastics diagnostics]]. [[Sound Emission Analysis|Sound emission analysis]], as a quasi-non-destructive testing method, in principle enables the evaluation of the damage kinetics of [[Fibre-reinforced Plastics|fibre-reinforced plastics]] (see also: [[Fracture Behaviour|fracture behaviour]]). Furthermore, with the aid of [[Frequency Analysis|frequency analysis]] of the recorded [[Sound Emission|sound emissions]], it is possible to correlate the [[Deformation Mechanisms|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|hybrid methods]] in [[Polymer Diagnostic|polymer diagnostic]]). [[Sound Emission Analysis|Sound emission analysis]] exploits the fact that the [[Acoustic Emission|acoustic emissions]] generated by the sudden release of elastic energy stored in the [[Material & Werkstoff|material]] are directly related to the underlying causes, thereby enabling them to be attributed to the [[Deformation Mechanisms|damage mechanisms]].

===Experimental method===

A polypropylene reinforced with 20 % by mass of short glass fibres ([[Plastics – Symbols and Abbreviated Terms|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 [[Fibre–Matrix Adhesion|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|crack propagation]] under [[Impact Loading Plastics|impact loading]] was carried out at the [[Instrumented Charpy Impact Test|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-Procedure MPK-ICIT|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 [[Specimen|specimens]] with dimensions of 80 x 10 x 4 mm³ (''L'' x ''W'' x ''H'') are used. The [[Notching|introduction]] of the [[Notch|notches]] was carried out using a manual notching device, whereby a metal blade (razor blade) is pressed into the [[Specimen|test specimen]] with a constant feed rate. The metal blades used produced a [[Notch Sensitivity|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|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 [[Standard Atmospheres|room air conditioning]]. The test was carried out using an instrumented pendulum impact tester (see: [[Impact Loading Pendulum Impact Tester|impact loading pendulum impact tester]]) with a work capacity of 4 J at maximum drop height. Force measurement is achieved using semiconductor [[Strain Gauge|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|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|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|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 [[Specimen|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 [[Specimen|test specimen]] positioned on the [[Support Distance|support]] with the sensor applied.

{| border="0"
|[[File:IKBV_SEA_1a.JPG]]
|
{| border=0
|-valign="top"
|width="25px"|A –
|instrumented pendulum hammer with a 4 J impact energy at maximum drop height
|-valign="top"
|B –
|specimen
|-valign="top"
|C –
|attached acoustic sensor using beeswax as the coupling medium
|}
|}
{|
|- valign="top"
|width="50px"|'''Fig. 1''':
|width="600px" |Experimental setup for coupling the [[Instrumented Charpy Impact Test|ICIT]] with the [[Sound Emission Analysis|AE]]
|}

As a result of investigations carried out to optimise the [[Frequency Analysis|frequency analysis]], the [[Inertial Load|inertial load]] was successfully filtered out of the measurement signal.

To carry out the [[Sound Emission Testing|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 Emission|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 [[Frequency Analysis#Wavelet transform|wavelet transform]], together with the load–time diagram (''F''–''t'' diagram) for PP/20.

[[File:IKBV_SEA_2.JPG]]
{|
|- valign="top"
|width="50px"|'''Fig. 2''':
|width="600px" |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|crack propagation]] and low [[Crack Propagation Energy|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 Emission|acoustic emissions]] are recorded at different times depending on the [[Ashing Method|glass fibre content]]. At the force maximum, i.e. the onset of unstable [[Crack Propagation|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 [[Specimen|test specimen]] is thus converted into mechanical and acoustic energy during [[Crack Propagation|crack propagation]]. The onset of [[Sound Emission|sound emissions]] is recorded well before ''F''gy. The '''wavelet transform''' (see: [[Frequency Analysis|frequency analysis]]) of the [[Sound Emission|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 (Δ''f''1 to Δ''f''3) could be derived. The frequency range Δ''f''1 also occurs during the [[Fracture|fracture]] of the test specimen. A specific assignment of the [[Deformation Mechanisms|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 [[Notching|notched]] [[Specimen|test specimens]] in the [[Environmental-SEM (ESEM)|environmental-SEM (ESEM)]].

[[File:IKBV_SEA_3.JPG]]
{|
|- valign="top"
|width="50px"|'''Fig. 3''':
|width="600px" |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 Emission|sound emissions]] first occur and to demonstrate [[Micro-Damage Limit|micro-damage limit]], the [[ICIT – Stop Block Method|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|crack tip]] as a result of [[Deformation#Plastic deformation|plastic deformation]] is to be understood as the onset of material damage. The [[Deformation#Plastic deformation|plastic deformation]] is characterised on the [[Fracture Surface|fracture surface]] as a [[Stretch Zone|stretch zone]] with the stretch zone height (SZH) and stretch zone width (SZW). Due to the [[Fracture Surface|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|crack opening]], [[Stretch Zone|stretch zone]] and [[In-situ Tensile Test in ESEM with AE|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 Emission|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 [[Sound Emission Analysis|SEA]] both before the transition from elastic to elastic–plastic material behaviour and during the [[Fracture|fracture]] of the test specimen, as can be seen in '''Figures 4a–b'''. By limiting the deflection, it was demonstrated that the [[Acoustic Emission|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|fibre–matrix adhesion]]). Due to stress concentration at the notch root and facilitated by microstructural damage caused by the [[Notching|introduction]] of the [[Notch|notches]] using a metal blade, acoustic emissions can be induced before ''F''gy is reached, i.e. before the transition from [[ICIT – Nonlinear Material Behaviour|elastic to elastic–plastic material behaviour]]. As the damage progresses further, [[Sound Emission|sound emissions]] are only recorded at the point of unstable [[Crack Propagation|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|crack propagation]] that the elastic energy stored in the [[Specimen|test specimen]] is released.

[[File:IKBV_SEA_4.JPG]]
{|
|- valign="top"
|width="50px"|'''Fig. 4''':
|width="600px" |''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|Deformation mechanisms]]
* [[Frequency Analysis|Frequency analysis]]
* [[Hybrid Methods|Hybrid methods]]
* [[Hybrid Methods, Examples|Hybrid methods, examples]]
* [[Instrumented Charpy Impact Test|Instrumented Charpy impact test]]
* [[Polymer Diagnostic|Polymer diagnostic]]
* [[Sound Emission Analysis|Sound emission analysis]]

==References==

{|
|-valign="top"
|[1]
|[[Bierögel, Christian|Bierögel, C.]]: Zur Problematik der Schallemissionsanalyse an verstärkten Thermo- und Duroplasten. PhD Thesis, [https://de.wikipedia.org/wiki/Technische_Hochschule_Leuna-Merseburg Technischen Hochschule „Carl Schorlemmer“ Leuna-Merseburg (1983)]
|-valign="top"
|[2]
|Bierögel, C.: Hybrid methods of polymer diagnostics. In: [[Grellmann,_Wolfgang|Grellmann, W.]], [[Seidler,_Sabine|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)
|-valign="top"
|[3]
|Krietsch, T.: Schallemissionsanalyse struktureller Versagensprozesse in faserverstärkten Polymeren. PhD Thesis, Technische Universität Berlin (1999)
|-valign="top"
|[4]
|Block, J.: Detektion von Schädigungsgrenzen in kohlenstoffaserverstärkten Kunststoffen mittels Schallemissionsanalyse. PhD Thesis, Universität Kassel (1988)
|-valign="top"
|[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)
|-valign="top"
|[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)
|-valign="top"
|[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
|-valign="top"
|[8]
|Oluschinski, A., Schoßig, M., Bierögel, C., [https://www.researchgate.net/profile/Wolfgang-Grellmann Grellmann, W.]: [[winIKBV]], [https://de.wikipedia.org/wiki/Polymer_Service_Merseburg Polymer Service GmbH Merseburg] (2009-14)
|-valign="top"
|[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)
|-valign="top"
|[10]
|Schoßig, M., Bierögel, C., [https://de.wikipedia.org/wiki/Wolfgang_Grellmann 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)
|-valign="top"
|[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
|-valign="top"
|[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)
|}

[[Category:Fracture Mechanics]]
[[Category:Hybrid Methods]]
[[Category:Instrumented Impact Test]]

ICIT with AE - Revision history