Tensile Test and Sound Emission Analysis

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Tensile test and acoustic emission analysis

Introduction

In the quasi-static tensile test, the sound emissions occurring during loading on single-notched test specimens are used to evaluate the damage kinetics. Due to the use of notched test specimens, it is possible to specify a defined distance between the acoustic emission source and the sensor position and thus derive 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 the sound emission, different experimental conditions apply and the test was carried out in accordance with ISO 527-1 [1].

Experimental

A polypropylene (PP/20) reinforced with 20 wt.-% short glass fibres was investigated. Due to the non-polar nature of polypropylene, maleic anhydride was used as a coupling agent to optimise the bonding of the fibres to the matrix (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 % by mass. For this reason, Echtblau was used as the nucleating agent. Injection-moulded multi-purpose test specimens in accordance with ISO 527-2 [4] with a total length l₃ of 170 mm were available for the investigations.

The depth of the notch made with a metal blade (see also: notch geometry) was 2 mm with a notch radius of 0.3 µm and a sensor–notch distance of 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 measuring system (VALLEN-SYSTEME GMBH, ICKING, GERMANY) 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 as well as 150–650 kHz. Impedance matching during application of the sensor to the test specimen surface was achieved using beeswax as an adhesive, and constant contact pressure was ensured by using a clamp. Figure 1 shows a clamped test specimen equipped with the acoustic sensor.

A – Acoustic broadband sensor, applied to the test specimen with coupling medium B – Clamp attached C – Safety cable to catch the sensor and prevent damage

Fig. 1:

Test setup for sound emission measurements in the tensile arrangement

Example

The distribution functions for the peak amplitude values A_p, the event duration t_ED and the cumulative rate representation of the energy E_AE are shown for PP/20 together with the force-traverse path diagram in Figures 2a–c.

Fig. 2:

Representation of the distribution functions of the amplitude values Ap, the event duration tED and cumulative rate representation of the energy EAE, as well as the division into three acoustically different ranges for PP/20 (a–c) [5]

For PP/20, unstable crack propagation was determined and, in comparison with the unnotched test specimens (results not shown), the insertion of a sharp notch results in a lower strength level due to the formation of a triaxial stress state and the higher deformation rate at the notch tip.

From the distribution functions, three acoustically different areas can be derived on the basis of the hit density, which are illustrated in the graphical representation in Figure 2 by vertical lines and were adopted for the event duration t_ED and for the rate representation of the energy E_AE. The amplitudes and event duration in areas II and III are characterised by an overlap of the values in the areas preceding them (Table 1). Area I is characterised by low acoustic activity, and the transition from area II to area III shows a disproportionate increase in acoustic emission. Before the ultimate failure of the material, most sound emissions per unit of time are detected with the highest amplitude values and maximum energies, which can be attributed to the increase in material damage.

Table 1: Assignment of amplitude and event duration values to the acoustic ranges for PP materials

acoustic range	corresponding amplitudes	corresponding event duration
I	40–50 dB	< 20 $\mu$s
II	50–68 dB	20–200 $\mu$s
III	> 68 dB	> 200 $\mu$s

To interpret the results, SEM images of the corresponding fracture surfaces of PP/20 were taken in order to qualitatively evaluate the adhesion conditions and damage mechanisms. Figures 3a–b show an overview and a detailed image. Glass fibre breaks (i), numerous pulled-out glass fibres not wetted with matrix material (ii), holes resulting from pull-out (iii) and severely plastically stretched matrix webs (iv) are visible. Based on the fracture surfaces, it cannot be clearly determined whether the glass fibres broke during the manufacturing process or as a result of unstable crack propagation.

Fig. 3:

REM image (a) and detailed section (b) of the fracture surface of PP/20; i – fibre fracture, ii – fibre not wetted with matrix material, iii – hole resulting from pull-out, and iv – plastically stretched matrix bridges between the glass fibres

It is not possible to assess the adhesion conditions on the basis of the fracture surfaces obtained in quasi-static tests, as there is an impermissible influence, i.e. uncovering of the fibres during the pull-out. In this case, preparation must be carried out at a high test speed and/or at low temperatures [6, 7]. An evaluation of the adhesion conditions can be carried out, for example, on fracture surfaces obtained from the instrumented Charpy impact test (ICIT). If the fibres are well bonded to the matrix, force is transferred between the matrix and the fibre during loading. In contrast to impact/dynamic loading (see: impact loading plastics), the deformation and thus energy absorption of the matrix in the quasi-static test is greater, as illustrated by the strongly plastically deformed matrix areas on the fracture surface.

See also

• Tensile test
• Sound emission
• Sound emission experimental conditions
• Sound emission analysis
• Sound emission testing

References

[1]	ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
[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]	ISO 527-2 (2025-06): Plastics – Determination of Tensile Properties – Part 2: Test Conditions for Moulding and Extrusion Plastics
[5]	Schoßig, M.: Bewertung der Schädigungsmechanismen von kurzglasfaserverstärkten Polyolefinen durch simultane Aufzeichnung der Schallemissionen unter quasistatischer und dynamischer Beanspruchung. Dissertation. Martin-Luther-Universität Halle-Wittenberg (2010), (ISBN 978-3-8348-1483-8); see AMK-Library under B 1-21) Content as pdf
[6]	VDI 3822 Blatt 2.1.2 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Processing
[7]	VDI 3822 Blatt 2.1.10 (2024-07): Failure Analysis – Significant Instrumental Analysis Methods for Failure Analysis of Products Made of Plastics