Pennsylvania Edge Notch Tensile (PENT) Test

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Pennsylvania Edge Notch Tensile (PENT) Test

Characterisation of slow crack growth

The Pennsylvania Edge Notch Tensile (PENT) test (Fig. 1), developed by N. Brown and his colleagues [1] and standardised in ISO 16241 [2], induced the same type of quasi-brittle fracture (see: fracture types) that can occur in plastic pipes after long-term practical use [3–5]. It is therefore used for the accelerated characterisation of slow crack growth (SCG). However, due to the high resistance of modern PE materials to SCG – as in the Full Notch Creep Test (FNCT) – the modification of the PENT test or the development of completely new tests, such as the Crack Round Bar (CRB) test or the Strain Hardening Test, is currently needed.

Test specimen and notching

The test specimen geometry corresponds to that of single-edge notched tensile test specimens (SENT-specimens), which are machined either from compression-moulded plates or tubes. For tubes, the test specimens can be sampled either axially (Fig. 2a) or perpendicularly (tangential, Fig. 2b) to the extrusion direction, enabling to analyse the impact of orientation. The notch is produced by pressing a fresh metal blade into the material at a constant velocity of 330 μm/min (see: notching). Selection of the notch depth is done in such a way that the failure time is minimised, but no pronounced plastic flow occurs across the remaining test piece cross-section. Both the width and thickness of the test specimen and the side grooves are selected in such a way that the fracture occurs under conditions of predominantly plane strain state.

Parameters of the PENT Test

The kinetics of the failure process is observed at a constant nominal stress of 2.4 MPa and a temperature of 80 °C in air.

Fig. 1:

Pennsylvania Edge Notch Tensile (PENT) test: Experimental test setup (a) and schematic representation (b).

The crack opening displacement (see: extended CTOD concept) is measured using a light microscope having a resolution of approximately 2 μm. The time most important for distinguishing between different PE-HD types regarding their long-term failure is the fracture time t_f. The minimum slope in the linear part of the COD‒time diagrams (Fig. 2c) represents the stable crack propagation rate and is another important parameter for describing the kinetics of slow crack growth (SCG). Another mean parameter is the time t_i until the initiation of SCG. Compared to the Full Notch Creep Test (FNCT), the fracture mechanics-based PENT test provides significantly more information due to its multi-parameter description (see: levels of knowledge in fracture mechanics) of the crack propagation and fracture process.

Fig. 2:

Notched test specimens machined from tubes in the axial direction (a) and in the tangential direction (b), and schematic crack opening displacement (COD) vs. time (t) crack resistance curve (c).

See also

• Full Notch Creep Test (FNCT)
• Slow Crack Growth
• Strain Hardening Test (SHT)

References

[1]	Lu, X., Brown, N.: A Test for Slow Crack Growth Failure in Polyethylene under a Constant Load. Polymer Testing 11 (1992) 309–319
[2]	ISO 16241 (2005-02): Notch Tensile Test to Measure the Resistance to Slow Crack Growth of Polyethylene Materials for Pipe and Fitting Products (PENT)
[3]	Nezbedová, E., Kučera, J.: Experimentelle Methoden zur Charakterisierung des Bruchverhaltens von HDPE-Rohren. In: Grellmann, W., Seidler, S. (Eds.): Deformation und Bruchverhalten von Kunststoffen. Springer, Berlin Heidelberg (1998), pp. 91–98, (ISBN 3-540-63671-4; E-Book (2014): ISBN 978-3-642-58766-5; see AMK-Library under A 6)
[4]	Nezbedova, E., Hodan, J., Kotek, J., Krulis, Z., Hutar, P., Lach, R.: Lifetime of Polyethylene (PE) Pipe Materials – Prediction using Strain Hardening Test. In: Grellmann, W., Langer, B. (Eds.): Deformation and Fracture Behaviour of Polymer Materials. Springer, Berlin (2017) pp. 203–210, (ISBN 978-3-319-41877-3; E-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19) https://springer.com/book/10.1007/978-3-319-41879-7
[5]	Lach, R., Nezbedova, E., Langer, B., Grellmann, W.: Schnelle Abschätzung des mechanischen Langzeitverhaltens moderner Werkstoffe für Kunststoffrohre mittels des einachsigen Zugversuchs. In: Frenz, H., Langer, J. B. (Eds.): Fortschritte in der Werkstoffprüfung für Forschung und Praxis. Prüftechnik – Kennwertermittlung – Schadensvermeidung, (ISBN 978-3-9814516-7-2; see AMK-Library under A 20), Proceedings „Werkstoffprüfung 2017“, 30.11./01.12.2017, Berlin, pp. 259–264 (ISBN 978-3-9814516-7-2; see AMK-Library under A 20)