Full Notch Creep Test (FNCT)

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Full notch creep test (FNCT)

Experimental Method

For a long time, the Full Notch Creep Test (FNCT) (Fig. 1), standardized in ISO 16770 [1], has extensively been used in Europe for the accelerated characterization of slow crack growth (SCG).

Test specimen and characteristic parameter

The test specimen is a prismatic bar (machined from tubes or compression-moulded plates) having a size of 10 × 10 × 100 mm3 with a 1.5 mm-deep circumferential notch, where a nominal stress of 4–5 MPa in a surface-active aqueous medium (temperature: 80–95 °C) is applied. Only the failure time is analysed. The failure time in the FNCT ranges from about ten hours for mono-modal PE types to about a thousand hours for bimodal PE 100 types and in some cases more than a year for new PE 100-RC types (RC – Resistant to Cracking). The high resistance of modern PE materials to SCG requires a modification of the FNCT or the development of completely new tests, such as the Crack Round Bar (CRB) test or the Strain Hardening Test (SHT).

Comparison with other methods for describing slow crack growth

Compared to other SCG tests currently used specifically for PE pipes to be loaded with internal pressure, the FNCT was selected early for standardization within the ISO framework (see also: accreditation and certification). This was done against the backdrop that this test is relatively simple and sufficiently sensitive to structural variables (see: microscopic structure) in PE materials [2, 3]. However, it should always be noted that the validity of data obtained from the FNCT must be critically evaluated due to the large standard deviation of the measured values. Since the introduction of ISO 16770, several round robin tests have therefore been carried out with the aim of reducing the standard deviation of the measured values. Compared to the fracture mechanics-based Pennsylvania Edge Notch Tensile (PENT) test, the low depth of information provided by the FNCT has a negative impact too.

Fig. 1:

Schematic representation of the Full Notch Creep Test (FNCT)

See also

• Slow crack growth
• Pennsylvania edge notch tensile (PENT) test
• Strain hardening test (SHT)

References

[1]	ISO 16770 (2019-09): Plastics – Determination of Environmental Stress Cracking (ESC) of Polyethylene – Full-Notch Creep Test (FNCT)
[2]	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) 203–210 (ISBN 978-3-319-41877-3; see AMK-Library under A 19)
[3]	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, „Werkstoffprüfung 2017“, 30.11./01.12.2017, Berlin, Proceedings pp. 259–264 (ISBN 978-3-9814516-7-2; see AMK-Library under A 20)