PENT test versus time to rupture curve for HDPE

Session 11A
4:10 pm

Eva Nezbedova, Polymer Institute Ltd.

The new type of PE 100 (PE 100 RC) should achieve the life time over 100 years. Conventional hydrostatic pressure test to qualify a new resin must last 104 hours (ISO 1167). In last ten years two accelerated tests that produce the same type of brittle fracture that occurs in pipes after long time in service were developed and standardized (PENT test – ISO 16241 and FNCT – ISO 16770). In contrast to FNCT test PENT enables to follow the kinetics of slow crack growth (SCG) and estimates some relevant parameters: (i) time for SCG initiation – ti, (ii) rate of SCG – d(COD)/dt and (iii) time to failure – tf. The rate of SCG is evaluated as the slope of the linear part on curve COD vs. time measured during the PENT test. The time for SCG initiation and the rate of SCG can be estimated in relatively short time in comparison to the time to failure and can distinguish the difference grades of HDPE with respect to resistance against SCG.

SEN specimen geometry and single method of loading type that is used in PENT test enable to calculate another parameter namely stress intensity factor, K. The region of stable crack growth (SCG) can be explained by exponential relationship between d(COD)/dt and the calculated stress intensity K factor.
In the contribution these approaches in combination with numerical simulation of slow crack growth are used to describe the part of time to rupture curve that is responsible for service lifetime. These experimental and theoretical procedures were used for one grade of PE 100 and three grades of PE 100 RC materials. The results shown a pronounced difference in rate of SCG between the PE 100 and PE 100 RC and confirmed the possibility to use this procedure for describing the brittle part of time to rupture curve.

Eva Nezbedova, Scientific worker with Polymer Institute
Pavel Hutař, Scientific worker with Institute of Physics of Materials
Zdenek Knesl, Scientific worker with Institute of Physics of Materials

Polymer Institute Ltd
Czech Republic