PSI - Issue 43

Lukáš Trávníček et al. / Procedia Structural Integrity 43 (2023) 148 – 153 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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Nomenclature a ini initial crack length CRB cracked round bar D

diameter of a CRB specimen inner diameter of a pipe specimen outer diameter of a pipe specimen

D in D out Δ F Δ K I F min F max Δ σ

force loading range stress loading range

stress intensity factor range minimum applied force

maximum applied force FNCT full notched creep test HDPE high density polyethylene l length of a CRB specimen N f number of cycles to failure NPT notched pipe test PENT pennsylvania edge notch test R loading ratio in a cyclic test SCG slow crack growth t thickness of the pipe wall

100 years are generally expected today. During their service life, the structural reliability of piping systems is essential as premature failures may cause severe material loss (Nayyar 2000). For the long-term applications of polyethylene pipes, the lifetime is connected to the resistance against crack initiation and slow crack growth (SCG). The traditional experimental testing method is based on internal pressure test standardized in ISO 9080 (ISO 2012). However, with the development of polyethylene pipe grades comes higher SCG resistance of the materials, for which the internal pressure test meets its time limits. This fact led the pipe community to develop accelerated testing methods to obtain the material ranking of the SCG resistance in shorter times. Among the most common methods are Notched Pipe Test (NPT) (Beech 2013), Pennsylvania Edge Notch Test (PENT) (Lu 1992) and Full Notch Creep Test (FNCT) (Fleissner 1998). All these accelerated tests are carried out at elevated temperatures of T = 80°C and/or under the influence of aggressive chemical environment to accelerate the crack initiation and SCG. These tests provide a significant reduction in testing times in comparison with the hydrostatic pressure test, but for modern polyethylene pipe grades (such as PE100RC) they still require times of several months. Therefore, Cracked Round Bar (CRB) test, that was recently standardized as a ranking method for polyethylene pipes (ISO 2015), was developed (Pinter 2007, Frank 2014, Arbeiter 2015). The CRB test is carried out on cylindrical specimens with a razor-sharp circumferential notch in the middle. The acceleration of crack initiation and SCG is done by cyclic loading that is much more effective than the special aggressive environment or high temperature. The important fact is that the cyclic loading does not change the mechanism of failure (Pinter 2013) and therefore, it is suitable for the acceleration of the SCG (Frank 2019). Moreover, the results are obtained within a few days even for the most durable materials. It follows from the above that the CRB test is a quick, robust, and effective tool to quantify the resistance against SCG of polymer materials. The specimens for all accelerated tests are typically manufactured from plates produced by compression molding, which is a different process that the one used for polymer pipes production – extrusion. For polymer pipes producers, the direct extrusion of the specimens for accelerated testing would lower the cost and save time. In this respect, the main aim of the presented work is to classify different types of polyethylene pipe grades of PE80, PE100 and PE100RC using new extruded pipe specimens in the tests and compare the results with classic CRB tests. The experiments of new pipe specimens were carried out under the same conditions as CRB tests, so the acceleration of the crack initiation and SCG was achieved by cyclic loading. The comparison as well as the usability of the new pipes specimens is discussed here, based on the obtained results.