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Characterization of Crack Initiation and Slow Crack Growth Resistance of PE100 and PE100-RC pipe Grades with Cyclic Cracked Round Bar Tests

 

Andreas Frank, Isabelle J. Berger, Florian Arbeiter, Gerald Pinter

Papers # 2014 Chicago

During the past years the Cyclic Cracked Round Bar (CRB) Test has been developed and meanwhile standardized as a new and modern test method for a quick material ranking of polyethylene pipe grades by their resistance against slow crack growth (SCG). As an additional benefit this test method allows a very precise determination of crack initiation which provides further lifetime relevant information of the material. The objective of the current paper is a demonstration of the practical application of the Cyclic CRB Test for a quick material ranking. On the one hand, the study of the crack initiation and SCG behavior of ten PE 100 and PE 100-RC pipe grades from five different raw material supplier demonstrate that there are significant differences in the material performance, even within one material class. On the other hand, based on the failure curves of 35 different PE pipe grades the results show that, at least at room temperature, the performance of PE 100 and PE 100-RC has an overlap.

Pressurized pipes of polyethylene (PE) used for gas and water applications are designed to fulfill operating times of at least 50 years [1-6]. As a result of developments and improvements of the raw materials, particularly in the bimodal molecular mass distribution and in the controlled implementation of short chain branches, an increase in minimum service expected [5]. The classification of PE pipe grades is based on the long-term failure behavior of internal pipe pressure tests at different temperatures and extrapolation methods as described in EN ISO 9080 [7] or ASTM D2837 [8]. The long-term failure mechanisms of pressurized PE pipes have been studied comprehensively and are well known as crack initiation and quasi-brittle slow crack growth (SCG) [2, 3, 9-11]. Internal pipe pressure tests on PE pipe grades typically last several months or even years. In practice testing of pipes that do not fail after duration of 104 hours (approx. 13.5 months) are usually stopped. Based on such tests the minimum required strength (MRS) to ensure pipe lifetimes of at least 50 years is determined and leads to a classification of the materials as PE 63 (MRS=6,3 MPa), PE 80 (MRS=8 MPa) or PE 100 (MRS=10 MPa). For PE 100 which also pass the requirements according PAS 1075 [12] an additional classification PE 100-RC (RC=Resistant to Crack) has been defined.

The appreciated material improvements of the raw material suppliers have increased the failure times of internal pipe pressure tests significantly resulting in time consuming and expensive test procedures. Especially for modern grades of the classification PE 100 and PE 100-RC no quantitative information about the relevant quasi-brittle failure region can be determined with internal pipe pressure tests anymore. To meet the demand for an accelerated material characterization and to rank different materials by their SCG resistance, several laboratory tests like the Notched Pipe Test (NPT) [13-15], the Pennsylvania Edge-Notch Test (PENT) [16-18], the Notched Ring Test (NRT) [19] or the Full Notch Creep Test (FNCT) [20-22] have been developed. All this test methods are usually conducted at elevated temperatures of T=80 °C or even under the influence of stress cracking liquids (FNCT). Although the mentioned tests provide a significant acceleration of testing times, characterization of modern PE pipe grades is still related to long testing times of several months up to years. According to further time reduction in the characterization of SCG properties, recent studies have shown promising results 2 Copyright © 2014 by PPCA for two new test methods, the Strain Hardening Modulus [23-26] and the Cyclic Cracked Round Bar (CRB) Test [27-47] which has been recently standardized by the Austrian Standards Institute in ONR 25194 [48].

The current paper investigates ten PE 100 and PE 100-RC pipe grades from five different raw material supplier which are currently available in the market with the Cyclic CRB Test. On the one hand, the results demonstrate the practical application of the Cyclic CRB Test for a quick material ranking by the crack resistance of the materials. On the other hand, the additional information about crack initiation also allows a more detailed investigation of the materials. Moreover, based on Cyclic CRB Tests of 35 different PE pipe grades in the laboratory of the authors the generated data demonstrate that different failure ranges are determined depending on the PE classification and that, at least at room temperature, the failure performance of PE 100 and PE 100-RC has an overlap.

Related keywords : crack, pe100, material, based, slow, resistance, mpa, different.
Please note that the whole article content is available on PPCA website only

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