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Fast comparison of different polymeric pipe materials: extending the use of the cyclic CRB- test (ISO 18489)

 

G. Pinter, F. Arbeiter, A. Frank

Papers # 2016 Berlin

The cyclic cracked round bar test has recently been added as a full-fledged ISO standard (ISO 18489) for comparison of polyethylene pipe grade materials’ slow crack growth resistance. However, possibilities of this test are yet far from being exhausted. Seeing, that polyethylene is only used in approximately 30% of polymer pipe applications, it is vital to extend the use of ISO 18489 to other materials. First experimental studies on further pipe grade materials, such as cross-linked polyethylene, polypropylene, polybutylene, polyvinylchloride and polyamide, showed promising results.

This conference contribution is based on work published at Polymer Testing (Elsevier Ltd. - doi:10.1016/j.polymertesting.2015.05.008) under the name of “Cyclic tests on cracked round bars as a quick tool to assess the long term behaviour of thermoplastics and elastomers”. 

The cyclic cracked round bar (CRB) test has recently been added as a full-fledged ISO standard (ISO 18489) for comparison of polyethylene (PE) pipe grade materials’ slow crack growth (SCG) resistance. However, possibilities of this test are yet far from being exhausted. Seeing, that PE is only used in approximately 30% of polymer pipe applications, it is vital to extend the use of ISO 18489 to other materials.

Aim of this work is to show the principle applicability of this test method for other polymeric pipe grade materials, besides PE. First experimental studies on further pipe grade materials, such as cross-linked PE (PE-Xa), Polypropylene (PP), Polyvinylchloride (PVC-U) and Polyamide (PA12), showed promising results. Due to the lack of comparable data, simple fracture curves (-t) might not be sufficient for material comparison. Therefore, material damage behaviour and failure has been studied in detail during the tests and post-mortem. To gain insight into damage development, crack-opening displacements have been studied using extensometers. Fracture mechanisms and resulting surfaces have been inspected using state of the art scanning electron microscopy. 

Depending on the material in question and testing conditions, quasi-brittle failure could be achieved for most materials within several hours to days of testing. A clear differentiation of material classes was possible, using fracture curves. Additionally, crack-opening data could be used to determine crack initiation times and showed significant differences between materials. Whereas clear indications of step-wise crack growth could be found, for example in PE and PA12, other materials like PP showed more continuous damage propagation. Correlating with crack development, fracture surfaces also showed noteworthy differences, depending on the material. Summarizing, cyclic CRB-tests, as suggested in ISO 18489, look promising as a tool for fast material comparison, outside of the PE pipe grade material family.

Related keywords : Durability, CRB Test, Fatigue, Pipe Materials.
Please note that the whole article content is available on PPCA website only


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