Cyclic CRB Tests – A Quick and Reliable Tool for Ranking of PE Pipe Grades

Session 9A
11:40 am

Gerald Pinter, Montauniversitat Leoben

It is well known that crack initiation followed by slow crack growth (SCG) is the most important long-term failure mechanism in pressurized polyethylene (PE) pipes. Due to this, strong efforts have been put into the development of fracture mechanics tests, which are able to simulate SCG behavior of pipes in laboratory tests and to obtain information on the long-term behavior of PE pipes within a reasonable time-frame. SCG tests evaluated according to the Full Notch Creep Test (FNCT), the Pennsylvania Notch Test (PENT), the Notched Pipe Test (NPT) and the Cone Test, among others, were introduced and are widely used throughout the industry as well as the scientific community. However, testing of materials from newest generation with these methods exceeds practicable time-frames.

One way to accelerate SCG even in high performance PEs and at room temperature is the application of fatigue loads. In recent years the authors could establish a procedure based on cyclic tests with Cracked Round Bars (CRB), to characterize the SCG behavior of PE-pipe materials within less than a week at 23 °C by measuring the failure times at different initial loads. Currently they were able to improve the method by using extensometers for detecting the crack initiation time, which gives another reduction of testing time of about 50 %.

In this work the big potential of the cyclic CRB method for a quick and reliable quality assurance tool is described. The method is able to rank PE pipe grades reflecting the expectations from their molecular and morphological structure and even the impact of minor lot to lot variations on SCG and long term failure could be identified. Tests on different machines within one lab and also correlations of results from different labs could prove the reliability of the method.

G. Pinter (1*), A. Frank (2), A. Redhead (2), R.W. Lang (3)
1) Montanuniversität Leoben, Austria
2) Polymer Competence Center Leoben, Austria
3) Johannes Kepler University of Linz
*) corresponding author