Laurent
Developments in Materials for Pipes # 2001 Munich
The development of improved toughness properties of PE pipe resins agrees with the implementation of modern PE pipe installation technologies. Especially the increased resistance to Stress Cracking and to Slow Crack Growth allows contemplation of improved economics, longer lifetime and higher safety. However the development of such properties requires reliable and predictive test methods, which are adapted to actual resistant materials. Several currently practised accelerated test methods have been applied to a series of bimodal copolymer PE 100 resins. These methods consist of the Notched Pipe Test (NPT), the Full Notch Creep Test (FNCT), the Cone Test, the pressure test under Point Loading and the Tensile test (‘Natural Draw Ratio’). Overall, a fair coherence has been demonstrated among the results of the different tests. Major variations over 1 decade in terms of failure time were detected between good and very good PE 100 resins. In the case of the latter, even the most severe tests lead to excessive testing time. The differences in performance evidenced are associated with differences of morphology of the fracture surface. The observations are coherently interpreted based on the critical voiding stress, which determines the length of the plastic zone that forms at notch tip. A relation has been established between the resistance to SCG and the ‘natural draw ratio’ as obtained with a simple tensile test. It is anticipated that the ‘natural draw ratio’ may serve as quality control and predictive information within one family of resins.
