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Evaluation and comparison of standard and accelerated slow crack growth determination methodologies: effect of the comonomer type influence

 

Papers # 2018 Las-Vegas

Last methodologies for fast evaluation of the SCG resistance on polyethylene pipe materials are analyzed in this work and compared with traditional PENT and FNCT tests. Strain Hardening (SH), Crack Round Bar (CRB) and Accelerated Creep Test (ACT) will be good alternatives for evaluating PE100RC resins. Moreover, SH and CRB are good methods for evaluating and ranking PE resins from PE80 to PE100RC. A significant influence of the type of comonomer is observed in any of the tests analyzed.

Slow Crack Growth (SCG) resistance is one of the most critical long-term mechanical properties that must be controlled and evaluated for determining the final performance of the polyethylene (PE) pipes. The main reason for this claim lies on the fact that SCG is the only failure mechanism that normally appears in current pipe networks. Last generation of multimodal polyethylene resins with very high resistance to SCG has been developed during last years. Standard methodologies for SCG evaluation like Pennsylvania Notch Tensile (PENT) Test (ISO 16241 - ASTM F1473) or Full Notch Creep Test (FNCT) (ISO 16770) have become insufficient due to the long time needed (more than one year) with the consequent unwanted thermal aging-effects. 

During last years, different accelerated and short-time methodologies have been developed for SCG determination on compression molded plaques being the most promising and used: Strain Hardening (SH) determination (ISO 18488), Cracked Round Bar (CRB) Test (ISO 18489) and Accelerated Creep Test (ACT) (Analogous to ISO 16770). 

In the present work, all these methodologies have been deeply evaluated and compared with the standard FNCT and PENT tests. Moreover, the effect of the comonomer type on the test results has been analyzed using Ziegler-Natta and Chromium commercial resins that use 1-butene and 1-hexene comonomer respectively. Finally, considering that this topic is under standardization discussion, a different SH critical value for PE100RC materials as function of the comonomer type used has been proposed.

Related keywords : Slow Crack Growth, Accelerated methods, Strain Hardening Test, Comonomer effect..
Please note that the whole article content is available on PPCA website only


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