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Evaluating strain hardening (SH) and cracked round bar (CRB) test methods with respect to molecular structure of materials and correlation to notched pipe test (NPT)

 

Suleyman Deveci, Dongyu Fang

Papers # 2016 Berlin

Latest developments in polyethylene production (hexene co-monomer and bi-modality) have increased the material’s Slow Crack Growth (SCG) properties, which is a time dependent brittle type failure under low stress levels and one of the most important long term properties of pipe grade polyethylene materials. Despite the employed acceleration factors, current standard test methods for evaluating the SCG properties of polyethylene materials, such as Notched Pipe Test (NPT), Full Notched Creep Test (FNCT) and Pennsylvania Notched Test (PENT), take unfeasible time to reach a brittle failure at standard test conditions.

Several test methods have been under development phase to understand slow crack growth resistance of pipe materials comparably at shorter times without using aggressive chemicals to accelerate the failure. Strain hardening modulus and crack round bar tests are one of these recently developed and published standard test methods. However, those test methods still need better understanding with respect to the materials molecular structure and comparative studies with existing slow crack growth test methods such as Notched Pipe Test (NPT) are of great interest to the industry.

This study discusses the correlations of morphological, molecular weight, molecular weight distribution and rheological properties of different PE materials with their slow rack growth resistances obtained from the strain hardening (SH) test, crack round bar (CRB) test and their correlations with notched pipe test (NPT).

Related keywords : Polyethylene pipes, SCG, NPT, CRB, Strain Hardening Modulus.
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