Polypropylene multi-layer-pipes - 100 year lifetime assessment using linear elastic fracture mechanics

 

Papers # 2016 Berlin

Applying scientifically well established and investigated linear fracture mechanic methods (LEFM) the kinetics of the slow crack growth (SCG) on polypropylene layers and coextruded interfaces on multi-layer-pipes could be investigated. Taking effects of ageing and stress relaxation during the service lifetime into consideration, it was possible to predict the lifetime of such a pipe of at least 100 years. 

Co-extruded multi-layer pipes out of polypropylene have been widely used in nonpressure sewerage applications for several years. Due to good material properties and generally low mechanical loads not many critical failures are noticed up to now. For the lifetime assessment over 50 years internal pressure tests are performed at the moment to determine, whether SCG is threatening the integrity of the whole structured wall pipe.

This paper presents results achieved on a co-extruded multi-layer polypropylene pipe with a middle-layer consisting of mineral reinforced polypropylene block copolymer, taking into consideration, that the pipe is subjected to ageing effects and due to relaxation the stress level is reduced during the lifetime. 

In particular LEFM were applied, characterizing the SCG on the polymer materials and the coextruded interfaces. The crack initiation and crack growth as well as the SCG kinetics was investigated by using static methods (Single-Edge-Notched Bend, SENB) and cyclic fatigue tests with different loading ratios (Cracked Round-Bar, CRB).

By using these results it was possible to show, that the phenomenon of SCG can occur in both, reinforced and un-reinforced polypropylene block copolymer. Evaluating the kinetics of the SCG, it was possible to predict the lifetime of such a pipe of at least 100 years. 

Related keywords : Polypropylene, Pipes, Lifetime, Linear Fracture Mechanics.
Please note that the whole article content is available on PPCA website only


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