Structure-Property Relationships of Polyamide 12 Grades Exposed to Rapid Crack Extension.

Mario Messiha,Gerald Pinter,Florian Arbeiter,Andreas Frank,Jan Heimink

doi:10.3390/ma14195899

Mario Messiha, Gerald Pinter + Show 3 more

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https://doi.org/10.3390/ma14195899

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Abstract

Thermoplastic materials have established a reputation for long-term reliability in low-pressure gas and water distribution pipe systems. However, occasional Slow Crack Growth (SCG) and Rapid Crack Propagation (RCP) failures still occur. SCG may initiate only a small leak, but it has the potential to trigger RCP, which is much rarer but more catastrophic and destructive. RCP can create a long, straight or meandering axial crack path at speeds of up to hundreds of meters per second. It is driven by internal (residual) and external (pressure) loads and resisted by molecular and morphological characteristics of the polymer. The safe installation and operation of a pipe throughout its service lifetime therefore requires knowledge of its resistance to RCP, particularly when using new materials. In this context, the RCP resistance of five different polyamide (PA) 12 grades was investigated using the ISO 13477 Small-Scale Steady State (S4) test. Since these grades differed not only in molecular weight but also in their use of additives (impact modifiers and pigments), structure-property relationships could be deduced from S4 test results. A new method is proposed for correlating these results more efficiently to evaluate each grade using the crack arrest lengths from individual S4 test specimens.

Highlights

To determine the resistance against Rapid Crack Propagation (RCP) of newly developed plastic pipe grades, the Full-Scale (FS) test, standardized in ISO 13478, has become the “gold standard” for product qualification
This study has highlighted relevant structure-property relationships regarding the resistance against Rapid Crack Propagation (RCP) in morphologically different polyamide (PA) 12 grades
An obvious improvement of the RCP resistance was observed with increasing molecular weight MW, while the incorporation of inorganic pigments reduced the rapid fracture toughness of PA12

Summary

Introduction

To determine the resistance against Rapid Crack Propagation (RCP) of newly developed plastic pipe grades, the Full-Scale (FS) test, standardized in ISO 13478, has become the “gold standard” for product qualification. It measures a critical pressure value (pc,FS ) above which RCP can occur under operating conditions for pressurized pipes in service. Since several FS tests are needed to evaluate the RCP behavior of a given material, this method is expensive and time-consuming These disadvantages were mitigated by the development of an accelerated laboratory test, the so-called Small-Scale Steady State (S4) test standardized for thermoplastic pipes in ISO 13477 [1]. To better understand underlying physical processes, detailed post-mortem examination of fracture surfaces was carried out

Experimental

Results and Discussion

Conclusions and Outlook