A systematic investigation of fracture in polyamide 12

Mario Messiha

Research output: ThesisDoctoral Thesis

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Abstract

Plastic pipelines used for gas and water transportation exhibit a notable record as long-term operating applications. However, failure does happen. In that manner, slow crack growth (SCG) under certain conditions is considered to be the most critical failure mode. As steel pipe systems operating at high pressure levels are increasingly replaced by modern pipe grade materials, such as polyethylene (PE) and polyamide (PA), the understanding of fundamental fracture mechanisms in order to effectively evaluate the resistance against SCG, has become of indispensable value. Yet, when it comes to materials other than PE, only few studies regarding the relevant long-term failure mechanisms have been performed. Hence, a key element within this study was to examine possible fracture and micro deformation mechanisms as well as structure property relationships that promote slowly extending cracks in PA, particularly PA12. This was done with the aid of an accelerated tool to characterize SCG performance of a material – the cyclic cracked round bar (CRB) test. In contrast to its non polar paradigm PE, PA is additionally influenced by the ability to build hydrogen (H) bonds. Therefore, influences of H bonds on SCG were examined using a sequential debonding fracture model. It was demonstrated that the additional energy needed to dissociate effective H-bonds becomes the dominating source of energy which has to be overcome. This is the case, if at least 45 % of all H-bonds crossing the crack plane engage in the fracture process and are actively resisting SCG in contrast to a theoretically non physically cross linked PA. Special focus was also put on the formation of crazes as predecessors to cracks as well as crack initiation and propagation resistances. PA12 grades of increasing molecular weight (MW) showed an increasing disentanglement resistance due to hydrogen bond effects leading to higher initiation and propagation resistances, whereas plastic zone sizes do not change considerably. An impact modification, on the other hand, promotes the development of notably larger plastic zones and higher SCG resistance, whereas pigmented grades exhibit a reduced plastic zone size, rendering a lower amount of dissipated energy before physical crack initiation and during SCG. Though, SCG may initiate only a small leak, it has also the potential to trigger rapid crack propagation (RCP) under certain conditions, which is much rarer but can be more catastrophic and destructive. Characteristic feature of RCP in critical applications, such as pressurized pipes or vessels, is recognized by a dynamic crack propagating at high-speeds of up to several hundred meters per second. The external loading situation as well as the molecular and morphological structure of the material are known to affect the crack behavior. Thus, safe installation and operation during service lifetime of components prone to RCP, cannot be assured without sound knowledge about the resistance against RCP, particularly when considering the use of new materials, such as time and temperature dependent plastics. In this context, the RCP resistance of different PA12 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 established. A new S4 evaluation concept was further proposed to increase the data yield while reducing the necessary number of tests. Yet, even a modified S4 test becomes too uneconomic and time consuming to check for changes in RCP behavior, in order to advance material development. Based upon mathematical formulations of the adiabatic decohesion model proposed by Leevers and elasto dynamic fracture mechanics theory, an analytical ranking parameter R** was introduced which showed strong cor
Translated title of the contributionEine systematische Untersuchung von Brüchen in Polyamid 12
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Pinter, Gerald, Supervisor (internal)
  • Fischer, Jörg, Assessor B (external)
  • Frontini, Patricia, Co-Supervisor (external), External person
  • Kopp, Jean-Benoit, Assessor A (external), External person
Publication statusPublished - 2022

Bibliographical note

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Keywords

  • Polyamide 12
  • Slow crack growth
  • Rapid crack propagation
  • Fracture mechanics
  • Structure-property relationships

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