Polymer fracture mechanics extended to large plastic deformation and mixed mode loading

Anja Gosch

Research output: ThesisDoctoral Thesis

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Abstract

The proper characterization of occurring fracture mechanisms in polymers, the design of sufficient testing methods, and its theoretical aspects are gaining more and more in importance. However, there are still less investigated areas of research within polymer fracture mechanics, such as elastic plastic fracture mechanics (EPFM) and mixed mode fracture of bulk polymers. In order to increase the knowledge in these areas, a detailed scientific study was conducted in the present work. In the past, a lot of effort was put into the assessment of testing procedures in the area of EPFM, in order to characterize ductile polymers with large plastic deformations. However, with the procedures available for polymers it can still be challenging to evaluate reproducible fracture parameters. Therefore, several testing methods of EPFM were applied in combination with scatter reduction procedures in this thesis to evaluate the fracture behaviour of polymers. Additionally, the evaluation of size-independent fracture parameters was evaluated by the application of a testing scheme developed by the European Structural Integrity Society (ESIS TC 4). The determined fracture initiation parameters showed size dependent values, whereby, the calculated crack growth resistance curve (J-R curve) displayed one overlapping curve without any sign of size dependency. Furthermore, the crack initiation parameters were correlated with occurring fracture processes for the different specimen sizes. In the future, the conducted measurements on different specimen sizes can be used in combination with numerical simulations for component design with more complex geometries. In the case of mixed mode loading, several specimen configurations and testing procedures were developed within this study to characterize bulk polymeric materials. Mixed mode I/III fatigue fracture tests were conducted and compared to pure mode I fatigue results. A significant life-time reduction was observed for mixed mode I/III loaded samples in comparison to pure mode I loading. The results led to the development of an equivalent stress intensity factor range Keq, which takes both loading cases into account. The introduction of Keq is especially beneficial, to assess different mixed mode problems using one fracture mechanical parameter. During mixed mode loading the crack flanks can stay close, which may result in wear abrasion and friction. This was also detected in the current work, by an increasing surface temperature during the mode I/III fatigue tests. For the investigation of quasi-static mixed mode conditions, measurements of pure mode I and pure mode III were conducted on thin-walled specimens. The obtained results of this study can be used to improve life-time estimation of mixed mode loaded applications. Additionally, the observed experimental test approaches can act as starting point for further measurements of mode II loaded cracks.
Translated title of the contributionBruchmechanik von Kunststoffen erweitert für plastische Deformation und Mixed Mode Lasten
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Baldi, Francesco, Co-Supervisor (external), External person
  • Pinter, Gerald, Supervisor (internal)
Publication statusPublished - 2021

Bibliographical note

embargoed until null

Keywords

  • Polymer fracture mechanics
  • J-Integral
  • Mixed Mode

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