Eigenspannungsanalyse und Schädigungsverhalten von verstärkten Polypropylen-Rohrwerkstoffen

Translated title of the contribution: Residual stress analysis and damage behaviour of reinforced polypropylene pipe materials

Manuel Schwab

Research output: ThesisMaster's Thesis

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Abstract

For pipe manufacturers it is essential to guarantee a lifetime of their products of at least 50 years. Each of their new developed pipe materials has to pass a tedious approval process. Mechanical long term properties of pipes as well as residual stress are amongst the most important values. Another significant parameter, which also has to be considered, is the damage behaviour of the materials, in order to avoid failure during their expected lifetime. In this work the properties of four different polypropylene (PP) types were investigated. Examined materials consisted of a pure PP, a wollastonite and two talcum and chalk reinforced materials. The mechanical long term properties (tensile creep modulus and time-based poisson’s ratio) of these materials were determined via the usage of the “Stepped Isothermal Method” (SIM) and compared with the results of conventional creep tests. The results showed that the creep modulus after 50 years of talcum and chalk reinforced materials had the highest values. The unfilled polypropylene displayed the smallest value and the wollastonite reinforced PP was in-between. In comparison to the conventional creep tests the graphs were very similar and comparable. The evaluation of the poisson’s ratio revealed that in the beginning of the experiments the variable stays at a constant level and starts to slightly increase during the tests. In addition the residual stress in axial and radial direction of extruded pipes, made out of the four materials, was investigated. The wollastonite reinforced material showed differences between residual stress in axial and radial direction. This may be attributed to the needle-shaped geometry and orientation of the mineral reinforcement. Stress levels in radial and axial direction were comparable for the other examined materials, most likely caused by the higher homogeneity. The talcum and chalk reinforced PP samples had higher residual stress levels compared to the other ones. The damage behaviour of wollastonite reinforced polypropylene was investigated by using “Cracked Round Bar” (CRB) specimen in cyclic fatigue tests. Therefore, the propagation of the visible damage zone was investigated as a function of applied maximum load and R-ratio. It became apparent that the length of the damage zone after the same number of cycles was bigger for higher maximum forces. For the same maximum force and different R-ratio the damage zones were smaller in the beginning for lower R-ratios, most likely caused by the acting mean stress. However, bigger force amplitudes at lower R-ratios force the damage zone to grow faster as the test continues. Ultimately, this leads to a failure at a lower number of cycles compared to tests with a higher R-ratio. In order to implement a crack propagation law, depending on the damage propagation, a compliance calibration was done. Due to the low crack opening during the experiments, leading to difficulties with differentiation between damage propagation, signal noise and temperature effects, it was not possible to find an explicit correlation.
Translated title of the contributionResidual stress analysis and damage behaviour of reinforced polypropylene pipe materials
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Arbeiter, Florian, Co-Supervisor (internal)
  • Pinter, Gerald, Supervisor (internal)
Award date26 Jun 2015
Publication statusPublished - 2015

Bibliographical note

embargoed until 06-05-2020

Keywords

  • polypropylene
  • SIM
  • creep modulus
  • poisson’s ratio
  • residual stress
  • fatigue
  • damage zone

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