Bestimmung bruchmechanischer Eigenschaften als Funktion der Temperatur am Beispiel eines PTC-Werkstoffes

Translated title of the contribution: Determining fracture mechanical properties as a function of temperature of a PTC-material

Eugenia Sharova

Research output: ThesisDiploma Thesis

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Abstract

Barium titanate - based electro-ceramics show a phase transformation at the so-called Curie temperature. Due to this transformation they change their electrical, but also their mechanical properties. For materials exhibiting a positive temperature coefficient (PTC materials), which are based on barium titanate, the high-resistance region above the Curie temperature with respect to mechanical properties is poorly investigated. In this work, the strength of a selected PTC material was tested with the biaxial “Ball-on-3-Balls” test (B3B test), both at room temperature and at 200 °C, i.e. well above the Curie temperature. A slight increase in strength could be observed with a concurrent significant decrease of the Weibull modulus. Furthermore, the influence of air moisture and test speed has been studied. It emerged that the strength decreases strongly with increasing humidity. In addition, smaller testing rates lead to lower strength values by up to 22 %, which proves clearly subcritical crack growth. Another important fracture mechanical property for ceramics is the fracture toughness. Based on the request to determine the fracture toughness on original components and moreover at different temperatures as simply as possible, the methodic main objective of this work was the modification of the existing standardized “Surface-Crack-in-Flexure” method (SCF method) on disk-shaped samples in combination with the B3B test. First the procedure was analyzed at room temperature on a well-studied linear elastic reference material, namely silicon nitride. In comparison with conventional measuring techniques for toughness, the results of the modified SCF method agree well with the fracture toughness-values for silicon nitride. Finally, the fracture toughness measurement was performed both at room temperature and well above the Curie temperature of the non-linear-elastic PTC material. Both the standard, as well as the modified SCF method yielded matching toughness values. Due to a specific fractographic feature at 200°C, the initial crack sizes and therefore the fracture toughness values could be determined with surprisingly low scatter.
Translated title of the contributionDetermining fracture mechanical properties as a function of temperature of a PTC-material
Original languageGerman
QualificationDipl.-Ing.
Supervisors/Advisors
  • Supancic, Peter Hans, Supervisor (internal)
Award date13 Dec 2013
Publication statusPublished - 2013

Bibliographical note

embargoed until null

Keywords

  • PTC
  • strength
  • B3B test
  • fracture toughness

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