Oberflächenrisse unter Kontaktbelastung

Translated title of the contribution: Contact loading of surface cracks

Gernot Kolb

Research output: ThesisDiploma Thesis

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Favourable properties like a low density, high strength and stiffness combined with a good wear resistance up to high temperatures make structural ceramics candidate materials for ap-plications like tools for metal forming or balls in bearings. In such applications the compo-nents often do not fail by sudden brittle fracture but by growth of preexisting surface flaws due to contact loading. In this thesis the crack growth of artificially introduced (by Knoop indents) surface cracks in silicon nitride plates was investigated using acoustic emission detection. Contact loading was realised by pressing a ball onto the plate. Several practical aspects had to be investigates prior to the tests. Suitable settings for the acoustic emission detection, a practical surface finish of the plates and a possibility for the marking of the contact position were identified. The size and shape of surface cracks intro-duced with loads between 5 kg and 20 kg were determined. Long cracks (crack depth much smaller than crack surface length) were produced by aligning several indents in a row. These surface cracks were stressed by normal loading with a ball at various distances until an acoustic signal was detected. The load necessary for acoustic events increased with increasing distance of the loading point from the crack. For some of the cracks, an elongation was found at the surface. A simple fracture mechanical analysis showed, that not all cracks reach a mode I stress intensity factor that is sufficiently high for crack growth. Unfortunately, further fracture mechanical analyses regarding mixed-mode loading conditions were not possible since no information on mode II and mode III stress intensity factors was available. The obtained results showed that it is possible to grow surface cracks by contact loading and to detect the crack growth acoustically as well as by microscopic observations.
Translated title of the contributionContact loading of surface cracks
Original languageGerman
  • Lube, Tanja, Supervisor (internal)
  • Danzer, Robert, Co-Supervisor (internal)
Award date13 Dec 2013
Publication statusPublished - 2013

Bibliographical note

embargoed until null


  • silicon nitride
  • acoustic emission
  • contact loading
  • crack growth

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