P 09/01


Mechanical Effects of Clusters in Macro-Porous Ceramics

For porous ceramics used in process technology, fracture toughness and structural stability are essential. A thorough understanding of the interplay between microstructure characteristics and mechanical properties is the basis for a material development, which is focused on applications. The mechanical properties of macro-porous ceramics usually show large variations in experiments also at constant porosity. The prediction of macroscopic mechanical properties is based on homogenization methods. Usual homogenization methods cannot forecast these variations, because they are based on macroscopic mechanical models predicting properties as functions of the averaged porosity. These variations are usually ascribed to random accumulated pores, so-called clusters. Clusters introduce peak stresses under mechanical load. These peak stresses are critical for crack initiations. There is currently no consistent definition of a cluster. Usual cluster definitions include topological properties and no mechanical interactions between pores. In this project we work on stochastic relations between the tendency of building clusters inside a pore distribution and the macroscopic failure behavior. Building on this, we aim on a cluster definition based on spatial proximity as well as local stress peaks. Therefore we investigate theoretically and experimentally correlations between

  • structure characterizing parameters of average statistics and effective properties,
  • structure characterizing parameters of extreme value statistics and local stress concentrations and
  • cluster characterizing parameters of topology and effective as well as local properties.

  • These correlations are theoretically analyzed with a suitable number of representative volume elements based on Finite Element Method (FEM). Compression tests with perforated ceramic discs in terms of physical models and with application orientated macro-porous ceramics validate the FEM results. Analyzed in-situ photography and X-Ray tomography images document the local failure behavior during these tests.

    Contact: Hochrainer, Krummrich