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3. T. Fett, G. Martin, D. Munz and G. Thun, J. Mater. Sci. 26 (1991) 3320.
4. T. Fett and D. Munz, in Proceedings of the Conference on Fracture Processes in Brittle Disordered Materials, Noordwijk, The Netherlands, 19?21 June 1991.
5. Idem, Kernforschungszentrum Karlsruhe Report 4940 (1991).