[en] This report discusses the influence of defects on the failure behavior of ceramic materials under four-point bending stress. In this connection various Si₃N₄ and SiC materials with and without artificially introduced defect particles (Fe, WC, Si, pores) were examined by the following non-destructive test methods: photoacoustic microscopy, scanning laser acoustic microscopy, microfocus roentgenoscopy and ultrasound transit-time measurements. Finally, a four-point bending test and a fracture-mechanical evaluation of the fracture-incuding defects were carried out at the Institute for reliability and failure studies in mechanical engineering of the University of Karlsruhe. According to the type of stress the samples predominantly failed in the case of defects in the surface zone of the side in tension. Among the ndt methods applied the photoacoustic microscopy as a typical surface testing method could predict most of the fracture-inducing defects (30-50 %) without causing destruction. In this connection a different detection sensitivity which corresponds to the thermal reflection factors became apparent according to the type of defect. Furthermore the reports describes the results of some preliminary tests on ndt of green ceramics. In these investigations both the microfocus roentgenoscopy test and the roentgen computed tomography showed a high potential of detecting inhomogeneities and defects in green Si₃N₄ and SiC components. (orig.)