The fracture toughness behavior during crack growth (KR‐curve behavior) of duplex ceramics is investigated. Different types ofKR‐curves can be distinguished depending on the microstructural designs of these materials which are characterized by the volume fraction and size of the dispersed pressure zones, and by their effective volume expansion. According to theirKR‐curve behavior, duplex ceramics can be subdivided into two groups consisting of “short‐range” and “long‐range” toughened materials. The experimental results are discussed regarding the appearance of different toughening mechanisms which are documented by crack path micrographs. An unusual toughening effect, a “crackbranching chain reaction,” is documented by in situ observations. The critical stress to nucleate the observed process zone development is calculated and compared with the internal stress intensity factorKiwhich has been previously proposed for these materials and with