Fracture toughness of brittle honeycombs depends on cell microstructure, specimen and geometries. A microstructure coefficient in the KIC expression of brittle honeycombs can not be found analytically. In this paper, a finite element program is utilized to numerically determine the coefficient. In practice, fracture toughness can be measured from conducting a three-point bend or uniaxial tension test. Specimen geometry restrictions of three-point bend test for honeycombs are examined and proposed here. Meanwhile, fracture toughness of honeycombs under the two loading geometries is compared; results show that KIC measured from uniaxial tension test is smaller than that from three-point bend test if the KIC formulations for solid materials are employed. As a result of that, the KIC formulation of three-point bend test is modified for honeycomb-like materials.
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