A special test fixture has been developed for fracture mechanical testing of brittle materials inside an environmental scanning electron microscope. The fixture loads a double cantilever beam specimen with pure bending moments and provides stable crack growth. Crack growth is detected by in situ observation and acoustic emission. As an example, crack growth in a cubic‐phase yttria‐stabilized zirconia is detected easily by in situ observation of the crack‐tip region. Many fracture toughness measurements are obtained for each specimen, giving high confidence in the measured fracture toughness value. In situ observation is useful for the study of toughening mechanisms and subcritical crack‐growth behavior and to sort out erroneous measurements (e.g., due to crack branching).

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