Fracture from artificial spherical pores, as well as natural defects, in alumina in a grain-size range of 0.8-9.2 µm has been studied experimentally and compared with a fracturemechanics model. Results from fracture-strength measurements have been combined with detailed fractographic analysis to elucidate the ensuing crack instability. Two existing models of possible crack configurations have been extended and contrasted. The semicircular crack as well as the circumferential crack both are described as flaws in the stress-concentrating field of a spherical pore. Surface correction terms afforded by the presence of the pore have been incorporated. A comparative computation shows that fracture occurs more likely from the semicircular crack configuration than the circumferential crack configuration.

Neutron powder diffraction was used in operando to determine the macroscopic strain and piezoelectric coefficient as a function of applied electric field in a technically relevant actuator material. We were able to individually investigate the two coexisting phases in the material and reveal the origin of maximized strain at phase boundaries. Insight into the strain mechanisms with unprecedented detail gives evidence that, on average, the classic inverse piezoelectric effect does not apply for polycrystalline materials.

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