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Fracture in Ferroelastic Ceramics

Final Activity Report Summary - FERROFRAC (Fracture in Ferroelastic Ceramics)

The mechanical and fracture behaviour of notched LaCoO3 based perovskites under compressive loading at room temperature was analysed. The fracture mechanism of these materials could be explained by considering strain incompatibility and high shear stresses in the lateral directions of the samples. The residual tensile stresses which were produced by strain irreversibility at the notch tip were not large enough to generate direct crack growth as compared to elastic ceramics.

While the majority of the mixed ionic electronic conducting (MIEC) ferroelastic perovskites exhibited a single softening followed by hardening during loading, we reported an unusual dual softening in (La0.9Sr0.1)0.95Cr0.85Ni0.05Mg0.1O3 perovskite because of a pressure-induced first order phase transition from orthorhombic Pnma to rhombohedral phase during loading. The effects of temperature and cyclic incremental loading on the deformation behaviour of LSCNM were studied.

The experimental investigation of room temperature creep at different stresses in polycrystalline LaCoO3 based oxides under compression was carried out. A new phenomenological approach for ferroelastic creep was proposed to identify the most important parameters that affected creep strain over different time periods. An analytical expression was obtained instead of the previously used power law, allowing for the equilibrium strain at constant stress to be estimated. An expression was also proposed to calculate the characteristic time of creep and recovery processes. The equilibrium creep diagrams for LaCoO3 and La0.8Ca0.2CoO3 perovskites were determined.

Finally, a general scheme for describing the mechanical behaviour of ferroelastic materials was developed. It was possible to predict the equilibrium stress-strain curve of ferroelastic materials using this scheme. Relevant results were anticipated to guide the design and modelling of components using ferroelastic materials, such as solid oxide fuel cells and ferroelectric ceramic actuators.