Photoelastic effects in $Pb(Mg1/3Nb2/3)O3-29%PbTiO3$ single crystals investigated by three-point bending technique

Photoelastic effects in an unpoled $Pb(Mg1/3Nb2/3)O3-29%PbTiO3(PMN-29%PT)$ single-crystal beam have been investigated using three-point bending experiments. A linear relationship between the applied load and the measured displacement was observed up to a proportional limit of $∼30MPa$⁠. Beyond this proportional limit, yielding was observed. Samples were loaded as high as 77 MPa without fracture. Young’s modulus $Y⟨001⟩∼1.9×1010N/m2$ was determined directly from the initially linear region using beam theory. The photoelastic fringe order versus fiber stress plot also displays an initially linear region up to a proportional limit of $∼20MPa$⁠, suggesting that optical measurements are a more sensitive measure of the onset of microplasticity than mechanical measurements. Residual photoelastic fringes associated with yielding were completely removable by annealing above the Curie temperature, implying that plastic deformation occurs by reversible processes such as domain switching and phase transformation. The stress-optical coefficient for unpoled $PMN-29%PT$ determined from the initially linear region of the fringe order versus fiber stress curve is $104×10−12Pa−1$⁠. This value is large and comparable with the stress-optical coefficient of polycarbonate, making unpoled $PMN-29%PT$ single crystal a good candidate for optical stress sensors and acousto-optic modulators.