Photoelastic effects in an unpoled Pb(Mg1/3Nb2/3)O-3-29PbTiO3 (PMN-29PT) 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 similar to 30 MPa. Beyond this proportional limit, yielding was observed. Samples were loaded as high as 77 MPa without fracture. Young's modulus Y-similar to 1.9x10(10) N/m(2) 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 similar to 20 MPa, 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-29PT determined from the initially linear region of the fringe order versus fiber stress curve is 104x10(-12) Pa-1. This value is large and comparable with the stress-optical coefficient of polycarbonate, making unpoled PMN-29PT single crystal a good candidate for optical stress sensors and acousto-optic modulators.
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