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A method to non-intrusively measure stresses both at and below the surface of structural components is developed. Based on the piezo-spectroscopic effect in optical fluorescence, this technique affords high-accuracy stress measurement with spatial resolution on the order of one micron. The theoretical basis for the piezo-spectroscopic effect is reviewed, and several concepts relevant to its application are established. The applicability of fluorescence spectroscopy to stress measurement is demonstrated on two sapphire-fiber reinforced composites, one with γ-TiAl and the other with α-Ti as the matrix. The residual stress distributions along the fibers are measured using a through-focusing procedure, and a mean volume stress is determined using a transmission fluorescence configuration. The effective temperature below which residual stresses can no longer be relieved by creep and yielding is determined using an elastic model. In the TiAl-matrix composite, this temperature is approximately 400‡C below the processing temperature, indicating extensive stress relaxation during cooling. The elastic solution fails to provide a reasonable estimate of the stress-free temperature for the Ti-matrix system. This is attributed to extensive matrix yielding, and a complete treatment will require incorporation of inelastic constitutive behavior into the model.
stress measurement, residual stress, piezo-spectroscopy, fluorescence, sapphire, ruby, TiAl matrix, Ti matrix
Graduate student researcher and professor, University of California, Santa Barbara, CA