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Nonlinear ultrasonics is suggested as a new approach for the effective evaluation of material degradation. As its quantification, the parameter /spl beta/ is introduced on the basis of nonlinear elasticity, and a new method to measure the parameter /spl beta/ using bispectral analysis is proposed. Then, the correlation between /spl beta/ and material degradation is investigated. From the results for several mild steel (SS41, SS45) specimens that were degraded by stretching and cyclic loads, it was confirmed that the parameter /spl beta/ has a strong correlation with material degradation. As another practical application, the evaluation of the aging degradation in a high temperature material is tried. For this, Cr-Mo-V specimens that are generally used in turbine rotors in power plants were prepared, and the variation of /spl beta/ caused by aging time was investigated. For comparison, the fracture appearance transition temperature (FATT) of the specimen was measured, and its behavior showed good agreement with /spl beta/. In addition, for all of the experiments, no noticeable change in attenuation and sound velocity in the same specimens with change of degradation were observed. From these results, it may be concluded that nonlinear ultrasonics could be applied to the quantitative evaluation of material degradation.