Acoustoelastic Spectrum Method for Nondestructive Residual Stress Measurements

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The spectrum method for acoustoelastic stress analysis was studied experimentally. In main experiments, this method was applied to analyze plane stress distributions in two notched aluminum specimens. In the measurement, ultrasonic transverse waves were sent in the thickness direction, and their echoes were analyzed by FFT. First, the amplitude spectrum was used to determine the difference of two transverse velocities, and polarization directions. Since the orthogonality of polarization directions thus determined was not so satisfactory in the stressed state as in the stress-free state, the use of averaged orthogonal directions was shown necessary in this stress analysis. Applying these results to the acoustoelastic law of slightly orthotropic materials, the normal stress difference and shear stress were obtained, and each normal stress was subsequently determined by the shear difference method. Secondly, the normal stress difference was determined from the phase spectrum, and this new approach was confirmed very significant for further development of the spectrum method. Finally the availability of a small size transducer in the spectrum method was also confirmed. Though the stresses treated in these experiments were not residual ones, the obtained results are useful for nondestructive mesurements of residual stresses. For residual stress analysis, another experiments were carried out by using weld, and shrink-fit specimens of steel, and it was confirmed that the shear stress and normal stress difference were nondestructively determined by the spectrum method.