Ultrasonic Sizing of Cracks in Web Geometries

Evaluation of the critical nature of interior cracks in turbine rotor component web regions in order to assess the remaining service life of the parts requires accurate determination of the crack sizes in order to perform fracture mechanics analysis. This analysis is important both in order to retire critically defective parts and in order to return components to service if detected flaws are sub-critical. The purpose of this paper is to evaluate several techniques for sizing internal cracks in planar geometries

Estimates of Eddy Current Response to Subsurface Cracks from 2-D Finite Element Code Predictions

Using a two dimensional finite element code, the response of a U-core eddy current probe was computed for a subsurface flaw in a stainless steel medium. Next, using a three dimensional scattering model, the change in coil impedance was calculated for the same situation. From a comparison of these two results, it was concluded that the two dimensional finite element code overestimates the eddy current sensor response for the practical problem at hand by a factor of 10. This agreed well with the result obtained using an approximate technique described in this paper to estimate the true response from two dimensional calculations. Application of such desensitization factor should allow the two dimensional calculations to be effectively used in design studies

Ultrasonic Sizing of Cracks in Web Geometries

Evaluation of the critical nature of interior cracks in turbine rotor component web regions in order to assess the remaining service life of the parts requires accurate determination of the crack sizes in order to perform fracture mechanics analysis. This analysis is important both in order to retire critically defective parts and in order to return components to service if detected flaws are sub-critical. The purpose of this paper is to evaluate several techniques for sizing internal cracks in planar geometries.</p

Estimates of Eddy Current Response to Subsurface Cracks from 2-D Finite Element Code Predictions

Using a two dimensional finite element code, the response of a U-core eddy current probe was computed for a subsurface flaw in a stainless steel medium. Next, using a three dimensional scattering model, the change in coil impedance was calculated for the same situation. From a comparison of these two results, it was concluded that the two dimensional finite element code overestimates the eddy current sensor response for the practical problem at hand by a factor of 10. This agreed well with the result obtained using an approximate technique described in this paper to estimate the true response from two dimensional calculations. Application of such desensitization factor should allow the two dimensional calculations to be effectively used in design studies.</p

Application of Diffraction Corrections to the Absolute Measurement of Scattering Amplitudes

Ultrasonic scattering measurements are influenced both by the measurement geometry and the properties of the flaw. This paper discusses a set of correction factors which account for the measurement geometry effects and allow the scattering properties of the flaw to be directly related to the measured data. As shown in the inset of Fig. 1, theories for ultrasonic scattering often assume plane wave illumination and predict the scattering amplitude, A, which defines the far field radiation that would be observed in an unbounded elastic medium. This scattering amplitude is a function of frequency and the angles and polarizations of the incident and scattered fields. The diffraction corrections are designed to allow results of measurements in complex geometries to be related to these unbounded medium scattering amplitudes.</p