CRACK DETECTION METHOD FOR OPERATING SHAFT

The presence, size and location of a crack in a shaft is determined by comparing actual measured natural frequencies of the operating shaft system with the results of an analytical model. From a multi-station analytical model of an uncracked operating shaft system, natural frequencies and associated mode shapes are derived. A suspected axial location of a crack is defined, and a natural frequency of interest is selected which has an associated mode shape exhibiting significant localized bending at the suspected axial location of the crack and at a site of response measurement. The analytical model is modified to include a representation of an asymmetric crack at the suspected crack location. A predicted split and downward shift of a lateral natural frequency of interest as a function of crack depth and/or a predicted downward shift of a torsional natural frequency of interest as a function of crack depth is calculated from the modified model. The actual shaft system is subjected to an operating force excitation, and vibrational response measurements are taken with strain gages and then output through a short range telemetry system. A fast Fourier transform analyzer derives frequency response spectra from the measurements which indicate the actual natural frequencies of the shaft system. A comparison of actual natural frequency(s) in the region near the natural frequency of interest with frequency values predicted by the modified model is employed to determine the presence and severity of a crack in the shaft.