Crack detection method for shaft at rest
First Claim
1. A method for detecting a crack in a shaft system under test, comprising the steps of:
- utilizing a multi-station structural dynamics model representative of the shaft system under test without cracks to derive natural frequencies of an uncracked shaft system, each natural frequency having an associated mode shape representative of shaft system deflection at the natural frequency of each point along a longitudinal axis of the shaft system;
defining a probable axial location of a crack and selecting from among the natural frequencies derived from the model a natural frequency of interest having an associated mode shape which exhibits significant localized bending at said probable axial location of the crack and at a site of response measurement;
modifying the model to include a representation of a crack at said probable axial location;
employing said modified model to calculate effect of said representation of a crack upon the natural frequency of interest as a function of crack depth;
introducing an excitation force at an excitation site on the shaft system under test and taking measurement of vibrational response of the shaft system to said force at the site of response measurement;
processing said measurement to determine an actual natural frequency of the shaft system under test in a region near the natural frequency of interest; and
comparing said actual natural frequency to the calculated effect of the representation of a crack upon the natural frequency of interest in order to determine the existence and severity of a crack in the shaft system under test.
1 Assignment
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Accused Products
Abstract
The presence, size and location of a crack in a shaft is determined by comparing actual measured natural frequencies of the shaft system with the results of an analytical model. From a multi-station analytical model of an uncracked 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, and a predicted split and downward shift of a lateral natural frequency of interest 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 excitation force, and vibrational response measurements are taken. A fast Fourier transform analyzer derives a frequency response function from the measurements which indicates 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.
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Citations
46 Claims
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1. A method for detecting a crack in a shaft system under test, comprising the steps of:
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utilizing a multi-station structural dynamics model representative of the shaft system under test without cracks to derive natural frequencies of an uncracked shaft system, each natural frequency having an associated mode shape representative of shaft system deflection at the natural frequency of each point along a longitudinal axis of the shaft system; defining a probable axial location of a crack and selecting from among the natural frequencies derived from the model a natural frequency of interest having an associated mode shape which exhibits significant localized bending at said probable axial location of the crack and at a site of response measurement; modifying the model to include a representation of a crack at said probable axial location; employing said modified model to calculate effect of said representation of a crack upon the natural frequency of interest as a function of crack depth; introducing an excitation force at an excitation site on the shaft system under test and taking measurement of vibrational response of the shaft system to said force at the site of response measurement; processing said measurement to determine an actual natural frequency of the shaft system under test in a region near the natural frequency of interest; and comparing said actual natural frequency to the calculated effect of the representation of a crack upon the natural frequency of interest in order to determine the existence and severity of a crack in the shaft system under test. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27)
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28. A method for detecting a crack in a shaft system under test, comprising the steps of:
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employing a multi-station analytical model of the shaft system to determine a natural frequency of interest for a crack at a designated location and to predict a shift of said natural frequency of interest as a function of crack depth; measuring an actual natural frequency of the shaft system under test at rest in response to a vibration inducing force excitation, said actual natural frequency being in the region of the natural frequency of interest; and comparing said actual natural frequency to the predicted shift of the natural frequency of interest to determine a correlation therebetween, whereby such a correlation indicates the existence of a crack in the shaft system and the severity thereof. - View Dependent Claims (29, 30)
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31. A method for detecting a crack in a shaft system under test, comprising the steps of:
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(a) employing a multi-station analytical model of the shaft system to determine a first natural frequency of interest for a crack at a designated location and to predict a split and shift of said first natural frequency of interest as a function of crack depth for lateral analysis; (b) measuring actual natural frequencies of the shaft system under test at rest in response to a lateral vibration inducing force excitation, said actual natural frequencies being in a region near the first natural frequency of interest; (c) comparing said actual natural frequencies to the predicted split and shift of the first natural frequency of interest to determine a correlation therebetween; (d) employing a multi-station analytical model of the shaft system to determine a second natural frequency of interest for the crack and to predict a downward shift of said second natural frequency of interest as a function of crack depth for torsional analysis; (e) measuring an actual natural frequency of the shaft system under test at rest in response to a torsional vibration inducing force excitation, said actual natural frequency being in a region near the second natural frequency of interest; and (f) comparing said actual natural frequency to the predicted downward shift of the second natural frequency of interest to determine a correlation therebetween. - View Dependent Claims (32)
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33. A method for detecting a crack in a shaft system under test, comprising the steps of:
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introducing a force excitation at an excitation site on the shaft system under test; measuring vibrational response of the shaft system to said excitation at a site of response measurement; processing said measured vibrational response to determine an actual natural frequency of the shaft system under test in a region near a natural frequency of interest, said natural frequency of interest having been derived from a multi-station structural dynamics model representative of the shaft system under test without cracks, said natural frequency of interest having an associated mode shape which exhibits a region of high bending at a probable axial location of a crack and at the site of response measurement; and comparing said actual natural frequency to one of (a) the natural frequency of interest and (b) an earlier similarly determined actual natural frequency of the shaft system, in order to identify the presence of a crack in said shaft system. - View Dependent Claims (34, 35, 36, 37, 38, 39)
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40. In a method for determining presence and severity of a crack in a shaft system under test, an improvement comprising the following sequence of steps:
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utilizing a multi-station structural dynamics model representative of the shaft system under test without cracks to derive natural frequencies of an uncracked shaft system, each natural frequency having an associated mode shape representative of shaft system deflection at the natural frequency; defining a probable axial location of a crack and selecting from among the natural frequencies derived from the model a natural frequency of interest having an associated mode shape which exhibits significant localized bending at said probable axial location of the crack and at a site of response measurement; modifying the model to include a representation of a crack at said probable axial location; and employing said modified model to calculate effect of said representation of a crack upon the natural frequency of interest as a function of crack depth, such that a measured actual natural frequency of the shaft system under test in a region of the natural frequency of interest can be compared to the calculated effect of the representation of the crack upon the natural frequency of interest in order to determine presence and severity of a crack in the shaft system under test.
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41. A method for detecting a crack in a shaft system under test, comprising the steps of:
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subjecting the shaft system to a vibration inducing force excitation at an excitation site; measuring frequency response of the shaft system to said force excitation at a site of response measurement; identifying in said frequency response an actual natural frequency in a region near a natural frequency of interest, said natural frequency of interest being derived from a multi-station structural dynamics model representative of the shaft system under test; and comparing said actual natural frequency to the natural frequency of interest in order to determine the existence of a crack in the shaft system. - View Dependent Claims (42, 43, 44, 45, 46)
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Specification