Method of an apparatus for selecting the optimum test frequency in eddy current testing

Claim 1

In an eddy current testing apparatus for determining a metallurgical characteristic of material samples; the combination including a substantially constant current, electrical signal source, a single eddy current device having a single primary constant field-producing coil and a single secondary sensing coil with provision for placing the sample in said single device so as to generate an eddy current field between said primary and secondary coils, an electrical narrow pulse signal generator connected to said signal source across a resistor in series with said single primary coil and capable of producing independent of frequency an electrical pulse signal when the electrical current signal from said signal source and the field from said primary coil passes through zero between cycles, an electrical integrator circuit coupled to said secondary coil capable of shifting said signal phase angle by 90* and compensating for variation in the signal voltage amplitude generated across said secondary coil due to electrical signal frequency changes, the signal from said integration circuit is thereby a signal the magnitude and phase angle of which is dependent only upon the eddy current characteristics of said test specimen, a signal detection circuit coupled to said integration circuit and said pulse signal generator circuit, said detection circuit capable of producing a direct current signal proportional to the magnitude of the signal from the integrating circuit at the instant the pulse signal representing the zero crossing of the current through said primary coil and through the zero crossing of the produced field is received from said pulse signal producing circuit, and means for measuring the direct current so produced coupled to said detection circuit, said last mentioned means thereby indicating the magnitude of said resistance component so produced whereby the metallurgical characteristics of the test sample is indicated.

Claim 12

A method of comparing a metallurgical characteristic of unknown material sample to that of a standard sample, comprising placing the standard sample in the field of a coupling device, subjecting the coupling device to uniform current electrical signals over a wide range of signal frequencies, measuring the resistance component of the output signal at various predetermined frequencies, selecting that frequency at which the resistance component of the signal is maximum, and comparing the magnitude of the resistance voltage component of the standard sample under test at that frequency with the resistance voltage component of the unknown sample of same size and shape at the same selected frequency to thereby determine the variation of the eddy current characteristic of the unknown material under test from the eddy current characteristic of the standard sample at the same frequency whereby the metallurgical variation between the standard sample and the unknown material sample is determined.

Claim 13

A method of comparing a metallurgical characteristic of unknown material sample to that of a standard sample, comprising placing the standard sample in the field of a coupling device, subjecting the coupling device to uniform current electrical signals over a wide range of signal frequencies, measuring the resistance component of the output signal at various predetermined frequencies, selecting that frequency at which the resistance component of the signal is maximum, and comparing the magnitude of the resistance voltage component of the standard sample under test at that frequency with the resistance voltage component of the unknown sample of same size and shape at the same selected frequency to thereby determine the variation of the eddy current characteristic of the unknown material under test from the eddy current characteristic of the standard sample at the same frequency whereby the metallurgical variation between the standard sample and the unknown material sample is determined.