Electricity: measuring and testing – Magnetic – With compensation for test variable
Patent
1993-04-20
1995-02-21
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
With compensation for test variable
324233, 324240, G01N 2790
Patent
active
053919882
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for nondestructively detecting flaws such as hairline cracks and defects within a conductive object undergoing testing and, more particularly, to method and apparatus capable of distinguishing surface flaws from internal, deep flaws.
BACKGROUND OF THE INVENTION
There is a related art apparatus for detecting flaws such as hairline cracks and defects or the kind of material of a test object using a sensor comprising a ferrite core on which primary and secondary coils are wound. This sensor is moved while being kept in contact with the surface of the conductive test object. A rectangular-wave current is supplied to the primary coil to induce eddy currents in a minute region on the surface of the inspected object. The eddy currents induce a magnetic field, which is detected by the secondary coil. The pulse duration of the voltage pulses detected by the secondary coil are measured. In this way, the flaws or the kind of material is detected. The principle of this detection utilizes the fact that the attenuation time of the eddy currents induced on the surface of the test object, i.e., the pulse duration, differs depending on the flaws such as hairline cracks or defects and on the kind of material.
The peak value and the pulse duration of the voltage pulse detected by the secondary coil are affected greatly by variations in the distance between the test object and the sensor. This makes it impossible to discern the sensor output signals indicating the flaws such as hairline cracks or defects and the material. Hence, the reliability of detection of flaws has presented problems.
Accordingly, it has been important for the flaw detection method and apparatus of this kind to reduce the effect of the aforementioned variations in the distance between the sensor and the object. For this purpose, various contrivances have been made. One example has been proposed by the present Applicant and is disclosed in Japanese Patent Laid-Open No. 316655/1989. In particular, the secondary coil output signal produces positive and negative driving currents in response to signal variations caused by variations in the distance between the sensor and the object. The driving currents activate driving coils disposed around a permanent magnet which is connected to one end of the sensor. In this manner, the distance between the sensor and the object is kept constant. However, the above-described driving currents must be supplied constantly. Also, large amounts of current must be supplied. Consequently, a large amount of electric power is consumed. In addition, the apparatus lacks portability. Furthermore, the responsiveness is a problem.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a flaw detection method and apparatus using eddy currents which cancel the effect of variations in the distance between a sensor including primary and secondary coils and an object undergoing testing only by analyzing the signal of the voltage pulse induced in the secondary coil while directly using the prior art principle employing the attenuation characteristics of eddy currents without mechanically varying the sensor, whereby only the flaws, such as hairline cracks and defects within the object, are detected correctly.
A flaw detection method using eddy currents according to a first embodiment of the invention uses a ferrite core on which primary and secondary coils are wound. A rectangular-wave current is supplied to the primary coil to induce a voltage pulse in the secondary coil. The pulse duration of the induced voltage pulse is measured at two separate time instances corresponding to two different threshold voltages. The effect of the variations in the distance between the ferrite core and the conductive test object is canceled, using both pulse durations. As a result, only correct flaws such as hairline cracks or defects within the object are detected.
Threshold voltages used for measurement of the pulse duration of the pulse voltage induce
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Kabushiki Kaisha Nihon Hihakai Keisoku Kenkyusho
MES Testing & Research Center Co., Ltd.
Mitsui Engineering & Shipbuilding Co. Ltd.
Strecker Gerard R.
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