Electricity: measuring and testing – Magnetic – Calibration
Reexamination Certificate
1997-09-29
2001-09-04
Snow, Walter (Department: 2862)
Electricity: measuring and testing
Magnetic
Calibration
C324S238000, C324S225000
Reexamination Certificate
active
06285183
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for determining surface defects on metal components due to corrosion and damage. More specifically, this invention relates to a method for quantifying the volume loss caused by surface corrosion and damage by measuring the magnitude of eddy current responses on the surface of the substrate.
BACKGROUND OF THE INVENTION
Corrosion damage is a significant threat to the safe operation of both military and commercial aircraft. Failure to detect and correctly quantify corrosion damage can lead to catastrophic failure of various aircraft components. Such failures can lead to the loss of the aircraft as well as loss of lives. Corrosion damage is an even more significant problem with military aircraft due to the extreme environments in which they must operate.
The relevant art is replete with systems for determining damage to metal structures. One such system includes a thermographic system for detecting exfoliation corrosion on aircraft skins. However, this technique is limited to flat surfaces and only presents images that are visually compared with surrounding areas. Traditional impedance plane eddy current techniques are also used to evaluate corrosion loss on aircraft skins. The eddy current response from a test sample is compared to the eddy current response from a reference standard. The result is a subjective evaluation of whether or not the test sample is better or worse than the reference standard. The technique is limited by the inability to produce corrosion reference standards with quantifiable defects. As a result, assessment of corrosion damage in an aircraft metallic structure has required a good understanding of the physics involved and extensive experience of a specialist in complex and time consuming inspection techniques currently available in the industry. In other words, corrosion damage determination has been highly subjective and quality depends to a large degree upon the individual conducting the evaluation.
Conventional eddy current inspection methods are typified by U.S. Pat. No. 5,510,709 to Hurley. This patent discloses an eddy current surface inspection array probe and a method for detecting cracks and flaws in aircraft skin metal immediately surrounding rivets without requiring the removal of rivets or manual scanning. The method comprises positioning a probe concentrically around the rivet. The sense coil pairs of the probe either provide a zero reading corresponding to no cracks or defects or a non-zero reading corresponding to one or more cracks or defects in the surface. Thus, this system is used to locate only two dimensional anomalies such as cracks in the surface around the rivet. Furthermore, the method described in this patent does not provide a quantified measurement of the metal loss on the surface around the rivet.
Another method of detecting surface corrosion is described in U.S. Pat. No. 5,491,409 to Flora et al. The Flora invention permits the detection of surface corrosion on metallic structures which are insulated by a coating or cover, or covered with marine growth. Excitation coils are wrapped around a magnetizing yoke which carries an alternating current in order to produce an alternating magnetic field by the magnetized yoke. The alternating magnetic fields induce an eddy current which runs through the metal component between the legs of a yoke and a pair of magnetic flux sensors are differentially connected beneath one or more of the excitation coils to sense a signal response. Thus, defects located on the surface can be detected. Nevertheless, Flora is only of interest and does not provide a method of quantifying the volume loss of these defects.
Moreover, the great majority of damage to structural members due to corrosion originates from corrosion damage within the fastener holes. However, none of the current inspection methods mentioned are applicable to evaluating corrosion damage in fastener holes.
SUMMARY OF THE INVENTION
The present invention provides a method for the direct measurement and quantification of the volume loss along the surface of a substrate and thus more accurately depicts the surface profile of a surface than conventional processes which can only provide comparative measurements of a surface profile or two dimensional anomalies on a test surface. In addition, the method of the invention can be used with various types of materials and can be used to measure volume losses even on non-planar surfaces such as around fasteners or inside fastener holes where corrosion defects often originate. The method of the invention allows a contour map of the surface of substrate to be produced illustrating the material lost due to corrosion or damage. Advantageously, the method of the invention allows for the early determination of metal loss due to corrosion and damage thereby allowing replacement or repair of the test substrate prior to failure of the substrate.
The method of the invention comprises inducing eddy currents in a test substrate, measuring the magnitude of the eddy current within the substrate at a plurality of locations on the surface of the test substrate, and converting the measured eddy current magnitudes at the locations to corresponding volume losses on the surface of the substrate using the eddy current magnitude measurements of a reference substrate having defects of predetermined volume loss. Typically, the measurements of the eddy current magnitude on the test surface are converted by multiplying a normalized eddy current magnitude for a sector on the surface of the test substrate both by a predetermined calibration factor (C
f
) representing volume per unit area eddy current magnitude and the surface area of the sector to provide volumetric measurements of the metal loss in the sector. The calibration factor (C
f
) is determined by measuring the magnitude of the eddy current produced for a plurality of defects having predetermined volume losses on the surface of a reference substrate.
In a specific method embodiment of the invention, the volume loss on the surface of a metal test substrate is measured by first calibrating the system using a reference substrate and then using the system to test the surface of a substrate. The system calibration comprises inducing an eddy current in a reference substrate having a surface comprising a plurality of sectors of predetermined surface area and predetermined volume loss such that at least a portion of the sectors have no volume loss and at least a portion of the sectors have a positive volume loss. The magnitude of the eddy current produced in the reference substrate is measured on the surface of the substrate at a plurality of locations within each sector and the eddy current magnitudes measured for the locations in the sectors having no volume loss are averaged to provide a threshold level (T). The threshold level (T) is then subtracted from the measured magnitudes for the locations in the sectors having positive volume loss to provide a normalized eddy current magnitude for each location. Preferably, the normalized eddy current magnitudes measured at each of the locations in the sector having the greatest volume loss are summed to provide a normalized eddy current magnitude for the sector of greatest volume loss. The predetermined volume of the sector of greatest volume loss is then divided by both the normalized eddy current magnitude for the sector of greatest volume loss and the surface area of the sector of greatest volume loss to produce a calibration factor (C
f
) representing the measured volume per unit area eddy current magnitude.
Once the process is calibrated, a surface is tested by inducing eddy currents in the test substrate and measuring the magnitude of the eddy current produced within the substrate on the surface of the substrate at a plurality of locations within sectors of predetermined surface area. As in the calibration process, the locations of no volume loss are averaged to produce a threshold value (T
test
). The measured eddy current magnitudes at the locations are decreased b
Collingwood Michael R.
Keener Steven G.
Alston & Bird LLP
McDonnell Douglas Corporation
Snow Walter
LandOfFree
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