Eddy current test probe with circumferential Segments and method of testing material surrounding fastener holes

• US 4,495,466 A
• Filed: 04/08/1982
• Issued: 01/22/1985
• Est. Priority Date: 04/08/1982
• Status: Expired due to Fees

• Current Assignee: Northrop Corporation
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Lakin, Kenneth M.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/366,732
• Time in Patent Office: 1,020 Days
• Field of Search: 324/200, 324/228, 324/234, 324/235, 324/238-243
• US Class Current: 324/242
• CPC Class Codes: G01N 27/904 with two or more sensors

Abstract

An instrument having a multi-segmented cup core of a high permeability material with a center post. The outer wall of the cup contains equidistant holes and slots through which a plurality of individual windings are constructed to form a circular array of pick-up coils. A primary winding or search coil is constructed around the center post. Excitation of the search coil induces eddy currents in adjacent multi-layered bolt hole regions which in turn produce a voltage in each of the pick-up coils. As a crack in the material under test will cause an abnormal distribution of eddy currents, a comparison of the voltage measurements of the specimen under test with those of a reference sample will detect the presence of a crack and its corresponding orientation.

Claims

1. A method of testing specimens with fastener holes in multi-layer material which may have sealant material between layers using an instrument comprising a cup core formed of high permeability material having an outer cylindrical wall and an inner center post with the outer wall being joined to the center post at one end and the other end terminating with a rim so as to form an open end of a cup, the outer wall containing a plurality of equidistant openings extending completely through the outer wall, and along the cylindrical length from the rim to a plane perpendicular to the cylindrical axis through the outer wall, to thereby provide a plurality of segments in the form of peripheral posts around the outer wall, with a plurality of segments in each quadrant, with the segments having planes of symmetry through the cylindrical axis which have equal angles between them completely around the cylinder, there being a search coil wound around said center post, and a pick-up coil wound around each segment, the pick-up coils being substantially the same size and shape, at substantially the same location on the respective segments;

said method comprising the steps of;

exciting the center post winding with alternating current of a selected operating frequency;

scanning the pick-up coils and measuring the voltage of each individual pick-up coil with the instrument centered on a fastener of a flawless reference specimen;

scanning the pick-up coils and measuring the voltage of each individual pick-up coil with the instrument centered on a fastener of a specimen to be tested with the same excitation voltage; and

comparing the voltage of each individual pick-up coil with the corresponding voltage from the reference specimen whereby flaws in the specimen being tested are indicated by discrepencies between the two voltage readings and flaw orientation is determined by identification of the segment coil;

said frequency being lowered for testing deeper layers of material.

2. The method according to claim 1, wherein the instrument has sixteen of said segments, and the frequency for testing deeper layers of material is approximately 400 Hertz.

3. The method according to claim 1, wherein the instrument is centered on a fastener of a specimen by connecting the center post winding to a bridge circuit, the bridge circuit being also connected to a null detector, exciting the center post winding via the bridge circuit with a voltage at a centering frequency which is substantially higher than said operating frequency, flaw and other effects being cutoff due to skin effects at the centering frequency, and positioning the instrument so that the bridge is balanced with no error voltage indicated by the null detector.

4. A method of testing specimens with fastener holes in multilayer material which may have sealant material between layers using an instrument comprising a cup core formed of high permeability material having an outer cylindrical wall and an inner center post with the outer wall being joined to the center post at one end and the other end terminating with a rim so as to form an open end of a cup, the outer wall containing a plurality of equidistant openings extending completely through the outer wall, and along the cylindrical length from the rim to a plane perpendicular to the cylindrical axis through the outer wall, to thereby provide a plurality of segments in the form of peripheral posts around the outer wall, with a plurality of segments in each quadrant, with the segments having planes of symmetry through the cylindrical axis which have equal angles between them completely around the cylinder, there being a search coil wound around said center post, and a pick-up coil wound around each segment, the pick-up coils being substantially the same size and shape, at substantially the same location on the respective segments;

said method comprising the steps of;

centering the instrument on a fastener of a specimen to be tested by connecting the center post winding to a bridge circuit, the bridge circuit being also connected to a null detector, exciting the center post winding via the bridge circuit with a voltage at a very high centering frequency, and positioning the instrument so that the bridge is balanced with no error voltage indicated by the null detector;

exciting the center post winding with alternating current of a selected relatively high frequency;

measuring the voltage of each individual pick-up coil

exciting the center post winding with alternating current of a selected lower operating frequency;

scanning the pick-up coils and measuring the voltage of each individual pick-up coil

comparing the voltage of each individual pick-up coil as measured with the lower operating frequency with the corresponding voltage as measured with said relatively high frequency, the two sets of voltages being measured with the same instrument placement, whereby flaws in the specimen being tested are indicated by discrepencies between the two voltage readings and flaw orientation is determined by identification of the segment coil;

said lowered operating frequency being lowered further for testing deeper layers of material.