Detection of surface cracks near a workpiece fastener hole by means of a rotatable probe

• US 4,445,089 A
• Filed: 03/02/1982
• Issued: 04/24/1984
• Est. Priority Date: 08/21/1978
• Status: Expired due to Term

• Current Assignee: The secretary of state for defence in her Britannic Majesty's Government of the United Kingdom of Great Britain And Northern Ireland
• Original Assignee: The secretary of state for defence in her Britannic Majesty's Government of the United Kingdom of Great Britain And Northern Ireland
• Inventors: Harrison, David J.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US06/354,041
• Time in Patent Office: 784 Days
• Field of Search: 324/202, 324/207, 324/219-221, 324/228-240, 324/242, 324/243, 324/260-262, 324/200
• US Class Current: 324/238
• CPC Class Codes: G01N 27/9006 Details, e.g. in the structure or functioning of sensors
• CPC Class Codes: G01N 27/9046 by analysing electrical signals

Abstract

Apparatus for the detection of cracks or other defects near the surface of a conducting engineering material of interest includes a magnetic probe having two sites which in operation act as magnetic poles on the same side of the probe such that an external magnetic field extends between them principally in a region adjacent to that one side of the probe, means for rotating the probe about an axis spaced from the mid-point between the two sites, whereby when the probe is adjacent to the surface of an engineering material of interest occupying the region with the axis substantially perpendicular to the surface the magnetic field is able to penetrate the surface and rotate beneath it about the axis causing energy associated with the magnetic field to be dissipated in the material, and means for detecting a change caused by a defect in the material, to the dissipated energy.

Claims

1. Apparatus for the detection of cracks near the surface of a workpiece and adjacent to a fastener hole therein comprising:

a magnetic probe including a magnetic core having a coil mounted thereon, the core having two arms forming magnetic poles and a link portion between them, said coil being mounted on a first one of said arms, said arms and said link portion having a cross-sectional thickness perpendicular to its axis which is small compared to the spacing between said arms, whereby in operation with the surface of the workpiece adjacent to said poles a magnetic field extends between said poles and penetrates said workpiece, said magnetic field being concentrated in the region of both of said poles, the lateral spread of said magnetic field being restricted by the cross-sectional thickness of said arms and said line portion to a width which is small compared to the spacing between said poles;

a rotational stepping drive arrangement for rotating said probe in a stepwise manner about an axis of rotation substantially parallel to said arms and displaced from said first arm which axis may in use be made coincident with the axis of said fastener hole, whereby said first arm and said coil are rotationally stepped about said axis of rotation;

means for energizing said coil;

detection circuit means for generating at the end of each step of said stepped rotation a first signal representative of the energy of said magnetic field dissipated in said workpiece by detecting the energy supplied to said coil;

reference signal generator means for generating a second signal representative of the energy dissipated in the defect free regions of said workpiece;

comparator means for comparing said first and second signals and for providing a third signal representing any difference between said first and second signals;

an analog-to-digital converter for digitizing said third signal,

a digital store for storing said third signal in digital form while said probe is rotating; and

a display, having an input from said store, for displaying a graphical plot of the variation of said third signal with the rotational position of said probe.

2. Apparatus as claimed in claim 1 wherein said stepping drive arrangement comprises means for stepping the position of said first arm around an arcuate path and returning said first arm to a starting position by rotating said arm in the reverse direction of said stepped rotation.

3. Apparatus as claimed in claim 2 wherein the time taken for a forward rotation of said first arm is greater than 1 second.

4. Apparatus as claimed in claim 1 wherein said core is substantially E-shaped, one of the outer arms of said core being said first arm carrying said coil, the axis of rotation of said probe passing through the middle arm of said core, the other outer arm of said core carrying a second coil acting as a reference signal coil, said reference signal generator being a circuit for detecting the voltage across said second coil.

5. Apparatus as claimed in claim 1 wherein said core comprises two mutually orthogonal E-shaped members having a common central arm, each of the outer arms of said core carrying a coil, the energizing means being connected to each of the coils and the comparator means being arranged to compare the voltages across opposite pairs of said coils.

6. Apparatus as claimed in claim 1 wherein the apparatus comprises a computer device incorporating said store which computer device also includes a pattern recognition processor for comparing said third signal with a pre-programmed set of data characteristic of a signal due to a crack in said workpiece.

7. A method for detecting cracks near a fastener hole provided in the surface of a conducting workpiece including the steps of:

(a) locating a rotatable probe above said hole with the axis of rotation of said probe substantially coincident with the axis of said hole, said probe including a magnetic probe including a coil mounted on a magnetic core, the core being shaped so as to provide two arms forming separated magnetic poles, said arms having at said poles a cross-sectional area which is small compared with the distance between said poles to form a field concentrated in the region of said poles, one of said poles being on one side of said probe and the axis of rotation passing through the other of said poles, the probe, when energized creating said magnetic field extending between poles principally in a region adjacent to that one side of the probe and in the region adjacent to one side of said hole, said coil being mounted on said arm on said one side of said probe;

(b) energizing said coil to provide said magnetic field;

(c) rotating said probe by means of a stepping drive arrangement about its axis of rotation in a step-wise fashion; and

(d) generating at the end of each step of stepped rotation a first signal representative of the energy of said magnetic field dissipated in said workpiece by detecting the energy supplied to said coil;

(e) generating a second signal representative of the energy dissipated in defect free regions of said workpiece;

(f) comparing said first and second signals and providing a third signal representing any difference between said first and second signals;

(g) digitizing said third signal,

(h) storing said third signal in digital form while said probe is rotating; and

(i) displaying a graphical plot of the variation of said third signal with the rotational position of said probe.