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

Claim 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.

Claim 8

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.

Claim 9

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.