GB2361065A

GB2361065A - Flaw detecting probe and method

Info

Publication number: GB2361065A
Application number: GB0013715A
Authority: GB
Inventors: Andrew Michael Pratt
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-04-05
Filing date: 2000-06-07
Publication date: 2001-10-10
Also published as: GB0008221D0; GB0013715D0

Abstract

A probe and method for detecting flaws (12) in electrically conductive materials (18) by scanning (10, 11) large areas of the material comprises one or more long narrow electrically conductive elements (1-4) held close to but not contacting the material being inspected. The conductive elements are scanned over the material and the conductivity of the material is monitored as an impedance to alternating current in the element. Flaws in the material can be detected as they create a local change in the conductivity and hence change the impedance in the element passing over the flaw. A single element may be used but a plurality of elements may be connected to form a symmetrical bridge network so as to be insensitive to changes in proximity to the material, only to flaws.

Description

2361065 1 Flaw Detecting Probe and Method This invention relates to

non-destructive testing of electrically conductive materials.

Finding flaws in materials by using eddy currents is a well established method. However as the area within which flaws can be detected is local to the probe the surface has to be scanned completely, usually in a raster or zig zag pattern, so that the probe centre comes within detecting distance of any flaws. When carried out by hand this can be tedious and time consuming if a large area has to be inspected.

According to the present invention there is provided a flaw detecting probe and method for electrically conductive materials, the probe comprising one or more long narrow conductive elements which are held close to but not in electrical contact with the material being inspected. The impedance to alternating current in the conductors being affected by the material being inspected, when the conductors are scanned across the material, flaws in the material will change the impedance of the conductive elements.

Specific embodiments of the invention will now be described by way of examples with reference to the accompanying drawing.

Figure 1 shows in plan view looking down on a four element probe placed on a material to be inspected. For clarity the electrical connections are not shown. The elements are approximately 10Omm long.

Figure 2 shows an end elevation of Figure 1.

Figure 3 shows how the four parallel elements of Figures 1 and 2 are connected to form a bridge network.

Figure 4 shows a suitable electronic circuit for detecting changes in impedance of the conductive elements.

Figure 5 shows a two element probe.

Figure 6 shows a four element probe formed to the contours of a particular component in this case the tyre bead seat of an aircraft wheel.

Figure 7 shows a one element probe.

The first example to be described is a four element probe as shown by Figures 1, 2, and, 3 this probe is for inspecting flat surfaces. The four elements 1, 2, 3, and 4 are mounted on a nonconductive former 5 and their proximity to the surface of the material being inspected 18 is set by the insulating spacer 6. The four elements are electrically connected to form a bridge network as shown by Figure 3. An oscillator 7 provides the alternating current drive to the bridge network. The frequency of oscillation in this example is approximately 100 kHz. The frequency used is determined by the depth of penetration required in the material being inspected. When the probe is in contact with the surface all four elements 1, 2, 3, and 4 are affected equally by the currents induced in the material, and the output voltage from the bridge at the detector nodes 8 and 9 is low. The probe is scanned in a direction 10 or 11 at 2 right angles to the elements. If for example the direction of the scan is 10 then when a flaw 12 passes under the leading element 1 the bridge will be unbalanced as the impedance of the element 1 will be affected by the local decrease in conductivity due to the flaw 12. The voltage across the detector nodes 8 and 9 will change and be amplified by the ac amplifier stage 13. The ac signal is then rectified to dc by the stage 14. As the probe is passing over a flaw this dc signal will be changing with respect to time as each of the elements 1, 2, 3, and 4 pass over the flaw in turn. The changing dc level is passed by the high pass filter 15 which in this case is set at approximately 6 Hz. The cut off frequency is determined by the required scanning speed for the probe. The output of the high pass filter is then amplified to drive the meter 16 by the variable gain amplifier 17.

The probe can be constructed with only two elements in close proximity with the material being inspected. The other two elements are fixed impedances to complete the bridge network, Figure 5.

The probe can be constructed to conform to the shape of a particular component being inspected. Figure 6 shows a four element probe to inspect the tyre bead seat of an aircraft wheel.

The probe can be constructed with only one element where the oscillator and detector circuits have common connections as Figure 7. However without using a bridge network the single element is more sensitive to changes in proximity to the surface due to paint thickness variation or probe movements.

3

Claims

A flaw detecting probe and method for electrically conductive materials, the probe comprising one or more long narrow conductive elements which are held close to but not in electrical contact with the material being inspected, the impedance to alternating current in the conductive elements being affected by the material being inspected, when the conductive elements are scanned across the material being inspected, flaws in the material being inspected will change the impedance of the conductive elements.
2 A flaw detecting probe and method for electrically conductive materials as claim 1 comprising several elements, connected as a symmetrical bridge network, the bridge being unbalanced by the effect of a change in local conductivity due to a flaw in the material being inspected, the bridge remaining in balance for changes in proximity which affect all elements.
3 A flaw detecting probe and method for electrically conductive materials substantially as described and illustrated in the accompanying drawings.