GB2350900A

GB2350900A - Surface flaw detecting probe for electrically conductive materials

Info

Publication number: GB2350900A
Application number: GB9913124A
Authority: GB
Inventors: Andrew Michael Pratt
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-06-08
Filing date: 1999-06-08
Publication date: 2000-12-13
Also published as: GB9913124D0

Abstract

A surface flaw detecting probe for electrically conductive materials comprises conductive strips 3, 4, 5, 6, positioned in close proximity to the surface 1 being tested so as to form with the area of material next to each strip an electrical transmission line. The propagation time of high frequency signals along the transmission lines is increased when current is diverted around such a flaw. Transmission lines may be operated in pairs 3 and 5, 4 and 6, so that electrical connection to the material 1 is unnecessary. The use of two separate pairs allows discrimination between flaws and changes in proximity to the surface 1. The probe may be contoured to match the shape of the surface being inspected.

Description

2350900 SURFACE FLAW DE1EMNG PROBE FOR ELECTRICALLY CONDUMVE MATERIALS

Tlis invention relates to Non Destructive Testing.

There are several well developed methods for detecting flaws such as cracks in the surface of materials for example penetrant, magnetic particle, eddy current, ultrasonic, and radiographic flaw detection. None of the methods mmbine rapid scanning of large areas coupled with low cost and freedom from health and safety problems.

According to the present invention there is provided a surface flaw detecting probe for electrically conductive materials compdsing an arrangement of electrically conductive strips positioned in close proximity to the surface being tested so as to form with the area of the material next to each strip an electrical transmission line, the propagation time for high frequency signals along the transmission line being increased by diversion of the current in the material surface around any flaw crossing the conductive strip.

A specific embodiment of the invention will now be described by way of an example with reference to the accompanying drawing in which:- Figure 1 shows in perspective a flaw detecting probe for scanning flat surfaces. The material being inspected 1 is shown cut away for clarity.

Figure 2 shows the electrical circuit used for measuring the changes in propagation times. Two similar oscillators 8 are used, the details are only drawn for one, the other is shown as a box.

Figure 3 shows in perspective a flaw detecting probe for inspecting a concave radius such as the tyre bead scat on an aircraft wheel to show how probes can be contoured to shape of the item being inspected.

Referring to the drawing the flaw detector comprises four electrically conductive strips 3, 4, 5, and 6 mounted on a non conductive former 2 that conforms to the shape of the surface being inspected 1. The four strips are held off the sirface by a layer of suitable insulating material 7. The individual strips now form electrical strip transmission fines with the surface immediately below them- When a high frequency signal (in this case 301fib) is transmitted along an individual line it will travel with a speed determined by the permittivity of the insidating material. Should a flaw such as a cfack be present under an individual transmission line such that the crack length crosses the strip the propagation delay along the transmission line will increase due to the current in the surface of the material being diverted around the flaw. In order to drive signals into and out of the transmission lines without connecting directly to the surface of the material the strips are oonnected in pairs to coaxial cables 10, 11, 12, and 13 as shown in Figure 2. Each coaxial cable has its core connected to one strip of a pair and its screen to the other. The characteristic impedances of each of the transmission lines in the pair are now presented in series as a balanced load at one end of the strips and a generator at 2 the other. The coaxial cables are wound with five turns on ferrite toroids to form balanced to unbalanced transformers 9 where they connect to the strips.

Referring to the circuit diagram two similar oscillators 8, use the coaxial cables and the strip transmission fines as feedback paths. Each oscillator will run at a frequency where the electrical length of the feedback path is one half wavelength.

The outputs of the oscillators 14 and 15 are mixed together in a standard circuit not drawn, and the difference frequency monitored on a spech= analyser or a radio receiver capable of resolving conflimous wave transmissions. It is desirable to adjust the oscillators to run at slightly different frequencies.

11w surface of the material is scamed for flaws by moving the assembly at right angles to the strip lines. Flaws will cause differential changes in the oscillator frequencies due to changes in propagation delay as the flaw passes under a strip line. Any effect that is common to both oscillators such as change in the proximity to the surface say due to paint thickness change will not produce a differenfial change in frequency.

Claims

A surface flaw detecting probe for electrically conductive materials comprising an arrangement of electrically conductive strips positioned in close proximity to the surface being tested so as to form with the area of the material next to each strip an electrical transmission line, the propagation time for high frequency signals along the transmission line being increased by diversion of the cuff ent in the material surface around any flaw crossing die conductive strip.
2 A surface flaw detecting probe for electrically conductive materials as in claim 1 where the conductive strips are arranged to operate in electrically balanced pairs so that electrical connection to the material being tested is not required.
3 A surface flaw detecting probe for electrically conductive materials as in claims 1 and 2 where two pairs of conductive strips one pair carrying a signal that is independent of the signal carried by the other pair can discriminate between flaws causing a difference in propagation delay between the pairs, and changes in proximity causing a common change in propagation delays.
4. A surface flaw detecting probe for electrically conductive materials substantially as described herein with references to Figures 1-3 of the accompanying: drawing.