GB2040584A - Method and apparatus for introducing electromagnetic ultrasound into electrically conductive material - Google Patents

Abstract

In a method for carrying out non- destructive testing in electrically conductive material (10) an ultrasonic pulse is emitted from a coil (5) located in the magnetic field of an electromagnet (1), and ultrasonic waves are generated in the core (11) and the test material (10) which interact with a receiver coil (6) in the field of another magnet (2). The core (11) of the magnet (1) is made entirely or partially of a composite material consisting substantially of solid particles of magnetic material and elastic bonding agent, the composite material having a strong ultra-sound attenuating effect. According to one embodiment, the outer portion (15, 16) of the core (11) is made of said composite material and in other embodiments the peripheral portions or the entire core (18) is made of said composite material. <IMAGE>

Description

SPECIFICATION Method of introducing electromagnetic ultrasound into electrically conductive material, and apparatus therefor This invention relates to a method of introducing electromagnetic ultra-sound into electrically conductive material.

The invention further relates to an ultra-sound attenuated electromagnet, which like the method is intended to be used primarily for carrying out non-destructive testing in electrically conductive material by electromagnetic ultra-sound.

At electromagnetic ultrasonic testing ultrasound is introduced free of contact into an electrically conductive material by means of a strong magnetic field, which is generated by an electromagnet, in which a transmitter coil is provided, which is fed with current of supersonic frequency.

A transmitted and reflected ultrasonic wave is received in the material by means of a receiver, which also includes an electromagnet, which generates a strong magnetic field, in which a receiver coil is provided. In said receiver coil a signal is generated when the test material vibrates in the magnetic field of the receiver.

When currents flow through a transmitter coil and in the test material, they exercise forces on the electromagnet of the transmitter, especially in the core of the magnet. This causes ultra-sound to be introduced into the core. The ultra-sound is reflected against the defining surfaces of the core, which implies that it takes a relatively iong time until the ultra-sound has decayed completely. The magnetic field generated by the electromagnet of the transmitter, consequently, vibrates in the same way and, therefore, the magnetic field induces a voltage with corresponding variations in the receiver coil. Hereby, thus, an interference arises in the receiver coil which substantially deteriorates the conditions of reception.

In certain cases also the core of the electromagnet of the receiver can start to vibrate, which additionally disturbs the reception conditions.

The present invention reduces or entirely eliminates these interferences.

The present invention relates to a method as referred to in the attached claim 1 and has characterizing features defined by said claim.

The invention further relates to a device referred to in the attached claim 2 which substantially has the characterizing features defined by said claim 2.

When electromagnetic ultra-sound is applied to non-magnetic test material, the signal/noise ratio is unfavourable, thereby rendering it additionally difficult to detect a received ultrasonic echo.

One example is defect indication in steel workpieces or products, which are heated to above the Curie-temperature.

The present invention offers essential advantages by improved reception conditions which, of course, are improved also in the case of magnetic test material.

The invention is described in greater detail in the following, with reference to the accompanying drawing, in which Fig. 1 schematically shows an electromagnetic testing equipment with an embodiment of the invention applied thereto, Fig. 2 shows a core for an electromagnet according to a first embodiment, Figs. 3, 4, 5 show a core according to other embodiments.

In Fig, 1 two electromagnets 1, 2 with associated windings 3, 4 are shown. The first electromagnet 1 is intended to generate a strong magnetic field for transmitting an ultrasonic signal by means of the transmitter coil 5. The second electromagnet 2 is intended to generate a strong magnetic field for receiving an ultrasonic echo by means of the receiver coil 6.

In this example the windings 3, 4 of the electromagnets 1, 2 are fed from the same generating set (not shown) through the conductors 7. The transmitter coil 5 is fed via the conductors 8 from a generator (not shown) which generates supersonic frequency. The receiver coil 6 is connected via conductors 9 to amplifiers and detecting circuits. The function of this electromagnetic testing equipment is in principle briefly as follows.

The electromagnets 1, 2 are activated, whereafter a mostly sinus-shaped ultrasonic pulse is emitted from said generator to the transmitter coil 5. The current in the transmitter coil induces a correspondingly varying current into the surface of the test material 10. As a magnetic field B exists in a conductor and at the same time a current I flows in the conductor (i.e. test material), a force F, the so-called Lorent-force, is formed on the conductor which is equal to F=l x B where the dimension of I is A/m2, B Tesla and F N/m3 In this case the force is oscillating with supersonic frequency, whereby elastic ultrasonic waves are introduced into the test material.

When the ultrasonic waves arrive at the receiver, according to Faraday's induction law a corresponding electric signal is generated in the receiver coil, which is located in a magnetic field from the second electromagnet 2.

As mentioned above, at known devices of this kind vibrations of supersonic frequency occur in the core of the electromagnet 1 of the transmitter due to currents in the transmitter coil 5 and test material 10. The magnetic field hereby generated by the core also vibrates, and thereby this magnetic field induces a voltage with corresponding variations into the receiver coil 6.

In Fig. 1 the two electromagnets have been shown lying relatively close to each other. The distance between them, however, within normal limits for such testing, is of no significant importance for eliminating the aforesaid problems.

The core 11 of the first electromagnet 1 in Fig.

1 is designed as described below with reference to Fig. 2, whereby the aforesaid problems are eliminated. In Fig. 2 a core 11 according to a first preferred embodiment of the invention is shown, where the yoke 12 and the legs 13, 14, for example, are laminated. The outer part of each pole shoe of the core 11 consists of a portion 1 5, 16 of a material having a high ultra-sound attenuating effect. The said material 15, 1 6 is a composite material consisting substantially of small solid particles of magnetic material surrounded by an elastic bonding agent Such a composite material has a strong attenuating effect on ultra-sound.The bonding agent in the composite material consists of a material, which readily conducts ultra-sound from the core into the portions 15, 16 preferably an epoxy resin, for example such one which is marketed under the registered trademark Araldite. The solid particles preferably consist of iron powder whereby good magnetic permeability can be maintained.

Through said portions 15, 1 6 ultra-sound induced into the core 11 is reflected against the defining surfaces of the core and flows into said portions 15, 16 owing to said good ultra-sound contact. The ultrasonic waves, however, are attenuated in the portions 15, 16 and, therefore, only a very small part of ultra-sound having arrived in the portions flows again into the core 1 3. The result, thus is that ultra-sound induced into the core rapidly is decayed.

A further advantage of said attenuating material is, that the elastic bonding phase is an electric isolator, whereby the formation of eddy currents is prevented.

According to a second embodiment of the invention, shown in Fig. 3, also the yoke consists of a portion 17 of said material, serving partly to attenuate induced ultra-sound and partly to increase the magnetic cross-section of the yoke.

According to a third embodiment of the invention, shown in Fig. 4, the outer walls 17 of the entire magnet core 11 consist of said ultrasound attenuating material.

Said last-mentioned embodiment gives rise to a higher attenuation of induced ultra-sound than the embodiments according to Figs. 2 and 3.

At the aforesaid embodiments the core is designed so that said attenuating material has a thickness of 1 to 20 mm, preferably 3 to 5 mm.

The thickness, however, must be adapted to the strength of the magnetic fields and the strength of the ultrasonic signal.

According to another preferred embodiment of the invention, shown in Fig. 5, the entire core 11 is made of said attenuating material. According to this embodiment the core preferably is made of a material consisting of two or more size fractions of iron powder and said elastic bonding agent. By using iron powder of different sizes a higher proportion iron is obtained in the core. The magnetic saturation limit of the core hereby is up to 70 per cent of the saturation limit of a conventional core.

Several embodiments have been described above. Further embodiments, however, can be imagined, for example a core consisting of iron lamellae bonded together by said attenuating material or consisting of iron rods bonded together by said material.

In most cases it is sufficient to provide the electromagnet 1 of the transmitter with a core 11, 18 according to the present invention. In certain cases, however, it is necessary also to provide the electromagnet 2 of the receiver with a core according to the invention.

The advantages of the present invention are essential, because interferences in the reception caused by the core 11, 1 8 of the electromagnet 1 of the transmitter are reduced substantially or eliminated entirely, and at the same time eddy current formation is prevented.

The present invention must not be regarded restricted to the aforesaid embodiments.

The composite material, for example, may be another solid magnetic material and an elastic bonding agent.

Claims (8)

1. A method of introducing electromagnetic ultra-sound into electrically conductive material in non-destructive testing, at which a magnetic field is generated and a coil therein is supplied with a pulse of supersonic frequency, wherein said magnetic field is caused to be generated by means of an electromagnet comprising an ultra-sound attenuated magnet, which entirely or partially is made of a composite material consisting substantially of solid particles of magnetic material and an elastic bonding agent.

2. An ultra-sound attenuated electromagnet, intended primarily for use in devices to carry out non-destructive testing in electrically conductive material by means of electromagnetic ultra-sound, comprising a core and a winding, wherein the core entirely or partially is made of a composite material consisting substantially of solid particles of magnetic material and an elastic bonding agent.

3. An ultra-sound attenuated electromagnet as defined in claim 2, wherein said composite material substantially consists of iron powder and an epoxy resin.

4. An ultra-sound attenuated electromagnet as defined in claim 2 or 3, wherein at least a portion constituting the outer portion of each pole shoe of the core is formed of said composite material.

5. An ultra-sound attenuated electromagnet as defined in claim 2, 3 or 4 wherein the outer walls of the entire core consist of said composite material.

6. An ultra-sound attenuated electromagnet as defined in any one of the preceding claims wherein the core is designed so that said composite material has a thickness of 1 to 20 mm, preferably 3 to 5 mm.

7. An ultra-sound attenuated electromagnet as defined in any one of the claims 2, 3, 4 or 5 wherein the entire core is made of a composite material comprising two or more size fractions of iron powder and a bonding agent.

8. An ultra-sound attenuated electromagnet substantially as herein described with reference to the accompanying drawings.