Method and means for transmitting elastic waves

Nov. 17, 1959 i. l. joy 2,913,602

METHOD AND MEANS FOR TRANSMITTING ELASTIC WAVES

Filed Nov. 3, 1955 2 Sheets-Sheet 1

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Nov. 17, 1959 i. l. joy 2,913,602

METHOD AND MEANS FOR TRANSMITTING ELASTIC WAVES Filed. Nov. 3, 1955 2 Sheets-Sheet 2

<img file="US2913602A_D0002.tif" />

2,913,602

Patented Nov. 17, 1959

United States Patent Office

2,913,602

METHOD AND MEANS FOR TRANSMITTING ELASTIC WAVES

Ivan L. Joy, Topeka, Kans.

Application November 3,1955, Serial No. 544,741

Claims. (Cl. 310—8.3)

This invention relates to a method and means for transmitting elastic waves between a transducer and a body under test and more particularly is concerned with improving the efficiency of elastic wave transmission through transducer holders that are intended for use in instances wherein there is relative movement between the transducer and the body under test.

Transducer holders of the type referred to above are characterized by a confining chamber in which the transducer is mounted for intimate contact with a suitable liquid couplant. At least a portion of the chamber wall is made of elastic wave transmitting material and this wall portion is located in the path of the elastic waves and is adapted for coupling to the entering surface of the body under test.

The principal object of the invention is to increase the efficiency of elastic wave transmission through transducer holders of the type described above.

Other objects and advantages of the invention will be apparent during the course of the following description.

In the accompanying drawings forming a part of this specification and in which like numerals are employed to designate like parts throughout the same,

Figs. 1 and 2 are sectional views of two different transducer holders of the general class to which the present invention is applicable; and

Fig. 3 is a fragmentary sectional view showing a modification of the holder of Fig. 2.

A variety of coupling liquids are known to be effective for transmitting elastic waves between a transducer and a solid body and each exhibits its own particular advantages and disadvantages. For instance; castor oil has the property of high attenuation and hence is useful in situations wherein it is desired to avoid standing waves; silicone oil is inert and hence finds application when holders using rubber diaphragms are employed; and water is cheap and plentiful.

According to the invention, it has been found that the elastic wave transmitting ability of any of these or similar coupling liquids is importantly enhanced by mixing an appropriate quantity of metallic particles therewith. This includes such substances as white lead, powdered iron, powdered aluminum or aluminum oxide, and presumably any other powdered metallic substances.

In tests metal particles having a diameter less than one-thousandth of an inch have been employed and this appears to be the most desirable arrangement. This specific disclosure, however, is purely illustrative and is not intended to impose undue limitations upon the range of particle sizes.

In the case of any of these substances it has also been noted that standing wave problems are eliminated and while this seems anomalous in view of the increased transmission characteristics, this phenomenon is apparently due to the presence of a large number of interfaces provided by the dispersed metallic particles.

' In general, the transmission characteristics of the coupling liquid mixture improve as the proportion of solid particles increases, however, such increases ultimately reduce the ability of the crystal holder to adapt itself to the surface contours of the body under test and/or to accommodate crystal movements. For the types of transducer holders discussed herein a ratio by volume of one part solid particles to two or three parts coupling liquid has been found satisfactory. Obviously the specific ratios recited are not critical and may be varied in accordance with the requirements of the particular application.

Referring now to the drawings, and particularly to Fig.

1, there is shown a transducer holder 10 that is intended for use in the ultrasonic testing of joint bars 11, such as are used to connect adjacent rails, and other similar objects which are characterized by the fact that the surface 12 through which the ultrasound enters is of a rough and irregular contour. Only the end portion of the joint bar 11 is shown in the figure.

The transducer holder comprises a base 13 which may be made of Bakelite or some such similar insulating material, a piezoelectric crystal 14 mounted on the base, a rubber diaphragm 15 for confining coupling liquid for contact with the crystal, a handle 16 connected to the base 13 by suitable screws 17, and a pair of wires 18 for supplying potential variations to the crystal.

The front face of the base is formed with a stepped recess that provides an annular shoulder 20 and the crystal is mounted on the base by cementing the outer marginal portions thereof to the shoulder 20. The arrangement provides an air gap 21 behind the crystal 14 and a couplant confining chamber 22 in front of the crystal. The base is also provided with a bore 23 to facilitate filling the compartment 22 with couplant 25 and back flow is prevented by sealing the bore with a screw 24.

The wires 18 enter the holder through a coaxial connector 26 mounted in the free end of the holder 16, pass through an axial bore 27 formed in the handle and thence through the Bakelite base 13 for connection to the front and rear faces of the crystal. For this purpose the crystal faces are provided with suitable coatings of silver, copper, or other electrically conductive material.

In accordance with the invention the couplant 25 in chamber 22 may consist of a mixture of a coupling liquid of any chemical composition that will not be destructive, of the metallic coating on the front face of the crystal 14 or to the rubber diaphragm 15 and metal particles as described above.

In testing joint bars and the like with the holder 10, it is desirable to first clean the surface 12 through which the ultrasound is to pass and apply a suitable wetting liquid thereto. The wetting liquid may be oil or water and preferably should not be harmful to the rubber diaphragm 15. The rubber diaphragm is preferably onehalf wave length thick or a multiple thereof, this wave length being selected in relation to the speed of propagation of the ultrasound through the diaphragm.

The handle 16 of the holder is grasped and the diaphragm 15 placed against the surface 12 of the body under test and effective ultrasonic contact between the joint bar 11 and diaphragm 15 is readily obtained by applying a slight pressure. This slight pressure insures that the rubber diaphragm conforms to the irregular contours of the rough surface 12 and achieves the desired ultrasonic contact with but a minimum of coupling liquid between the diaphragm and the test part.

Since the holder 10 is applied to the testing of bodies having rough entering surfaces, it is necessary to vary the orientation of the crystal relative to the entering surface in order to select experimentally an angle of incidence such that the ultrasound will be directed through the entire length of the object under test. The rubber diaphragm responds to the slight pressures imposed and closely conforms to the contour of the entering surface

2,913.

. 3 at all times. These necessary changes in the contour of the rubber diaphragm as caused by the contour of the entering surface of the body under test limit the ratio of metal particles to coupling liquid - that may -be employed in the confining .chamber .22. Should an unduly high proportion . of metal particles be employed,', the ability of the rubber diaphragm. to . follow the contour of the test surface would ultimately .be limited by the increased rigidity of the couplant mixture.

It is believed that the ability of metal particles. to· increase the elastic wave transmission efficiency of. a. coupling liquid depends in part upon the maintaining, of a homogeneous mixture. In the broadest aspect of the invention, of course, the solid particles may be mixed in a coupling liquid whether, the liquid is confined as in.the present case or whether it is freely flowing, and in either case, will enhance the efficiency of transmission. However, in a holder wherein the coupling liquid is confined the vibrational energy of the elastic . waves passing through the couplant mixture tends to maintain the particles in suspension and therefore produce a couplant medium of the highest transmission, efficiency.

It has been found that the thickness of the chamber 22 in the direction of elastic, wave propagation should not exceed one-quarter of an inch. When this figure is exceeded attenuation increases unduly and limits the effectiveness of the arrangement.

Fig. 2 illustrates another form of transducer holder 40 which is of the variable angle type and which is used in the testing of axles or other surfaces 41, the entering surface 42 of which is more regular than the end surfaces of joint bars. The holder 4® consists of a metal shell 43 to which is secured a mounting block 44 on which a rubber diaphragm 45 is secured for defining therewith a confining chamber 46 in which the crystal 47 is mounted for angular movement. The crystal 47 is cemented at its outer marginal portions to an annular shoulder 49 that is provided by forming a stepped recess in the front face of a Bakelite base 48. With this arrangement the crystal is mounted with its back face bonded by an air gap 50 and with its front face in open communication with the couplant chamber 47.

The. base. 48 is mounted for angular movement and for this purpose is hinged at one end to a projectnig arm of a bracket 51 that is secured to the mounting block .45 and has its opposite end associated with a screw 52 that is threadedly received in the mounting block. In addition, the base is spring biased , by a coil spring 53 and thus is urged inwardly by advancing the screw 52 . and is urged inwardly by the action of the spring .5.3 when the screw is retracted.

The crystal is actuated by potential variations supplied over the wires 54 which pass through the mounting block 45 and through the confining chamber for contact with metallic. coatings on the front and rear faces of the, crystal.

In testing solid bodies with the holder 40. the metal frame 43 is normally maintained stationary relative to the entering surface of. the body under test and the positioning of the screw 52 is varied to vary the angle.of orientation of the crystal 47, With this arrangement the crystal is able, to scan a solid body simply by varying, the angle of orientation of the crystal. Similar variable angle-crystal holders find use in applications wherein, it is desired to test a body with ultrasound that is transmitted at a. plurality of angles of incidence in order to insure the detection of all of the flaws in the body irrespective of the plane in which they are -oriented.

In this instance the crystal 47 is shown curved, and this permits the beams of ultrasound 55 to be focused to facilitate the inspection of particular regions; within, the object and also to intensify the beam and permit the detection of smaller defects. - -The couplant mixture is designated 56 and once again must: have a sufficiently low proportion of solid particles:

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- 4-.as to permit the base 48 to move freely under the control of the screw 52 and bias spring 53. Here again the passage of the ultrasonic energy through the coupling mixture maintains the solid particles in suspension with5 in the coupling liquid and increases the efficiency of transmission.

No insulating problems-have- arisen from the use of metal particles in the couplant mixture. Such problems can be avoided completely, however, by partitioning the 10 chamber 46 and as shown in the fragmentary view of Fig. 3, this may be done .by connecting an auxiliary diaphragm 60 between the side walls of diaphragm 45 . and the base 48. This auxiliary diaphragm is preferably of thin rubber material and yields sufficiently to accom15 modate all necessary movements of the pivoted base 48.

Another advantage of the diaphragm 60 is that it limits the confining chamber to the region through which the ultrasound must pass. This is desirable from the standpoint of controlling the ratio of the couplant 20 mixture.

It will be noted that in the case of the holder of Fig. 1, the movement of the crystal relative to the test surface was necessitated by the roughness of this surface whereas in the case of the holder of Fig. 2, the move25 ment of the crystal accomplished a scanning of the test part and/or permitted detection of defects regardless of their plane of orientation. There are other applications wherein the crystal moves relative to the test surface for still a different reason, such as in the . testing of rail ‘ 30 wherein the crystal moves longitudinally along the rail in order-to progressively test successive portions of the rail. Changes in rail contour require movement of the crystal relative to the rail surface.

The invention also finds important application in trans35 ducer holders for the testing of rail since many of these holders also utilize confining chambers for the couplant and here again. there is the particular advantage that, the passage of the ultrasound through the confined coupling mixture maintains the solid particles in suspension. ⁴⁰ and achieves a maximum of transmission efficiency.

It should be understood that the description of the preferred form of the invention is for the purpose of complying with section 112, title 35, of the U.S. Code and that the appended claims should be construed as ⁴⁵ broadly as the prior art will permit.