Patent GB-959029-A (GB)
Title: Method and apparatus for testing materials
Inventor: BRADFIELD GEOFFREY
IPC: G01N29/11
Filed: 1960-02-10
Abstract: 959,029. Ultrasonic testing methods and devices; ultrasonic wave couplings. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 10, 1961 [Feb. 10, 1960; May 21, 1960; May 23, 1960], Nos. 4740/60, 18006/60 and 18157/60. Headings H4D and H4J. In a method of examining the surface of a specimen, a beam of ultrasonic waves is directed against the surface and the intensity of reflected energy is measured over a range of angles of incidence. It is stated that the reflection of ultrasonic waves at a solid-liquid interface drops to a low value at a certain critical angle, which is the angle (A) at which a surface wave is generated at the interface ; sin A = C 1 /C SR  where C 1  is the wave velocity in the liquid and C SR  is that of the surface wave and is mainly dependent on the shear wave velocity in the solid and its Poisson's ratio. A goniometer for measuring A accurately comprises Figs. 1, 2, a tank 11 with distilled water to level 12 below which a window 13 of cylindrical fused quartz in the tank wall carries an X-cut quartz disc 14 generating vibrations in thickness mode. The axis 15 of the ultrasonic beam is horizontal, and just below its point of intersection with the vertical axis of the tank a specimen table 17 is mounted to be normal to, and rotatable about, the tank axis. To provide in addition for specimen rotation about an axis perpendicular to that of the tank, support 18 is added. Further arrangements are provided for specimens of various shapes. The reflected beam is received by a fuzed quartz prism 24 on which a Y-cut quartz crystal transducer 27 is mounted. Specimen table 17 is rotated by gearing from knob 32, and to enable the rotatable receiving means to be kept in the reflected beam it is carried by 2 : 1 gearing from that of table 17. The angle of beam incidence is measured by scale 42 and vernier 43; vernier 44 attached to the specimen table may be added. To ensure constant velocity of sound in the water, tank 11 is double-walled to permit circulation of a heating or cooling medium via cooks 49 and thennoatatic control is added. To examine specimens not conveniently totally immersible, e.g. concrete blocks, a semi-cylindrical bolster 54 of polymethyl-methacrylate resin is interposed between transmitter 55 and receiver 56, and specimen 57, a film of oil 58 being interposed. The transmitter and receiver are each mounted on a coupling shoe 59 as described in Specification 772,083; damping backing 61 is provided. The transmitter and receiver are of X-cut quartz. Means for simultaneous adjustment of transmitter and receiver are described, Figs. 7, 8 (neither shown). Measuring Poisson's ratio.-To measure Poisson's ratio, R, a beam of longitudinal waves is transmitted through a bolster into a specimen of square cross-section, the beam lying in a plane normal to the specimen face and parallel to the specimen cross-section, once at such an angle A 1  that a beam of longitudinal waves enters the specimen at an angle of 45 degrees to the normal so that after reflection at successive faces it emerges into the bolster at the point where it entered the specimen and proceeds in the bolster at an exit angle equal to A 1  but on the other side of the normal, and another time at such an angle A s  that a beam of shear waves enters the specimen at an angle of 45 degrees to the normal, measuring the angles A 1  and A s , and evaluating R from specified equations. In an alternative method, a pulsed beam is used and the travel times T 1 , of longitudinal waves in the specimen, and T s , of shear waves in the specimen, are measured; it is stated that R = (T s /T 1 )<SP>2</SP>-2 2(T s /T 1 )<SP>2</SP>-2. Arrangements for carrying out the methods are described, Figs. 22, 23 (neither shown). Examination for anisotropy, Fig. 9. The apparatus of Fig. 6 can be moved to any part of the surface of a large flat specimen and rotated in azimuth. To reduce variations due to oil film thickness change with specimen surface finish, a block 81, Fig. 9, of polymethyl-methacrylate resin incorporates semicylindrical bolsters 54a, 54b with normal axes and with transmitter assemblies 55a, 59a, 61a, 55b, 59b, 61b, and receiver assemblies 56a, 59a, 61a, 56b, 59b, respectively. As block 81 is rotated about a vertical axis two sets of amplitudes of reflected signals are obtained, the ratio of which depends on the ratio of the orthogonal properties of the specimen. Repetitive examination of large specimens, Fig. 10. For examination of curved plates of toughened glass 84, the range of angular adjustment necessary is small. The specimen is immersed in a water tank 82 and transmitter and receiver supports 87 are movable by gearing not shown. Temperature control as in Figs. 1 and 2 may be added. Combined transmitter and receiver.-In the above apparatuses the receiving means may be the transmitting transducer as described in Specification 766,982 and a reflecting surface to return the beam is added. Beam splitting.-To increase measuring precision, beam splitting is used. A transducer array as described in Specification 772,083 may be employed, Fig. 12 (not shown) ; or a system comprising two adjacent transmitting crystals, the relative phases of which are switched periodically in synchronism with relative position displacement of resulting signals on a cathode-ray tube display, Fig. 11 (not shown). A ceramic piezo-electric bimorph element may be utilized, Figs. 14, 15 (neither shown). Transducer array elements may be fed via transistor circuits, Fig. 13 (not shown). Further applications.-A conical beam may be arranged for examining an inset bore in a specimen, Figs. 20, 21 (neither shown). Since the value of critical angle depends on the depth to which the incident beam penetrates the - surface layer of a specimen and this depth varies with the testing frequency, such testing provides additional information. The invention may be employed for determination of the Y and Z axes of a slab of X-cut crystalline quartz or of single crystal germanium or silicon; of the depth of cold work in a specimen; for monitoring the stress in toughened glass, by choosing a frequency at which the penetration is down to the zero stress plane and measuring the critical angle; for investigating the anomaly in the surface condition of a specimen fatigued to breaking by bending; for measuring the properties and thickness of platings and coatings, and the adhesion of the layer to the underlying material; for testing aircraft skinsin situ. Typical experimental results are quoted. A gating circuit may be used to measure amplitude at a centre section of a wave train. The third Provisional Specification refers to Specification 841,512. Specification 766,982 also is referred to.