Patent GB-1001857-A (GB)
Title: Method and apparatus for flaw detection by ultrasonic means
IPC: G01N29/04
Filed: 1962-12-20
Abstract: 1,001,857. Ultrasonic inspection. B. W. O. DICKINSON. Dec. 20, 1962, No. 48196/62. Heading H4D. In an arrangement for the detection of flaws in a member (e.g. pipes or plates) in which ultrasonic waves are introduced into the member at an angle to the longitudinal axis and received after reflection at a flaw the transmitting and receiving transducers are coupled to the member by near trapezoidal shaped wave directors of such configuration that ultrasonic waves which are not reflected at a flaw and which enter the receiving transducer wave director are dispersed by the latter in such direction that they do not actuate the receiving transducer. Thus, as shown in Fig. 1, two wave directors 14 associated with respective transmitting and receiving transducers T,R are positioned in contact with opposite end walls of a pipe 11 the arrangement being such that some of the incident pencil of waves A are reflected by a longitudinal flaw, such as C, along a right-handed helical path 28 and enter the receiving wave director at an angle which allows them to impinge on the receiving transducer R. Waves not reflected from the flaw continue in the direction (a left-handed helical path) defined by the angle of incidence and therefore enter the receiving wave director at such an angle that they do not impinge on transducer R. Fig. 2 shows the configuration of the wave directors and illustrates the reflected wave path 28 impinging of the transducer 16 whereas the non-reflected wave path 26 is directed away from the transducer. In the arrangement of Fig. 1 the apparatus 18 provides the ultrasonic drive for the transducer T and where waves from the pencil A having different angles of incidence are reflected from the flaw a corresponding number of received pulse signals e.g. 71 . . . 73 are obtained these being of different amplitude and spaced in time in accordance with the different number of turns of each helical path. In practice the transmitter wave director is adjusted such that only one helical loop is made by the reflected waves and the peak value of the received pulses is then related to the radial thickness of the flaw whilst the pulse length is determined by the length of the flaw. When it is merely desired to ascertain whether or not a flaw is present continuous waves may be utilized instead of pulses. When, however, the location of the flaw is to be determined pulse transmission is necessary. It is also necessary to know the length and diameter of the pipe together with the group velocity of the ultrasonic wave train which latter is determined by the pipe wall thickness. The latter and the pipe diameter are assumed to be known and constant but, in the case of production line testing, the legnth may vary. This is measured by conventional techniques utilizing transmitting and receiving transducers 44, T,R (Fig. 3) and by measuring the transmission time between the transducers 16, T, and R and knowing the angle of incidence of the ultrasonic waves (from the setting of the wave directors) the position of the flaw may be computed in a device 49. The oscilloscope screen 21 may also be photographed by a camera 46. In a modification the receiving wave director 14<SP>1</SP> (Fig.4) is formed to direct received waves from a number of different directions on to respective receiving transducers R 1  . . . R 3  so that by serially selecting the latter to determine which receives the maximum signal the angle and number of helical loops in the flaw reflected wave may be determined. The transmitting wave director may be formed similarly to the receiving wave director. In a development of Fig. 4 the receiving wave director is formed as a near semi-circular plate provided with a plurality of " flats " on the curved surface to receive the transducer, the " flats " being marked with the angle at which waves are received (Fig. 5, not shown). Other techniques of measurement, all utilizing wave directors, involve a single transmitting transducer and a plurality of receiving transducers the arrangement also including an " accept " " reject " device for the pipes being tested (Fig. 7, not shown), and an arrangement in which two transmitting transducers directing waves into the pipe in opposite directions are employed together with an oppositely directed pair of receiving transducers (Fig. 10, not shown). In all of the above arrangements it is assumed that the flaw C is generally parallel to the longitudinal axis of the pipe (e.g. in a longitudinal weld). If, however, the flaw is substantially perpendicular to the axis the transmitting and receiving wave directors must be positioned as shown in Fig. 11. For the inspection of the pipes already installed the wave directors may be fixed to the outer side walls, e.g. by welding (Fig. 12, not shown) or they may be located by a special " shoe " which is secured to the pipe by means of a chain (Figs. 13 and 14, not shown). Fig. 15 shows an arrangement in which the receiving wave director is provided with two receiving transducers respectively responsive to the flaw reflected wave 28 and to the unreflected wave 26 so that since the intensity of the latter is constant the ratio of the two waves is a measure of the flaw. In a further arrangement the location of a flaw in e.g. a flat plate 48 (Fig. 16) is determined by the production of perturbations in the received displayed signal when a source of selective spot heating (e.g. an oxy-acetylene torch 78), which varies the local thickness, density and elastic properties of the material, is incident on the location of the flaw. This heating technique may also be employed in conjunction with the embodiments described above. Fig. 18 shows an arrangement in which waves other than those reflected by a flaw C are prevented from reaching the receiving wave director 14 by uniformly heating the edge DE and then employing selective spot heating to determine the location of the flaw. Instead of heating, the same effect, but on a reduced scale, may be produced by cooling e.g. by the application of liquid nitrogen. In an alternative arrangement, Fig. 19, perturbations in the displayed signal may be produced by the attenuating effect on sonic waves of a jet of liquid (e.g water) which conducts sound waves away from the material 11 and produces a reduction in amplitude of the displayed pulses when it impinges over the area of the flaw. Instead of employing electro-acoustic transducers for producing the ultrasonic wave trains a mechanical device comprising a powder actuated gun may be employed (Fig. 20, not shown). In all of the embodiments the coupling between the wave directors and the test material may be improved by the use of materials such as aluminium, copper or lead or ordinary lubricating grease. The apparatus according to the invention may also be employed as a permanent installation to monitor thermally insulated pipes &c. for local or general temperature changes and also to monitor pipes carrying corrosive fluid lined with an inert substance for failure of the latter.