GB1066090A - Method for ultrasonically testing a weld - Google Patents

Abstract

1,066,090. Ultrasonic flaw-detecting; ultrasonic couplings. FORD MOTOR CO. Ltd. Oct. 15, 1964 [Dec. 6, 1963], No. 42057/64. Headings H4D and H4J. In an ultrasonic pulse-echo method for testing a weld between two sheets of metal, in which a single transmitting- receiving transducer transmits a pulse train towards the weld zone and receives a first echo pulse train from the metal surface nearest the transducer and a second echo pulse train from the weld zone, the duration of the transmitted pulse train is made short to prevent overlap of the two echo pulse trains. The transducer (Fig. 6, not shown) comprises a piezoelectric crystal or ceramic wafer (17-4) backed with vibration - damping material (17-9) and coupled through oil or water (17-6) to a workpiece 19, and is mounted in a housing (17-1, 17-2, 17-3). The measurement cycle:-The discharge of a capacitor (15-3) (Fig. 7) in a pulse generator circuit 15 applies a sharp voltage pulse to the transducer which then emits an acoustic pulse comprising oscillations at the natural frequency of the piezo-electric wafer (17-4) (e.g. 2À25 to 25 M/cs.). Because of the heavy damping by the material (17-9), a few cycles only (e.g. 1 to 5) of the oscillation occur in the acoustic pulse. The resulting first, second and subsequent echo pulses are received by the transducer, then amplified, shaped and passed through a noiselimiter in a unit 21 to provide a video signal which is displayed on a C.R.O. (cathode-ray oscilloscope) 25. The measurement cycle is repeated at a frequency determined by a lowfrequency (e.g. 100 to 1000 c/s.) oscillator in a control unit 13 which provides trigger signals for the pulse generator 15 and for the C.R.O. timebase. Gating and measurement of selected echo pulses:- A synchronizer circuit 27-1 and gate generator 27-2 are triggered by the video signal and gate an amplifier 27-3 such that the video signal from unit 21 is fed to a read-out circuit only after the first echo pulse train has been received, thus eliminating disturbing effects due to variations in probe coupling. The gating signal from generator 27-2 may be delayed and varied in length in order to pass echo pulses in a selected time interval to the read-out circuit. In a measurement cycle, the echo pulses from video amplifier 27-3 are integrated by a unit 27-4 and the resultant signal is fed to a peak-reading meter and alarm circuit 27-5, 27-6, 29 to give a read-out representing the acoustic energy reflected by the weld zone in the multiple reflections occurring during the gated interval. Alternatively, the integrator 27-4 can be by-passed, allowing the peak-detector 27-5 to detect the maximum voltage of the video signal from amplifier 27-3 in the gated interval. A large proportion of reflected acoustic energy indicates a poor weld. After each measurement cycle, the peakdetector 27-5 is reset by a trigger pulse from the control unit 13. The C.R.O. 25 may be dispensed with or used to facilitate adjustment of the read-out circuits or to monitor the gate signals from generator 27-3.