Effect of the Size of a Flat-Bottom Hole in a Cylindrical Specimen on the Bottom-Signal Level

Abstract

The effect of a change in the size of a flat-bottom hole on the level of a bottom signal measured with a normal probe at an end surface of a cylindrical specimen is considered. It is shown that this effect depends on the specimen's radius-to-length ratio. It is established that the difference in the levels of a bottom signal and a signal from a hole in the specimen is in good agreement (within 0.5 dB) with the corresponding difference obtained from the calculated DGS diagram.During ultrasonic inspection of forged pieces, rods, etc., with normal probes aimed at the tuning of the sensitivity of flaw detectors and evaluation of the equivalent area of flaws, a common practice is to use cylindrical calibration blocks with artificial reflectors in the form of flat-bottom holes, the planes of which are oriented perpendicularly to the ultrasonic-beam propagation [1]. Such reflectors are manufactured in the form of holes with flat profiles at different depths, axially symmetric with respect to the specimens. Reliable time selection of signals from flat-bottom reflectors against the background of a bottom signal from the specimen's surface, which is opposite to the surface of emission, and reception of a longitudinal wave can be achieved at typical distances from the reflectors to the former surface (the depth of the hole) of 15-20 mm. An example of specimens with such reflectors is a KCO-2 kit of calibration blocks (GOST (State Standard) 21397-81) used in ultrasonic testing of intermediate products and articles from aluminum alloys [2]. Analogous specimens are also produced from steel with allowance for the requirements of the above standard [1]. Specimens undergo primary and periodic certification (upon production and during service, respectively) to check the correspondence of their parameters to the required standards. In order to ensure the uniformity of measurements in the determination of the echo-signal amplitudes from the specimen's flatbottom reflectors, it is necessary to choose a reference signal, the amplitude of which must be compared to the signal amplitudes from the reflectors. For a KCO-2 kit, it was proposed to use an echo signal from a steel ball in an immersion bath as the reference signal, which is used to adjust the bath's elements and calibrate the flawdetector's sensitivity [3]. The normalized signal levels obtained are compared to the rated values, the tolerable deviations from which are ± 2 dB at most. The necessity of performing rather laborious measurements using a special-purpose setup makes the monitoring of signal levels from the reflectors impracticable not only during operation but also during periodic certification of specimens by metrological agencies.One of the methods for substantially simplifying the determination of the correspondence of signals from flat-bottom reflectors to the rated values is their comparison with the levels of the bottom signals in the same specimens, which are measured at such a position of a normal probe in which the maximum signal from a given reflector is achi...