Acoustic nondestructive testing of rods using multiple reflections

Abstract

A procedure of the pulse echo-mirror method for testing rods by Pochhammer waves that uses multiple reflections of the transmitted pulse from flaws and free end faces of these rods is presented. It is shown that the sensitivity of the method can be increased considerably. The experimental studies of multiple reflections from an artificial flaw are confirmed theoretically.It is known [1] that the sonication of articles with plane-parallel surfaces in the form of plates results in a series of multiply reflected echo pulses that follow each other at distances equal to the plate thickness on the flaw detector's display. The echo-signal amplitude decreases with an increase in the number of reflections for several reasons: damping of the ultrasound in the plate material in accordance with the exponential law, divergence of the ultrasonic beam, and losses upon reflection at the test object-transducer boundary.Losses caused by divergence can prevail when testing thick-walled objects and low-damping materials. During contact with metal, reflection losses are high for piezoelectric probes with an acoustically hard protector, slightly smaller for piezoelectric transducers with an acoustically soft protector, and almost absent for the contact-free excitation method, for example, using electromagnetic-acoustic transducers.If a plate contains one minor flaw that only slightly distorts the echo pulse from the back wall, for instance, a layer with some finite permeability, the reflection from this flaw will be observed after each of the multiple reflections from the back wall. The amplitude of the echo signal from the flaw changes in a different manner than the amplitude of the reflection from the back wall. It was noted in [1] that the location with the highest sensitivity to a flaw is located at the end of the near-field zone, while in the long-field zone, the amplitude of the echo pulse decreases inversely to the square of the distance.Despite some increase in the sensitivity to flaws in the zone of multiple reflections, this fact has not found any practical use.It was shown in [2] that Pochhammer waves propagate along a rod with minimal losses. Pochhammer waves propagate only in one direction (along a rod); they are planar and are attenuated only due to damping in the material of the test object. Damping of the S 0 mode of a Pochhammer wave in the region of the minimum velocity dispersion (at a low frequency) caused by internal losses is low, which makes it possible to observe a series of multiple reflections in sufficiently long rods. Figure 1 shows the functional diagram of the experimental setup for observing the pattern of multiple reflections in a rod. Short acoustic pulses of symmetric mode S 0 are excited in a rod ( 1 ) of length L from one of its ends using an electromagnetic-acoustic (EMA) transmitter ( 3 ). After passing through the rod, the pulses are detected by an EMA receiver ( 4 ), which converts the acoustic oscillations into electrical pulses that are displayed on an electronic oscilloscope ( 6 ). The genera...

Method and Apparatus of Measuring Velocity and Sound Attenuation Coefficient in Bulk Materials Based on the Analysis of the Structure of Sound-Insulation Materials on the Basis of Perlite

“…Measured average attenuation coefficient US perlite 25 kHz frequency is 19 dB/cm or in relative units 2.185 Np, that far exceeds the damping factor at the same frequency in the air 5.42 dB/cm (0.624 Np) [11][12][13].…”