GB1184333A - Method and apparatus for Acoustic Analysis - Google Patents

Abstract

1,184,333. Acoustic testing of materials. ROCKWELL CORP. 6 March, 1967 [7 March, 1966], No. 10524/67. Heading H4D. An arrangement for detecting variations in the specific acoustical impedance of a workpiece caused by structural discontinuities therein utilizes the variations which such discontinuities produce in an acoustical standing wave pattern created in the workpiece. The invention is described in connection with the testing of panels of sandwich construction utilizing electroacoustic probes comprising either piezo-electric or magnetostrictive elements and adapted to operate in an immersed liquid coupling 24 (Fig. 1) with the panel 6 or in contact therewith via a thin liquid coupling (Fig. 2). In operation, the probe is excited by an oscillator operating in the range 2À5 to 80 Kh to set up a standing wave which acoustically energizes the workpiece throughout its entire thickness so that the latter loads the probe the extent of the loading being a measure of the distance of the discontinuity from the probe (i.e. distance is inversely proportional to, loading) whilst the position of the probe indicates the location of the discontinuity. Fig. 5 shows one arrangement for carrying out the invention involving an additional crystal or magneto-strictive element 134 structurally secured to the main probe element 30 so that both vibrate in unison. The output of element 134 is then fed-back to control the amplitude of the drive produced by oscillator 114 and current changes indicated by the meter 118 are utilized to detect changes in the load on the element 30 resulting from discontinuities in the workpiece. In a modification of this arrangement, the read-out means comprising a meter or a cathode-ray tube is connected to the oscillator via a variable filter, Fig. 8 (not shown). In another arrangement, the probe 3 (Fig. 6) of the kind shown in either Figs. 1 or 2, is connected to a bridge circuit, which is of the resistance-inductance type when the probe comprises a magneto-strictive element and of the .resistance-capacitance type when the probe comprises a piezo-electric element (an example of the latter being described with reference to Fig. 9, not shown) an oscillator 114 driving the probe and an amplifier 116 feeding an indicating meter 118. A further arrangement employs an FM signal generator supplying the probe via a self-balancing bridge connected to a null detector, Fig. 7, not shown. In the operation of all arrangements the apparatus is first calibrated with the probe in contact with an area of the workpiece known to be free from defects to produce an initial reading to which subsequent test readings are related. Typical readings at defect free areas and at areas having defects at different distances from the probe are shown in Figs. 3 and 4 for a honeycomb and a laminated panel construction respectively. It is stated that instead of a meter read-out, other means such as Lissajous patterns on a cathoderay tube or digital read-out means may be employed.