GB1084792A - Improvements in the non-destructive testing of materials - Google Patents

Abstract

1,084,792. Sonic testing of materials; transistor Wien-bridge circuits. BRITISH CAST IRON RESEARCH ASSOCIATION. May 4, 1964 [May 3, 1963], No. 17697/63. Headings H3T and H4D. In an arrangement for determining physical qualities of materials by exciting test samples to resonance and measuring the resonant frequencies the onset of resonance is indicated positively by a " resonance," " non-resonance " indicator preferably comprising a bi-stable circuit arranged to energize resonance and non-resonance indicators in its respective states. Fig. 1 shows one embodiment in which a test sample 3 (e.g. a bar of cast iron) is excited to resonance by a transducer 1 and the resulting vibrations reconverted by transducer 2 the output of which is fed via amplifier 6 and phase splitter 7 to a detector 8. Transducer 1 is driven via amplifier 4 by the output of a circuit 5 which is arranged as a Wien bridge and operates either as a tunable oscillator with feedback provided via switch S in the " Manual " position or with S in " Auto " position as an amplifier having its input connected to an appropriate phase of the output of phase splitter 7. Initially, when a number of nominally similar test samples are to be analysed, the measurement on the first sample is made with the switch in the " Manual " position, the frequency of 5 being adjusted until resonance, as indicated by maximum reading on a voltmeter 10 supplied from detector 8, occurs. This frequency is then read on a digital frequency meter 12. Switch S is now put in the " Auto " position and the other samples substituted for the first sample the arrangement then being selfoscillating. In this mode of operations the output of detector 8 is supplied as a control signal to a bi-stable circuit 11 the latter remaining in one of its two states (in which a " non- resonance " indicator is energized) until the level of the control signal exceeds a predetermined value. Such level is only attained when the test sample 3 resonates and circuit 11 in response to this level then reverses and resonance is indicated by the extinction of the " non- resonance " indicator and the energization of a " resonance " indicator. The resonance frequency for each sample is then read on meter 12 and the general operation of the system is monitored by a cathode-ray display tube 9 the orthogonal deflections of which are respectively controlled by one phase of the output of phase splitter 7 and by an output from circuit 5. Details of the circuitry of the blocks 4 to 11 are shown in Figs. 2 and 3. The " resonance " and " non-resonance " indicators comprise lamps L 1 , L 2 and the circuit 5 consists of two transistors T 9 , T 10 connected as a super-alpha pair to give high output impedance together with a transistor T 11 . The frequency at which the circuit operates is determined by capacitors C 1 , C 2 and resistors R 1 , R 2 and to ensure that oscillation occurs when switch S is in the " Auto " position two such positions " Auto 1 " and " Auto 2 " giving opposite phases of the output of phase splitter 7 are provided. In Fig. 4 (not shown), the resonant frequency of an unknown test-piece (23) is compared in comparator (20) against that of a standard (23a) and the frequency difference (if any) utilized to control a selected stage in the manufacture of the unknown-e.g. a heat treatment furnace (28) having a servo-controlled fuel valve (29). In this arrangement each of the units A. B may be substantially identical to the arrangement of Fig. 1 to 3. An arrangement utilizing a single transducer (31) operating both as an exciter of and detector of resonance in the test-piece 3, the arrangement being, otherwise, generally similar to that of Fig. 1, is described with reference to Fig. 5 (not shown). In another embodiment (Fig. 6, not shown), the exciting transducer (43) is driven by a pulse generator (49) connected to frequency meter (48) synchronized via amplifier (45) from the vibration detecting transducer (42) and connected to (43) via amplifier (44), phase adjuster (47) and a gate (46) which remains open only when the pulse intervals lie between pre-set limits thus limiting the band-width. This embodiment may be varied to operate as a continuous wave system by replacing (46) by a servo-transmitter (47), by a servo-controlled sweep frequency generator and (49) by a detector. Experimental results on typical test samples including the use of the invention to distinguish between materials having external similarities of weight, size and appearance (e.g. plastics) are given. Suitable transducer constructions are described with reference to Figs. 7 and 8 (not shown).