Patent GB-1185471-A (GB)
Title: Improvements in or relating to the Testing of Solid Materials for Cracking.
Inventor: JARRATT ARNOLD HALL
IPC: G01N3/00
Filed: 1966-05-12
Abstract: 1,185,471. Acoustic testing of materials. BRITISH PETROLEUM CO. Ltd. 14 April, 1967 [12 May, 1966], No. 21090/66. Heading H4D. [Also in Division G1] A method of and apparatus for testing for fracture of a sheet of solid material involves subjecting the sheet to a bending stress, concurrently changing the temperature of the sheet in a controlled manner, sensing electro-acoustically for the occurrence of a fracture in the sheet and automatically indicating the temperature at which such fracture occurs. The invention, which is described in connection with testing sheets of bitumen for fracture as the temperature is lowered, the noise produced on fracturing being sensed by a microphone, is also applicable for use with materials which become brittle and produce an acoustically detectable noise on fracturing as a result of a rise in temperature. In carrying out the invention a metal plaque 10, Fig. 2, is coated with a 0À5 mm. thickness of bitumen and positioned between the jaws 18 of a bending device which is operated by a motor driven cam to produce a cyclic flexing of the plaque (Fig. 1, not shown). When a fracture occurs in the bitumen sample as a result of such flexing the resulting sound is sensed by a microphone 36 which controls via circuitry to be described the production of an indication of the temperature at which such fracture occurs. In order to prevent spurious operation due to noise produced by bitumen breaking away from the plaque where it contacts the jaws 18 a small amount of the bitumen is removed from the edges of the plaque. For controlling the temperature of the bitumen, the bending apparatus is positioned in a glass tube 24 contained in a cooling chamber 30 through which an air stream, controlled by a magnetic valve 28 and cooled by passage through a coil 26 immersed in a bath of acetone and carbon-dioxide at - 70‹ C. flows. For calibration purposes a mercury thermometer (22, Fig. 1, not shown) is positioned adjacent to the plaque 10 following considerations illustrated with respect to Figs. 4A and 4B (neither shown) to permit of exact repetition for successive sample tests. For sensing the temperature a resistance thermometer 32 is wound around the glass tube 24. This is connected in a bridge circuit 40 (Fig. 5) which includes a potentiometer RV1 the shaft of which is driven by a synchronous motor and carries a temperature indicating scale the calibration of which is adjusted by calibrating resistors in the bridge circuit. In operation, when the motor driving the potentiometer is energized the unbalance of the bridge which is produced results in a phase change in the signal supplied from the bridge to amplifier stages T 1 -T 4  the output of which is supplied to a transistor T 5  the collector of which is supplied with a reference signal in the form of a negatively-going half-wave rectified voltage of 50 c.p.s. On phase coincidence of this reference and the bridge output voltage T 5  conducts and causes an increase in the average current flow through a transistor T 6  which results in the energization of a relay RL1 and the closing of the contacts a, a and b, b. Contacts a, a control the valve 28 (Fig. 2) and, when closed, allow cooling air to be blown through the chamber 30. When the resistance of thermometer 32, which changes with the cooling effect, restores the bridge balance, contacts a,a open and the air flow is interrupted. In order to prevent an overshoot of temperature in chamber 30 the closing of contacts b,b causes capacitor C 1  to charge towards the supply rail voltage and thus by increasing the base bias of transistor T 5  causes an increase in its collector current which results in T 6  being switched-off. The contacts of relay RL1 then open and C 1  discharges via R 1 , R 2 , R 3  thus.causing the current through T 5  to reduce and allowing T 6  to again conduct and close the relay contacts. In this manner short bursts of cooling air are blown through chamber 30 when ever the bridge circuit is unbalanced so that the cooling rate is slowed down. The output of the microphone in response to the noise produced by a fracture of the bitumen is supplied via a switch SW1 (which selects one microphone from a number of alternative sample chambers) an amplifier stage T 7 -T 10  (the gain of which is adjustable by means of RV 2 ) to trigger a mono-stable multivibrator T 12 , T 13  the output pulse of which is supplied via amplifier T 14 , T 15  to control relay RL2. The output pulse has a period of approximately two seconds which is sufficiently long to allow the contacts of RL2 to close one set of the contacts thus providing a holding circuit. The operation of the other contacts then results in the completion of the circuit of an indicator lamp L and via relay RL3, in the interruption of the power supply to the motor of the bending apparatus and to the motor driving the potentiometer and the activation of an alarm bell B. The temperature at which the fracture has occurred is then indicated on a scale by means of a pointer attached directly to the shaft of the potentiometer RV1. A reset switch SW2 allows the relays RL2 and RL3 to be de-energized before the next test cycle is commenced.