Patent GB-1222852-A (GB)
Title: Apparatus for measuring the damping of vibrations in visco-elastic materials
IPC: G01N3/38
Filed: 1968-09-06
Abstract: 1,222,852. Testing physical properties of materials. STAMICARBON N.V. Sept. 6, 1968 [Sept. 8, 1967], No.42615/68. Heading G1S. [Also in Division G4] In apparatus, for measuring the damping of vibrations in visco-elastic materials, comprising a scale 11 and means for indicating the amplitude of vibrations of a movable arm 8 connected to a test specimen of the material to be examined, (a) the scale 11 is provided, on each side of a zero point, with a series of graduations 40 spaced according to a logarithmic scale so that the interval between any two successive graduation marks is directly proportional to the distance of one of them from the zero point, and (b) a counter mechanism 16 determines the decrease of the number of graduation marks passed in successive vibration periods. The specimen may be subjected to torsion, deflection or tension and compression. For the torsional vibration test described it may be a strip or tube and has its upper end clamped to a shaft 2, suspended in an air-cushioned bearing 3 and carrying (i) inertia arms 7 to which weights may be fixed when measurements are required at lower frequencies; (ii) the indicator arm 8; (iii) a drive for applying impulses until the amplitude is suitable and steady and a damping count can begin; (iv) a bracket for inserting a rod of known stiffness when testing materials of high damping and low stiffness values e.g. non-vulcanized, slightly cross-linked rubbers at room temperature, the added stiffness thus increasing the range of measurement. The lower end of the specimen is clamped in a holder, vertically movable on a hinged arm, Fig. 1 (not shown) and the holder may be rotated in a horizontal plane for zero setting. During a test, a predetermined temperature is accurately maintained in a heat-insulated housing for the apparatus. The scale 11 on the arm 8 has logarithmically spaced light-transmitting marks 40 increasing in width with their distance from the zero point, Fig. 5, in preference to makers of uniform width, Fig. 6 (not shown). By means of an objective lens 13 an image of the filament of a lamp 12 is projected on the scale 11 and a photo-transister 14 is illuminated each time the image falls on a graduation mark. With the specimen vibrating freely, a unit 19, Fig. 4, resets the counter circuit 16 when a push button applies a pulse on line 18 and as soon as the zero point of the scale passes, the phototransistor 14 feeds a pulse over line 20 to a control unit 21 which opens gates 22, 23 and 24. If the degree of damping is so great that the two successive vibration periods for the test are not completed before the amplitude becomes too small for a zero point pulse to be followed by a first graduation mark pulse, the second of the consecutive zero point pulses will cause resetting and the test will have to be repeated with a wider initial amplitude. So long as a first graduation follows the zero point its pulse closes the gate 22 and inhibits the next zero point pulse from causing resetting. During the first vibration period the first pulse train is fed over line 25 through gate 23 to the positive input of circuit 16 and at the end of this period a control pulse closes gate 23 and opens gate 28 so that the second pulse train is fed to the negative input of circuit 16. A final control pulse closes all the gates to terminate the measurement. The gate 24 is open while both trains are counted for clock pulses from an oscillator 26 to be fed to a time counter 27 throughout the two successive periods. These periods may correspond to an addition and a subtraction (i) half cycle, (ii) whole cycle or (iii) pair of cycles with control signals at (i) the second and third zero point pulses, (ii) third and fifth or (iii) fifth and ninth respectively with the zero pulses stored in a flip-flop memory and the memory transmitting the control pulses in the second and third alternative modes of operation, any of which may be selected by a control knob which, at the same time, selects a suitable frequency for the oscillator 26 so that the counter 27 always indicates the cycle time in seconds, while the circuit 16 controls a display indicating the logarithmic decrement.