Patent GB-900272-A (GB)
Title: Measure ment of dynamic properties of elastomers and like flexible materials
IPC: G01N3/32
Filed: 1960-01-19
Abstract: 900,272. Vibration tests. LORD MANUFACTURING CO. Jan. 19, 1960 [Jan. 26, 1959], No. 1876/60. Class 40 (1). [Also in Groups XXXV and XXXVI] Apparatus measuring the dynamic properties of elastomers comprises means for measuring the components of a first voltage derived from an accelerometer responsive to the acceleration of the elastomer, an accelerometer producing a second voltage in phase with one component of the first voltage, a first operational amplifier multiplying the second voltage by a variable coefficient and a second operational amplifier adding the first voltage and the output of the first amplifier, whose output is adjusted in magnitude and sign in accordance with the variable coefficient to produce a minimum output of the second amplifier, so that the output of the first amplifier represents one component of the first voltage and that of the second amplifier represents the other component of such voltage. In Fig. 1, bodies 1 of an elastomer under test are bonded to opposite sides of a plate 2, carrying a mass 3, and to outer plates 4, carried by a shake table 5 oscillated by exciter 6. An accelerometer 7 measures the acceleration of the shake table, and a further accelerometer 8 measures the acceleration of the mass 3 due to the accelerating force transmitted through the elastomer body 1 and resolvable (Fig. 2) into vectoral component 9, due to the elastic modulus of the test material and proportional to the relative displacement of the table 5 and mass 3; and vectoral component 10, due to the damping modulus of the test material. The voltage output of accelerometer 7 is amplified, phase inverted at 15, and added at 16 to the amplified voltage output of accelerometer 8 yielding a signal which is multiplied at 17 by a coefficient α variable by control 19 and is added at 18 to yield a voltage signal which, after distortion filtration at 21, is measured on a valve voltmeter 23 and displayed on an oscilloscope 22. Control 19 is adjusted to minimize the output voltage, at which it is shown mathematically that this represents the acceleration of mass 3 due to the damping characteristic of the test material, the voltage signal at the multiplier output represents the acceleration due to its elastic characteristic, and the voltage signal at the output of adder 16 represents the acceleration of the mass 3 relatively to the shake table 5. In a modification measuring mechanical impedance (Fig. 3, not shown), defined as the ratio of force applied to a structure to resulting velocity thereof, an exciter unit incorporating a force pick-up is applied to a test structure to develop signals representing the force applied, F, and resultant acceleration x. The latter is inverted and multiplied by coefficient α (which may be positive or negative, as determined by switching) and is added to the amplified former signal to produce a signal which is applied after filtration to a valve voltmeter and oscilloscope. Coefficient α is adjusted to minimize this signal, and it is shown mathematically that the amplified force signal represents where Fr is the reactive component of force; Fd is the damping component;  the adder output Fα#x represents the damping force Fd and the adder input represents Fr=α#x. The reactive and damping components of the mechanical impedance are given by F/# and Fr/# respectively.