R I G I D D Y N A M O M E T E R FOR M E A S U R I N G LOADS BETWEEN 50 AND 2 0 0 0 N V. F. G a i d u c h e n y a

a n d A. I. S a v i n k o v

UDC 531.781

The principal requirement imposed upon the sensitivity of instruments for micromechanical tests is high rigidity, corresponding to the condition K M >> dP/dAt0, where K M is the rigidity coefficient of the machine and Al 0 is the elastic deformation of the sample under the load P; dP/dA10 is the rigidity coefficient of the sample K0 [4]. In other words, the "machine-sample ~ system should have as low as possible a compliance 0 = K0/K M ~ 0. In existing Soviet-made machines of this kind, the requirement of a high rigidity is satisfied only by the DubovRegel' relaxometer incorporating a photoelectric dynamometer (elasticity 0.002 y/N) [1]. Even in this instrument, however, the inadequate rigidity of the dynamometer (K = 50.107 N / m ) produces an error of about 20% of the measured values when recording stress-relaxation curves [2]. Disadvantages of this relaxometer include the high cost of the optical-mechanical system of the dynamometer and the fatigue of the photocells, leading to a zero drift. In this paper we shall describe the construction of a rigid dynamometer based on wire strain-gauge resistors in conjunction with a high-sensitivity electronic circuit, comprising a preliminary stage of amplification and a system for recording the measured stresses (Fig. 1). The elastic element of the dynamometer is a hollow cylinder made of 35KhGSA steel, the internal diameter of which, d o = 14 mm, is only restricted by the minimum area needed to attach a strain-gauge bridge composed of wire converters. The range chosen for the working loads of the dynamometer, 50-2000 N, is divided into six practical measuring ranges with maximum measured stresses of 50-2000 N. The absolute deformation of all the elastic elements in the working part of the length (equal to l 0 = 20 ram) is no greater than 0.2 g for the nominal load. The construction of the proposed dynamometer and the elastic element is illustrated in Fig. 2. The converters employed are strain-gauge resistors of the PKP type made of PKTT-0.2 Constantan wire, each having a resistance of 400 ~ and a working base of 20 ram. The working resistors are glued to the elastic element; a high-resistance shunt resistor is placed in the body, which protects the electrical circuit from external electrical and mechanical effects. This arrangement ensures a high sensitivity for small deformations of the elastic etement, and also provides for stability of the measured-load readings at temperatures of 40-50~ and a relative humidity of 85%.

Fig. 1. Block diagram of the electrical strain-gauge dynamometer. 1) Dynamometer; 2) voltage amplifier and cathode follower; 3) electronic potentiometer ~PP-09M3; 4) supply transformer of measuring circuit; 5) voltage stabilizer.

For the maximum load on the dynamometer, and on the assumption of a linear temperature dependence of the elastic modulns, the Poisson coefficient, the strain sensitivity, and the crosssectional dimensions over the range :k50~ the bridge disbalance voltage is given [3, 4] by the expression

2,66 :\Ud = - ~ t - - Us sK,

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 50-51, June, 1971. Original article submitted June 26, 1969.

J

i9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever ] without permission of the publisher. A copy of this article is available from the publisher for $15.00.

881

p

.,,

tO0

/'U

8O

I

60 I

r I

I

~0

tJ Fig. 2. 1) Cylindrical elastic element; 2) protective screen and body of the dynamometer; 3) points for attachment of the straingauge converter; 4) bushing for fixing the protective screen and the body of the dynamometer.

~o

o

0,/

O,2

0,3 ~t, mm

Fig. 3

where AUd is the maximum disbalance voltage of the strain-gauge bridge for the nominal loadr Us is the supply voltage of the strain-gauge resistors (6 V): G is the relative deformation of the resistors (10 -5 cm): K is the strain sensitivity of the resistor material [4]. Thus if we neglect the terms of secona and higher orders of smallness in the temperature changes and also the elastic imperfections of the material of the elastic element and the material of the converter wires, we obtain AUd = 60 gV. The output signal of the strain gauge is required to be reproduced in *load-time" coordinates in a secondary recording device. By way of a secondary recording device, we chose a potentiometer of the ~PP-09M3 type, with a uniform scale marked in a hundred divisions, the recording being effected on a strip-type diagram. For the normal operation of the ~PP-09M3, the minimum voltage at the input of the amplifier Uin = 6/zV. The minimum bridge disbalanee voltage of the dynamometer AUdmin is no ffeater than 0.6 #% ten times smaller than the voltage required. In order to achieve preliminary amplification to 6 ~tV, a special two-stage rheostat-transformer amplifier with a 6N2P tube was developed; the first stage constituted a volrage amplifier with'an amplification of Ka = Uout/ Uin = 34. The second stage was made in the form of a cathode follower with substantial negative voltage feedback. The transmission factor of the stage is given by the conditions [5, 6]

SRL Kt -

-- 0.85.

1 ~ SR L

where S is the transconductance of the tube; RL is the anode load resistance; Uout = A~minKaKt = 0.6.34.0.85 - 17..'2 #V; i.e., the output voltage required for the EPP-09M3 is covered by a factor of 2.86 times. Atthe input of the voltage amplifier is a potentiometer, to ensure smooth regulation of the amplification. The sensitivity threshold of the amplifier is no greater than 0.2 /~V, the error is about 0.2%. At the output of the cathode follower is an output transformer with a 1 : 15 ratio, the voltage from this being passed to the input of the U~-119 amplifier of the EPP-09M3. The measuring circuit consists of three four-armed equilibrium bridges, strictly balanced with respect to resistive and reactive components: the dynamometer bridge, the zero-set bridge, and a compensation bridge with a slide wire. The disbalance voltage of the dynamometer 2xUd is compensated by the voltages of the compensation bridge The bridges are supplied from separate six-volt windings of a low-voltage transformer based on a toroidal armature. The measuring circuit provides for the possibility of regulating the zero and the sensitivity during the experiment.

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TABLE 1

TABLE 2

Nonlinearity Load, N

Time zero 1Change in drift in I sensitivity lyr

in % 50 100 200 500 I000 2000

--0,2 --0,193 --0, 185 --0,179 --0,18 - - 0 , 186

0,29 0,24 0,281 0,289 0,283 0,292

0,25 0,251 0,254 0,25 0,25 0,25

Material Material Steel Aluminum Silicon. Roetr salt

E, 10,0 Pa 2,1 8

13,2 4,8 0,7--0,8

O=Ko/KM 0,027 0,013--0.11 0,018 0, 0 0 6 0,0018

In order to ensure the required characteristics of the amplifier, a high-stability anode voltage supply source was developed. The amplifier tubes were heated with a dc voltage derived from a rectifying bridge based on germanium diodes. In the Standard U~-119 amplifier of the EPP-09MS, the one-half-period rectifier based on the second half of the 6N9S tube Ls was eliminated. The anodes of the U~-119 tubes were also fed from the voltage stabilizer. The zero drift experienced on operating the dynamometer with the electronic circuit indicated was no greater than 0.1% of the value of the working soale over a period of 12 h. The quality factors of the dynamometer after one year's operation in the load-measuring system are presented in Table 1. The class of accuracy of the series of dynamometers under considerations is no lower than 0.2. The characteristic frequency of the vibrations of the unloaded dynamometers for the loads in question lies between 1.6 and 6.5 kc/sec. The class of accuracy of the whole stress-measuring system is no worse than 1.5. However, experience gained over a year's work showed that, subject to accurate calibration before each experiment, the error of the whole system was no greater than 0.5% of the measuring limit. The calculated rigidity coefficient of the dynamometer Kd equals 1010 N/m, and correspondingly the elasticity Ud = 0.0001 ~ / N , which is 20 times smalter than the Ud of the Dubov-Regel' photoelectric dynamometer. Since the rigidity coefficient of the system KM = 7.4.109 N/m, the compliance factor of the whole loading system when testing materials with a wide range of elastic properties is smaller than unity (see Table 2). This indicates a substantial reserve of rigidity in the system, so that the latter may also reasonably be employed for studying various dynamic processes taking place in the sample under toad (twinning, fault formation, yield teeth, etc.). The potentialities of the instrument are illustrated by the compression diagram of a KRS-5 crystal (Fig. 3), which is characterized by instantaneous rises and falls in the load P, arising as a result of a process of fault formation, accompanied by plastic deformation A1. The error in the recorded relaxation stresses of an LiF crystal (relative to the true ones) arising from the intrinsic relaxation of the elastic element of the dynamometer (without considering "plastic shears" in the test machine) is no greater than 1.5%. Thus the sensitivity and accuracy of the proposed electronic dynamometer recommend the latter as a universal instrument for studying various mechanical properties of small samples of a wide range of materials. LITERATURE 1,

2. 3. 4. 6. 6.

CITED

G. A. Dubov, Author's abstract of dissertation, Moscow State University (1958). N. K. Rakova, Author's abstract of dissertation, Moscow State University, Moscow (1967). K. Perry and G. Lisner, Fundaments of Stress Measurement [Russian translation], IL, Moscow (1957). Z. Ruzga, Electrical-Resistance Strain Gauges [in Russian], Gos4nergoizdat, Moscow (1961). G. B. Voishvillo, Low-Frequency Tube Amplifiers [in Russian], Svyaz'izdat, Moscow (1989). E. O. Fedoseeva, Amplifying Devices [in Russian], Iskusstvo, Moscow (1961).

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