MECHANICAL MEASUREMENTS

NEW T E S T I N G I N S T R U M E N T W I T H A F L O A T

(UDC 681.2 : 532.135)

B. M. G e r s h k o v i c h

Translated from Izmer i t e l ' naya Tekhnika, No. 6, pp. 13-16, June, 1965

D Y N A M O M E T E R

Instruments with dynamometer mechanisms suitable for measuring re la t ive ly large deformations in a sample, which are produced by small external forces, are required for evaluat ing the theologica l properties of elastoplast ic

and dispersed materials . Such measurements are necessary not only because plastics exhibit in their l iquid-f lowing condit ion a nominal plastic strength which is almost a mi l l ion t imes smal ler than that of meta l , but also because they are preferable for determining the theo log ica l parameters of cer ta in types of plastics. It is known [1] that polymers exhibit their e las t ic i ty and viscosity patterns more c lear ly in sufficiently diluted solutions than in their solid state, since in the first instances stresses are determined with the masking of in termolecular interactions al- most comple t e ly e l imina ted . Thus, the testing carried out with sufficiently small , main ly sheer, stresses, i .e . , with- out destroying the structure, is sui table for evaluat ing and ca lcu la t ing a whole series of rheologiea l parameters.

Existing tensile test machines and instruments for exper iment ing with plastics have imperfect dynamometer mechanisms and, therefore, provide low precision and restrict the range in testing samples which deform at low stresses.

E lec t r ica l viscosimeters [2] have an adequate precision, but they have a very compl ica ted circui t and a large ove r -a l l size, which l imits their wide appl icat ion, espec ia l ly in factory laboratories.

Test instrument PPS-1 with a float dynamometer designed by the author of this a r t ic le and V. I. Tarasov, and made and tested at the Al l -Union Scient i f ic Research Institute of New Building Mater ia ls (VNIINSM) is re la t ive ly s imple and has several advantages.

The instrument is intended for evaluat ing the theo log ica l parameters of plastics with various viscosities and consistencies, starting with 0.3 and up to 10 s N ' sec /m z. These parameters include deformations in simple and plastic shears, dynamic viscosity and ve loc i ty gradients with respect to shear stresses, nominal plastic strength (maximum shearing strength), cohesion, adhesion, surface tension, tensile strength, elongation, compression et al.

The k inemat ic schemat ic of the instrument is shown in Fig. 1. Feed screw 11 is rotated by motor 9 through bel t drive 8 ( i= 2, 1, 0.5) by means of worm gear 7 (i = 60). The feed screw has a keyway with a key fixed to the casing, as wel l as a bracket connected through bottom roller 13 to the cord of the drum.

The screw is displaced ver t ica l ly with its table 6 along cy l indr ica l guide 10.

Rod 12, which is coupled to the table and, hence, to the source of power through the tested sample, is sus- pended at its upper end from a float, with its lower end resting against a Teflon roller on frictionless bearings and engaging by means of its rack with gear 14.

In the absence of deformations in the sample, i .e . , in the absence of reversible (elast ic) and irreversible de- formations (elastoplast ic yielding), the f r ic t ion-force vector which passes through the effect ive axis of the device with the sample is directed towards the loading-force appl ica t ion point. In such a case the friction force does not

affect the precision of the dynamometer . When a load is appl ied to the sample, producing in it e las t ic deformations or yielding, the f r ic t ion-force vector is st i l l directed towards the loading-force appl ica t ion point, whereas the rod with the sample which is fixed to it is affected by a force pointing in the opposite direct ion. Thus, the friction force produces a measurement error when the rod is being displaced and, therefore, it is necessary to make this force con- stant and reduce it to a minimum.

The dynamometer mechanism implements this task in addit ion to its main objec t ive [3]. The mechanism consists of float 1 (see Fig. 1), tank 2 with knob 3, Teflon bearings 19, guide20 and lid 21.

499

6

Fig. 1

The recording mechanism consists of a light drum

4 rotated on frietionless bearings by a flexible cord taken over a roller which is mounted on the axle of gear 14, a

roller fixed to axle 16 of the drum, dial 5, two pointers

22, and carriage 18 with pen 15 and counter-balance 17.

The pen is connected by flexible cords with the rod and the table.

The operation of the float dynamometer used in

this instrument consists essentially of the following. The

float which is suspended on the rod in the liquid balances

its own weight, that of the moving components connected

to it and of the sample-carrying device. It also compen-

sates various errors due to the friction produced by the dis-

placement of the rod. Thus, at the beginning of testing

the load on the sample and, hence, the stresses in it are

equal to zero, which is very important in testing with small loads. The float is submerged by the action of the

motive power through the tested sample. The resulting Archimedean force reacts on the sample by pushing the

float upwards. The float is displaced along the guide

with a min imum liquid friction, since the float bearings

are subjected to a load parallel to the rod, and not to a

radial load which would have produced Coulomb friction whose value depends on the normal force. The design of

this dynamometer is of particular interest, since even the

smallest friction produced by the displacement of the

float does not depend on the loading of the rod. Thus, the friction in the bearings at various load produces a

constant systematic measurement error, which can easily be excluded in the processing of measurement results.

Moreover, another advantage of the float dynamometer

consists of the possibility of exerting on the sampte a con- tinuous, smooth and inertialess load which varies linearly.

The force acting on the sample and measured by the dynamometer can be represented as

p = S h p g ~ - - cr - - F f ,

where S is the effective area of the float, h is the immersion depth of the float under the effect of an external force, p is the density of water, o is the surface tension of water, Ff is the liquid friction force due to the displacement of the bearings with respect to the guide.

The instrument's constant error is o = 82.5 �9 10 -4 N for a float diameter of 7.5 cm, bearing diameters of 1-2 cm

and a coefficient of surface tension of water of 72.8 �9 10 -a N/m. Force Ff also forms part of the constant error, but it

is of even a lower order than o, Therefore, taking into consideration the constants of the instrument, we find that

force P depends only on h.

The instrument's technical data are: maximum loading force of ~ 3 N; rates of loading the sample of 8 �9 10 -4,

16 �9 10 -4 and 21 �9 10 -4 m/sec; error in measuring the force of 0.7-0.8%; a threefold amplif icat ion for the recording scale; motor type A O L B 1 2 - 4 , N = 80 W, n ~ 141 rad/see; over- all size of 582 x 565 x 1300 mm and a mass of 60 kg.

The instrument's equipment includes a control desk for starting and stopping the instrument and controlling the

heating of the sample.

The instrument operates with special devices for gripping the tested sample. These devices are fixed to the

table and the rod. Let us examine the instrument's operation in particular types of testing.

500

I P

k \ \ \ ' f . . \ \ ~

Fig. 2 Fig. 3

F

7 P

2

Tensi le testing. The sample is fixed with one grip to the rod, and with the other to the table. The motor provides

through the worm gear and the feed screw a rec iprocat ing ver t ic le movement to the table . The lat ter drags behind it the sample with the rod, immerses the float and increases the e jec t ing Archimedean force, which, in its effect is equivalent to raising the load on the sample.

The table , which moves at a given speed, conveys that

speed to the rod through the sample, provided the lat ter is not e longated. Therefore, the record ing-mechanism drum is made to rotate through its coupling to the rod, whereas the pen which is coupled both to the rod and the table remains stat ionary and draws a c i rcumference (a straight l ine when the mi l l ime t e r graph paper is straightened out).

The sample is e longated with a suff iciently large load. Then, the tab le and the rod and, hence, the grips of the sample are provided with different d isp lacement speeds, since the table together with the lower grip wil l be affected by a single movement , whereas the rod by two movements . Together with the upper grip it is displaced by the feed-screw axia l force which acts upon it through the tested sample. The second movement of the rod is

provided by the Archimedean force which exceeds the sample ' s resistance to the externa l force and, therefore, this movement is in the opposite di rect ion to that of the table . The resulting ve loc i ty vector of the rod is smal ler than that of the table when the sample is e longated. Therefore, the pen carr iage is displaced and the pen plots the " load- e longat ion" curve.

Evaluation of theologica l parameters by means of coax ia l longi tudina l ly-d isp laced cylinders consists of the following. The tab le is provided with an a t tachment (Fig. 2) consisting of casing I (external cylinder) , internal cyl inder 2, base 3, moving bot tom 4, e l e c t r i c a l heater 5 and thermocouple 6. The tested compound is placed into the casing, and the internal cyl inder is ~screwed" into it and then fixed to the rod. Any superfluous compound is c leaned off the lower face of the internal cyl inder by means of the moving bottom in order to e l imina te adhesion. A recording instrument is used for testing compounds whose viscosities exceed 100-130 N. s ec /m ~. The displace-

ment of the internal cyl inder in testing liquid systems is read by means of a rule and an index. The effect ive height of the cylinders is 40 mm and the internal d iamete r of the ex te rna l cyl inder is 20 mm. The externa l cylinders are exchangeable . Their d iameters are chosen in order to provide a c learance between cylinders of 1.5 to 0.25 mm.

At the beginning of the cyl inders ' d i sp lacement due to the downward movement of the table , the loading on the sample is equal to zero, since the float balances the weight of the internal cyl inder and of the compound layer. The force act ing on the sample is then increased, but it is s t i l l smal l and the sample is subjected only to e las t ic deformations. When the loading is sufficiently high, the internal cyl inder begins to move at a constant speed with respect to the external one, thus producing a stationary e las toplas t ic y ie ld ing of the sample. The interact ion of the d i f ferent ia l recording device is then the same as for an e longat ion of the sample. The d i sp lacement t ime of the internal cylinder, or the intervals between the pen recordings are evaluated on a stop watch.

Computat ions are made according to the formulas [4]:

P 113 r2 "

~I- 2.~Lv rl '

P 0 = ~ r l ;

g ~-0

N

In r2 r I P

E---- 2~L ' ) J r '

where ~ is the dynamic viscosity; P is the force in N act ing on the tested sample; L is the effect ive height in cm of the cylinders; rl, r 2 are the effect ive radi i in cm of the external and internal cylinders; 6) is the nomina l plast ic strength (yield value); g is the shearing ve loc i ty gradient; E is the shear modulus; X is the d isp lacement in cm of the internal cylinder; v is the d isp lacement ve loc i ty of the internal cylinder; r is the tangent ia l tension.

501

~ P

S am pie ' .~ ----~.

P b

/N<Z / Sample

Fig. 4

It wil l be seen from the formulas that the region of plast ic y ie ld ing is l imited to an e lementa ry layer d i rec t ly adjacent to the surface of the internal cylinder. For the remaining compound layers which consist of coaxia l cyl- inders and have a larger shearing area the specific shearing force is insufficiently large in order to produce stationary yielding. This in turn indicates that the beginning of the internal cyl inder ' s d isplacement determines O, if we have in mind the ratio between the e jec t ing force which produces that d i sp lacement and the effect ive shearing area.

Evaluation of theologica l parameters by ver t ica l p l ane -pa ra l l e l plates is carried out by means of an at tach- ment (Fig. 8) consisting of beaker 1, and two cy l indr ica l jaws 2 and 8, between whose fluted surfaces tested sample 4 is placed in advance. Jaw 3 is fixed to the rod by bracket 5. The load is applied to the sample after its p lacing in a thermostat. The table is either lifted or lowered. The upper jaw can be displaced with respect to the lower one. The shear deformation starts when the load exceeds its c r i t i ca l value. Since the direct ion of the loading force coincides with the shearing plane, and skewing is e l iminated owing to the coax ia l i ty of the cy l indr ica l components,

the applied force and deformation are ca lcu la ted from the measured t ime and the readings on the d ia l and the drum by means of the following formulas

P S Ph

~1 = - 7 - ; E - - E S X '

where S is the effect ive plate area in cm2; h is the gap in cm between the plates (thickness of the sample); X is the jaw displacement in cm.

Evaluation of adhesion, cohesion and surface tension is carried out by means of a cone (Fig. 4a), ring (Fig. 4b) and swivell ing end plates (Fig. 4c). I n a l l three cases the weight of the upper part of the device and of the sample is first balanced by the float. The loading of the sample is then started from zero.

The testing of the instrument has revealed its high precision, sensit ivity and s impl ic i ty in appl icat ion.

1.

2.

3. 4.

L I T E R A T U R E C I T E D

P. A. Rebinder and L. V. Chumakova, "Mechanico-s t ructura l (elastoplast ic) properties of solutions and methods for measuring them." In a col lec t ion ent i t led "Advances in the Chemistry and Technology of Polymers" [in Russian], Goskhimizdat , Moscow (1957). E. E. Kalmykova and N. V. Mikhai lov, Zav. lab. , No. 8 (1958). VNIINSM, B. M. Gershkovich, Author's Cer t i f ica te No. 169861 of November 9, 1964. D. M. Tolstoi , ZhFKh, 5, 5 (1934).

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