UDC 678:5.39. 432 EFFECT

OF L I Q U I D

M E D I A ON F A T I G U E

STRENGTH

OF P O L Y A M t D E S

S, B. Ratner and N, I. Barash Mekharlika polimerov, Vol. 1, No. 1, pp, 124-127, 1965 The resuIts of an experimental study of polyamide fatigue strength are presented. Multiple flexing tests were performed on specimens rotating at 3000 cycles/min with a symmetrical load cycle in various media, including air (with and without blowing), volatile liquids (petroleum ether, ethanol, water), glycerin, and trans~ former oii. Fatigue strength is determined from (oN - log N) curves for 108 cycles. Dissipation of heat is found to be the main factor determining the effect of slightly aggressive media on fatigue strength. V a h e s of the ratio fatigue strength/static strength (coefficient K) are presented. potyamides are widely used in machine building for parts working under conditions of continually varying stress and directly exposed to various liquid media. Thus, polyamides have replaced nonferrous metals and alloys in the fabrication of gears, bushings, gaskets, impeller blades, and other parts for mixers, pumps, separators, and simiIar apparatus. An Lnvestigation of the effect of liquid media on the fatigue failure of polyamides is therefore of considerable interest. The fatigue strength of poIyamides was studied in multiple flexing tests on a rotating specimen for a given stress amplitude and a symmetrical load cycle. Testing was carried out on a cantilever-type UKI-t0M machine, using the method described earlier in [1, 2]. The fatigue strength of the materials was determined from curves of (oN - log N) as the stress corresponding to a fatigue life of 106 load cycles. The rate of stressing was 8000 cycles/min. The liquid medium was introduced in the manner recommended by Yu. F. Klimehuk [8]: drops of liquid applied to the surface of the rotating specimen were retained in a lightweight plastic cup loosely attached to one side of the working part. The static strength was determined in tension, as the polyamide specimens did not faii in bending. Therefore, the fatigue coefficient obtained on the basis of N = 106 cycles was higher than usual [1, 2]. The tests were carried out in three stages: 1) air-dry specimens were tested in air: a) with blowing; b) without blowing; 2) air-dry specimens were tested in iiquid media: a) volatiles (petroleum ether, ethanol, water); b) non-volatiles (transformer oil, glycerin): .3) specimens that had swolien in a medium were tested: a) in that medium; b) in air. Swelting took place at room temperature and lasted 1000 hours. In fatigue failure the liquid medium affects, firstly, the initial (static) strength o 2, and, secondly, dissipation of the heat released by repeated deformation of the specimens. Moreover, there are several less obvious factors affecting the process of fatigue failure, The combined effect of all these factors except o I is reflected by K, the fatigue strength coefficient, in the equation

where o N is the fatigue strength for a

given number

of deformation

cycles N.

T a b l e t shows the role of the three factors referred to above, separately and combined, in retation to polyamide68. The following conclusions may be drawn from the data of Table 1: 1. The fatigue strength of the initial unswollen polyamide tested in petroleum ether, water, and alcohol is higher than in air, although the static strength of polyamide-68 swollen in the same media is reduced. This behavior is due to cooling of the specimen following rapid evaporation of the volatile media at its surface. 2, This is confirmed by the fact that in atl these cases the fatigue coefficient (58-72) is not tess than for a forced stream of air (K = g6). Consequently, the increase in fatigue strength is due to cooling. 3. A non-volatile liquid medium (glycerin, oil) therefore has no effect; K = S6-41%, just as in the fatigue failure of a dry specimen without blowing (K = 41%). 4. If fatigue failure occurs in a medium in which the specimen was first swollen, then the fatigue strength exceeds that of the initial material, even if the static strength is reduced as a result of swelling, 5. However, if after swelling in volatile liquids the specimen is tested in air without blowing, then on account of the absence of cooling its fatigue strength wiI1 be less than in the previous case. 87

The effect of the above media on nylon-6 (Table 2) is virtually the same. The static strength of this material decreases after swelling for 1000 hours. The fatigue strength and the coefficient K of the unswollen material tested in highly volatile media is considerably higher than for difficultly volatile liquids. Although the fatigue strength and the coefficient K TABLE

1

Variation in strength properties of polyamide-68 under the action of liquid media

Swelling %

Medium

Air

(gas)

--

<0.01 <0,0t

Glycerin Transformer oil Petroleum ether Ethanol Water

' !

In Tables 1 and 2:

0.44 2,54 0,74"**

iI Fatigue strength, 1 Coefficient K, Static. i kgf/cm~ % tensile t failure strength, I in air I in medium l in air] in medium kgf/cm 2 / ununswollen swollen swollen swollen 603 644 576

250* 272 275

2-~5 270

340** 245 220

41.* 44 48

40 47

56** 41 36

587 575 512

200 172 170

395 265 355

350 436 355

34 30 33

67 46 69

58 72 59

--

* fatigue failure of initial material in air medium; • * ditto, in stream of air (blowing); • ~' weight increase compared with air-dry state of specimen.

for nyton-6 in fatigue tests with air blowing are higher (23%) than in air with no air circulation (17%), blowing has less effect than in the case of polyamide-68. On the other hand, the coefficient K for nylon-6 with blowing is the same as for difficultly volatile media (K ~ 22%), This difference is due to the greater sensitivity of nylon-6 to atmospheric oxygen [4] and to the more rapid oxidation of the material due to the motion of the air. The authors therefore tested with nitrogen blowing also. This increased the coefficient K to 27%, which is close to the value for highly volatile liquid m e dia (28-30°), TABLE 2 Variation in strength properties of nylon-6 under the action of liquid media

Medium

Air (gas) Glycerin Transformer oil

Petroleum ether Ethanol Water Nitrogen (gas)

Swelling,

9

Fatigue strength, I Coefficient K, kgf/cm z l % Static failure tensile strength, in air t in medium fin air 1 in medium kgf/cm 2 unswollen sw le n swollen swollen 738 658 613

122" 130 130

164 165

176"*

17"

0,352 0,354

175

20

t55

21

0.031 6.27,0 7.100"**

655 369 562

130 128 102

176 162 190

206 220 205 200

20

27

35 18

44 34

23**

25

23 21 28 30 28 27

The decisive factor in the action of slightly aggressive liquid media on the fatigue strength of polyamides must therefore be the conditions of heat dissipation. This also applies to the fatigue failure of any plastic. In the light of these data it is easy to understand the increased fatigue strength of GRPs tested in petroleum ether [5]. Unfortunately, the appropriate conclusions about the effect of liquid media on fatigue strength [5] cannot be drawn merely from the confirmation of a fact of this kind without taking into account the effect of the medium on the initial (static) strength and, particularly, on the heating of the specimen during fatigue testing. REFERENCES 1. S, B. Ratner, A. V. Stinskas, and Yu. G. Gil'gendorf, Plast, massy, 9, I960. 2. A. V. Stinskas, Candidate's thesis, Karpov PhysiocochemicaI Inst., Moscow, 1968, 88

3. Yu. F. Klimchuk, Report to 2nd All-Union Conferenceonthe Use of PIasticsin Machine Building, Riga, 1964. 4. G. Khopff, A. Myuller, and F. Venger, Polyamides [in Russian], Moscow, 1958. ~5. L. N. Arsen'eva, Candidate's thesis, Gubkin Petroleum Chemistry Inst., Moscow, 1964, 12 October 1964

Moscow

89