Prec To ehee tests on natural and synthetic rubbers. Tear resistance values obtained with the angle specimen were slightly less influenced by grain than values with the A S . T.M. specimens, but no correlation hetween values was found. Coefficient of variation, criterion used for comparing precision of the three methods, showed precision of angle specimen tests was consistently better than other tests.
T .
HE Winkelmann tear test with the ercscent specimen has
been widely used in the ruhher industry. The A.S.T.M. ( 1 ) has adopted this test using tw'o forms of the crescent specimen with a single nick in each as recommended by Poulcs (6). Recently Buist (SI demonstrated that the test result obtained with the A.S.T.M. type B crescent specimen is very sensitive to the depth of the nick. He found that the same blade setting did not make the same depth of nick in all stocks. To overcome this difficulty he developed a cutting machine in which the hlado setting could be changed to secure, by cut and try, a nick depth of exactly 0.02 inch as specified by A.S.T.M. ( 1j.
grain-in arriving a t each publishcd chefficient of variation. The prcsent authors have conducted tear tests on 14 stocks using the anglc spccimcn and both A.S.T.M. type A and typc B Specimens, and have test.ed not lcss t.han 32 specimcns of each shape from each stock (16 serogs grain and 16 along grain). The dies uscd in this statistical study and the specimens cut therewith are illust.rated inFigure 1. The dimensions of tho specimens are given in Figuros 2,3, and 4. The stocks test.cd are formulated in Tablo I and thoir tensile properties are list,cdin Table 11. Natural ruhbm and sevcn synthetic rubbers wcrc uscd in t.he preparation of those various stocks.
Figure 1. Dies and Specimens for Tear Tests
The stocks wrere taken off a mill or calender a t 0.1-inch thickness and molded into sheets 9 X 12 inches, with a nominal thickness of 0.08 inch. The direction of the grain, if any, urns arranged to be in the longer dimension of the sheet. From each of four sheets of every stock excepting stocks 2, 3, and 4,four specimens were cut parallel to the grain direction and four specimens vere cut a t right angles to the grain direction with each of the three dies shown in Figure 1. Specimens were prepared in this manner from eight sheets of each of stocks 2, 3, and 4. Thus far each die, not less than 16 specimens of every stock were tested aerpss grain and not less than 16 specimens were tested along gram. The apparatus manuftlotured by the Akron Equipment, Campany were used for cutting the nick in the A.S.T.M. specimens. The razor blades in these apparatus were set a t 0.02-inch extension bevand the bearine surface of their reswctive holders.
DETAILED PROCEDURE
Left.
Greves A.S.T.M. type A A.S.T.M. fypeB
Center.
Right.
specimen were &l$ in self-tighteniig clamps. All tear tests aid the tensile tests previously mentioned mere conducted a t 82"
Just previous to the publication of Buist's work, a paper by Graves (4)described tests on a n angle tear specimen and gave data and observations t o show that this specimon had the following advantages over the crescent specimcn:
+
5" F.
RESULTS
The results of the tests were calculated as the force in pounds per inch thickness required to tcar the specimen. The individual results srenot given h e r e b e c a u s e of their voluminosity, but the arithmetic means of the results are given in Table 111. The data in Table I11 show that the tear resistance values obtsincd +-7 / with the angle specimen w e r e slightly less influenced by grain than the tearresistFigure 2. A.S.T.M. Type A ance values obspecimen
1. No nick is required. 2. Test results are more reproducible. 3. Pulling stress is more concentrated a t the point of rupture. The present authors desired to check the better precision (reproducibility) claimed for the test with the angle specimen, since this feature has the most interest to testing litboriltorics. An increase in the precision of the test enables smaller differences in tear resistance between stacks to be recognized and reduces the number of specimens which must he testcd in order to secure a r e . liable average value for any stock.
i
EXTENT O F TESTS
Graves (4)limited his statistical study t o four stocks, tested only the A.S.T.M. type B crescent specimen in comparison with his angle specimen, and tested only 5 specimens-dl wit,h the I
Present address, Shell Development Co., Emeryville, Calif.
436
437 -
Table I f .
/' 1.L.
!
s t ,i,!k
I
4 5 6 7 8 9 10 11 12 13 14
>
2.7 *
4.5.'1'.\1. 'l'jpe H Specimen
Figure 3 ,
+dined n i t h tht, A S T 11 5pccinienq. These data also shon that the A S . T 11. type A specimen ,generally gave value, which w r e gieatest in magnitude, followed in turn by the \ 3.T 11. type B spwimen and the angle specimen. There
Table 1. Composition of Storks Tested 4
1
Hel-ea Gasket Sruoked .beet
..
100.0
1;R-S
Perbunan 26 Zinr oxide Alicronex Standard
5 0
100.0 5.0
....
....
70.0
.
.
3 .I)
3 0
1' 0 3.0
....
. .
8 0
.
10.0 10.0
1.0
,
....
.
...
0 5 0.5 0.5 2 0
0.75
2 0 0 4
0 4
....
0 6
.... 3.0 .... .
15.0 15.0
...
.
.
2.0
....
3.0
....
.
.
....
.
.
.
.... ....
I
100.0
. 5.0
. . .
10 0 20 0
10 0
.... ....
100.0
....
....
....
. . .
. . .
. . .
. . .
....
..
4.0
...
50 0
1 0
....
50.0 20 0 1 0 10 0
0 5 30 0
....
... ,
10 0 25.0
2.0
....
....
...
10.0 2 0
1.0 1 75
....
....
1 75 60/274
GR-31 Hose
.... .... ....
5,0
1 0 50 310
80 310
i z i :0
1.0 10.0
....
1.0
....
....
..
2 '0
....
1.75
ioo:o
12
00.0
....
Hose
11
....
1.2
Hose
10
Hose
....
1.75
.... . . .
14 Thiokol
9 Seoprene
GR-;\I
0.3
. . . ....
13
8
G8-N
i .'o
.... .... . .
T hiokol
0.1 25/310
.
5 ,0 4'0
BT
90.0 10.0
.
30.0 2.0 10.0 3.0
....
..
2.0
FA
. . . .
.
.... ..
iio.0 10.0
'60'0
... ..
. . ...
80.0
..
Tensile Streugth
a-ker asker asket Gasket Gasket Hose Hose Gasket Gasket Gasket Hose Hose Hobe Hohe
2920 1410 1790 2310 2170 2080 2010 1840 1490 1800 1720 2610 1020 1420
l,,
I.ltiniate Elongation v ,c
790 700 680 590 700 580 280
750 :330 880 350 380 160 3611
Modulus a t 200% Elongation LI,.!J*.i,i.
40
140 230 440 250 1080 165U 270 ,570 110 1090 1330
ikli
seemed to be n o colieldtion between the vdlues ohtaintd bv the three methods; consequently the procetluie using the angle specimen should not be employed as a replacement for an A S . T.11. procedure in any case requiring a reliance on previously accumulated tear resistance data. d study of the tear re Table I11 in comparison with the tensile 6 7 data in Table 11 leads to the conclusion Perbunan Perbunan 26 26 that there lvas no correlation 1)et.rqeen Hose Hose tear resistance as determined by any of .... .... the three methods on the one hand and 100.0 1oo:o tensile strength, ultimate elongation, or .5 . 0 5.0 modulus on the other hand. An out50.0 .... standing example of lark of coirelation .... .... 40.0 80.0 was stock 13. Even though this stock . . . 40.0 1.0 1.0 had a tensile stiength oi only 1020 ... 2.0 pounds per square inch and a n ultimate ... .... . . .... elongation of only 160rc, its tear resist10.0 10.0 ance by any of the thiee methods was 10.0 .. far supeiior to the corresponding tear .... . . .... .... resistances of stocks 1, 0 , and 10, nhirh . . . 10 0 had much better tenvle properties. ... 10.0 ....
....
0.1 20/310
Gasket
Thiokul d T Thinkiil F.1
60/280
20 310
FK
GR-AI GR-11
....
25,310
Gasket
I'erbuiian 26 Perbunan26 Perhiinan26 Perhunan26 Hycar OR-15 Seoprene F H GR-11
40/300
2 0 f290
Hycar OR-15 Gasket 100.0
F.
.... ..
, .
2.0
. . ....
... 1.0 3.0
. . .
1
Cure, minute per
...
. .
8 I1 8 (1
.
60.0
. .
1.0 3.0 2 ,5 . . .
3tork Y o
Hycar OR-1.5 Seoprene FR GR-A2 Thiokol ST Thiokol LP-2 Thiokol F A Zinc oxide
.... . .
ioo: 0
5.0
(MPC) S t a t e s B (FF)
Pelletex (SRF) Therrnax (SIT) dtearir dcid Heliozone Cottonseed oil Saftolen 510 Cuniar P-10 Piccocizer 30 Bardol Dibutyl phthalate Dibenzyl sebacate Tributosyethyl pliubphate Plastirizer SC .ige Rite Resin D Seozone .1 Captas .iltax Tuadb Thiones Diphenyl guanidine Sulfur 3elenium Cure, minute per E.
5
Perbunan Perbunan 26 26 Gasket Gasket
.ipplirnrion
I.h./ay.
0.3' RAD. , I t - . - -
Hubher
SO.
-
Tensile Properties
.... ....
0.5
,
.
. . ... ... ...
....
. . . ...
....
401287
30/300
60/260
i:5
. . . ..
60.0
....
0.5
.... ...
... 0.3 0.1
.... ....
40/800
PRECISION OF TESTS
The criterion used for comparing the precision of the three methods was t.he coefficient of variation. The coefficient of variat,ion is the standard deviat,ion of a set of values divided by the arithmetic. mean of the set and tlspreased as a percentage. The standard deviation is the root mean square of the individual deviations from the arithmetic mean. The coefficients of variation calculated from the tear resistance values obtained with each of three types of specimens are given in Table IT'. These coefficients s h o t,hat ~ the precision of the test using the angle spwinien was consistently better than the precision of the other tests except in the c a s r of stock 7 a(+rossgrain, stock 0 along grain, stock 13 both directions, and stock 14 both directions. The arithmetic means and the medians of the coefficients determined in each direction TT-ith each specimen show a distinct superiority for the angle specimen. The medians of the cwefficients art' perhaps thti tirttrr basis
VOLUME
438
NO
1 2 3 4 6 6 7 8 9 10 11 12 13 14
Rubber Hevea
GR-S
GR-S Perbunan26 Perbunan26 Perbunan26 Perbunan26 Hycar OR-15 NeopreneFR GR-M
GR-AI
GR-M Thiokol ST Thiokol FA
Type Across grain 198 230 235 297 316 379 183 212 87 158 356 273 233 271
A Specimens Along grain Diff +1 197 234 -4 +13 222 299 -2 +4 312 363 197 216 -4 93 -6 155 +3 270 327 266 -33 263 +8
ki:
5;:
Type B Specimens Across .4long grain grain Diff 149 147 +2 227 215 +11 221 214 4-7 253 -2 251 225 223 1-2 -3 -16 179 179 0 74 89 -15 121 115 +6 361 257 +lo4 -47 266 313 232 241 -9 272 281 -9
% %
Angle Specimens Across Along grain grain Diff. 104
101
+3
148 168 215 186 236 153 159
139 166 205 181 225 160 150
+2 9
Q7
+IO +5 +11 -7 +9 II -1
09
1-
1-
101 238 216 183 290
101 203 212 230 303
28,1
Figure 4.
Since the completion of these tests the authors have been advised bv Graves ( 6 ) that the angle specimen used in this work is not the same as the angle specimen originally designed ( 4 ) . The angle specimen used by the authors was copied after the drawing given in Graves' paper. Several critical dimensions were missing from this drawing, so that it was necessary to scale the dimensions directly from the drawing. The original angle specimen as employed by Graves is shown in Figure 5 . It will be noted that the points of difference between this specimen and the specimen used by the authors are the length and the outside radius at the 45' bend.
Stock So.
Angle Specimen
for this comparison because the anomalous values obtained with stocks 13 and 14 have less influence on the mcdisns than t,hey do on the arithmet,ic means. Buist and Kennedy ( 3 ) pointed out that testing with the anglr specimen gives a n indication of the force t o initiate a tear, whereas testing with the crescent specimen determines the force to continue tearing. From this standpoint t,het e t using the angle specimen seems to be more important, since a tear must be st,arted before it can be continued. This fact together with the greater precision of the test using the angle specimen are strong arguments for its adoption in lieu of the present A.S.T.11. tear tests. I n conclusion, it should be emphasized that, only one angle die was used in this study. I t could well be that the variation intro-
ADDENDWM
0 +35 +4 -47 -13
Table IV.
li-- --Y
NO. 7
duced into test by using more than one angle die would decrease the precision of this test below that of the test with the crescent specimens. The use of more than one die in the case of the latter specimens would probably not affect the precision, since it is the nick in the crescent specimens which is the critical factor.
Table 111. -4rithmetic Mean of Tear Resistance Values, Pounds per Inch Thickness Stock
19,
Coefficient of Variation of Tear Resistance Values
Rubber Hevea
GR-S GR-S
9 10 11 12 13 14
Perbunan Perbunan Perbunan Perbunan Hycar OR-15 Keoprene FR
26 26 26 26
GR-RI GR-R.1 GR-RI Thiokol S T Thiokol FA Arithmetic mean Median
Type A Specimens Across Along grain grain 4 9 4.5 7 7 13.4 10 2 11.1 8.7 10 3 11.7 13 8 9 0 7.0 5 6 7.5 6 2 10.9 12.5 6.7 13.1 8.9 7.9 9 1 9 0
Type B Specimens Across Along grain grain 4 1 8 7 10 9 8 5 11 2 10 3 6 8 6 3 7 1 5 8 7 4 10 7 7 2 6 0
7 3 7 13 6 7 11 8
4 7 3 7 5
8
4 9 8 7 9 15 19 10
9 0 8 2
5.3
Angle Specimens Across Along grain grain 2 7 2 7 61 4 4 5 0 6 3 4 0 4 9 5 4 4 3 4 7 1 6 4 5 8 0
.
4.3
8 7 9 2 2 3
6 9 7 1 6.8 19 0 12 5 9 3
4 5 0 9 15 5 12 2
9 5
8 7 7 2
6 0 4 7
8 8
6 3 6 3
3.1 10 3 1 4 12 11
8 3 7 6 9 5
4:
The authors believe that their angle test specimen gives essentially the same results as t,he true Graves specimen, since the small difference in lehgth will have no effect. on the results, and the difference in radius would be expected to have a negligible effect. This statement is not made with the intention that the authors' angle specimen shall be adopted in lieu of the true Graves specimen, but' merely to justify the application of the conclusions of this work t o t.he true Graves specimen.
* LITERATURE ClTED
(1) Am. Soc. Testing Materials. Method D624-44. (2) Buist, J. M.,Trans. Inst. R u b b e r Ind.. 20, 155 (1945); Rubber Chem. Tech., 18, 486 (1945). (3) Buist. J. M., and Kennedy, R. L., India R u b b e r J., 110, 809
' A d 100 R A D
I1 : Figure 5 .
224
Angle Specimen Designed by GraFes
(1946). (4) Graves, F. L., India R u b b e r W o r l d , 111, 305 (1944); Rubber Chem. Tech., 18,414 (1945). ( 5 ) Graves, F. L., private communication. (6) Poules, I. C., I n d i a R u b b e r W o r l d . 103, 41 (February 1941). THEopionions or assrrtatiuns in this article are those of the authors and are not t o be construed as official or reflecting the views of the S a v y Department or the naval service a t large.
