Julv 15. 1930
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Autographic Stress-Strain Curves of Rubber at Low Elongations‘ A. A. Somerville, J. M. Ball, and L. A. Edland R . T . VANDERBILT CO., 230 PARK4vE., NEWYORK,N. Y.
The usual methods of obtaining stress-strain data X A S L I U C H as rubber ruler or pointers, and the through the use of dumbbell-shaped test pieces and compounders are devotother recording the dial load Schopper rings are briefly discussed. A new test piece ing a considerable on receiving a verbal signal. is then described through the use of which stressFigure 1 shows such a test amount of laboratory work strain curves are drawn autographically, from which toward o b t a i n i n g s t r e s s being made by two operators stresses are read quite accurately at the lower portion strain data and discussing in an unusual position. of the curve even down to 10 per cent elongation for these data in detail in their An i m p r o v e d o n e - m a n either pure gums or heavily loaded stocks. Various technical papers. it w o u l d method m a k e s use of the factors affecting the position of the stress-strain curves seem worth while that stressspark device by which a hole are considered, such as variation in cures, speed of strain measurements should is burned through the recordtesting machine, temperature and humidity, together be made with some accuracy. ing papcr by electrical conwith the effect of mill grain and flaws in the test piece. F u r t h e r n i o r e , stress-strain tact made by the foot a t There is also a brief discussion of the effect of increased each 100 per cent increment measurements made to date loadings of carbon black. of elongation. do not give any information Figure 2 shows a c o m a t low elongations of the test piece. The method here to he discussed furnishes a iiieaiis of parison of tn-o stress-strain curves-of a typical tire-tread determining fairly accurately the stress a t extremely low stock made by each of these two methods. Sotice that elongations. KOatteiiipt i q macle to correlate the shape of a the curve made froin data obtained by the two operators shows a higher modulus due to delay in recording the corstress-strain curve or the magnitude of the i i i o d ~ d derived ~i~ therefrom with the practical service t o be obtained from an rect reading. The Schopper method of employing a ring-shaped test article in n-hich the rubber compound in question is used. piece gives an autographic curve to the breaking point, readable from about 200 or 300 per cent elongation upward, but not very accurate a t lower elongations. Such curves have been obtained on rings 6 inches (15.2 cm.) in circumference slipped over four horizontal pulleys. Figure 3 shows the curve for the same tire tread stock in comparison with the two curves already given obtained on dumbbell test pieces. The curve breaks off short, showing the low tensile-stretch characteristics of ring test pieces.
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K h e n a college freshman is given the problem of determining Young’s modulus of steel, he may be handed a piece of wire 20 feet long. H e attaches one end of the wire to a point near the ceiling and hangs increasingly heavy weights to the 800
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Usual Methods of Obtaining Stress-Strain Data
A common method of obtaining data for constructing the stress-strain curve of rubber might be called t h e “hand and voice” method, in which two operators are employed, one following the bench marks of the dumbbell test piece with a Received April 15, 1930. Presented before the Division of Rubber Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930. 1
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Figure 2-One Operator vs. T w o Operators, Using D u m b bell Test Pieces a n d Tread Stock
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other end. H e can read accurately the resulting increases in length of the wire by means of a finely divided scale read through a telescope. By this method a beginner obtaiiis something very near the true value for Young's modulus. Is it surprising that' various people testing rubber startingwith a piece 1 inch (2.5 cm.) long get different results? I t is true that rubber stretches much more than steel! so it i3 hardly necessary or feasible to start with a piece of rubber 20 feet long; but there is no reason why we cannot start with a piece of rubber 10 inches (25 em.) long, and the results given in this paper have been obtained on a test piece of this length. Actually the test piece in question is a loop 10 inches (25 em.) long, or 20 inches (50 cm.) long if opened out! molded square '/4 by '/4 inch (6 by 6 min.), as shown in Figure 4. It is stret'ched over single collarbutton jaws. Such an enlarged test piece has the following advantages: (1) It has about six times the cross section of the ordinary dumbbell or ring test piece, and therefore tends to overcome or mask friction of the machine, and for a very small elongation gives a relatively large reading in the dial. (2) Each loop is molded, eliminating all subsequent cutting. The gage of the molded loop need not vary greatly, but if variation is Figure found it may be compensated for by the adjustment of weight on the machine arm. (3) Ordinary flaws in the molded loop are of no consequence. (4) Every part of the test piece is stretching a t a constant speed throughout the entire test.
Preparation of Rubber Compounds
Six compounds were prepared containing increased carbon black from pure gum u p to 50 parts of black to 100 parts of rubber, according to the following formulas:
x Smoked sheets Pale crepe Zinc oxide Sulfur Antioxidant Stearic acid Pine t a r Accelerator Carbon black
A-l
A-2
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2 25 2.23 0.7
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I n the mixing of these compounds A and A-5 were designed t o cure at the same rate and then blended in the proper proportions to give the four intermediate compounds. All the compounds were designed to have the same rate of cure, and that this was actually the case is shown by the tensile-time curves in Figure 5. All the compounds reach their maximum tensile in about the same time. The 50-minute cure was chosen for the stress-strain curves shown below. It might be well to point out that mixing two stocks in a 50-50 ratio does not necessarily give a stock having physical properties exactly midway between those of the two individual stocks. I n the above case the addition of 10 parts of carbon black produces less effect than might ordinarily be expected.
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VARI.4TION IN CCRE-Figure 6 s h o w the stress-strain curves of the pure gum compound a t increasing cures drawn autographically using the IO-inch loop test piece. Figure 7 is the analogous series of curves for compound 6-4, containing 40 parts of carbon black to 100 parts of rubber. SPEEDOF TESTIXG ~ ~ S C H I N E -8 Fshows ~~U the~effect ~ of v a r p n g the speed of the testing machine on the stress-strain curve for pure gum, and Figure 9 is the analogous curve for the tread stock containing 40 parts of carbon black. S o t e that in these cases the low speed of 2 inches ( 5 cm.) per minute gives the softer curves. Since with the IO-inch (25em.) loop a stretch of 10 inches is required to give a n elongation of 100 per cent it is clear that a speed of 20 inches (50 em.) per minute for the machine is equivalent to a speed of
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only 2 inches per minute for a I-inch (2.5-cm.) length of the rubber sample, and that a speed of 2 inches (5 em.) per minute for the machine is equivalent to only 0.2 inch (5 mm.) per minute for a I-inch (2.5-cm.) length. When dumbbell test pieces are stretched a t the ordinary speed of 20 inches (50 cm.) per minute, the actual rate of separation of the 1-inch (2.5-em.) bench marks begins at about half that speed and the rate of stretch of the I-inch portion gradually decreases as the elongation increases. Therefore a I-inch length of the dumbbell test piece separates much faster than a corre-
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I N D U S T R I A L d S D E S G I S P E R I S G CHEJfISTRY
July 15, 1930
sponding length of a 10-inch (25-cm.) loop. dl1 these considerations have a bearing on the relative position of stressstrain curves obtained on a compound a t two different speeds. It is actually possible to obtain cuives such that the one obtained a t the lower speed indicates a stiffer stock than that obtained at the higher speed. MILL Gum-Figures 10 and 11 shox the effect of mill grain on the stress-strain curves of pure gum and tread stock, respectively. The curves obtained show that the stocks are slightly stiffer when the loops are cut in a direction lengthwise with the grain. FLAWS IN TESTPIECE-Figure 12 shows trro curves, one obtained with a perfect loop and the other with a loop containing severe flaws. It is evident that the flaws do not a p preciably influence the shape or position of the curve in any way. HuMmTF-Figures 13 and 14 show curves for pure gum and tread stock, respectively, run at two different humidities, 45 and 96 per cent. The effect of humidity is nearly negligible. A relative humidity of 45 per cent is one of the conditions adopted as standard by the Physical Testing Committee for conditioning the test samples and it does have an effect on uncured stock. TEUPERATuRE-FigUreS 15 and 16 show curves for pure gum and tread stock, respectively, drawn a t three different temperatures. As previously pointed out ( I ) , a loaded cornpound shows considerable spread between the 0" and 100" C. stress-strain curves, whereas a properly cured pure gum stock inay show very little, if any, spread, and may even show a
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crossing of these two curves whereby the stress a t 100" C. IS actually greater a t a given elongation than the stress a t 0" C. Figure 15 shows that the pure gum compound being tested has practically identical stress-strain properties a t the two temperatures u p to an elongation of at least 300 per cent. INCREASED LOADISGSOF CARBOSBL.icr
