From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analytical report on the dynamic fatigue life curves for a 50 durometer rubber stock.
| Identifier | ExFiles\Box 178\2\ img016 | |
| Date | 15th January 1940 | |
| JANUARY 15, 1940 ANALYTICAL EDITION 21 Linear Dynamic Fatigue Life Curves for a 50 Durometer Stock Figure 8 is a plot of experimental results obtained on a rubber stock which has a reading of 50 on a type A Shore durometer. The data are for rubber worked indoors in artificial lighting and in the absence of oil, grease, or other deteriorating chemical agents. The per cent linear strain at minimum length in the oscillation is plotted as abscissa. The dynamic fatigue life in terms of the number of cycles for complete rupture is plotted as ordinate on a logarithmic scale. On a linear scale the difference in height between each maximum and its corresponding minimum would be greatly enhanced. For the 25 per cent oscillation, for example, the fatigue life at the minimum is about 6,000,000 cycles, while at the maximum in the extension region it is over 600,000,000 cycles or more than 100 times as great. The graph shows results for a series of different constant oscillation strokes varying from 25 to 350 per cent. Each experimental point represents numerical averages of from 1 to 20 sample breaks. The graph itself contains results on 450 samples of the 50 Shore durometer stock. For small oscillation strokes, such as 25 or 50 per cent, there are two maximum fatigue regions, one in extension and the other in compression. For large oscillations—for example, 300 per cent—there is no definite compression maximum. For a given minimum strain length, the larger the oscillation stroke, ΔL, the lower the dynamic fatigue life of the rubber. The fatigue life hump in the extension region shifts towards the origin as the oscillation stroke is increased. This last point is partly due to choice of variables, since for a given stock the per cent elongation at break is a fairly definite quantity and as the oscillation stroke increases the Lmin. for which there will be a break in the first cycle decreases. No corrections have been made in this graph for rubber temperature variations resulting from fatiguing samples of different sizes or shapes. However, when such corrections are made to, say, a rubber temperature of 100° F.{Mr Friese}, the general nature of the curves remains the same. All rubber stocks considered (which include stocks of hardnesses varying from 30 to 80 Shore durometer, type A) have similar fatigue life curves for constant strain conditions of oscillation. In general, for the same strain conditions of oscillation a harder rubber stock will have a lower fatigue life than a softer stock. The compound formula for the 50 durometer stock whose experimental dynamic fatigue life curves are given in Figure 8 is the following: Rubber 100 Carbon black 39 Zinc oxide 5.5 Stearic acid 1.5 Pine tar 4 Antioxidant 0.6 Retarder 0.2 Accelerator 0.8 Sulfur 2.9 Effect of Temperature on Dynamic Fatigue Life Figure 9 shows how the temperature of the rubber during oscillation affects its fatigue life. The graph is a composite one FIGURE 5 (Upper) RUBBER SAMPLES MOUNTED IN SLOW-SPEED FATIGUE MACHINE (Center) RUBBER SAMPLES MOUNTED IN HIGH-SPEED CONSTANT-LOAD FATIGUE MACHINE (Lower) RUBBER SAMPLES MOUNTED IN HIGH-SPEED DYNAMIC FATIGUE MACHINE | ||