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).
The fatigue testing of rubber under various conditions of strain, oscillation, compression and extension.
| Identifier | ExFiles\Box 178\2\ img057 | |
| Date | 25th March 1940 guessed | |
| -2- The fourth slide illustrates various conditions of initial strain and constant oscillation cycles. If the unstressed length is L_o we may compress it and apply a compression cycle; or we can have a com- pression-extension cycle, or an extension cycle with zero strain at one point as I previously demonstrated with the piece of rubber; we can have the piece in considerable extension at all times. We have developed a number of machines for investigating the whole range of possible cycles and one of these is shown in the next slide. (slide No.6). Here the same vertical oscillation is being applied sim- ultaneously to about 35 samples which have different minimum extens- ions as shown in slide 7. The next picture shows a high speed fatigue machine which applies similar cycles 3500 times per minute. The effect of minimum strain is very large and extremely impor- tant. The next slide shows the typical behaviour of rubber under the com- plete range of minimum strains for an oscillation cycle equal to 25% of the unstressed length. (slide 10). The curve covers a region from ex- tremen compression to high extension. At the left side of the diagram we can have sufficient compression to cause the piece to break in one cycle. At the extreme right we have so great an extension that again the piece breaks in one cycle. Between these extremes we have a very wide range in life. Instead of getting a good fatigue resistance at our "com- monsense" point of zero strain it is practically the worst condition that can be chosen. If we start with the rubber under a minimum exten- sion of about 200% the dynamic fatigue life will be at least a hundred- fold greater than when minimum strain is zero. Slide 11 is a more complete diagram for a standard rubber stock in which the effect of variations in the minimum strain is shown for eight different oscillation cycles. Note that the life is shown on a logarithmic scale in kilocycles and megacycles. The upper curve is for the 25% oscillation cycle shown in the previous slide. The lower curves are for progressively greater oscillation cycles. The behaviour is sim- ilar in all the curves and in all cases the range in dynamic fatigue life is very large. As a final example compare the life at 200% osc- illation cyclesfor minimum strains of zero and of 200%. At zero min- imum strain the life is about 20 kilocycles. At 200% minimum strain it is 15 megacycles or 750 times as great. Needless to say, differences of this magnitude mean that designs for the use of rubber should be giv- en very careful study to take advantage of the regions of maximum life. These effects are not confined to simple compression or extension for they are equally evident in studies of shear. If we have the simple shear mounting shown in slide 19 we can apply the constant oscillation cycles with different minimum strains as shown in A, B, and C. In dyn- amic fatigue C has 15 times the life of B. In the 2nd and 3rd columns, lateral extension and compression have been added. Again there is a large variation in life which can be understood when the forces are resolved to show the minimum strain during the oscillation cycle. | ||