The elastic properties of vulcanized rubber present a difficult problem to generalize, mainly because of the large number of compounds in common use. There are good reasons for many kinds and grades, and a characteristic common to all is large elastic strain. Even in the best grades of rubber these strains are not perfectly elastic, but this is also true of almost all other materials, including spring steels. An elastic strain theory will be derived for a specific group of rubber compounds, namely, spring stocks. These are dense, strong rubbers, highly elastic within the working stress range, and capable of supporting loads under continuous vibration and shocks. To be of real value, the strain law should closely predict strain for the range of working stress, or more generally for the elastic range, and should indicate the trend at very high stresses where inelasticity becomes more prominent. Figure 1 shows a cylindrical element of rubber having an area A0 and length h when unstressed. If a uniform stress is imposed perpendicular to the end area, the length changes to H and the area to A. These symbols apply whether the stress is tension or compression. The usual definition of strain, i.e., change in length due to stress divided by the unstressed length, is comprehensive and descriptive for the large strains in rubber.

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