5 TENSILE STRESS STRAIN

After indentation hardness, the most common type of stress strain measurement is that made in tension. The ability of rubber to stretch to several times its original length is one of its chief characteristics but it is worth noting that at least as many rubber products are used in compression or shear as are used in tension. Besides being of relevance for products strained in tension, tensile stress/strain properties have been taken since the beginning of the industry as a general guide to the quality of a rubber; being sensitive to filler or plasticiser content as well as to mixing and curing efficiency.

Standard methods for determining tensile properties of rubbers have evolved gradually and are now in a well-defined state. Essentially, dumb-bell shaped, or less often ring, test pieces are strained at a constant rate of traverse and force and corresponding extension recorded. The force readings are expressed as stresses by reference to the original cross-sectional area of the test piece.

A typical tensile stress/strain curve for rubber is shown in Figure 8.8 (this also illustrates the differential strain between the inside and outside of a ring test piece which is discussed later.) It can be seen that there is no linear elastic portion as is usual with, for example, metals, and rubber technologists do not normally measure a modulus as such but quote the stress at various percentage elongations, commonly at 100%, 200% etc. If a figure for Young's modulus was required, this could...