Natural rubber test pieces

In free vibration methods, the rubber test piece, with or without an added mass, is allowed to oscillate at the natural frequency determined by the dimensions and viscoelastic properties of the rubber and by the total inertia. Due to damping in the rubber, the amplitude of oscillations will decay with time and, from the rate of decay and the frequency of oscillation, the dynamic properties of the test piece can be deduced. 

Any rubber test piece with or without added mass has a natural or resonant frequency of vibration determined by the dimensions and viscoelastic properties of the rubber, the total inertia of the system, and the mode of deformation. If constant force amplitude cycles are applied to the rubber and the frequency varied, the resulting deformation cycles will have a maximum value when the applied frequency equals the resonant frequency of the test piece system. 

The rotary-platform, double-head or Taber abrader, unlike those mentioned above, was not developed by the rubber industry but was intended for very general use. It is of the form (d) in Figure 11.4 but uses a pair of abrasive wheels. Although the degree of slip cannot be varied, the Taber is in other ways a very versatile apparatus. It uses a simple flat disc as the test piece which could, if necessary, be fabricated from more than one piece. The force on the test piece and the nature of the abradant are very readily varied and tests can be carried out in the presence of liquid or powder lubricants. When using the usual type of abrasive wheel, a refacing procedure is carried out before each material tested. 

Vulcanized rubber was ozonized as follows. Natural rubber was compounded with the ingredients shown in Table III and cured at 141 °C. for 13 minutes. The vulcanizates were cut off from the sheets with JIS No. 1 dumbbell cutter to obtain the specimens for the ozone crack test. The test pieces were exposed to an oxygen atmosphere containing 0.01% ozone under an elongation of 50%, and a time, t0, required for the initial crack formation was measured. 

A long-term natural ageing programme was started in 1958 when 19 rubber compounds were exposed at 3 locations. The final sets of test pieces were withdrawn in 1998 giving a total of 40 years of natural ageing. The results of the physical tests carried out at intervals over the 40 years have been published [1]. 

In a so called heal buildup test the prime object is to induce a temperature rise in the lest piece, to measure its magnitude, and to study its effects. The heating is a result of the viscoelastic nature of polymers some of the work done in stressing the material is dissipated by viscous forces between molecules and converted to heat energy. Compression, shear, or some combination of them is normally used with relatively bulky test pieces. The tests are almost exclusively applied to rubbers and were generally intended to be relevant to tires. The geometries used are inevitably arbitrary and involve superimposing dynamic strain or stress cycles onto a prestress or strain. 

Lubrication of the test pieces is now standard practice in order to eliminate one obvious source of variation. The more uniform flattening of the test piece also eases measurement of thickness after release from compression. However, there remain specifications in which set is determined in the absence of lubricants. It has also become common practice with general-purpose rubbers to measure compression. set after just one day at 70°C, which for sulfur-vulcanized elastomers can be a. sensitive measure of the state of cure. Higher test temperatures are specified for special-purpose and speciality synthetic rubbers, but the one-day test has remained popular, not least as a cla.ssification criterion and grade requirement in such specifications as ASTM D2000 and the British Standard series of material specifications for individual rubber types. Tests seldom last more than seven days, and recovery is usually confined to the standard. 30 minutes after release, during which time the test piece cools to standard laboratory temperature if taken from an oven. The short-term nature of the test and the absence of isothermal conditions during recovery has been questioned by Birley and other workers [43]. 

FIGURE 10.30 Growth of an edge crack in a test piece of a natural rubber vulcanizate stretched repeatedly to 46% extension. (From Greensmith et al. (1963).)... 

FIGURE 10.31 Fatigue life versus depth of initial cut for test pieces of natural rubber and SBR stretched repeatedly to 50% extension. (From Lake and Lindley (1964).)... 

FIGURE 10.33 Fatigue life for test pieces of natural rubber versus minimum extension, min-Ae, denotes the additional strain imposed repeatedly. (From Cadwell et al. (1940).)... 

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