Subject rubber specimens

Ozone chamber testing of materials deformed by alternating load has - for various reasons - never become established in practice to the same degree as testing under static mechanical conditions. It is simpler, more focused, and easier to subject rubber products to practical testing by mounting them in a vehicle or in an appropriate mechanical apparatus than to subject rubber specimens to the dynamic ozone chamber test in the testing chamber [232]. 

Equation (3.32) is general and is not restricted to any particular state of stress. Let us derive a stress-strain equation for a rubber specimen subjected to a constant uniaxial stress. The deformation along the stress is denoted X. It may also be assumed that the transverse deformations are equal Aj = A3. The assumption that the volume remains constant during deformation can be formulated as follows ... 

For the case of a rubber specimen subjected to a constant uniaxial stress, the following true stress (cr)-strain (A) expression can be derived ... 

When the butyl rubber was compounded with up to 30 percent of polyisobutylene, which, lacking the unsaturated isoprene units, did not enter into the cross-linking reaction, the tensile strengths were, of course, considerably reduced. They were found nevertheless to be accurately represented by the same equation, (53), provided merely that Sa is taken as the fraction of the composite specimen consisting of network chains subject to orientation. Thus, in this case... 

BS 4735. Assessment of Horizontal Burning Characteristics of Small Specimens of Cellular Plastics and Cellular Rubber Material When Subjected to a Small Flame, BSI, London. 

Fracture initiated in the tensile tested ABS samples, as noted also by Truss and Chadwick from either surface flaws or from internal flaws. Figure 33a shows an SEM picture of the tensile fracture surface of a sample broken at a comparatively high deformation rate of 12.7 cm/min. The fracture surface is unlike that of SAN (Fig. 27 a) or that of rubber modified polystyrene (Fig. 3 a). Fracture, for this specimen, has developed from both a surface source and from an internal source and fine radial flow lines emanate from both sources. The slow growth region adjacent to the source tends to develop a conical shape as has been noted This is probably a result of localized shear formation. In ABS specimens subject to creep deformation at low values of stress, the creep strain is found to be due almost entirely to shear but, at higher stresses, shear is accompanied by crazing Crazes can also be induced... 

The principle of these testers is that the specimen can be subjected to controlled stresses in two orthogonal directions (biaxial testers) or three orthogonal directions (triaxial testers). In the case of the triaxial testers, two of the orthogonal stresses are usually equal, normally generated by liquid pressure in a pressure chamber. The specimen is placed in a cylindrical rubber membrane and enclosed by rigid end cups. The specimen is consolidated isotropically, i.e. by the same pressure in all three directions which leads to volumetric strain but little or no shear strain. This is followed by anisotropic stress conditions, whereby a greater axial stress is imparted on the specimen by mechanical force through the end cups. In the evaluation of results it is assumed that the principal stresses act on horizontal and vertical planes, and Mohr circles can be easily drawn for the failure conditions. 

BS 4755. Laboratory method of test for assessment of the horizontal burning characteristics of specimens no larger than 150 mm x 50 mm x 15 mm (nominal) of cellular plastics and cellular rubber materials when subjected to a small flame. 

Thermomechanical Analysis (TMA) can be defined as the measurement of a specimen s dimensions (length or volume) as a function of temperature whilst it is subjected to a constant mechanical stress. In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored. Many materials will deform under the applied stress at a particular temperature which is often connected with the material melting or undergoing a glass-rubber transition. Alternatively, the specimen may possess residual stresses which have... 

Annulus Test An ozone resistance test for rubbers that involves a flat-ring specimen mounted as a band over a rack, stretched 0 to 100%, and subjected to ozone attack in the test chamber. The specimen is evaluated by comparing to a calibrated template to determine the minimum elongation at which cracking occurred. 

FIGURE 14.5 FT-Raman spectra of the surface layer ofAnnco iron specimen subjected to friction against various polymer materials studied. (A) polysulfide rubber, (B) SBR vulcanizate (C) ebonite/100 N (D) polysulphone. 

Following the manner of presentation in the previous sections, the subject of viscoelasticity will be explained by reference to experiments familiar to the practitioner of rubber technology. This is a rather unorthodox approach and different from the usual one, which begins with an introduction of the theory. The experiment is tensile stress-strain measurement. In the rubber industry tensile measurements are routinely performed with crosslinked specimens. Here, we are concerned with gum-rubber behaviour. Therefore, we must perform the measurements with uncrosslinked specimens. First, compression-moulded specimens must be prepared they require special attention, which will be described next. 

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