Glass tensile measurements

Very recently, Williams, Billington Pearson (1992) have examined the effect of reinforcement by silver or silver-tin alloy on the mechanical properties of three glass-ionomer cements. Measurements of compressive, flexural, tensile (measured by the diametral compressive procedure) and shell strength are given in Table 5.17. These results show that the effect of reinforcement varies from cement to cement but, in general, increases it. 

A constant strain rate of 10% per minute was employed. Polymers were isolated by air drying or freeze drying and molded at 150°C into dumbbell speciments for the tensile measurements. Glass transition temperatures (Tg) were measured by differential scanning calorimetry. 

The cooling rate should preclude the formation of permanent stresses exceeding ]/20—l/40th of the glass tensile strength, i.e. approximately 1.5 —3,0 MPa which corresponds to an optical path difference of 50—lOO nm cm in birefringence measurements (see later). 

Lin and coworkers [41] also investigated the static tensile strength and fatigue behavior of long glass-reinforced semicrystalline polyannide and amorphous polycarbonate composites. The static tensile measurement at various tanperatures and tension-tension fatigue loading tests at various levels of stress amplitudes were studied. 

Mieck et al. [13], and Mukherjee et al. [14] on different types of flax and pineapple fibers. Their results show that tensile strength of flax fibers is remarkably more dependent on the length of the specimen than are usual glass fibers (Fig. 3). In contrast, tensile strength of pineapple fibers is less dependent on the length. The dispersion of the measured values is located mainly in the range of the standard deviation. 

Little information is available on other tests of strength. Isolated measurements give a flexural strength of 24-5 MPa (0ilo, 1988) and a tensile strength of 13-6 MPa (Kent Wilson, 1971). These values lie within the range of those recorded for glass polyalkenoate cement. Translucency is easily achieved as values for the inverse property of opacity show (Table 6.10). 

Tensile strength of gel-derived fibers have been measured. It has been found (8) that gel-derived fibers are a little weaker than silica glass fibers, especially when the diameter is large, as shown... 

In order to ascertain these properties in a reproducible manner, very specific test geometry must be used since it is necessary to know the stress distribution at predefined, induced cracks of known length. For example, the measured tensile or compressive strength of a series of glass bar... 

When determined relatively soon after placement, reported shear bond strengths of glass-ionomers are of the order of 3-7 MPa [213,220], However, because the material fails cohesively, this is actually a measure of the tensile strength. As the cement matures, the tensile strength increases, and failure rates in clinical service are very low [221],... 

In the same way that close-packed directions in a crystal have larger refractive indices, so too can the application of a tensile stress to an isotropic glass increase the index of refraction normal to the direction of the applied stress. Uniaxial compression has the reverse effect. The resulting variation in refractive index with direction is called birefringence, which can be used as a method of measuring stress. 

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