Fracture tear test piece

Equation (10.5) is more generally applicable than Eq. (10.6) because it is not restricted to linearly elastic materials. It constitutes a criterion for tensile rupture of a highly elastic material having a cut in one edge of length, /, in terms are of the fracture energy, Gc- Two important examples of test pieces of this type are (1) the ASTM tear test piece for vulcanized rubber (ASTM D624-54) and (2) a typical tensile test piece that has accidental small nicks caused, for example, by imperfections in the surface of the mold or die used to prepare it. 

Eisele et al24 describe the so called tear analyzer using a strip test piece with a cut in one edge cycled in tension, which can be considered the classic geometry for obtaining fracture mechanics data on rubbers. This sophisticated instrument introduces nothing new in concept but has a temperature controlled chamber and can operate at different frequencies, pre-strains and strain amplitudes, with automatic compensation for set. 

We must recognize, however, that a tear that begins to propagate from an initial cut or flaw will soon develop a characteristic tip radius r of its own, independent of the sharpness of the initiating stress raiser (Thomas, 1958). It is therefore more appropriate to treat the product r as a characteristic fracture property of the material. Indeed, Irwin (1948, 1957) proposed that fracture occurs for different shapes of test piece and under varied loading conditions at a characteristic value of a stress intensity factor, Kc, defined as... 

Rupture of a tensile test piece may be regarded as catastrophic tearing at the tip of a chance flaw. The success of the WLF reduction principle for fracture energy, G, in tearing thus implies that it will also hold for tensile rupture properties. Indeed, a/, and may be calculated from the appropriate value of G at each rate and temperature, using relations analogous to Eqs. (10.6) and (10.7). The rate of extension at the crack tip will, however, be much greater than the rate of extension of the whole test piece, and this discrepancy in rates must be taken into account (Bueche and Halpin, 1964). 

Because the tear force in this case is a direct measure of the fracture energy Gc and is independent of the elastic properties of the material and of the length of the tear, this test piece is particularly suitable for studying the effects of composition and test conditions on Gc [17-22]. 

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