Pieces tear test

It is important to recognize that the fracture energy, Gc, is not a constant value for a particular material it depends strongly on the temperature and rate of tear that is, the rate at which material is deformed to rupture at the tear tip, as discussed in Section 10.5.2. Nevertheless, several critical values may be distinguished. The smallest possible value is, of course, twice the surface free energy, about 50 mJ/m for hydrocarbon liquids and polymers (Tarkow, 1958). 

Values of this order of magnitude are indeed observed for fracture induced by ozone, when the function of the applied forces is merely to separate molecules already broken by chemical reaction, as discussed in Section 10.8. 

Another critical value is that necessary to break all the molecules crossing a plane, in the absence of any other energy-absorbing processes. This minimum energy requirement for mechanical rupture is found to be about 50 J/m it is treated in the following section. Finally, there are the considerably larger values found in normal fracture experiments, ranging from 100 to 100,000 J/m. These are described in Section 10.5. 

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Figure 8-19. Forms of tear test piece. Solid line, original shape broken line, shape after...

Tear test pieces are normally stamped from sheet with an appropriate die (see Chapter 4) and then a nick cut if required, although with the Delft test piece both operations are usually done simultaneously. 

Figure 8-21. Cross section of die-cut tear test pieces, (a) Nicked crescent test piece (b) Delft test piece, w = effective width of untom portions.

Tear test pieces are gripped and stretched to break in a machine of the same type as used for tensile tests and, hence, the comments made in Section... 

There are three main configurations for the tear test. In the first two a tensile force is applied to the sample. In one configuration, the force is vertical to the plane of the sample, and in the second it is at right angles. In the third style, an element of shear is also introduced. Figure 9.3 shows the basic configuration of the tear test pieces. 

In nicked tear test pieces, the result is strongly affected by the size and quality of the nick. When any comparisons are made, it should be remembered that some old published data for crescent tear strength will be for a previous standard nick size of 0.5mm. and in most instances this will be reflected in a higher test value. 

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. 

Figure 5.2 DUferent types of tear test-pieee geometry (a) trouser, (b) pure shear, (c) angle and (d) split tear test-pieces.

For the split tear test-piece, the tearing energy is given by ... 

Figure 5.9 Photos showing split tear test-piece to simulate crack propagation in the direction of molecular orientation in knotty tearing.

Crack growth measurements using the split tear test-piece provide further experimental evidence that tearing energy is a measure of true crack growth... 

Comparison between split-tear and other test-piece, dc/diV vs tearing energy, T. The strain imposed on the spht-tear test-pieces ranged from 25% to 60%. 

The tearing energy of a split tear test-piece is computed using Equation (5.7), discussed in Section 5.2. 

Figure 5.28 shows the crack growth characteristics of unfilled NR obtained from split tear test-pieces and compared with the results published by Lake et The results are presented in the form of crack growth per cycle... 

Split-tear test pieces were prepared by die stamping on a thin flat sheet of vulcanized rubber of uniform thickness. The test piece was clamped by a grip, and a dead load apphed to the other end of the test piece via a fiictionless pulley. A dead load, which... 

Azemi, S. and Bijarimi, M. P. (2004). Cyclic crack growth measurement using split-tear test piece. J. Rubb. Res. 

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