Doubletorsion testing

A specific problem with the single edge notched beam test is the process of prenotching the specimen before testing. In addition the procedure has been criticized by Evans (1974) as not being entirely satisfactory for porous specimens. For such specimens Evans (1974) considered the double-torsion method first derived by Outwater and Jerry (1966) and developed by Kies and Clark (1969) to be more appropriate as it eliminates the need to measure crack length.

The specimen is a rectangular plate (Fig. 16) with a narrow groove extending its full length supported on four hemispheres. The load is applied by two hemispheres attached to the upper platten. Controlled precracks are introduced in the specimen by preloading until a 'pop in' is observed (a pop in is a momentary decrease in load and is an indication of crack growth). It should be noted that the groove is necessary to help guide the crack and ensure it remains confined within the groove itself. The critical stress intensity factor is then calculated from the load F required to cause catastrophic crack propagation leading to failure by the expression:

Whshm

where u is the Poisson's ratio and /, h, h„, W, and Wn are the dimensions of the specimen given in Fig. 16.

The double-torsion method has been used only for microcrystalline cellulose (Avicel PH101) and Sorbitol 'instant' by Mashadi and Newton (1988). As with the single edge notched beam specimens, measurements varied with specimen porosity and using linear extrapolation Mashadi and Newton (1988) calculated values of K,Co of 1.81 and 0.69 MPam1/2 for the two materials, respectively. The higher results obtained for these two materials compared with the data obtained for single edge notched beams (1.21

Fig. 16 Geometry for the double torsion method for measuring critical stress intensity factor. F, applied load; h, plate thickness; W, plate width; hn and Wn, distances as shown.

Fig. 16 Geometry for the double torsion method for measuring critical stress intensity factor. F, applied load; h, plate thickness; W, plate width; hn and Wn, distances as shown.

and 0.47 MPam1/2 respectively) have been explained in terms of the specific geometries and stress uniformity of the two techniques. However, it is known that values of kic determined from double-torsion techniques are generally greater than those from notched beam specimens (Evans, 1974).

A specific practical problem of the double-torsion method for pharmaceutical materials is the preparation of the specimen and the very large compaction pressures needed to produce specimens of low enough porosity. For microcrystalline cellulose, Mashadi and Newton (1988) were only able to produce specimens of greater than 25% porosity.

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