Page 381 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C008.fm Page 361 Monday, May 23, 2005 5:59 PM

Fracture Testing of Nonmetals 361

(a)

(b)

FIGURE 8.4 Razor notching of polymer specimens: (a) razor-notched guillotine, and (b) razor sawing.
Photographs provided by M. Cayard.

In order to minimize material damage and residual stresses that result from razor notching,
Cayard [4] recommends a three-step procedure: (1) fabrication of a conventional machined notch,
(2) extension of the notch with a narrow slitting saw, and (3) final sharpening with a razor blade
by either of the techniques described above. Cayard found that such an approach produced very
sharp cracks with minimal residual stresses. The notch-tip radius is typically much smaller than
the radius of the razor blade, apparently because a small pop-in propagates ahead of the razor notch.
While the relative softness of plastics aids the precracking process, it can cause problems during
testing. The crack-opening force that a clip gage applies to a specimen (Figure 7.8) is negligible