Page 327 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C007.fm Page 307 Monday, May 23, 2005 5:54 PM

Fracture Toughness Testing of Metals 307

(a)

(b)

FIGURE 7.11 Simultaneous measurement of crack-mouth-opening displacement (CMOD) and load-line
displacement on an SE(B) specimen. The CMOD is inferred from a clip gage attached to knife edges, while
the load-line displacement can be determined from a comparison bar arrangement; the bar and outer coil of
the LVDT remain fixed, while the inner rod moves with the specimen.

7.1.5 SIDE GROOVING

In certain cases, grooves are machined into the sides of a fracture toughness specimens [7], as
Figure 7.12 illustrates. The primary purpose of side grooving is to maintain a straight crack front
during an R-curve test. A specimen without side grooves is subject to crack tunneling and shear lip
formation (Figure 5.15) because the material near the outer surfaces is in a state of low-stress triaxiality.
Side grooves remove the low triaxiality zone and, if done properly, lead to relatively straight crack fronts.