Page 372 - Mechanical Behavior of Materials

Section 8.6 Fracture Toughness Values and Trends 373

Figure 8.29 Compact specimens of two different sizes, b = 5.1 and 61 cm. (Photo courtesy of
E. T. Wessel, Haines City, FL; used with permission of Westinghouse Electric Corp.)
In addition to bend specimens, various other specimen geometries are used, such as the compact
specimen geometry of Fig. 8.16. For these, the thickness is usually t = 0.5b. Figures 8.29 and 8.30
are photographs of untested and tested compact specimens.
Fracture toughness testing of metals based on LEFM principles is governed by several ASTM
standards, notably Standard Nos. E399 and E1820. Similar tests are also done for other types of
material, as in Standard No. D5045 on plastics (polymers) and No. C1421 on ceramics. A situation
addressed in these standards is that K Q decreases with increasing specimen thickness t, as illustrated
by test data in Fig. 8.31. This occurs because the behavior is affected by the plastic zone at the crack
tip in a manner that depends on thickness. Once the thickness obeys the following relationship
involving the yield strength, no further decrease is expected:
2
K Q
t ≥ 2.5 (8.33)
σ o