Page 395 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C008.fm Page 375 Monday, May 23, 2005 5:59 PM

Fracture Testing of Nonmetals 375

(a)

(b)

FIGURE 8.17 Loading fixtures for DCB specimens: (a) end blocks and (b) piano hinges.

Figure 8.17 illustrates two common fixtures that facilitate loading the DCB specimen. The
blocks or hinges are normally adhesively bonded to the specimen. These fixtures must allow the
free rotation of the specimen ends with a minimum of stiffening.
The DCB specimen can be tested in Mode I, Mode II, or mixed-mode conditions, as Figure 8.18
illustrates. Recall from Chapter 2 that the energy release rate of this specimen configuration can
be inferred from the beam theory.
For pure Mode I loading (Figure 8.18(a)), elastic beam theory leads to the following expression
for energy release rate (see Example 2.2):
Pa
22
G = BEI (8.36)
I
I
where

2 Pa 3
EI = I (8.37)
3∆ I

The corresponding relationship for Mode II (Figure 8.18(b)) is given by

22
3 Pa
G = II (8.38)
II
4 BEI
assuming linear beam theory. Mixed-loading conditions can be achieved by unequal tensile loading
of the upper and lower portions of the specimens, as Figure 8.18(c) illustrates. The applied loads