Page 375 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

1656_C008.fm Page 355 Monday, May 23, 2005 5:59 PM

Fracture Testing of Nonmetals 355

(a)

(b)

FIGURE 8.1 Crack growth at a constant CTOD in a linear viscoelastic material: (a) crack-tip position at time
t o , and (b) crack-tip position at time t o + t ρ .

˙ a
where is the crack velocity. For many polymers, the time dependence of the relaxation modulus
can be represented by a simple power law:

Et() = E t − n (8.6)
1
where E and n are material constants that depend on temperature. If crazing is assumed to occur
1
at a critical strain that is time independent, the crazing stress is given by
σ cr Et() ε = cr (8.7)

Substituting Equation (8.5) to Equation (8.7) into Equation (8.4) leads to

t
K 2 Ic c cr 1 2 − 2 n
E = δε
 ρ  − n2
= δε E 2 c (8.8)
ccr 1  a ˙ 