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34
Figure 15. CTOD Design curve
Paris and Hutchinson have developed a detailed instability analysis that takes account
of structural compliance. Their approach has been incorporated into the EPRI elastic-
plastic fracture handbook. Dimensionless tearing modules are defined for the calculation
slopes of the crack driving force and material resistance, in the form:
2
R
JR
o
dJ E T
da
σ
=
(26)
where
J
R
is the
J
value according to the resistance curve. Value of applied
T
depends on
partial derivative for
J
at the place of nominal displacement:
2
j
T
o
E J
T
a
σ
Δ
∂⎛ ⎞
= ⎜ ⎟ ∂⎝ ⎠
(27)
The conditions for stable and unstable crack extension are as follows:
T
J
< T
JR
(stable)
T
J
≥
T
JR
(unstable)
Figure 16. Evaluation of growth instability
Figure 17: Comparison of tearing instability for
load or displacement control
Experimental
results
a+da
a+da
F
Δ
dΔ
Δ