Environmental Engineering Reference
In-Depth Information
the structure. The toughness of structures, in presence of inherent defects, is
evaluated through a fracture mechanics approach. While most of the codes
use a linear elastic fracture mechanics (LEFM) approach, small structures
and ductile materials require elastic-plastic fracture mechanics (EPFM)
formulations. The validity of LEFM compared to EPFM depends on the
plastic zone size as shown in Fig. 1.4 and, in general, LEFM is not applica-
ble when the plastic zone size is too large compared to either the crack size,
the uncracked ligament or the member height. 9 In very large structures and
relatively brittle materials where LEFM is valid, the stress fi elds are charac-
terized by stress intensity factor, K I , given by
KY a
I
a ,
[1.9 ]
σπ
σ
where a is half-crack length,
is applied nominal stress and Y is a geometry
factor which is a function of the ratio of crack length to its width ( a / w ). As long
as K I is lower than the plane strain critical fracture toughness K IC , the structure
with the crack can withstand the applied loads. In cases where LEFM is not
valid (Fig. 1.4) either crack tip opening displacement (CTOD) or elastic-plastic
fracture toughness (J-integral) can be conveniently adopted.
Although fracture toughness is a fundamental parameter characterizing
the fracture behaviour of cracked bodies, it is often more convenient to
use the ductile to brittle transition temperature (DBTT) measured using
the relatively simple Charpy impact tests, to study the effect of neutron
σ
K-field
￿ ￿ ￿ ￿ ￿ ￿
2 r 0
h
Far-field
Plastic
zone
a
( b - a )
b
1. 4 Stresses around a cracked body ( a
=
half-crack length, r 0
=
plastic
zone size and b
a
2r 0
=
remaining ligament).
 
Search WWH ::




Custom Search