Numero 1 2007 - IIS

E. Seib e M. Koçak - Fracture analysis of strength undermatched welds of thin-walled aluminium structures using FITNET procedure
The FITNET FFS approach uses the
methodology formerly known as the
SINTAP procedure [8] and extends it
with fully validated strength undermatched
welded thin-walled structures.
If the yield strength difference between
the base and weld materials is more than
10%, the FITNETT FFS Mismatch
Option provides an assessment route
accounting for the special features of
welds, as it was established within the
SINTAP procedure.
In the following, only the set of equations
for the Mismatch Option of the
Fracture Module will be given. For the
complete information on the different
analysis options within the FITNET FFS
Procedure, the reader is referred to [3].
The required input information, as
schematically illustrated in Figure 7, for
the application of the Fracture Module
to cracked welded structures will be
given subsequently, including the determination
of the weld metal tensile and
fracture properties.
4.1 FITNET FFS Procedure - Fracture
Module: Option 2: Weld strength
mismatch
The Fracture Module provides two
Material related input:
- tensile properties
of base and weld materials
- fracture resistance
complementary analysis routes: Failure
Assessment Diagram (FAD) and Crack
Driving Force (CDF). Since both routes
are based on the same set of equations,
their predictions are also the same.
Therefore, only the CDF route will be
presented in this paper. The CDF
expression in terms of the crack tip
opening displacement (CTOD), δ, is
given as:
with the elastic part of CTOD, δ e:
K denotes the elastic stress intensity
factor,
the parameter m (m = 1 for plane stress
and m = 2 for plane strain) is considered
a constraint parameter,
E’ = E for place stress and
E’ = E/(1-v 2 ) for plane strain (E =
Young’s modulus, v = Poisson’s ration),
and
FITNET Procedure
Fracture Module
Prediction of critical conditions:
- critical crack size
- maximum load level
Figure 7 - Required input information for the application of the FITNET FFS Procedure - Fracture Module.
(5)
(6)
is the ratio of externally applied load, F,
and the yield load, F Y, of the cracked
component which is a function of the
material’s yield strength, σ Y, of the
crack location and component/weld
geometry. Regarding the selection of E’,
the plane stress condition has been
chosen due to the fact of the thin sheet
material. It should be pointed out that
for v = 0.3, E’ for the plane strain case
differs only by a factor of 1.1
from the plane stress case, whereas the
variation of m between 1 and 2 is much
more pronounced. The plasticity correction
function, ƒ/(L r) is subdivided into
different options within the FITNET
FFS Procedure and is dependent on the
extent of the material data input and on
the case analyzed (homogeneous or
heterogeneous with strength mismatch).
For a strength mismatched configuration
(FITNET FFS Fracture Module Option
2), the plasticity correction function is
defined as:
Component or structure
related input:
- K-factor solution
- Yield load solution
(7)
Riv. Ital. Saldatura - n. 1 - Gennaio / Febbraio 2007
95