Page 311 - Handbook of Materials Failure Analysis

References 307

specimens and components if the mesh size is coupled to the microstructure and if the
above restrictions do not arise in the problem.
The nonlocal model, however, is much more powerful and versatile. The corre-
sponding restrictions with the use of the local formulation are overcome when the non-
local formulation is used. The results of the nonlocal model are also independent of
mesh size. In addition, the nonlocal models are able to accurately predict the scatter
in fracture toughness and its variation in the DBTT regime for specimens with various
crack-depth, thickness values and sizes. This is due to the ability of the nonlocal models
to predict extremely small extents of stable crack growth before the specimen fails by
unstable mode of cleavage fracture. In case of the specimen with shallow crack, the
fracturetoughnesstransitiontemperature T 0 aspredicted bythe nonlocalmodelislower
compared to thatofthe deeplycracked specimenand hence,the datafromthese typesof
specimens may be more appropriate in structural integrity analysis of components with
shallow surface cracks compared to those obtained from standard deeply cracked spec-
imens. Moreover, the nonlocal FE damage model can be reliably used to simulate the
fracture behavior of actual components with postulated cracks rather than resorting to
carrying out experiments (which may not be feasible in many situations) or using
standard deeply cracked specimen data with large values of crack-tip constraints
(which may not be representative of the actual crack-tip constraint in the component).
Design and safety analysis of dissimilar weld joints are an important issue for
ascertaining structural integrity of these joints which are unavoidable at several loca-
tions of thermal and nuclear power plants. In order to optimize the dimensions and
properties, one must have a tool to predict the performance of these joints during
actual and accidental loading conditions. To prove the safety of these joints, initial
cracks of different sizes are usually postulated at different locations, especially the
interfaces. Fracture mechanics method cannot be directly used because of difficulties
in defining the crack-tip loading parameters. The use of a porous plasticity yield
function for initiation and propagation of cracks in ductile materials have several
advantages, for example, the material stiffness degrades with progress of damage
and simulates a crack (material with negligible stress carrying capability) at the
material point with maximum plastic deformation and stress triaxiality. A FE formu-
lation of the Rousselier’s damage model has been used in this work to predict the
crack path and fracture resistance behavior of a SEB specimen made from the dis-
similar weld joint with an initial crack at the ferrite-buttering layer interface. The
micro-mechanical parameters have been determined for the individual material
and it was demonstrated that they can be safely used to predict the crack growth
in the specimen with dissimilar metal welded joint.

REFERENCES
[1] Needleman A, Tvergaard V. An analysis of ductile rupture in notched bars. J Mech Phys
Solids 1984;32:461–90.
[2] Kussmaul K, Eisele U, Seidenfuss M. On the applicability of local approach models for
the determination of the failure behavior of steels with different toughness. In: Mehta HS,