ASSESSMENT OF CRACK LIKE DEFECT IN THE DISSIMILAR
WELDED JOINT BY ANALYTICAL AND FINITE ELEMENT METHODS
Sz. Szávai, R. Beleznai, T. Köves
Bay Zoltán Foundation, Miskolc, Hungary
Comparative analysis of different analytical procedures and numerical
modeling for crack like defects in dissimilar metal weld of pipes and 3D parts is
presented in this paper. On the basis of the numerical calculation the applicability
of the ASME BPVC XI A and H appendix and the FITNET procedure have been
checked for given crack parameters and geometry and the reason of the deviations
have been investigated.
Since the fracture mechanical analysis is evident in case of nuclear pressure
vessels during in-service inspection, reliable and verified methods are required
for analyzing nuclear pressure vessel and its welds. For such cases when
minimum defect size - to be detected - is required for NDT evaluation the
solutions of different codes and procedures are widely used in spite of the fact
that those fracture mechanical solutions have been developed for simple
geometries like pipes or shell like components. However there are several
dissimilar metal welds at critical points which have no validated solution
moreover the critical points usually have complex 3D geometry and loading.
Due to the needs for verified methods for DMW comparative analysis have
been carried out involving different analytical procedures (ASME BPVC XI
H4221 and A3300, FITNET procedure) and numerical FE modeling for crack
like defects in dissimilar metal weld of pipes and 3D parts. One of the main
objectives of the project was to assess the applicability of the analytical K I
solutions of the ASME BPVC for
 complex geometry of VVER’s DMW
 mismatch materials like DMW
 mechanical and transient thermal loads
 Loading
o tension (100 MPa)
o bending (100 MPa)
 Dimensions
o Diameter of the DMW for
both case: 548 mm
o R/t: 0,136
 Crack size
o a/w: 0,25; 0,5; 0,75
o : 11,25°; 22,5°; 45°
Fig.1. DMW in VVER
ASME H appendix contains solution for pipes under axial tension and global
bending load. It has a wild crack geometry applicability range up to
circumferential crack but through wall stress distribution cannot be taken into
consideration. ASME A appendix can be applied for shell like components with
wildly variable crack geometry under tension, bending and through wall stress
distribution, however thick, highly curved geometries such as thick wall pipes are
out of the validity range. FITNET procedure has solution for thick wall pipes and
plates with semielliptical and circumferential or extended crack under tension,
bending and through wall stress distribution, but the applicability range are
smaller than the ASME has. Numerical calculations were used as a validation of
the analytical equations. KI and JI values have been calculated for real 3D
geometries with DMW under mechanical and transient thermal loading by FEM.
The models have been solved in MSC.MARC 2005r2, applying 3D-s 20 nodes
hexagonal elements.
Bending
Bending
80
80
70
70
60
60
50
ASME XI H app.
30
ASME XI A app.
20
FEM
40
KI
KI
50
40
ASME XI H app.
30
ASME XI A app.
20
FEM
Fitnet R6 for pipe
10
Fitnet R6 for plate
0
Fitnet R6 for pipe
10
Fitnet R6 for plate
0
0
20

40
60
0
40
20
60
a
Fig.2. KI calculation results for pipe with crack like defect under bending load
Based on the results it can be conclude that the ASME appendix H shows
good correlation with the FEM calculations for shallow cracks, but is becoming
conservative for deep cracks. ASME appendix A has good correlation with the
FEM results for shallow crack. The longer the crack is the more conservative
values can be obtained due to the linear parameter approximation. FITNET
procedure gives the closest results to FEM, but the values are smaller than the
numerically calculated ones - not conservative!
In case of complex load and geometry conditions, the stress distribution on
the wall is significantly different from the tension and bending of a straight pipe
so the ASME BPVC XI H appendix cannot be applied for the analyzed
geometries. The ASME BPVC XI A appendix gives conservative approximation,
however it cannot be recommended for more extended cracks. Thermal loads can
be handled as a through wall bending, so ASME BPVC XI A and FITNET
procedure appendix can be applied. Even if the FITNET procedure seems to be
applicable further numerical verification is needed with other FEM software.