4th International Congress
of Croatian Society of Mechanics
September, 18-20, 2003
Bizovac, Croatia
EXPERIMENTAL AND NUMERICAL ANALYSIS
OF CRACK TIP POSITION EFFECT LOCATED IN HAZ
Nenad Gubeljak, Dražan Kozak, Franjo Matejiček, Maks Oblak
Keywords:
Fracture toughness testing, heat affected zone, finite element analysis
1. Introduction
The heat affected zone (HAZ) is in many cases considered to be the preferred location for the
initiation and propagation of weld joint cracks. In order to evaluate the fracture toughness and fracture behaviour, the stress-strain field at HAZ cracks must be well understood.
In the last decade the extensive fracture toughness testing of weld heat-affected zones in structural
steels was performed [1, 2]. Many investigation of fracture behaviour shows that unpredictable
fracture behaviour of specimen with crack tip located in HAZ can occur. The reason for such behaviour may lie in the HAZ microstructure heterogeneity and existing of local brittle zones (coarsegrained heat affected zone - CGHAZ). Very important role plays also the yield stress ratio between
base metal and weld metal (so-called mismatch factor M=Rp0.2,WM/Rp0.2BM). It is expected that
strength overmatch should ensure protection effect from the yielding in the region of weld metal,
what extent the load carrying capacity [3]. However, this effect may lead to brittle fracture in the
case when the crack path deviated to brittle microstructure of welded joint. The situation becomes
more complex in the case when the crack tip is located in the region between weld metal with higher yield strength and softer base metal. Here the crack tip may be shifted from the interface between weld metal and CGHAZ, between CGHAZ and soft (fine-grained heat affected zone)
FGHAZ and finally between FGHAZ and base metal. The spreading of stress and strain fields in
the vicinity of crack tip, which is located in the HAZ, may be analysed very effective by finite element calculation [4-6].
The aim of this paper is to determine the effect of different crack tip position in HAZ on crack driving force and consequently on fracture behaviour of strength mismatch fracture toughness specimen.
2. The fracture toughness specimens and testing
High strength low alloyed HSLA steel (with thickness of 40 mm), corresponding to the grade
HT80, was used as the base metal (BM) in a quenched and tempered condition (Q+T). Three different overmatched X-grooved multi-pass welded joints are studied. The first was pure homogeneous with 21% higher yield strength related to BM (Fig. 1a). The second and the third joint were
heterogeneous with two root passes welded by wire, which ensures 13% overmatch with the root
height of 9 mm (Fig. 1b) and 14 mm (Fig. 1c), respectively. The rest passes were filled also by
electrode with 21% overmatch [3]. Mechanical properties were determined according to DIN
50125, using standard tensile specimens with section diameter of 5 mm, made from the root and
top region of the weld metal along the welding direction. The tensile tests were performed at room
temperature. The average values of tensile properties are given in Tab. 1.
1
60
1 b)
60
1 c)
14
M=1,21
9
40
M=1,21
40
M=1,21
40
60
1 a)
M=1,13
4
M=1,13
4
4
Fig. 1 Homogeneous 1 a), inhomogeneous 1 b) and 1 c) strength overmatch X-welded
Table 1. Mechanical properties of the base metal and weld metal
Material
BM
WMfill
WMroot
Temp.
°C
20
20
20
E
GPa
201
205
221
RP0.2
MPa
711
861
807
Rm
MPa
838
951
905
o
MPa
679
833
780
n
0.091
0.074
0.075
At
%
19.6
11.7
15.3
vE+
J
54-40C
56-10C
61-10C
M
1.21
1.13
Reference stress o and strain hardening exponent n given in the Tab. 1 obtained by fitting of
true stress-strain plot. They are needed for the determination of Ramberg-Osgood relation between
true stress and true strain.
The single edge notched bend (SENB) specimens with a machined surface notch tip completely
in the HAZ were extracted from the welded plates (Fig. 2). The thickness of the BxB specimens
was 36 mm. These specimens were used for the estimation of the specified microstructures with regard to fracture behaviour.
Fracture toughness specimens (a0/W ≈ 0,3) were fatigue pre-cracked in accordance with BS 7448
5 from the surface to a distinct welded joint microstructure, as shown in Fig. 3. The aim of producing the root layer with different heights by heterogeneous welded joint is to provide that crack
tip location will be changed from high strength filler metal to the root metal with medium strength.
The single specimen method was used. The DC potential drop technique was applied for stable
crack growth monitoring. The CTOD values were directly measured with a 5 clip gauge, developed by GKSS 8. The measuring points for CTOD (5) and CMOD (crack mouth opening displacement) are marked on the specimen surface, Fig. 3.
Figure 4. shows the records of loading as plots of load (F) vs. CMOD. The load saturates at a different levels of CMOD values, depending on the crack depth (a/W). In all specimens, except those
made from BM, an unstable fracture appeared after some amount of stable crack growth, as shown
in the F - CMOD plots.
Figure 2. Location of fatigue pre-crack tip
2
Figure 3. SENB specimen with heterogeneous welded joint cracked in the HAZ
The initiation of cleavage fracture is probably caused by crack tip position in the coarse-grained
microstructure of HAZ. The mechanism of cleavage fracture is related to existence of local brittle
zones (LBZ). The inclination of linear part of the F - CMOD diagram depends on the a/W ratio and
it does not depend on the interaction of single materials in the joint. On the Fig. 4 it is obviously
very different behaviour of the specimens with the almost the same crack length, but with the crack
tip, which is moving through the HAZ width. This is the proof of great scattering of the results
when the fracture toughness specimen is cracked in the HAZ.
250
200
F, kN
150
100
Ax1-4
Ax1t-3
50
Ax1t-1
Ax1-2
0
0,0
0,5
1,0
1,5
2,0
2,5
3,0
CMOD, mm
Figure 4. Experimentally obtained curves F vs. CMOD for BxB specimens with approximately the same crack depth, but with the different position of the crack tip
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3. Numerical analysis
Finite element analyses were performed to calculate the magnitude of maximum principal
stress, what may serve as a global parameter by assessment of material resistance with respect to
brittle fracture [5]. Distribution of stresses and strains could not be considered as a fracture toughness parameter, but it may help by establishing a fact about the reasons and mechanisms of fracture. However, in the case where crack tip is positioned in the HAZ, the accuracy of FE results is
strongly influenced by defining of proper material yielding law and correct modeling of geometry
as well. Some of input data is difficult to determinate i.e. shape and width of the HAZ, true mechanical properties of all HAZ regions, effective crack length etc. HAZ in this paper was modeled
as bimetal region with total width of 1,5 mm. Near the base metal a softer part of HAZ was supposed, while harder material was assigned to the coarse-grained HAZ in the vicinity of the overmatched weld metal. Yield strength as well as ultimate strength needed for the HAZ yielding law is
determined from empirical relations based on the microhardness [9]:
(1)
R
 3,1  HV  (0,1)n  80
p0,2
0,1
Rm  3,5  HV0,1  (1  n)  (12,5 
n
)  92
1 n
(2)
where HV0,1 presents Vickers microhardness and n is material hardening coefficient.
It has been noticed that during fatigue of the specimens, a significant plastification appears in
the vicinity of the crack. Because of that additional yielding, it was necessary to correct initial
crack length. Here plastic zone was calculated by using of known expression from Schwalbe:
K2
aeff  a0  rp  a0 
(3)
2     y2
where aeff is effective crack length, a0 is initial crack length, rp is plastic zone radius, K is stress intensity factor by maximal fatigue load and σy is yielding strength of the HAZ material. Adding of
plastic zone to initial crack length in the FE model move the crack tip from the fusion line to
CGHAZ region.
Figure 5 depicts coarse automatically generated finite element mesh consisted from 413 8-node
isoparametric plane strain elements with 1278 nodes. The first row of singular elements around
crack tip has the size of 50 μm, what is nearly the grain size. The crack line is almost 6 mm displaced from the welded joint symmetry line in the case when the crack tip is at the interface between BM and FGHAZ.
Validity of the FE model has been proved comparing measured load line displacement LLD
on the SENB specimen with homogeneous welded joint with the same calculated by finite element
method (Fig. 6). It has to be mentioned that the best agreement shown the model, which allows the
movement of the both specimen supports in the horizontal direction, what is adequate to the rollers
sliding. The ordinate in aforementioned diagram presents the ratio between the applied force F and
yielding force Fy. The yielding force for the homogeneous SENB specimen is defined as [10]:
B W 2 
a
Fy  A   y 
1  
S  W
2
(4)
where a, B, W and S are determined by specimen geometry, y is usually equal to yielding stress
Rp0,2 and the parameter A can be approximately determined from:
A  1,455
for a / W  0,296
a

A  1,455 - 3,141   0,31  
W

4
(5)
2
for a / W  0,31
F
Detail A
WM
CGHAZ
36 mm
FGHAZ
aeff
BM
Detail B:
Detail A:
1 mm
Detail B
5 mm
Figure 5. Finite element mesh for the specimen with crack on the interface FGHAZ-BM
0,9
0,8
0,7
F/F y
0,6
0,5
0,4
Experiment
0,3
BM-FGHAZ (FEM)
0,2
FGHAZ-CGHAZ (FEM)
0,1
0,0
0,00
CGHAZ-WM (FEM)
0,10
0,20
0,30
0,40
0,50
0,60
0,70
LLD, mm
Figure 6. Variation of FE results due to different crack tip position for the fracture toughness
specimen with homogeneous weld (M=1,21)
5
Due to dissimilarity of the materials in the interaction, the different values for local CTOD (5)
displacement left and right for 2,5 mm from the crack tip are present. This asymmetry is obviously
from the Fig. 7, where the 5 value in weld metal is greater than the 5 value in base metal for the
crack tip located at the middle of the HAZ.
0,09
CTOD (5)
0,08
0,07
d5 exp
0,06
d5 BM (FEM)
0,05
d5 WM (FEM)
0,04
d5 total (FEM)
0,03
0,02
0,01
0
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
F /F y
Figure 7. Asymmetry of 5 values for the position of the crack tip in the middle of HAZ
The main aim of numerical analysis was to get an insight into stress and strain distribution in the
vicinity of moving HAZ crack tip. Both, the position of the maximal stress and spreading direction
of the plastic zones should help by assessing of the influence of crack tip position on the fracture
behaviour of the specimen.
Figure 8 shows principal stress x, equivalent stress eq (von Mises) and equivalent plastic strain eq
corresponding to different crack tip position at HAZ in homogeneous and inhomogeneous weld
metals. Analyses show that the highest stress are occurred in high strength material where yielding
process deviated from the crack tip to low strength material due by mis-match effect. Figure 8, also
shows that x and eq for homogenous specimens do not show significant difference regarding to
crack tip position. A contrary, one can observe that in homogenous weld joint crack tip position
play important role to elastic-plastic strain eq behaviour at the vicinity of crack tip. Therefore only
a small difference at the crack tip position has a strong effect in crack behaviour in elastic and plastic behaviour of specimen. This can explain experimentally obtained different behaviour of specimens cut out from same weld plate and with similar crack depth in Figure 8.
Inhomogeneous weld joint with same crack tip position but different size of overmatch metal
(M=1.13) do not show any significant different in stress-strain behaviour of weld metals at the
crack tip. Hereby, the slightly over match root passes have no effect on fracture of specimens.
4. Conclusion
In this paper the different fracture behavior of specimens cut out same welded with crack tip in different regions of heat-affected-zone (HAZ) were analyses. The different fracture behavior is explained by numerical analysis using finite elements modeling, Numerical analysis shows that
changing of crack tip position has a significant effect on fracture behavior in elastic and plastic
loading range. Significantly low effect on fracture behaviour has changing of amount of overmatching weld metal in root layer. Obtained results give principles for idealization of finite elements modeling of fracture behavior of welded joints.
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x stress
eq stress
base metal
pl, eq strain
M=1,21
hard HAZ
yielding zone of
base metal
x, MPa
crack tip
yielding zone
of weld metal
eq, MPa
pl, eq, -
eq, MPa
pl, eq, -
yielding zone
of soft HAZ
yielding zone
of hard HAZ
1,5 mm
M=1,21
CGHAZ-FGHAZ interface
Weld metal-CGHAZ
soft HAZ
x, MPa
FGHAZ-BM interface
M=1,21
eq, MPa
x, MPa
pl, eq, -
crack tip
h (root) = 9 mm
M=1,13
eq, MPa
hroot=9 mm
x, MPa
pl, eq, -
h (root) = 14 mm
eq, MPa
x, MPa
base metal
M.=1,21
h root=14 mm
M=1,13
Figure 8. Stress and strain fields in the vicinity of the moving crack tip
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pl, eq, -
5. References
[1]
Fairchild, D.P., "Fracture Toughness Testing of Weld Heat-Affected Zones in Structural Steel, Fatigue
and Fracture Testing of Weldments", ASTM STP 1058, H. I. McHenry and J. M. Potter, Eds., American Society for Testing and Materials, Philadelphia, 1990, 117-142.
[2] Fairchild, D.P., Theisen, J.D. and Royer, C.P., "Philosophy and Technique for Assessing HAZ Toughness of Structural Steels Prior to Steel Production", Paper OMAE-88-910, Seventh International Conference on Offshore Mechanics and Arctic Engineering, Houston, TX, February 1988
[3] Gubeljak, N.,"Fracture behaviour of specimens with surface notch tip in the heat affected zone (HAZ)
of strength mis-matched welded joints", International Journal of Fracture 100, 1999, pp 155-167
[4] Thaulow, C., Toyoda, M., "Strength mis-match effect on fracture behaviour of HAZ", IIW Doc. X – F
– 033 -96, Reinstorf – Lüneburg, 1997
[5] Thaulow, C., Ranestad, Ø., Hauge, M., Zhang, Z., Toyoda, M. and Minami, F., "FE calculations of
stress fields from cracks located at the fusion line of weldments", Engineering Fracture Mechanics, Vol.
57, No. 6, 1997, pp. 637-651
[6] Matejicek, F., Gubeljak, N., Kozak, D. and Koçak. M., "Stress-Strain State at the Vicinity of the Crack
Tip in Strength Mis-match Welded Joint", 13th European Conference on Fracture, San Sebastian, Proceedings on the CD-Rom, 1U.6, 2000
[7] BS 7448, "Fracture mechanics toughness test, Part 2. Method for determination of K IC, critical CTOD
and critical J-values of welds in metallic materials", TWI Abingdon Hall Cambridge, 1997
[8] GKSS, Displacement Gauge System for Applications in Fracture Mechanics, Patent Publication, Geesthacht, 1992
[9] Akselsen, O.M. and Rorvik, G., "Tensile properties of heat affected zone of medium strength low carbon C – Mn and 2.25Cr – 1Mo steels, Materials Science and Technology, Vol. 6, 1990, pp. 383-390
[10] Schwalbe, K.-H., "The Prediction of Failure Situations Using the CTOD Concept Based on the Engineering Treatment Model (ETM)", The Crack Tip Opening Displacement in Elastic-Plastic Fracture
Mechanics Workshop on CTOD Methodology, Geesthacht, April 23-35, 1985
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