Fatigue Strength of Fillet Welded Joint subjected to Plate ...
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Steel Structures 8 (2008) 163-169 www.ijoss.org
Fatigue Strength of Fillet Welded Joint subjected to Plate Bending
Biehn Baik*, Kentaro Yamada1, and Toshiyuki Ishikawa1
Department of Civil Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan1Department of Environmental Engineering and Achitecture, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Abstract
Fatigue tests have been carried out on three types of non load-carrying fillet welded joint subjected plate bending, such assingle-side fillet welded joint, T-shaped fillet welded joint and cruciform fillet welded joint. Fatigue failure of each welded jointhas been demonstrated. The test results show that fatigue crack forms flat semi-ellipse during crack propagation and propagatesto about 80% of plate thickness before failure. The fatigue strength and life recorded under bending test have been examinedand compared with the previous results obtained by tension test. The fatigue strength of the fillet welded joint under bendingis higher than that of the welded joint under tension.
Keywords: Fatigue test, Plate bending, Fillet weld, Crack propagation
1. Introduction
Fatigue cracks developed at weld joints subjected to
bending load have received much attention. For example,
an orthotropic steel deck plate has been suffering from
fatigue cracks at the welded joints, caused by the
repeatedly flexural deformation of deck plate due to the
increase of heavy vehicle loading. The fatigue failure due
to bending stress may affect unfavorably the fatigue
performance of welded structures. Therefore, this study
has been motivated by the concern about the fatigue
behavior of welded joints subjected to plate bending.
In this study, fatigue tests are carried out on three types
of non load-carrying fillet welded joint subjected to plate
bending, such as single-side fillet welded joint, T-shaped
fillet welded joint and cruciform fillet welded joint
subjected to plate bending. Fatigue failure of each type
welded joint is examined and fatigue crack propagation
behavior is investigated by correlating crack size with a
number of cycles obtained from the fatigue test. The
fatigue strength of each type welded joint is achieved and
compared with the previous results and the design
strength specified by JSSC recommendation (1995). The
fatigue strength change due to the plate thickness increase
is discussed. In addition, to evaluate the fatigue strength
analytically, one-millimeter method (Xiao et al., 2004) is
applied.
2. Fatigue test
2.1. Test specimen
Fatigue test specimens were made from 12 mm thick
plates of JIS-SM400A. These specimens had fillet welded
rib to the structural steel plate of 12 mm thick and 300
mm wide. Details of the specimens are shown in Fig. 1.
Three types of the specimens were used. The first one
was the single-sided fillet welded joint denoted by SS.
The second one was the T-shaped fillet welded joint,
Note.-Discussion open until February 1, 2009. This manuscript forthis paper was submitted for review and possible publication on July30, 2008; approved on August 30, 2008
*Corresponding authorTel: +81-52-789-4620; Fax: +81-52-789-1674E-mail: [email protected] Figure 1. Fatigue test specimen.
164 Biehn Baik et al.
denoted by SD, and the last one was the cruciform fillet
welded joint, denoted by CR. The welding was performed
by CO2 arc procedure. The mechanical properties and
chemical compositions of the steel are given in Table 1.
2.2. Testing condition
The fatigue test was carried out using a fatigue testing
machine generating a plate bending type of loading
(Yamada et al., 2007), as shown in Fig. 2. A cantilever-
type specimen was set first on frame bed and a vibrator
was installed on the plate to generate constant amplitude
vibration. The vibration was transformed into a bending
type loading to apply to a test specimen. To control stress
ratio, R, a set of springs was adjusted to a certain desired
level. The adjustment of these springs produced tensile
stress to the specimen surface before this test was
conducted under the stress ratio, R>0.
To monitor stress ranges, strain gages were placed 5
mm away from the weld toe, as shown in Fig. 3. The
strain gages, G2 and G4 at the center of each specimen,
were selected as the applied stress range indicator. To
detect crack initiation, some copper wires of 0.04 mm in
diameter were glued on the surface of weld toe. Dye
penetrant and beach mark techniques were applied to
shape crack path on fracture surface.
3. Test results
3.1. Crack initiation and propagation
Fatigue crack location is schematically illustrated in
Fig. 4. Most specimens developed fatigue cracks at the
weld toe and failed finally. All specimens had fatigue
cracks initiated at several points of weld toe, and then
coalesced into semi-elliptical shapes during crack
propagation. Finally, the cracks propagated to about 80%
of plate thickness until failure, which is monitored by
crack initiation at the plate back surface. Some typical
fracture surfaces are shown in Fig. 5.
3.2. Crack shape variation during propagation
Crack shapes marked on the fracture surface were
measured and plotted in Fig. 6. As can be seen, as the
crack depth, a, grows, the crack length, c, grows further.
This implies that the fatigue crack growth rate tends to be
faster in surface direction than in the depth direction.
Stress gradient due to bending may delay the crack
propagation in the depth direction, while local stress
concentration due to weld accelerates crack propagation
along weld line in the surface direction.
Plotted in Fig. 7 is the variation of aspect ratio, a/c, to
depth ratio, a/t, to characterize the crack shape changes.
The test results have the decrease of a/c at the early stage
of crack propagation and then maintain the lower values
until failure, because the welded joint specimens have
small and multiple cracks coalesced together at an earlier
time before forming long and shallow semi-elliptical
cracks. This leads the rapid crack propagation in the
surface direction and then the aspect ratio drops off.
3.3. Fatigue crack propagation life
The fatigue crack propagation life of each specimen
tested is shown in Fig. 8. Since each specimen was tested
Table 1. Mechanical properties and chemical compositions of steel
Thickness(mm)
Yield strength(MPa)
Ultimate tensile strength (MPa)
Elongation(%)
Chemical compositions (%)
C Si Mn P S
12 315 431 34 0.11 0.21 0.98 0.019 0.005
Figure 3. Strain gage location.
Figure 2. Test setup. Figure 4. Fatigue crack location.
Fatigue Strength of Fillet Welded Joint subjected to Plate Bending 165
in the different stress range, the equivalent number of
cycles corresponding to the stress range, 200 MPa, was
calculated in the following form,
Neq=(∆σ/∆σe)m·n (1)
where Neq is an equivalent number of cycles, ∆σe is 200
MPa, ∆σ is an applied stress range, m is 3.0, and n is a
number of cycles obtained from the test.
Plots in Fig. 8 are comparing the measured crack sizes,
a, and c, with a number of cycles for the specimens. The
fatigue crack propagation life of SD12 is slightly shorter
than that of CR12, but less different. However, the fatigue
crack propagation life of SS12 is longer than those of
others.
Shown in Fig. 9 is a comparison of fatigue crack
propagation life associated with a type of loading, tension
and bending. Compared are the test results of CR12 and
those of the cruciform joint specimen tested under tension,
which was made of the plate with 9 mm thickness and
200 mm width and the weld leg length is 6 mm (Kim,
2000). The crack depth is normalized by the plate
thickness, and the crack length is normalized by the plate
width. Obviously, bending load contributes to the longer
fatigue life for tension. Since the stress gradient due to
bending linearly decreases in the crack depth direction,
the crack propagation is gradually slower in the depth
direction and then the fatigue crack propagation life
becomes relatively longer in bending than in tension.
The remaining fatigue life of the specimens tested
under similar stress range, 200 MPa, for tension and
bending, is shown in Fig. 10. It is seen that the remaining
fatigue life is significantly longer in bending than in
tension during crack propagation.
3.4. Fatigue strength and life
The fatigue life of failure versus the recorded nominal
stress range data are plotted on S-N curve, as shown in
Fig. 11. Also compared is the design strength for bending
specified by JSSC, which recommends that fatigue strength
for bending be equivalent to 80% of fatigue strength for
tension, if the plate thickness of welded joint is less than
25 mm. It indicates that the as-welded fillet welded joint,
such as CR12, with JSSC-E for tension, if under bending,
has JSSC-D fatigue resistance. As can be seen, the test
results exceed the design strength curve for bending.
The regression S-N lines for each set of specimens are
calculated with the inverse of the slope being set at m=3
Figure 6. Experimental crack size variation.
Figure 7. Aspect ratio, a/c and crack depth ratio, a/t.
Figure 5. Fracture surfaces.
166 Biehn Baik et al.
as the following equation,
log N=c−m×log ∆σ (2)
where N is a number of cycles to failure, and ∆σ is a
nominal stress range in MPa. Results of regression
analyses for the test results are listed in Table 2. The
standard deviation s is calculated by taking log N as
variable, and the mean fatigue strength shown in the table
is the stress range at 2 million cycles to failure obtained
from regression analysis. The run-out data are not
included in the regression analysis.
As a result, the fatigue strength of SS12 was higher
than those of SD12 and CR12. The fatigue strengths at 2
million cycles of SD12 and CR12 were 28% and 22%
lower than that of SS12, respectively.
3.5. Comparison with previous test results
Several fatigue tests were carried out on welded joint
under bending in the past.
Shown in Fig. 12 is a comparison of the test results of
SS12 specimens with those of the single-sided fillet
welded specimens including the fatigue behavior of
welded joint of trough rib to an orthotropic deck plate
(Yamada et al., 2008). The specimens consisted of 6 or 8
mm thick attachment, which was fillet welded at a 78-
degree angle to the plate. Most of the specimens failed
Figure 9. Comparison of fatigue crack propagation life fortension and bending.
Figure 10. Remaining fatigue life for tension and bending.
Figure 8. Fatigue crack propagation life.
Fatigue Strength of Fillet Welded Joint subjected to Plate Bending 167
due to crack at weld root, but some failed due to crack at
weld toe. The data of toe failure are compared with the
test results of SS12. As can be seen, they show the same
fatigue strength, because both specimens have almost
same dimensions.
The test results of SD12 and the previous results
provided by Maddox (1996) and Tanaka et al. (1995) are
shown in Fig. 13. They show also about the same fatigue
strength because their geometric dimensions, for
example, plate thickness and weld size, are similar to
each other.
Shown in Fig. 14 are the test results of CR12 compared
with the previous results of the cruciform joint with K-
butt weld, which had two different plate thicknesses, 25
and 49 mm, tested by Fukuoka et al. (2006). The test
results of CR12 show about the same as those of the as-
welded specimens, in spite of the different dimension of
each specimen. The fatigue strength normally decreases
by the increase of the plate thickness in welded joint. The
JSSC specifies that the fatigue strength of welded joint
with over 25 mm thickness plate be corrected by using
the following equation.
(t≥25) (3)Ct
25 t⁄4=
Figure 11. Fatigue test results of fillet welded specimens.
Table 2. Fatigue strength at 2 million cycles
Specimentype
c sMean strength at 2×106
cycles (MPa)
SS12 13.1118 0.2806 186
SD12 12.6843 0.1548 134
CR12 12.7934 0.1653 146
Figure 12. Comparison of fatigue test results of SS12 andthe previous results.
Figure 13. Comparison of fatigue test results of SD12 andthe previous results.
Figure 14. Comparison of fatigue test results of CR12and the previous test results.
Figure 15. Fatigue strength variation due to plate thicknesschange.
168 Biehn Baik et al.
where t is the plate thickness in mm.
A comparison of fatigue strengths of three-type cruciform
welded joints at 2 million cycles is shown in Fig. 15. A
dotted line stands for the design strength for tension and
a solid line represents the design strength for bending,
relating to the plate thickness change. Although the
fatigue strength tends to decrease slightly by the plate
thickness increase, its variation is less significant. The test
results are higher than the design strength corresponding
to plate thickness change and bending.
4. Fatigue Strength Evaluation byOne-millimeter Method
4.1. Finite element model
To evaluate the fatigue strength of each type welded
joint by using 1 mm method, finite element analysis
(FEA) is necessary. The stress distribution in the expected
crack path direction is usually determined by two or three
dimensional finite element analysis. The entire analysis
was carried out using the COSMOS/M 2.9. Plane strain
model with four-node two dimensional elements are used
in mesh definition for each welded joint model in this
study. An example of FE model, SD12, is shown in Fig.
16(a), with meshing around weld toe region. Minimum
mesh size in the weld toe region is 0.05 mm. In the
present analysis, Young’s modulus, E=200 GPa and Poisson’s
ratio, v=0.3 are assumed.
4.2. Fatigue life prediction by 1 mm method
The stress distribution in the expected crack path
direction is shown in Fig. 16(b). The stress at 1 mm in
depth is taken as an indicator of global geometry of
welded joint and used as a measure of fatigue strength.
The stress concentration factor at 1 mm in depth is 1.0
on the non load-carrying fillet welded cruciform joints
subjected to tension (Xiao et al., 2004). The cruciform
joints consisted of the base plate and attachment, which
were about 10 mm thick and fillet size was 6 mm. The
FEA showed that the stress concentration at 1 mm in
depth along the anticipated crack path was close to being
uniform. The scatter of test data of these cruciform joints
was defined as reference detail, and then can be referred
to determine fatigue strength of the objective welded
detail.
One-millimeter method is used to evaluate fatigue
strength of fillet welded joint subjected to bending. Stress
at 1 mm in depth is obtained for each type of specimens
from the stress distribution. Using the stress at 1 mm in
depth and the regression S-N curves of the reference
detail, fatigue strength evaluation is carried out on each
type of specimen and the result is plotted in Fig. 17 with
the fatigue test data.
The predicted life range is in good agreement with the
test data, while SS12 has rather conservative prediction.
5. Summary
The fatigue behavior of fillet welded joints subjected to
bending is the focus of this study. Fatigue test has been
carried out on three types of the fillet welded joints
subjected to bending load and crack shape development
has been examined.
The test results have indicated that most fatigue cracks
form relatively flat semi-ellipses during crack propagation
and propagate to about 80% of plate thickness before
failure. The aspect ratio, a/c, decreases at the early stage
of crack propagation and then maintain the lower values
until failure, because small and multiple cracks coalesce
at an earlier time before forming long and shallow semi-
elliptical cracks.
A comparison of fatigue crack propagation life
associated with a type of loading, tension and bending,
shows that bending obviously contributes to the longer
fatigue life than tension.
At 2 million cycles, the fatigue strength of SS12 is over
Figure 16. Example of finite element model and stress distribution.
Fatigue Strength of Fillet Welded Joint subjected to Plate Bending 169
20% higher than those of SD12 and CR12. The test
results exceed the design strength curve for bending,
specified by the JSSC, which recommends that the
fatigue strength for bending be equivalent to 80% of
fatigue strength for tension, if the plate thickness of
welded joint is less than 25 mm.
Acknowledgments
I would like to express my gratitude to Dr. S. Yamada
of Topy Industries, Ltd. and Dr. T. Ojio of Meijo
University, for their valuable advice on carrying out the
fatigue tests for bending.
References
Fukuoka, T., Maeda, T. and Mochizuki, K. (2006). “Effect of
plate thickness and improvement by grinding on fatigue
strength of cruciform joint under bending”, IIW
Document XIII-2134-06, International Institute of
Welding.
Japanese Society of Steel Construction (JSSC). (1995).
Fatigue Design Recommendations for Steel Structures.
(in English)
Kim, I.T. (2000). “Fatigue of welded joints under combined
stress cycles”, Doctoral dissertation, Department of Civil
Engineering, Nagoya University, Nagoya, Japan.
Maddox, S.J. (1974). Fatigue of welded joints loaded in
bending, TRRL Supplementary Report 84 UC,
Crowthorne, Berkshire.
Miki, C., Mori, T., Sakamoto, K. and Kashiwagi, H. (1987).
“Size effect on the fatigue strength of transverse fillet
welded joints”, Journal of Structural Engineering, JSCE,
Vol.33A, pp.393-402.
Tanaka, M., Mori, T., Irube, T. and Miyasita, R. (1995).
“Fatigue strength of non load-carrying one side fillet
welded cruciform joints”, Journal of Construction Steel,
Vol.3, pp.403-410.
Xiao, Z. and Yamada, K. (2004). “A method of determining
geometric stress for fatigue strength evaluation of steel
welded joints”, International Journal of Fatigue, Vol. 26,
pp.1277-1293.
Ya, S. and Yamada, K. (2007). “Fatigue tests of welded
joints of trough to orthotropic steel deck plate in
bending”, Proceedings of the 62nd Annual meeting of
JSCE, pp.23-24, (CD-ROM).
Yamada, K., Ya, S., Baik, B., Torii, A., Ojio, T. and Yamada,
S. (2007). “Development of a new fatigue testing
machine and some fatigue tests for plate bending”, IIW
Document XIII-2161-07, International Institute of
Welding.
Yamada, K. and Ya, S. (2008). “Plate bending fatigue tests
for root crack of trough rib of orthotropic steel deck”,
Journal of Structural Engineering, JSCE, Vol.54A,
pp.675-684.
Figure 17. Fatigue strength evaluation by 1 mm method.