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FATIGUE TESTS ON CFRP STRENGTHENED STEEL PLATES WITH
DIFFERENT DEGREES OF DAMAGE
QIANQIAN YU
Doctoral candidate
Tongji University
Department of Building Engineering, Tongji University, Shanghai 200092, China
TAO CHEN
Lecturer
Tongji University
Department of Building Engineering, Tongji University, Shanghai 200092, China
XIANGLIN GU
Professor
Tongji University
Department of Building Engineering, Tongji University, Shanghai 200092, China
XIAOLING ZHAO
Professor
Monash University
Department of Civil Engineering, Monash University, VIC 3800, Australia
Abstract
Composite fiber patching techniques have been considered as alternatives to traditional
methods of fatigue crack repair in steel structures. In this study, a series of experiments were
conducted to verify the effectiveness of the carbon fiber reinforced polymer (CFRP) plates on
preventing fatigue crack propagation and extending fatigue life of steel plates. Specimens
were steel plates with center holes and different lengths of artificial cracks, representing the
degree of damage. They were repaired by notch filling and double-sided bonding. Crack
propagation was monitored by “beach marking” technique. Fatigue lives and failure modes
were observed. Experimental results show that the application of composite patches
substantially reduces the crack growth rate and prolongs fatigue life. The application of CFRP
repair to cracked plates extended the remaining fatigue life to various degrees with different
initial crack lengths.
Keywords: Carbon fibre reinforced polymer plate; fatigue damage; fatigue test; steel plate;
1. Introduction
Fatigue damage is a major concern for many infrastructure including steel highway bridges.
Previous research pointed out that major highway bridges experience about 100 million to
300 million stress cycles in their 100 year lifetime, which may cause fatigue failure [1]
.
Therefore, more and more attention has been paid to repairing and retrofitting these old steel
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structures in a more economical and environmental friendly way. Composite fiber patching
techniques have been considered as alternatives to traditional methods of fatigue crack repair
in steel structures. Adhesively bonded composite patch repairs have been successfully applied
in aeronautical industry while research on using CFRP materials to strengthen steel structures
in civil engineering is still in the early stage. Earlier research indicated that CFRP materials
retrofitting to steel structures can decrease stress intensity factors at fatigue crack tip which
reduces crack growth rate, thereby extend fatigue lives of damaged steel members.
Colombi et al. (2003) [2]
carried out fatigue tests on notched steel plates and found that fatigue
life of specimens reinforced with non-prestress CFRP strips was increased by a factor of
about three while the fatigue life of the ones reinforced with prestress CFRP strips was
extended by a factor of about five. Jones et al. (2003) [3]
conducted an experimental study to
investigate the effectiveness of applying CFRP overlays to prolong fatigue life of steel plates.
Test results show that two sided applications extend the fatigue life by as much as 115%.
Monfared et al. (2008) [4]
indicated that using high modulus CFRP overlays can prolong the
fatigue life of notched steel specimens. Liu et al. (2009) [5]
conducted an experimental study
on center notched steel plates strengthened with CFRP sheets. For double-sided repairs, the
repair scheme extends the fatigue life by 2.2–7.9 times compared to the non-strengthened
ones. Täljsten et al. (2009) [6]
investigated the fatigue behavior of steel plates with four
different configurations. It is shown that the fatigue life of non-prestressed test specimens can
have their fatigue life prolonged by 2.45–3.74 times. Other than retrofitting damaged steel
plates, research on strengthening other steel structures such as welded web gusset joint,
aluminum connections with CFRP materials has also been conducted [7]-[9]
.
It is found from literature review that study of specimens with different degree of damage
strengthened with CFRP materials is limited. Most previous study focuses on the fatigue
behaviour of steel plates notched with a very short initial crack. The objective of this study is
to investigate the effectiveness of CFRP strengthening method for steel plates with different
damage degree representing different stages of service life. Failure modes and corresponding
fatigue lives were recorded. Crack propagation was monitored by “beach marking” technique.
2. DESCRIPTION OF FATIGUE TESTS
2.1 Configuration of the Test Specimens
Notched steel plates have uniform dimensions of 500mm x 100mm x 8mm. The central notch
consists of a 10mm hole and two initial cracks of 0.2mm wide with three different lengths.
Here we use the ratio of initial crack length to the plate width to define the degree of damage,
i.e. the degrees of damage are 2%, 10% and 20% corresponding to initial crack length of 1mm,
5mm and 10mm, respectively.
The steel plates are reinforced on both sides by two CFRP plates with length of 200mm and
width of 40mm as shown in Figure 1(b) and the center notch is filled with 4 layers of 8mm
CFRP plate. A two-component viscous epoxy is used for bonding the laminate to the steel
plate.
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(a) Unreinforced specimen geometry
(b) Reinforced specimen geometry
Figure 1. Specimen geometry (Unit: mm)
2.2 Materials
Mild carbon steel (Q345) conforming to Chinese Standard GB 50017-2003 in the form of
rolled plates is used. The mechanical properties of the steel plates are determined through
tensile coupon tests as shown in Table 1. Retrofitting materials are CFRP plates and adhesives
with their property details listed in Table 3 and Table 4.
Table 1 Mechanical property of steel
TYPE ULTIMATE STRENGTH/MPa YIELD STRESS/MPa YOUNG’S MODULUS/MPa ELONGATION/%
Q345 406 279 1.82×105 32.33
Table 2 Chemical compositions of steel (%)
TYPE Mn C S Si P
Q345 1.52 0.16 0.004 0.22 0.017
Table 3 Mechanical properties of CFRP plate
TYPE THICKNESS/mm TENSILE STRENGTH/MPa YOUNG’S MODULUS/MPa ELONGATION/%
1.4T 1.4 3089 1.91×105 1.7
Table 4 Mechanical properties of adhesive
TYPE TENSILE STRENGTH/MPa YOUNG’S MODULUS/MPa ELONGATION/%
EFR-400 41.6 3320 1.53
2.3 Specimen Preparation
The surfaces of the steel plates were sandblasted to remove the rust and create a rough surface.
Then the substrates were cleaned with acetone to remove grease and dust to expose a fresh
chemically active surface to ensure better mechanical interlocking. The CFRP plates were
glued using wet lay-up method to the steel surfaces and finally the specimens were cured for
one week in room temperature.
R=4
CFRP plate
Adhesive
10
0
200
40
40
CFRP plate
250 250 500
50
50 10
0 R5
1
8
0.2 1,
5, 1
0
R5
1
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2.4 Fatigue loading
Tests were performed on an Instron 1434 servo hydraulic testing machine with a dynamic
capacity of 200kN (Figure 2). All the specimens were tested under uniform amplitude tensile
loading with a constant frequency of 10 Hz and stress ratio 0.1. The stress range was kept for
all the specimens as 110MPa in the nominal section of the un-reinforced specimen.
Figure 2. Fatigue test setup
2.5 Measurement of crack propagation
The technique of ‘‘beach marking” was adopted to monitor the crack propagation developing
with the fatigue cycles. Using this technique, low stress range for a short number of cycles is
inserted in the original applied cyclic loading.
3. FATIGUE TEST RESULTS
Totally 6 specimens are tested in the experiments and Figure 3 gives a summary of the test
results to date. The specimen nomenclature is as follows: CN-1, CN-5 and CN-10 are equal to
center notched specimen with crack length of 1mm, 5mm and 10mm respectively. The load
cycles corresponding to the low stress range which are considered to be relatively small
compared to the total fatigue life are taken away. The improvement of CN-1, CN-5 and
CN-10 is 97.2%, 95.3% and 176.9% respectively.
All the specimens failed at the middle of the steel plates when the fatigue crack reached a
certain length accompanying with CFRP plates debonding. Figure 4 describes the relationship
between damage degree and fatigue life improvement. Here Np,with CFRP is the fatigue rack
propagation life with CFRP, whereas Np,no CFRP is the fatigue propagation life without CFRP.
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Figure 3. Fatigue propagation life of tested specimens
Figure 4. Damage degree versus fatigue life improvement
From the observation of cross section, “beach marking” is effective to record the crack front
during the fatigue test. By measuring the crack length in the cross section, the curves
depicting the relationship between the crack length and fatigue cycle number for each
specimens are plotted in Figure 5. The crack length was one half of the distance between two
crack fronts in the same cycles. It is indicated from the curves that the longer the initial cracks
are, the faster the cracks propagate under fatigue loading. CFRP repairs sharply retard the
crack development.
Number of fatigle cycles
0 105 2x105 3x105 4x105 5x105
Hal
f cr
ack l
ength
'a' (
mm
)
0
10
20
30
Unstrengthened with a damage degree of 2%
Strengthened with a damage degree of 2%
Unstrengthened with a damage degree of 10%
Strengthened with a damage degree of 10%
Unstrengthened with a damage degree of 20%
Strengthened with a damage degree of 20%
Figure 5. Half crack lengths versus number of fatigue cycles
Damage degree
Np
,wit
h C
FR
P/N
p, n
o C
FR
P
0.0
.5
1.0
1.5
2.0
2.5
3.0
2% 10% 20%
1 2 3N
um
ber
of
fati
gue
cycl
es0
105
2x105
3x105
4x105
5x105
unstrengthened specimen
Strengthened specimen
CN-1 CN-5 CN-10
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4. CONCLUSIONS
Fatigue testing of tension was performed on 6 specimens. Experimental result shows that the
application of CFRP plates is a promising technique for the reinforcement of fatigue damaged
steel plates. Regarding the crack length measurement methods applied, ‘‘beach marking” is
proven to be a reliable method to trace crack propagation developing with the fatigue cycles.
Fatigue life was increased up by 95.3% to 176.9% when CFRP plate was applied to the
specimen with respect to that of the unpatched reference ones. It is evident that the application
of composite patches can be used to reduce crack propagation of existing cracks and prolongs
fatigue life. Different crack lengths are adopted to simulate the degree of damage. It is shown
that the strengthening method is useful for all stages of crack propagation and it seems better
to repair as early as possible based on the limited result.
ACKNOWLEDGEMENT
The authors acknowledge the supports of National Natural Science Foundation of China
(Grant Number 50808139), Shanghai Municipal Education Commission and Shanghai
Education Development Foundation (“Chen Guang” project, Grant Number 09CG16) and
Kwang-Hua Fund for College of Civil Engineering, Tongji University.
REFERENCES
[1] Tavakkolizadeh, M., Saadatmanesh, H., “Fatigue strength of steel girders strengthened with carbon fiber reinforced polymer patch”, Journal of Structural Engineering, Vol. 129, No. 2, February 2003, pp. 186-196.
[2] Colombi, P., Bassetti, A. and Nussbaumer. A., “Analysis of cracked steel members reinforced by pre-stress composite patch”, Fatigue Fracture Engineering Structures, Vol.26, January 2003, pp. 59-66.
[3] Jones, S.C., Civjan, S.A., “Application of fibre reinforced polymer overlays to extend steel fatigue life”, Journal of Composites for Construction, Vol. 7, No. 4, November 2003, pp. 331-338.
[4] Monfared, A., Soudki, K. and Walbridge, S., “CFRP reinforcing to extend the fatigue lives of steel structures”, CICE2008: Fourth International Conference on FRP Composites in Civil Engineering, July 2008, pp. 1-6.
[5] Liu, H.B., Al-Mahaidi, R. and Zhao, X.L., “Experimental study of fatigue crack growth behaviour in adhesively reinforced steel structures”, Composite Structures, Vol. 90, No. 1, March 2009, pp. 12-20.
[6] Täljsten, B., Hansen, C. S. and Schmidt, J. W., “Strengthening of old metallic structures in fatigue with prestressed and non-prestressed CFRP laminates”, Construction and Building Materials, Vol. 23, No. 4, April 2009, pp 1665–1677.
[7] Nakamura, H., Jiang, W., Suzuki, H., Maeda, K. and Irube, T., “Experimental study on repair of fatigue cracks at welded web gusset joint using CFRP strips”, Thin-Walled Structures, Vol. 47, No. 10, January 2009, pp. 1059-1068.
[8] Nadauld, J., Pantelides, C.P., “Rehabilitation of cracked aluminium connections with GFRP composites for fatigue stresses”, Journal of Composite Construction, Vol. 11, No. 3, May 2007, pp. 328-335.
[9] Pantelides, C.P., Nadauld, J., and Cercone, L., “Repair of cracked aluminium overhead sign structures with glass fiber reinforced polymer composites”. Journal of Composite for Construction, Vol. 7, No. 2, May 2003, pp. 118-126.