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Effect of prestressed CFRP patches on crack growth of centre-notched steel plates
Accepted Manuscript Effect of prestressed CFRP patches on crack growth of centre-notched steel plates Reza Emdad, Riadh Al-Mahaidi PII: DOI: Reference:
S0263-8223(14)00663-1 http://dx.doi.org/10.1016/j.compstruct.2014.12.007 COST 6067
To appear in:
Composite Structures
Please cite this article as: Emdad, R., Al-Mahaidi, R., Effect of prestressed CFRP patches on crack growth of centrenotched steel plates, Composite Structures (2014), doi: http://dx.doi.org/10.1016/j.compstruct.2014.12.007
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Effect of prestressed CFRP patches on crack growth of centre-notched steel plates Reza Emdad1, Riadh Al-Mahaidi1 1
Swinburne University of Technology, Hawthorn, VIC 3122, Australia, (*corresponding author: [email protected]) ABSTRACT
Carbon fibre reinforced polymers (CFRPs) are now accepted in the retrofit and repair industry as extremely efficient and capable of compensating for the loss of structural integrity, resistance and serviceability. Nevertheless, the application of these materials can be improved in various ways to meet structural requirements. One of the methods of improving the usage of CFRP sheets and laminates and benefiting from their full capacity is prestressing. The fatigue performance of steel plates retrofitted with CFRP patches was investigated in this research. The test samples included three main types: reference centre-notched steel plates (unrepaired), repaired centre-notched steel plates using CFRP patches and repaired centrenotched steel plates using prestressed CFRP patches. In the specimens in which prestressed CFRP patches were used, the patches consisted of 1 or 2 layers of CFRP sheets and were prestressed up to 25% or 50% of their ultimate capacity. All specimens were then tested under uniaxial tensile fatigue. In order to measure the crack propagation rate, the beach marking method was used. Two different double-block fatigue loads were studied: one causing a mean stress of 135MPa and the other with a mean stress of 150 MPa relative to the gross section of the steel plate.
KEYWORDS Fatigue, Steel, Crack propagation, Retrofitting, CFRP, Prestressing
1
INTRODUCTION
One of the main modes of failure in metallic structures is crack propagation due to fatigue. In order to overcome this catastrophic phenomenon different methods have been proposed. Some of the methods are: drilling stopper holes, welding cover plates (Eurocode 3 and AASHTO), and composite patching [1]. Researchers in this field have studied crack propagation in steel and aluminium members by performing the common tensile fatigue testing of notched plates. Efforts are made to slow the crack propagation rate or achieve crack arrest. One method of crack growth retardation is drilling holes, known as crack stoppers. This method removes crack tip singularity by crack-tip blunting and increases the fatigue life [2]. The issue with this method is that it requires exactness in locating the crack tip; otherwise a misplaced stop hole can make the case even worse. Another problem with this method is that the hole itself might cause a change in the strength of the section or initiate new cracks. In an early experimental research performed by Tavakolizadeh et al. [3]
the fatigue
behaviour of steel girders strengthened with CFRP patches was evaluated and showed that the design characteristics of structural steel could be improved using epoxy-bonded CFRP laminates. Aidoo et al. [4] used externally-bonded FRP reinforcement on the tensile surfaces of concrete girders. Their results showed that the fatigue life of such girders can be improved using FRP patches. In both of the aforementioned studies the fatigue behaviour of a structural member is studied. These researches show the effectiveness of CFRP strengthening on the fatigue life of structural members and were a start point of more research in this field. Before finding its way into civil engineering applications, this method was primarily tested and used in the aerospace industry for aircraft repairs [5].
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The patch repair method which has been utilised by researchers is strengthening the cracked member with fibre reinforced polymers [6-10]. Some researchers have proved the fatigue resistance of the bond under fatigue loading is even better than the steel cover plates (Eurocode 3) welded to the defected member [7, 11]. Composite patch repairs are clearly less damaging than the conventional methods of repair based on drilling holes in or attaching external metallic patches to the original structure. Experimental and numerical analysis of double strap joints under axial load was performed by Fawzia et al. [13]. The dominant failure mode was found to be the bond between the CFRP and steel. Zhao et al. [14] expressed the observed modes of failure of CFRPstrengthened steel structures. They investigated different experiments considering the effect of different adhesives and steel sections such as plates and hollow sections in compression tension and bending. Many researchers have studied the propagation of cracks in steel and aluminium under fatigue. Baker [15], Cheuk et al. [16] Colombi et al. [17], Lee et al. [18], Hosseini-Toudeshky [19], Sabelkin et al. [20], Liu et al. [6] Taljsten et al. [9] and Wu et al. [8] have all investigated the patch repair technique in retrofitting of steel or aluminium plates. Tensile fatigue testing of notched plates is a common test performed by these researchers. Baker [15] experimentally investigated fatigue crack propagation in cracked aluminium alloy repaired with Baron/epoxy patches. Mainly focussing on aircraft structures, he found this technique to be efficient and less damaging to the original structure [5]. Cheuk et al. [16] studied fatigue crack initiation and growth in metal-to-composite bonded joints both experimentally and numerically. The fatigue tests performed were all tension-dominated, and the crack propagation was measured optically. Colombi et al. [17, 21] considered the effect of prestressing on crack growth under a load range of 160kN to 400kN causing a stress amplitude of 80MPa. The CFRP strips were attached to the sides of the notch not covering 3
the crack. The debonding pattern around the crack was also investigated by them. Lee et al. [18], Hosseini-Toudeshky [19] and Sabelkin et al. [20] investigated single-side repaired aluminium plates. When single-side repair is used the crack growth rate would become slower on the repaired side compared to the unrepaired face, creating a non-uniform beach mark on the crack front. Liu et al. [22] tested double-strap joints under fatigue loading to measure the bond strength under cyclic loading. These researchers [6] also tested centrenotched steel (CNS) plates retrofitted with CFRP patches under tensile cyclic fatigue. They tested several repair configurations and improved the fatigue life up to 8 times that of unrepaired steel plate. They investigated 10mm plates and achieved this result with 5 layers of high modulus CFRP covering the whole width of the plate with a bond length of 250mm. Taljsten et al. [9] also showed the benefit of prestress. They tested 8mm steel plates under a tensile cyclic loading of 175kN and 15kN generating a stress of 97.5MPa (considering the cross section of an unrepaired specimen). CFRP laminates were used with a similar repair configuration as in [17] in which the crack was not covered. Wu et al. [8] utilised ultra-high modulus CFRP laminates in order to enhance the fatigue life of cracked steel plates. Several configurations were used and compared. The results showed that where accessible, covering the crack with CFRP has a better influence in the repair mechanism. Experiments performed to date have utilised different fibre reinforced sheets and laminates with different elasticity moduli. The effectiveness of the elasticity modulus in the repair of cracks in metallic members has been pointed out by many researchers and can be concluded from their experimental findings [6, 8, 9]. The benefits of the patch repair method for metallic cracks include the high strength-toweight ratio, the modulus of elasticity of CFRP patches which is comparable to that of steel, corrosion-proofing the parent structure, easy handling and shaping of the patches and fatigue endurance. The capability of being prestressed can be also added to its merits. Effective 4
prestressing of the patches can increase the remaining fatigue life (cracked life) of repaired metallic structures. Reduction of stress intensity factor occurs in the vicinity of the crack when benefitting from the passive effect of the patches. However, when prestressing is used the stress intensity factor decreases even more due to the introduction of a compressive force pushing the edges of the crack closer together. Previous findings indicate the enhancement of the remaining fatigue life of metallic structures which have undergone composite patch repairs. However, in order to achieve a unified model and guidelines in this area more experiments and numerical validation are required. The experiments carried out in this research are similar to those reported in [6]. The main difference is that instead of increasing the number of CFRP sheets in the patch, prestressing was applied. Another difference is that 6mm thick plates were utilised, while [6] used 10mm thick plates. This paper covers an experimental investigation on the repair of cracked steel plates. The specimens were tested under two different types of proportional cyclic loads differing in the stress amplitude.
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2.1
Materials and Specimens
Material Properties
Coupon tests were performed to verify the mechanical properties of the steel. The steel plates were all 6mm thick, and all steel plates used in this study were from the same batch. These properties were calculated in accordance with ASTM E8/E8M-11[23] and the results are shown in Table 1. Normal modulus CFRP sheets (CF230) supplied by BASF were used in this research. The mechanical properties as per the manufacturers’ datasheet are shown in Table 2.
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Araldite 420 was used as the adhesive to bond the CFRP to the steel surface. The properties of the adhesive were verified by Fawzia et al. [13] using a series of coupon tests (refer to Table 3). Table 1. Mechanical properties of steel
Table 2. Mechanical properties of the normal modulus CFRP
Table 3. Mechanical properties of Araldite 420 [13]
2.2
Specimens
Three types of specimens were tested in this research program: •
CNS plates without repair
•
Double-side patch repaired CNS plates
•
CNS plates repaired with prestressed patches (double-sided)
2.2.1 CNS plates without repair The focus of this research was to measure crack propagation versus number of cycles. Therefore, CNS plates were utilised. As mentioned above, the steel plates used in this study were 6mm thick. All steel plates had a hole in the centre and initial cracks each 1mm long starting from the two edges of the hole were introduced. The dimensions of the plates and the notches are shown in Figure 1.
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Figure 1. a)TH=6mm Steel specimen dimensions and notch detail (NTS), b)Double-sided CFRP strengthening configuration for all specimens
At least 4 strain gauges were attached to each specimen. Two of the strain gauges located on the steel plate (opposite face and opposite side of the crack) 10mm from the edges at the centre of the CNS plate and the other two were attached each on the CFRP on both sides of the specimen approximately located on the centre of the CFRP. Readings from the gauges were used to prove that the loading was concentric. The locations of the strain gauges on the CNS plate are shown in Figure 1. 2.2.2 Double-sided patch repaired CNS plates The same CNS plates were used and repaired using a double-sided patch configuration, as shown in Figure 1(a). The repair configuration included 1, 2 & 3 layer patch retrofitting. The CFRP layers were all 250mm long and 50mm wide. The specimen dimensions and repair configurations were designed based on previous studies [6], [8]. 2.2.3 CNS plates repaired with prestressed patches In order to benefit from the active effect of the CFRP patches rather than their passive effect, already studied by Liu et al. [6], this set of specimens was repaired using prestressed patches. As explained later, in order to prestress the patches a hydraulic actuator was used. The patches on both sides of the plates were prestressed simultaneously. Specimens were prepared using the same configuration as shown in Figure 1(b) with different prestress levels to measure the effect of prestress on their fatigue life.
2.3
Specimen Preparation
In order to attach the CFRP patches to the steel plate and benefit from an effective bond, surface preparation is required. Therefore, all steel plates were sandblasted and cleaned with acetone. This removes the rust and cleans the surface of dust and grease. Araldite 420, a two7
part structural adhesive, was used to bond the patch and the steel. This adhesive has shown a great influence on the bond behaviour of CFRP-strengthened steel members ([13], [6], [8]). Previous studies have recommended Araldite 420 for its workability and pot life as well as its high shear strength.
2.4
Prestressing
A prestressing technique was developed for this study in the SmartLAB at Swinburne University of Technology. An MTS actuator was used to apply the prestress force on the CFRP patches. This method was found to be more accurate than using hydraulic jacks. The reason is that hydraulic jacks amplify the prestress loss over time as there are always unavoidable oil leaks despite using shut-off valves for temporary load holding, while using a servo hydraulic actuator in force control mode, will compensate the prestress loss during the curing process. The prestressing process has four main steps, as explained below. Step 1: Tensioning of the CFRP patch In this step the CFRP patch is gripped in the jaws. The tensile force is then slowly applied using a load control test procedure until the required level of prestress is reached (Figure 2(a). In the experiments carried out in this research the CFRP patches were prestressed up to 25 % or 50% of their ultimate strength. The maximum load that a one-layer patch with 50mm width can withstand is about 20-25kN. By increasing the number of layers to two, this value will be almost doubled.
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Table 4 shows the prepared specimens with their names and descriptions. The prestress level reached is also provided.
Figure 2. Prestressing stages: a) applying tensile force to the patch. b)applying adhessive to the desired bond length c) cutting the excess CFRP patch.
Table 4: Specimen list
Step 2: Attaching the patch in tension to the CNS plate After the desired level of prestress is reached, the loaded patch is attached to the CNS plate with a 250mm bond length. This step must be performed carefully to ensure that the CFRP patch and the CNS plate are perfectly aligned. The centre of the patch is placed on the centre of the CNS plate so that the whole crack is covered symmetrically in both directions. As mentioned earlier, Araldite 420 was used for the means of bonding CFRP to steel. It must be noted that the surface preparation of the CNS plates was done as previously explained prior to this step. Step 3: Curing of the loaded specimen While the CFRP is under tensile load, the specimen is left for curing. According to the manufacturer’s datasheet for Araldite 420, the recommended time for curing is 7 days. In order to reach a faster curing time heat treatment could be used. The authors have used heat treatment in some previously tested specimens and have achieved similar results. In the results revealed in this study heat treatment for achieving faster cure times is not used, because the induced heat affects the strain gauge readings. Figure 3(a) shows the test setup and Figure 3(b) shows a specimen curing under load in the designed set-up. 9
Step 4: Unloading of the patch After the specimen is cured, the prestress load can be removed. Removing the load, results in partial dissipation of the tensile stress in the patch, as well as the build-up of compressive stress in the CNS plate. This causes the edges of the crack to become closer (artificial crack closure). The extra CFRP patch can then be cut, leaving the specimen as shown in Figure 2(c).
Figure 3. a) Designed test setup, b) Specimen curing under load in the designed prestressing set-up
3
FATIGUE LOADING
The specimens were tested under a tensile fatigue load in a closed-loop servo-hydraulic 250kN fatigue-rated MTS machine. Tensile fatigue loading for the study was set at the stress ratio of 0.1. The stress ratio is the ratio of the minimum stress to the maximum stress of the specimen during the fatigue test: ௫ 0.1 In order to measure the effect of loading on the fatigue life, two types of fatigue loads were used. The two sets of fatigue loads differed in the stress range. In the first set, the maximum force applied in the fatigue load block was 81kN (Figure 4(a)) and in the second load the maximum force applied in the fatigue block was 92kN (Figure 4(b)). The 81-8.1kN range applied a stress range of 135MPa and the 92-9.2kN load range applied a stress range of 153MPa on the specimens. All specimens were gripped at both ends, as shown in Figure 5. The fatigue load cycles were sinusoidal waveforms applied at the frequency of 10Hz.
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Load I:
Load II: ௫ 150.0
௫ 170.3
15.0
17.0
௫ 135
௫ 153
Measuring the crack propagation rate was the main aim of this research. For this reason the beach marking method [6, 8, 18, 24] was chosen among the many other existing methods. Some of the popular methods applicable on patch repaired specimens being used, other than the beach marking technique are: use of crack gauges and using a displacement transducer or clip gauge. In the beach marking method the specimen will be loaded under a double block fatigue load repeatedly until failure. The first block is the main load which the crack propagation is the result of this block. The second block is used to produce beach marks at the end of each loading block (first block). The second block has a smaller amplitude than the first block therefore will not contribute much to the propagation of the crack. The beach mark on the crack front is a result of the sudden stress intensity drop due to reduction in the stress amplitude. Applying crack gauges to the expected crack line, has been used by [25] and the results have been reported very close to the beach marking technique. In this method the gauge must be bonded to the steel were the crack propagation is expected. This method is suitable for plain steel members. When the steel member is strengthened with CFRP, proper care and action must be taken to ascertain the bond between the CFRP and steel is not weakened at this critical location of the crack gauge which is the initial point of debonding.
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Benefitting from displacement transducers and clip gauges is also another method that has been used by [9] to measure the crack opening displacement (COD). The crack propagation lengths are then calculated based on the measured COD. There are more methods that have been used in literature. Some of the methods are not suitable for application on members strengthened with CFRP patches. Each of the methods discussed earlier have their own advantages. The beach marking technique was preferred in this study for the reason that it is consistent for all specimens and simple to apply. Therefore, errors induced by technical instruments are minimised. As mentioned earlier, in order to benefit from the beach marking technique, a double-block fatigue load was used. As shown in Figure 4 the second block has a stress range equal to half of that of the first block with double the frequency, to create marks on the crack front. The created marks were then used for crack propagation measurements. All of the specimens were tested until failure.
Figure 4. Double-block fatigue load for measuring crack propagation; a) Load I: Stress range of 135MPa, b) Load II: Stress range of 153MPa Figure 5. Specimen gripped at both ends while under fatigue in the MTS machine
When the specimens were loaded, the notch which represented the crack initiation grew. The crack growth started from the centre of the specimen and propagated all the way through the width of the plate.
3.1
Strain measurements
As mentioned previously, the prestressing technique has four main stages. Strain gauges were attached to the CFRP and CNS plates in order to ascertain symmetry in the prestressing stage and while under cyclic fatigue loading. 12
Average values of the strains during the prestressing stage on the CFRP are shown in Figure 6(a). After the CFRP is attached to the steel and a bond is fully formed, as the CFRP is unloaded, the tensile strain in the CFRP is transferred to the CNS plate, building up compressive strain in the steel. Figure 6(b) and (c) show the strain values on the CFRP and the CNS plate during unloading respectively. As the results indicate, the final strain on the CNS plate matches the amount of compression load applied. Figure 6. a) Prestressing stage: Average value of strain in CFRP, b) Unloading stage: CFRP Relaxation, c) Unloading stage: Compressive strain build-up in CNS plate
Another factor which is important is the durability of the prestress in order to ascertain that the prestress will remain without any loss under different environmental and loading conditions. This was not within the scope of the present research. However, the authors measured the strains of the samples for 7 days in order to ensure that no further loss occurred. Loading conditions are other factors which can affect the stored tension. As mentioned in the next sections, despite the high frequency tensile loads applied in the experiments, the prestress effect persisted until the final cycles.
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4.1
Results and discussion
Mode of failure
4.1.1 Steel After testing all the specimens until failure under fatigue, the crack fronts of the specimens were studied. With the aid of the beach marking technique, crack propagation was measured 13
versus the number of cycles. The crack fronts of some of the specimens are shown in Figure 7. As the cycles increase, the crack starts propagating from the edge of the existing notch. The crack grows symmetrically towards the edges of the plate. The crack growth region is smooth at the beginning, representing very small crack tip yielding and stable crack growth. When the crack reaches the edges of the plate, a catastrophic fracture is seen due to the fracture of the steel plate Figure 7(a) and (f). The same failure mode is seen in the CFRP-strengthened CNS plates. The difference is in the number of cycles and the length of the stable crack growth region. As shown in Figure 7(b), (c) and (d), the CFRP-strengthened plates have a larger stable crack growth length, which results in a smaller fractured edge of the steel. This can be explained as a decrease in the stress levels at the crack and exhibits the capability of the CFRP in controlling the crack growth rate and its direct effect on reducing the stress in the vicinity of the crack.
Figure 7. Failure modes of specimens tested under tensile fatigue load
In CNS plates strengthened with prestressed CFRP patches, the same modes of failure occur, and the experiments showed that the catastrophic failure region is even smaller. In specimen D2LP50, this region does not exist (Figure 7(d)). This may mean that the effect of prestress exists even after the crack has grown all the way through the width of the plates. In other words, the crack growth increases as the debonded region expands, but the effect of prestress holds the two edges of the crack together up to the last cycle. This shows how effective prestress can be, with its influence lasting even to the final cycles.
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4.1.2 CFRP As understood from the experiments, two main types of failure occur in the patch: fibre breakage and debonding. From what has been discovered in the experiments, when the patch consists of one layer of CFRP, the failure mode is fibre breakage (Figure 7(b)). Furthermore, if the patch has more than one layer, the failure mode changes to debonding (Figure 7(c)). 4.1.3 Fatigue life evaluation The experimental results are shown in Table 5 and Table 6. The fatigue life is measured based on the number of cycles to failure for each specimen. The last column represents the fatigue life extension. This value is the ratio of the fatigue life of each specimen to the fatigue life of the bare plate. This column also shows the capability of CFRP patches (with/without prestress) in the enhancement of the fatigue life of cracked plates. Table 5 shows the results of the tests under cyclic load I (Figure 4(a)). Table 6 shows the cracked life of the CNS plates under cyclic load II (Figure 4(b)). The results show that fatigue life can be enhanced 3 to 35 times with CFRP patch repair and using the prestressing technique.
Table 5. Specimen fatigue results (stress range=135 MPa)
Table 6. Specimen fatigue results (stress range=150 MPa)
Comparison of the results of Table 5 and Table 6 shows that as the stress range increases the fatigue life undergoes a decrease. This can be explained using the well-known Paris Law Eq. (1):
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∆
(1)
In this equation: ௗ ௗே
is the rate of crack propagation per cycle
C and m are both material constants ∆ is the stress intensity factor variation in each cycle
5
Crack Propagation
The beach marks of some of the tested specimens are shown in Figure 7(e) and (f) and Figure 8. As expected, the beach marks were symmetrical on both sides of the hole. Since both sides of the plate are reinforced (Figure 7(e)), the crack propagation is uniform, whereas in singlesided repairs the crack growths on the repaired and unrepaired sides are different [18]. The uniformity of the beach mark curves in the prestressed specimens confirms that the patches on both sides of the plate were prestressed up to the same level. If this were not the case, an undesirable out-of-plane bending effect would have been applied, causing variation in the stress intensity factor in the thickness of the plate, resulting in a lower fatigue life. Therefore, when prestressing is applied, for maximum efficiency, extra care must be given to the equality of the prestress levels of the patches on both sides of the plate. The crack propagation starts from the edge of the notches and grows toward the edges of the plate. The notches conduct the direction of the growth by creating high stress concentration points. Figure 9 shows the stress distribution in an unreinforced CNS plate. The cracks are modelled using the newly-introduced fracture mechanics capability in ANSYS. An analysis has also been done in ANSYS to measure the effect of prestressing on the stress intensity 16
factors under the same experimental loads without considering the effect of CFRP. As seen in Figure 10 with increase in the prestress level the stress intensity factor reduces. This is while as the crack grows the stress intensity factor increases. This graph is a result of a parametrical analysis in ANSYS. As mentioned these results do not take into account the effect of the CFRP as the prestress is directly applied to the steel plate. Figure 8. Measurement of the crack propagation vs. number of cycles using image processing Matlab program a) D1LP12.5-Load(I), b) D2L-Load(I), c)D1LP25- Load(II)
Figure 9.a)CNS plate modelled in Ansys with defined cracks; Stress distribution of CNS plate under b) load(I) and c)load(II) (crack length=2a=15)
Figure 10. Effect of prestress level on the stress intensity factors of an unreinforced steel plate with crack growth
The crack propagation rate was slow at the beginning of the test and then increased in all specimens with the growth of the crack. In the specimens in which the notch was covered by a patch the rate was even slower. Prestressed patches also contribute to slowing the crack growth rate by affecting the stress intensity factor. As explained earlier, the beach marking technique is a method used for measuring the amount of crack propagation versus the number of cycles. In the double block fatigue load that is applied to the specimen, the first block contains a high amplitude load which causes the crack growth. The second block is an auxiliary load which creates striations on the crack front. Hence, after a specific number of the first type of load cycles (block 1), a specific number (much less than the first block) of the auxiliary cycles (block 2) is applied to the specimen, then the distance from the crack initiation point to the beach mark is measured. This technique is very accurate as it reflects the exact amount of the crack growth naturally on the crack front. However, what becomes important is finding the distance between the beach marks. For measuring this distance 17
(propagation) a Matlab [26] program has been used. The inputs of this program are a matrix containing the number of cycles in each block and a very high resolution photo. The high resolution photo helps in distinguishing the exact location of the beach mark from the point where the crack has initiated. Therefore at each beach mark two outputs are generated by the program: 1) the number of fatigue cycles to each beach mark; 2) the crack growth measured from the point of initiation (mm) to each beach mark (Figure 8). As stated, in the beach marking technique the second block of cycles cause the beach marks on the crack front. The more the blocks, the more the beach marks. Congestion of beach marks on the crack front increases the accuracy of the crack propagation versus number of cycles plot by recording the crack propagation for small intervals of crack growth. Therefore, in order to distinguish the beach marks and achieve accurate crack propagation curves, a high resolution photo of the crack front, the number of fatigue blocks and the number of cycles in each block is fed to the program. The crack propagation rate “da/dN” can be calculated, as it is equal to the slope of the crack propagation versus the number of cycles.
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Crack length versus number of cycles
In order to have a better understanding of the fatigue test results, the crack length versus fatigue life of the specimens were extracted from the beach mark locations and the cycle counter in the Multipurpose Testing software of the MTS device respectively. As explained previously, with the increase in the number of cycles, beach marks are produced on the crack plane of the steel plate. The number of cycles applied between each bold beach mark is the number of cycles required to propagate the crack equal to the distance between the marks. As explained earlier, the measurements were made using a Matlab image processing program for the sake of accuracy.
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The graphs were then plotted and the results are shown in Figure 11 and Figure 12. The peaks in these graphs show the total fatigue life at failure. The vertical axis is based on half crack length. This is assuming the total crack length is equal to 2a due to symmetry. The results show a rational sequence in both graphs (Figure 11 and Figure 12). With the increase of number of CFRP layers, the fatigue life increases, as expected. This effect can be described with Paris Law (Eq. 1). This equation shows that lower stress intensity factors result in higher fatigue lives. The stress intensity factor itself is a function of the applied stress Eq. (2) F √
(2)
Where; a is half of the crack length, F is the correction factor σ is the stress As the number of layers of CFRP increases, the stress level on the steel drops, resulting in an extended fatigue life. The specimens strengthened with CFRP exhibit a quite high increase in their remaining cracked life. A single layer of CFRP on each side increases the life about three times. An additional layer will increase the life almost 3.5 times. Increasing the layers to three, will result in an increase of almost 5 times compared to the fatigue life of the bare plate. When prestress is applied this ratio changes significantly. As presented in Table 5 and Table 6, prestress can increase the results from 4 to 35 times those of the unrepaired plate, depending on the number of layers used in the patch and the prestress level applied. 19
Comparing Figure 11 and Figure 12 , the effect of loading can be explained. As the load level is increased from 81-8.1kN to 92-9.2kN, the fatigue life decreases as a result of increasing stress intensity factors at the crack tips. The average ratio of the fatigue life of the specimens without prestress under a stress range of 150MPa (load (II)) compared to the specimens tested under load (I) with the stress range of 135MPa is 0.6. The effect of prestress seems to change this ratio due to change in the stress intensity factors.
From the column charts shown in Figure 13(a) and (b), the effect of CFRP layers and prestress can be observed. Chart (a) shows the results of the specimens under Load (I) and chart (b) is a result of Load (II) applied to the same specimens. This chart shows that the effect of prestressing in extending the fatigue life is much higher than adding additional CFRP layers. Figure 11. Crack propagation vs. number of cycles under load (I) for: a) All specimens, b) close up view (all except D2LP50)
These specimens were retrofitted with the same patches as the rest of the specimens. The main difference was that the patch was cured while under tension. Enhancement in the fatigue life of these specimens is considerably greater than that in the equivalent nonprestressed specimens (see Figure 11 and Figure 12). The specimens compared had the same crack geometry and were all fatigued under the same loading conditions. Figure 12. Crack propagation vs. number of cycles under load (II) for: a) All specimens, b) close up view (all except D2LP50)
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Specimen D2LP50 was prepared with a 50kN prestressed CFRP being applied to the CNS plate. Under load (I) and load (II) it exhibits a cracked life of 7,704,456 and 2,867,178 respectively. Comparing Figure 11(a) with Figure 11(b) and Figure 12(a) with Figure 12(b) shows the effectiveness of prestress levels in the retardation of crack growth.
Figure 13. Effect of CFRP strengthening and prestressing on fatigue life; a) under load (I), b) under load (II)
In [6], CFRP patches were used to strengthen 10mm thick steel plates. Comparing the results in this paper with those reported in [6], the effect of metal thickness can to some extent be explained. The CFRP patches have more effect on the stress reduction of the thinner plates. If we compare the unrepaired CNS plates in these two studies, the results are almost the same, showing that under the same stress level (amplitude=135MPa) the life cycles are equal. Wu et al. [8] also reported a similar fatigue life for un-strengthened 10mm plates under the same stress amplitude.
7
Conclusions
The outcomes of this paper are the result of an experimental program to study the benefit of the active effect of CFRP patches rather than their passive effect. In this research the effectiveness of prestressing on enhancing the fatigue life of steel members has been clearly established. •
Comparison of the results of the strengthened specimens with those of the bare plate reveals that an increase of 3 times is seen when the specimen is strengthened with 1 single layer of normal modulus CFRP sheet. 21
•
Adding an additional layer of CFRP increases the fatigue life of the plate up to 30%.
•
Prestressing the CFRP patches before attaching them to the notched steel plates has a considerable effect on their fatigue life. Comparing D2L50 with D2L represents an increase in the cracked life of about 10 times and 6.5 times under load (I) and load (II) respectively, which shows the difference between a prestressed specimen versus a non-prestressed specimen.
•
The experiments show that increasing the stress range by about 15% shortens the fatigue life by almost by %60 for the specimens benefitting from the passive effect only while under the same condition the life of specimens strengthened under a prestressed configuration is reduced %40 on average.
•
The crack propagation versus number of cycles for the cracked steel plates is shown in Figure 11 and Figure 12. These graphs are drawn in accordance with the beach markings on the crack front using an image processing Matlab program. The results show that this beach marking technique is more reliable in tracing crack propagation than methods based on visual inspection.
•
The prestressing technique used in this research was proven to be effective. This may mean that using actuators for application of prestress can be an effective tool to be used in future research as it reliable and simplifies the loading and unloading stages.
•
For specimens with the same number of layers, as the prestress increases, the crack growth rates that are represented by the slopes of the curves are lower at a given crack length, increasing the crack life of the specimen.
•
Effective and correct use of prestressing in general can lead to increased fatigue life of cracked members by applying a compressive force to the crack edges. This compressive force will impose artificial crack closure on the flawed structure. It must 22
be kept in mind that in certain cases where double-sided repair is not possible due to limited access or for any other reason, the feasibility of prestress must be studied. This is because application of the prestress force on one side can cause significant bending moments to the member leading to unforeseen failure.
8
Acknowledgements
The contributions and assistance of the technical staff in the Smart Structures Laboratory at Swinburne University of Technology is gratefully acknowledged. This research was partially sponsored by the Australian Research Council through a Discovery grant (DP1095466). The authors would also like to acknowledge Ms Fatemeh Jalali for her invaluable assistance in the experiments.
9 [1] [2]
[3]
[4]
[5] [6]
[7]
[8]
References U. Meier, "Composite Materials in Bridge Repair," Applied Composite Materials, vol. 7, pp. 75-94, 2000/05/01 2000. P. S. Song and Y. L. Shieh, "Stop drilling procedure for fatigue life improvement," International Journal of Fatigue, vol. 26, pp. 1333-1339, 12// 2004. M. Tavakkolizadeh and H. Saadatmanesh, "Fatigue strength of steel girders strengthened with carbon fiber reinforced polymer patch," Journal of Structural Engineering, vol. 129, pp. 186-196, 2003. J. Aidoo, K. A. Harries, and M. F. Petrou, "Fatigue behavior of carbon fiber reinforced polymer-strengthened reinforced concrete bridge girders," Journal of Composites for Construction, vol. 8, pp. 501-509, 2004. A. Baker, "Bonded composite repair of fatigue-cracked primary aircraft structure," Composite Structures, vol. 47, pp. 431-443, 12// 1999. H. Liu, R. Al-Mahaidi, and X.-L. Zhao, "Experimental study of fatigue crack growth behaviour in adhesively reinforced steel structures," Composite Structures, vol. 90, pp. 12-20, 2009. M. Bocciarelli, P. Colombi, G. Fava, and C. Poggi, "Fatigue performance of tensile steel members strengthened with CFRP plates," Composite Structures, vol. 87, pp. 334-343, 2009. C. Wu, X.-L. Zhao, R. Al-Mahaidi, M. R. Emdad, and W. Duan, "Fatigue tests of cracked steel plates strengthened with UHM CFRP plates," Advances in Structural Engineering, vol. 15, pp. 1801-1816, 10/01/ 2012. 23
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[25]
[26]
Q.-Q. Yu, X.-L. Zhao, R. Al-Mahaidi, Z.-G. Xiao, T. Chen, and X.-L. Gu, "Tests on Cracked Steel Plates with Different Damage Levels Strengthened by CFRP Laminates," International Journal of Structural Stability and Dynamics, vol. 14, p. 1450018, 2014/08/01 2014. "MATLAB," ed. Natick, MA: The MathWorks, Inc., 2013.
25
26
27
28
29
30
31
33
34
35
36
37
38
39
Table 7. Steel mechanical properties
CNS plates 1
Tensile Yield1 [MPa]
Elastic Modulus1 [MPa]
Plate Thickness1 [mm]
Poisson's Ratio2
335
212000
6
0.3
These properties are the average properties of 3 coupon specimens. 2
This value is assumed.
Table 8. Mechanical properties of the normal modulus CFRP
MBrace CF 230/4900
Description
Fibre reinforcement Fibre density (minimum) Fibre modulus MBrace fibre thickness Ultimate tensile strength
Carbon-high tensile 1.76 g/cm3 230 GPa 0.17 4900 MPa
Table 9. Mechanical properties of Araldite 420 [6]
Adhesive
Tensile Strength [MPa]
Shear Strength [MPa]
Ultimate Strain
Elastic Modulus [MPa]
Shear Modulus [MPa]
Poisson's Ratio
Araldite 420
28.6
36
0.024
1901
700
0.36
Table 10: Specimen list
#
Description
1 Reference plate (Non-reinforced) 2 Double-sided repair, 1 Layer of CFRP 3 Double-sided repair, 2 Layers of CFRP 4 Double-sided repair, 3 Layers of CFRP 5 Double-sided repair, 1 Layer of prestressed CFRP 6 Double-sided repair, 1 Layer of prestressed CFRP 7 Double-sided repair, 2 Layers of prestressed CFRP 8 Double-sided repair, 2 Layers of prestressed CFRP 9 Double-sided repair, 2 Layers of prestressed CFRP #L: Number of layers P##: Prestress level (kN)
40
Prestress Level Fatigue (kN) Load I,II 0 I,II 0 I,II 0 I,II 0 I 12.5 I,II 25 I 18 I,II 25 I,II 50 D: Double-sided repair
Specimen BP D1L D2L D3L D1LP12.5 D1LP25 D2LP18 D2LP25 D2LP50
Table 11: Specimen fatigue results (stress range= 135 MPa)
Prestress* Description
Prestress level (kN)
Stress level on CFRP (MPa)
BP D1L D2L D3L D1LP12.5 D1LP25 D2LP18 D2LP25 D2LP50
N/A 0 0 0 12.5 25 18 25 50
N/A 0 0 0 750 1500 560 750 1500
Stress level on steel (MPa) N/A 0 0 0 23.15 46.3 33.33 46.3 92.6
Failure mode of repaired specimens
Number of loading cycles to failure
Fatigue life extension ratio
Fibre breakage Debonding Debonding Debonding Fibre breakage Debonding Debonding Debonding
218,613 632,761 786,722 1,029,987 869,950 1,574,483 1,115,308 1,753,764 7,704,456
1 2.89 3.59 4.71 3.98 7.20 5.10 8.02 35.24
*Stress levels are calculated based on the gross area of an un-cracked and un-repaired specimen.
Table 12: Specimen fatigue results (stress range= 150 MPa)
Prestress* Description
Prestress level (kN)
Stress level on CFRP (MPa)
BP D1L D2L D3L D1LP25 D2LP25 D2LP50
N/A 0 0 0 25 25 50
N/A 0 0 0 1500 750 1500
Stress level on steel (MPa) N/A 0 0 0 46.3 46.3 92.6
Failure mode of repaired specimens
Number of loading cycles to failure
Fatigue life extension ratio
Fibre breakage Debonding Debonding Fibre breakage Debonding Debonding
133,368 362,554 441,187 703,384 608,857 786,345 2,867,178
1 2.72 3.31 5.27 4.57 5.90 21.50
*Stress levels are calculated based on the gross area of an un-cracked and un-repaired specimen.
41
S0263-8223(14)00663-1 http://dx.doi.org/10.1016/j.compstruct.2014.12.007 COST 6067
To appear in:
Composite Structures
Please cite this article as: Emdad, R., Al-Mahaidi, R., Effect of prestressed CFRP patches on crack growth of centrenotched steel plates, Composite Structures (2014), doi: http://dx.doi.org/10.1016/j.compstruct.2014.12.007
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of prestressed CFRP patches on crack growth of centre-notched steel plates Reza Emdad1, Riadh Al-Mahaidi1 1
Swinburne University of Technology, Hawthorn, VIC 3122, Australia, (*corresponding author: [email protected]) ABSTRACT
Carbon fibre reinforced polymers (CFRPs) are now accepted in the retrofit and repair industry as extremely efficient and capable of compensating for the loss of structural integrity, resistance and serviceability. Nevertheless, the application of these materials can be improved in various ways to meet structural requirements. One of the methods of improving the usage of CFRP sheets and laminates and benefiting from their full capacity is prestressing. The fatigue performance of steel plates retrofitted with CFRP patches was investigated in this research. The test samples included three main types: reference centre-notched steel plates (unrepaired), repaired centre-notched steel plates using CFRP patches and repaired centrenotched steel plates using prestressed CFRP patches. In the specimens in which prestressed CFRP patches were used, the patches consisted of 1 or 2 layers of CFRP sheets and were prestressed up to 25% or 50% of their ultimate capacity. All specimens were then tested under uniaxial tensile fatigue. In order to measure the crack propagation rate, the beach marking method was used. Two different double-block fatigue loads were studied: one causing a mean stress of 135MPa and the other with a mean stress of 150 MPa relative to the gross section of the steel plate.
KEYWORDS Fatigue, Steel, Crack propagation, Retrofitting, CFRP, Prestressing
1
INTRODUCTION
One of the main modes of failure in metallic structures is crack propagation due to fatigue. In order to overcome this catastrophic phenomenon different methods have been proposed. Some of the methods are: drilling stopper holes, welding cover plates (Eurocode 3 and AASHTO), and composite patching [1]. Researchers in this field have studied crack propagation in steel and aluminium members by performing the common tensile fatigue testing of notched plates. Efforts are made to slow the crack propagation rate or achieve crack arrest. One method of crack growth retardation is drilling holes, known as crack stoppers. This method removes crack tip singularity by crack-tip blunting and increases the fatigue life [2]. The issue with this method is that it requires exactness in locating the crack tip; otherwise a misplaced stop hole can make the case even worse. Another problem with this method is that the hole itself might cause a change in the strength of the section or initiate new cracks. In an early experimental research performed by Tavakolizadeh et al. [3]
the fatigue
behaviour of steel girders strengthened with CFRP patches was evaluated and showed that the design characteristics of structural steel could be improved using epoxy-bonded CFRP laminates. Aidoo et al. [4] used externally-bonded FRP reinforcement on the tensile surfaces of concrete girders. Their results showed that the fatigue life of such girders can be improved using FRP patches. In both of the aforementioned studies the fatigue behaviour of a structural member is studied. These researches show the effectiveness of CFRP strengthening on the fatigue life of structural members and were a start point of more research in this field. Before finding its way into civil engineering applications, this method was primarily tested and used in the aerospace industry for aircraft repairs [5].
2
The patch repair method which has been utilised by researchers is strengthening the cracked member with fibre reinforced polymers [6-10]. Some researchers have proved the fatigue resistance of the bond under fatigue loading is even better than the steel cover plates (Eurocode 3) welded to the defected member [7, 11]. Composite patch repairs are clearly less damaging than the conventional methods of repair based on drilling holes in or attaching external metallic patches to the original structure. Experimental and numerical analysis of double strap joints under axial load was performed by Fawzia et al. [13]. The dominant failure mode was found to be the bond between the CFRP and steel. Zhao et al. [14] expressed the observed modes of failure of CFRPstrengthened steel structures. They investigated different experiments considering the effect of different adhesives and steel sections such as plates and hollow sections in compression tension and bending. Many researchers have studied the propagation of cracks in steel and aluminium under fatigue. Baker [15], Cheuk et al. [16] Colombi et al. [17], Lee et al. [18], Hosseini-Toudeshky [19], Sabelkin et al. [20], Liu et al. [6] Taljsten et al. [9] and Wu et al. [8] have all investigated the patch repair technique in retrofitting of steel or aluminium plates. Tensile fatigue testing of notched plates is a common test performed by these researchers. Baker [15] experimentally investigated fatigue crack propagation in cracked aluminium alloy repaired with Baron/epoxy patches. Mainly focussing on aircraft structures, he found this technique to be efficient and less damaging to the original structure [5]. Cheuk et al. [16] studied fatigue crack initiation and growth in metal-to-composite bonded joints both experimentally and numerically. The fatigue tests performed were all tension-dominated, and the crack propagation was measured optically. Colombi et al. [17, 21] considered the effect of prestressing on crack growth under a load range of 160kN to 400kN causing a stress amplitude of 80MPa. The CFRP strips were attached to the sides of the notch not covering 3
the crack. The debonding pattern around the crack was also investigated by them. Lee et al. [18], Hosseini-Toudeshky [19] and Sabelkin et al. [20] investigated single-side repaired aluminium plates. When single-side repair is used the crack growth rate would become slower on the repaired side compared to the unrepaired face, creating a non-uniform beach mark on the crack front. Liu et al. [22] tested double-strap joints under fatigue loading to measure the bond strength under cyclic loading. These researchers [6] also tested centrenotched steel (CNS) plates retrofitted with CFRP patches under tensile cyclic fatigue. They tested several repair configurations and improved the fatigue life up to 8 times that of unrepaired steel plate. They investigated 10mm plates and achieved this result with 5 layers of high modulus CFRP covering the whole width of the plate with a bond length of 250mm. Taljsten et al. [9] also showed the benefit of prestress. They tested 8mm steel plates under a tensile cyclic loading of 175kN and 15kN generating a stress of 97.5MPa (considering the cross section of an unrepaired specimen). CFRP laminates were used with a similar repair configuration as in [17] in which the crack was not covered. Wu et al. [8] utilised ultra-high modulus CFRP laminates in order to enhance the fatigue life of cracked steel plates. Several configurations were used and compared. The results showed that where accessible, covering the crack with CFRP has a better influence in the repair mechanism. Experiments performed to date have utilised different fibre reinforced sheets and laminates with different elasticity moduli. The effectiveness of the elasticity modulus in the repair of cracks in metallic members has been pointed out by many researchers and can be concluded from their experimental findings [6, 8, 9]. The benefits of the patch repair method for metallic cracks include the high strength-toweight ratio, the modulus of elasticity of CFRP patches which is comparable to that of steel, corrosion-proofing the parent structure, easy handling and shaping of the patches and fatigue endurance. The capability of being prestressed can be also added to its merits. Effective 4
prestressing of the patches can increase the remaining fatigue life (cracked life) of repaired metallic structures. Reduction of stress intensity factor occurs in the vicinity of the crack when benefitting from the passive effect of the patches. However, when prestressing is used the stress intensity factor decreases even more due to the introduction of a compressive force pushing the edges of the crack closer together. Previous findings indicate the enhancement of the remaining fatigue life of metallic structures which have undergone composite patch repairs. However, in order to achieve a unified model and guidelines in this area more experiments and numerical validation are required. The experiments carried out in this research are similar to those reported in [6]. The main difference is that instead of increasing the number of CFRP sheets in the patch, prestressing was applied. Another difference is that 6mm thick plates were utilised, while [6] used 10mm thick plates. This paper covers an experimental investigation on the repair of cracked steel plates. The specimens were tested under two different types of proportional cyclic loads differing in the stress amplitude.
2
2.1
Materials and Specimens
Material Properties
Coupon tests were performed to verify the mechanical properties of the steel. The steel plates were all 6mm thick, and all steel plates used in this study were from the same batch. These properties were calculated in accordance with ASTM E8/E8M-11[23] and the results are shown in Table 1. Normal modulus CFRP sheets (CF230) supplied by BASF were used in this research. The mechanical properties as per the manufacturers’ datasheet are shown in Table 2.
5
Araldite 420 was used as the adhesive to bond the CFRP to the steel surface. The properties of the adhesive were verified by Fawzia et al. [13] using a series of coupon tests (refer to Table 3). Table 1. Mechanical properties of steel
Table 2. Mechanical properties of the normal modulus CFRP
Table 3. Mechanical properties of Araldite 420 [13]
2.2
Specimens
Three types of specimens were tested in this research program: •
CNS plates without repair
•
Double-side patch repaired CNS plates
•
CNS plates repaired with prestressed patches (double-sided)
2.2.1 CNS plates without repair The focus of this research was to measure crack propagation versus number of cycles. Therefore, CNS plates were utilised. As mentioned above, the steel plates used in this study were 6mm thick. All steel plates had a hole in the centre and initial cracks each 1mm long starting from the two edges of the hole were introduced. The dimensions of the plates and the notches are shown in Figure 1.
6
Figure 1. a)TH=6mm Steel specimen dimensions and notch detail (NTS), b)Double-sided CFRP strengthening configuration for all specimens
At least 4 strain gauges were attached to each specimen. Two of the strain gauges located on the steel plate (opposite face and opposite side of the crack) 10mm from the edges at the centre of the CNS plate and the other two were attached each on the CFRP on both sides of the specimen approximately located on the centre of the CFRP. Readings from the gauges were used to prove that the loading was concentric. The locations of the strain gauges on the CNS plate are shown in Figure 1. 2.2.2 Double-sided patch repaired CNS plates The same CNS plates were used and repaired using a double-sided patch configuration, as shown in Figure 1(a). The repair configuration included 1, 2 & 3 layer patch retrofitting. The CFRP layers were all 250mm long and 50mm wide. The specimen dimensions and repair configurations were designed based on previous studies [6], [8]. 2.2.3 CNS plates repaired with prestressed patches In order to benefit from the active effect of the CFRP patches rather than their passive effect, already studied by Liu et al. [6], this set of specimens was repaired using prestressed patches. As explained later, in order to prestress the patches a hydraulic actuator was used. The patches on both sides of the plates were prestressed simultaneously. Specimens were prepared using the same configuration as shown in Figure 1(b) with different prestress levels to measure the effect of prestress on their fatigue life.
2.3
Specimen Preparation
In order to attach the CFRP patches to the steel plate and benefit from an effective bond, surface preparation is required. Therefore, all steel plates were sandblasted and cleaned with acetone. This removes the rust and cleans the surface of dust and grease. Araldite 420, a two7
part structural adhesive, was used to bond the patch and the steel. This adhesive has shown a great influence on the bond behaviour of CFRP-strengthened steel members ([13], [6], [8]). Previous studies have recommended Araldite 420 for its workability and pot life as well as its high shear strength.
2.4
Prestressing
A prestressing technique was developed for this study in the SmartLAB at Swinburne University of Technology. An MTS actuator was used to apply the prestress force on the CFRP patches. This method was found to be more accurate than using hydraulic jacks. The reason is that hydraulic jacks amplify the prestress loss over time as there are always unavoidable oil leaks despite using shut-off valves for temporary load holding, while using a servo hydraulic actuator in force control mode, will compensate the prestress loss during the curing process. The prestressing process has four main steps, as explained below. Step 1: Tensioning of the CFRP patch In this step the CFRP patch is gripped in the jaws. The tensile force is then slowly applied using a load control test procedure until the required level of prestress is reached (Figure 2(a). In the experiments carried out in this research the CFRP patches were prestressed up to 25 % or 50% of their ultimate strength. The maximum load that a one-layer patch with 50mm width can withstand is about 20-25kN. By increasing the number of layers to two, this value will be almost doubled.
8
Table 4 shows the prepared specimens with their names and descriptions. The prestress level reached is also provided.
Figure 2. Prestressing stages: a) applying tensile force to the patch. b)applying adhessive to the desired bond length c) cutting the excess CFRP patch.
Table 4: Specimen list
Step 2: Attaching the patch in tension to the CNS plate After the desired level of prestress is reached, the loaded patch is attached to the CNS plate with a 250mm bond length. This step must be performed carefully to ensure that the CFRP patch and the CNS plate are perfectly aligned. The centre of the patch is placed on the centre of the CNS plate so that the whole crack is covered symmetrically in both directions. As mentioned earlier, Araldite 420 was used for the means of bonding CFRP to steel. It must be noted that the surface preparation of the CNS plates was done as previously explained prior to this step. Step 3: Curing of the loaded specimen While the CFRP is under tensile load, the specimen is left for curing. According to the manufacturer’s datasheet for Araldite 420, the recommended time for curing is 7 days. In order to reach a faster curing time heat treatment could be used. The authors have used heat treatment in some previously tested specimens and have achieved similar results. In the results revealed in this study heat treatment for achieving faster cure times is not used, because the induced heat affects the strain gauge readings. Figure 3(a) shows the test setup and Figure 3(b) shows a specimen curing under load in the designed set-up. 9
Step 4: Unloading of the patch After the specimen is cured, the prestress load can be removed. Removing the load, results in partial dissipation of the tensile stress in the patch, as well as the build-up of compressive stress in the CNS plate. This causes the edges of the crack to become closer (artificial crack closure). The extra CFRP patch can then be cut, leaving the specimen as shown in Figure 2(c).
Figure 3. a) Designed test setup, b) Specimen curing under load in the designed prestressing set-up
3
FATIGUE LOADING
The specimens were tested under a tensile fatigue load in a closed-loop servo-hydraulic 250kN fatigue-rated MTS machine. Tensile fatigue loading for the study was set at the stress ratio of 0.1. The stress ratio is the ratio of the minimum stress to the maximum stress of the specimen during the fatigue test: ௫ 0.1 In order to measure the effect of loading on the fatigue life, two types of fatigue loads were used. The two sets of fatigue loads differed in the stress range. In the first set, the maximum force applied in the fatigue load block was 81kN (Figure 4(a)) and in the second load the maximum force applied in the fatigue block was 92kN (Figure 4(b)). The 81-8.1kN range applied a stress range of 135MPa and the 92-9.2kN load range applied a stress range of 153MPa on the specimens. All specimens were gripped at both ends, as shown in Figure 5. The fatigue load cycles were sinusoidal waveforms applied at the frequency of 10Hz.
10
Load I:
Load II: ௫ 150.0
௫ 170.3
15.0
17.0
௫ 135
௫ 153
Measuring the crack propagation rate was the main aim of this research. For this reason the beach marking method [6, 8, 18, 24] was chosen among the many other existing methods. Some of the popular methods applicable on patch repaired specimens being used, other than the beach marking technique are: use of crack gauges and using a displacement transducer or clip gauge. In the beach marking method the specimen will be loaded under a double block fatigue load repeatedly until failure. The first block is the main load which the crack propagation is the result of this block. The second block is used to produce beach marks at the end of each loading block (first block). The second block has a smaller amplitude than the first block therefore will not contribute much to the propagation of the crack. The beach mark on the crack front is a result of the sudden stress intensity drop due to reduction in the stress amplitude. Applying crack gauges to the expected crack line, has been used by [25] and the results have been reported very close to the beach marking technique. In this method the gauge must be bonded to the steel were the crack propagation is expected. This method is suitable for plain steel members. When the steel member is strengthened with CFRP, proper care and action must be taken to ascertain the bond between the CFRP and steel is not weakened at this critical location of the crack gauge which is the initial point of debonding.
11
Benefitting from displacement transducers and clip gauges is also another method that has been used by [9] to measure the crack opening displacement (COD). The crack propagation lengths are then calculated based on the measured COD. There are more methods that have been used in literature. Some of the methods are not suitable for application on members strengthened with CFRP patches. Each of the methods discussed earlier have their own advantages. The beach marking technique was preferred in this study for the reason that it is consistent for all specimens and simple to apply. Therefore, errors induced by technical instruments are minimised. As mentioned earlier, in order to benefit from the beach marking technique, a double-block fatigue load was used. As shown in Figure 4 the second block has a stress range equal to half of that of the first block with double the frequency, to create marks on the crack front. The created marks were then used for crack propagation measurements. All of the specimens were tested until failure.
Figure 4. Double-block fatigue load for measuring crack propagation; a) Load I: Stress range of 135MPa, b) Load II: Stress range of 153MPa Figure 5. Specimen gripped at both ends while under fatigue in the MTS machine
When the specimens were loaded, the notch which represented the crack initiation grew. The crack growth started from the centre of the specimen and propagated all the way through the width of the plate.
3.1
Strain measurements
As mentioned previously, the prestressing technique has four main stages. Strain gauges were attached to the CFRP and CNS plates in order to ascertain symmetry in the prestressing stage and while under cyclic fatigue loading. 12
Average values of the strains during the prestressing stage on the CFRP are shown in Figure 6(a). After the CFRP is attached to the steel and a bond is fully formed, as the CFRP is unloaded, the tensile strain in the CFRP is transferred to the CNS plate, building up compressive strain in the steel. Figure 6(b) and (c) show the strain values on the CFRP and the CNS plate during unloading respectively. As the results indicate, the final strain on the CNS plate matches the amount of compression load applied. Figure 6. a) Prestressing stage: Average value of strain in CFRP, b) Unloading stage: CFRP Relaxation, c) Unloading stage: Compressive strain build-up in CNS plate
Another factor which is important is the durability of the prestress in order to ascertain that the prestress will remain without any loss under different environmental and loading conditions. This was not within the scope of the present research. However, the authors measured the strains of the samples for 7 days in order to ensure that no further loss occurred. Loading conditions are other factors which can affect the stored tension. As mentioned in the next sections, despite the high frequency tensile loads applied in the experiments, the prestress effect persisted until the final cycles.
4
4.1
Results and discussion
Mode of failure
4.1.1 Steel After testing all the specimens until failure under fatigue, the crack fronts of the specimens were studied. With the aid of the beach marking technique, crack propagation was measured 13
versus the number of cycles. The crack fronts of some of the specimens are shown in Figure 7. As the cycles increase, the crack starts propagating from the edge of the existing notch. The crack grows symmetrically towards the edges of the plate. The crack growth region is smooth at the beginning, representing very small crack tip yielding and stable crack growth. When the crack reaches the edges of the plate, a catastrophic fracture is seen due to the fracture of the steel plate Figure 7(a) and (f). The same failure mode is seen in the CFRP-strengthened CNS plates. The difference is in the number of cycles and the length of the stable crack growth region. As shown in Figure 7(b), (c) and (d), the CFRP-strengthened plates have a larger stable crack growth length, which results in a smaller fractured edge of the steel. This can be explained as a decrease in the stress levels at the crack and exhibits the capability of the CFRP in controlling the crack growth rate and its direct effect on reducing the stress in the vicinity of the crack.
Figure 7. Failure modes of specimens tested under tensile fatigue load
In CNS plates strengthened with prestressed CFRP patches, the same modes of failure occur, and the experiments showed that the catastrophic failure region is even smaller. In specimen D2LP50, this region does not exist (Figure 7(d)). This may mean that the effect of prestress exists even after the crack has grown all the way through the width of the plates. In other words, the crack growth increases as the debonded region expands, but the effect of prestress holds the two edges of the crack together up to the last cycle. This shows how effective prestress can be, with its influence lasting even to the final cycles.
14
4.1.2 CFRP As understood from the experiments, two main types of failure occur in the patch: fibre breakage and debonding. From what has been discovered in the experiments, when the patch consists of one layer of CFRP, the failure mode is fibre breakage (Figure 7(b)). Furthermore, if the patch has more than one layer, the failure mode changes to debonding (Figure 7(c)). 4.1.3 Fatigue life evaluation The experimental results are shown in Table 5 and Table 6. The fatigue life is measured based on the number of cycles to failure for each specimen. The last column represents the fatigue life extension. This value is the ratio of the fatigue life of each specimen to the fatigue life of the bare plate. This column also shows the capability of CFRP patches (with/without prestress) in the enhancement of the fatigue life of cracked plates. Table 5 shows the results of the tests under cyclic load I (Figure 4(a)). Table 6 shows the cracked life of the CNS plates under cyclic load II (Figure 4(b)). The results show that fatigue life can be enhanced 3 to 35 times with CFRP patch repair and using the prestressing technique.
Table 5. Specimen fatigue results (stress range=135 MPa)
Table 6. Specimen fatigue results (stress range=150 MPa)
Comparison of the results of Table 5 and Table 6 shows that as the stress range increases the fatigue life undergoes a decrease. This can be explained using the well-known Paris Law Eq. (1):
15
∆
(1)
In this equation: ௗ ௗே
is the rate of crack propagation per cycle
C and m are both material constants ∆ is the stress intensity factor variation in each cycle
5
Crack Propagation
The beach marks of some of the tested specimens are shown in Figure 7(e) and (f) and Figure 8. As expected, the beach marks were symmetrical on both sides of the hole. Since both sides of the plate are reinforced (Figure 7(e)), the crack propagation is uniform, whereas in singlesided repairs the crack growths on the repaired and unrepaired sides are different [18]. The uniformity of the beach mark curves in the prestressed specimens confirms that the patches on both sides of the plate were prestressed up to the same level. If this were not the case, an undesirable out-of-plane bending effect would have been applied, causing variation in the stress intensity factor in the thickness of the plate, resulting in a lower fatigue life. Therefore, when prestressing is applied, for maximum efficiency, extra care must be given to the equality of the prestress levels of the patches on both sides of the plate. The crack propagation starts from the edge of the notches and grows toward the edges of the plate. The notches conduct the direction of the growth by creating high stress concentration points. Figure 9 shows the stress distribution in an unreinforced CNS plate. The cracks are modelled using the newly-introduced fracture mechanics capability in ANSYS. An analysis has also been done in ANSYS to measure the effect of prestressing on the stress intensity 16
factors under the same experimental loads without considering the effect of CFRP. As seen in Figure 10 with increase in the prestress level the stress intensity factor reduces. This is while as the crack grows the stress intensity factor increases. This graph is a result of a parametrical analysis in ANSYS. As mentioned these results do not take into account the effect of the CFRP as the prestress is directly applied to the steel plate. Figure 8. Measurement of the crack propagation vs. number of cycles using image processing Matlab program a) D1LP12.5-Load(I), b) D2L-Load(I), c)D1LP25- Load(II)
Figure 9.a)CNS plate modelled in Ansys with defined cracks; Stress distribution of CNS plate under b) load(I) and c)load(II) (crack length=2a=15)
Figure 10. Effect of prestress level on the stress intensity factors of an unreinforced steel plate with crack growth
The crack propagation rate was slow at the beginning of the test and then increased in all specimens with the growth of the crack. In the specimens in which the notch was covered by a patch the rate was even slower. Prestressed patches also contribute to slowing the crack growth rate by affecting the stress intensity factor. As explained earlier, the beach marking technique is a method used for measuring the amount of crack propagation versus the number of cycles. In the double block fatigue load that is applied to the specimen, the first block contains a high amplitude load which causes the crack growth. The second block is an auxiliary load which creates striations on the crack front. Hence, after a specific number of the first type of load cycles (block 1), a specific number (much less than the first block) of the auxiliary cycles (block 2) is applied to the specimen, then the distance from the crack initiation point to the beach mark is measured. This technique is very accurate as it reflects the exact amount of the crack growth naturally on the crack front. However, what becomes important is finding the distance between the beach marks. For measuring this distance 17
(propagation) a Matlab [26] program has been used. The inputs of this program are a matrix containing the number of cycles in each block and a very high resolution photo. The high resolution photo helps in distinguishing the exact location of the beach mark from the point where the crack has initiated. Therefore at each beach mark two outputs are generated by the program: 1) the number of fatigue cycles to each beach mark; 2) the crack growth measured from the point of initiation (mm) to each beach mark (Figure 8). As stated, in the beach marking technique the second block of cycles cause the beach marks on the crack front. The more the blocks, the more the beach marks. Congestion of beach marks on the crack front increases the accuracy of the crack propagation versus number of cycles plot by recording the crack propagation for small intervals of crack growth. Therefore, in order to distinguish the beach marks and achieve accurate crack propagation curves, a high resolution photo of the crack front, the number of fatigue blocks and the number of cycles in each block is fed to the program. The crack propagation rate “da/dN” can be calculated, as it is equal to the slope of the crack propagation versus the number of cycles.
6
Crack length versus number of cycles
In order to have a better understanding of the fatigue test results, the crack length versus fatigue life of the specimens were extracted from the beach mark locations and the cycle counter in the Multipurpose Testing software of the MTS device respectively. As explained previously, with the increase in the number of cycles, beach marks are produced on the crack plane of the steel plate. The number of cycles applied between each bold beach mark is the number of cycles required to propagate the crack equal to the distance between the marks. As explained earlier, the measurements were made using a Matlab image processing program for the sake of accuracy.
18
The graphs were then plotted and the results are shown in Figure 11 and Figure 12. The peaks in these graphs show the total fatigue life at failure. The vertical axis is based on half crack length. This is assuming the total crack length is equal to 2a due to symmetry. The results show a rational sequence in both graphs (Figure 11 and Figure 12). With the increase of number of CFRP layers, the fatigue life increases, as expected. This effect can be described with Paris Law (Eq. 1). This equation shows that lower stress intensity factors result in higher fatigue lives. The stress intensity factor itself is a function of the applied stress Eq. (2) F √
(2)
Where; a is half of the crack length, F is the correction factor σ is the stress As the number of layers of CFRP increases, the stress level on the steel drops, resulting in an extended fatigue life. The specimens strengthened with CFRP exhibit a quite high increase in their remaining cracked life. A single layer of CFRP on each side increases the life about three times. An additional layer will increase the life almost 3.5 times. Increasing the layers to three, will result in an increase of almost 5 times compared to the fatigue life of the bare plate. When prestress is applied this ratio changes significantly. As presented in Table 5 and Table 6, prestress can increase the results from 4 to 35 times those of the unrepaired plate, depending on the number of layers used in the patch and the prestress level applied. 19
Comparing Figure 11 and Figure 12 , the effect of loading can be explained. As the load level is increased from 81-8.1kN to 92-9.2kN, the fatigue life decreases as a result of increasing stress intensity factors at the crack tips. The average ratio of the fatigue life of the specimens without prestress under a stress range of 150MPa (load (II)) compared to the specimens tested under load (I) with the stress range of 135MPa is 0.6. The effect of prestress seems to change this ratio due to change in the stress intensity factors.
From the column charts shown in Figure 13(a) and (b), the effect of CFRP layers and prestress can be observed. Chart (a) shows the results of the specimens under Load (I) and chart (b) is a result of Load (II) applied to the same specimens. This chart shows that the effect of prestressing in extending the fatigue life is much higher than adding additional CFRP layers. Figure 11. Crack propagation vs. number of cycles under load (I) for: a) All specimens, b) close up view (all except D2LP50)
These specimens were retrofitted with the same patches as the rest of the specimens. The main difference was that the patch was cured while under tension. Enhancement in the fatigue life of these specimens is considerably greater than that in the equivalent nonprestressed specimens (see Figure 11 and Figure 12). The specimens compared had the same crack geometry and were all fatigued under the same loading conditions. Figure 12. Crack propagation vs. number of cycles under load (II) for: a) All specimens, b) close up view (all except D2LP50)
20
Specimen D2LP50 was prepared with a 50kN prestressed CFRP being applied to the CNS plate. Under load (I) and load (II) it exhibits a cracked life of 7,704,456 and 2,867,178 respectively. Comparing Figure 11(a) with Figure 11(b) and Figure 12(a) with Figure 12(b) shows the effectiveness of prestress levels in the retardation of crack growth.
Figure 13. Effect of CFRP strengthening and prestressing on fatigue life; a) under load (I), b) under load (II)
In [6], CFRP patches were used to strengthen 10mm thick steel plates. Comparing the results in this paper with those reported in [6], the effect of metal thickness can to some extent be explained. The CFRP patches have more effect on the stress reduction of the thinner plates. If we compare the unrepaired CNS plates in these two studies, the results are almost the same, showing that under the same stress level (amplitude=135MPa) the life cycles are equal. Wu et al. [8] also reported a similar fatigue life for un-strengthened 10mm plates under the same stress amplitude.
7
Conclusions
The outcomes of this paper are the result of an experimental program to study the benefit of the active effect of CFRP patches rather than their passive effect. In this research the effectiveness of prestressing on enhancing the fatigue life of steel members has been clearly established. •
Comparison of the results of the strengthened specimens with those of the bare plate reveals that an increase of 3 times is seen when the specimen is strengthened with 1 single layer of normal modulus CFRP sheet. 21
•
Adding an additional layer of CFRP increases the fatigue life of the plate up to 30%.
•
Prestressing the CFRP patches before attaching them to the notched steel plates has a considerable effect on their fatigue life. Comparing D2L50 with D2L represents an increase in the cracked life of about 10 times and 6.5 times under load (I) and load (II) respectively, which shows the difference between a prestressed specimen versus a non-prestressed specimen.
•
The experiments show that increasing the stress range by about 15% shortens the fatigue life by almost by %60 for the specimens benefitting from the passive effect only while under the same condition the life of specimens strengthened under a prestressed configuration is reduced %40 on average.
•
The crack propagation versus number of cycles for the cracked steel plates is shown in Figure 11 and Figure 12. These graphs are drawn in accordance with the beach markings on the crack front using an image processing Matlab program. The results show that this beach marking technique is more reliable in tracing crack propagation than methods based on visual inspection.
•
The prestressing technique used in this research was proven to be effective. This may mean that using actuators for application of prestress can be an effective tool to be used in future research as it reliable and simplifies the loading and unloading stages.
•
For specimens with the same number of layers, as the prestress increases, the crack growth rates that are represented by the slopes of the curves are lower at a given crack length, increasing the crack life of the specimen.
•
Effective and correct use of prestressing in general can lead to increased fatigue life of cracked members by applying a compressive force to the crack edges. This compressive force will impose artificial crack closure on the flawed structure. It must 22
be kept in mind that in certain cases where double-sided repair is not possible due to limited access or for any other reason, the feasibility of prestress must be studied. This is because application of the prestress force on one side can cause significant bending moments to the member leading to unforeseen failure.
8
Acknowledgements
The contributions and assistance of the technical staff in the Smart Structures Laboratory at Swinburne University of Technology is gratefully acknowledged. This research was partially sponsored by the Australian Research Council through a Discovery grant (DP1095466). The authors would also like to acknowledge Ms Fatemeh Jalali for her invaluable assistance in the experiments.
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Table 7. Steel mechanical properties
CNS plates 1
Tensile Yield1 [MPa]
Elastic Modulus1 [MPa]
Plate Thickness1 [mm]
Poisson's Ratio2
335
212000
6
0.3
These properties are the average properties of 3 coupon specimens. 2
This value is assumed.
Table 8. Mechanical properties of the normal modulus CFRP
MBrace CF 230/4900
Description
Fibre reinforcement Fibre density (minimum) Fibre modulus MBrace fibre thickness Ultimate tensile strength
Carbon-high tensile 1.76 g/cm3 230 GPa 0.17 4900 MPa
Table 9. Mechanical properties of Araldite 420 [6]
Adhesive
Tensile Strength [MPa]
Shear Strength [MPa]
Ultimate Strain
Elastic Modulus [MPa]
Shear Modulus [MPa]
Poisson's Ratio
Araldite 420
28.6
36
0.024
1901
700
0.36
Table 10: Specimen list
#
Description
1 Reference plate (Non-reinforced) 2 Double-sided repair, 1 Layer of CFRP 3 Double-sided repair, 2 Layers of CFRP 4 Double-sided repair, 3 Layers of CFRP 5 Double-sided repair, 1 Layer of prestressed CFRP 6 Double-sided repair, 1 Layer of prestressed CFRP 7 Double-sided repair, 2 Layers of prestressed CFRP 8 Double-sided repair, 2 Layers of prestressed CFRP 9 Double-sided repair, 2 Layers of prestressed CFRP #L: Number of layers P##: Prestress level (kN)
40
Prestress Level Fatigue (kN) Load I,II 0 I,II 0 I,II 0 I,II 0 I 12.5 I,II 25 I 18 I,II 25 I,II 50 D: Double-sided repair
Specimen BP D1L D2L D3L D1LP12.5 D1LP25 D2LP18 D2LP25 D2LP50
Table 11: Specimen fatigue results (stress range= 135 MPa)
Prestress* Description
Prestress level (kN)
Stress level on CFRP (MPa)
BP D1L D2L D3L D1LP12.5 D1LP25 D2LP18 D2LP25 D2LP50
N/A 0 0 0 12.5 25 18 25 50
N/A 0 0 0 750 1500 560 750 1500
Stress level on steel (MPa) N/A 0 0 0 23.15 46.3 33.33 46.3 92.6
Failure mode of repaired specimens
Number of loading cycles to failure
Fatigue life extension ratio
Fibre breakage Debonding Debonding Debonding Fibre breakage Debonding Debonding Debonding
218,613 632,761 786,722 1,029,987 869,950 1,574,483 1,115,308 1,753,764 7,704,456
1 2.89 3.59 4.71 3.98 7.20 5.10 8.02 35.24
*Stress levels are calculated based on the gross area of an un-cracked and un-repaired specimen.
Table 12: Specimen fatigue results (stress range= 150 MPa)
Prestress* Description
Prestress level (kN)
Stress level on CFRP (MPa)
BP D1L D2L D3L D1LP25 D2LP25 D2LP50
N/A 0 0 0 25 25 50
N/A 0 0 0 1500 750 1500
Stress level on steel (MPa) N/A 0 0 0 46.3 46.3 92.6
Failure mode of repaired specimens
Number of loading cycles to failure
Fatigue life extension ratio
Fibre breakage Debonding Debonding Fibre breakage Debonding Debonding
133,368 362,554 441,187 703,384 608,857 786,345 2,867,178
1 2.72 3.31 5.27 4.57 5.90 21.50
*Stress levels are calculated based on the gross area of an un-cracked and un-repaired specimen.
41