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Fatigue performances of the cracked aluminum-alloy pipe repaired with a shaped CFRP patch
Thin–Walled Structures 111 (2017) 155–164
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Fatigue performances of the cracked aluminum-alloy pipe repaired with a shaped CFRP patch ⁎
Jinchun Liua, Meijun Qina, Qilin Zhaob, Li Chena, , Pengfei Liub, Jiangang Gaoa a State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact, PLA University of Science and Technology, Nanjing, Jiangsu 210007, China b College of Field Engineering, PLA University of Science and Technology, Nanjing, Jiangsu 210007, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Aluminum-alloy pipe CFRP patch Fatigue Stress intensity factor Surface treatment
To fully understand performances of the cracked aluminum-alloy pipe repaired with CFRP patches, a series of fatigue and quasi-static tests were conducted on commercial available type-7005 pipes. The pipe damaged with artificial cracks was wrapped around by shaped CFRP patches. Two bonding surface treatment methods, i.e. screw-thread and mechanical grinding, were proposed and applied in the repairing process. The fatigue life, residual stiffness and cyclic creep of the repaired specimen were tested, respectively. A fine numerical model was built in ANSYS and validated by the test data. The stress intensity factor was calculated by the developed FE model to reveal the failure mechanism of fatigue tests. The influences of patch length and number of layers were also evaluated and discussed. Test results showed that the fatigue life of the wrapped specimen with the screwthread surface treatment was 22.18 times of that of the bare specimen, and 23.13% longer than that of specimen treated by mechanical grinding. It is concluded that the CFRP patch is capable of improving fatigue performances of the cracked aluminum-alloy pipe. The screw-thread surface treatment provides better fatigue effects than mechanical grinding. Optimized design in the shaped CFRP patch benefits achieving the best fatigue performances of repaired aluminum-alloy pipes.
1. Introduction Bonding with the composite material is a popular technology to repair the damaged aluminum-alloy components, which has the advantages of high quality, efficient and low cost. This method is capable of significantly improving fatigue performances of the repaired component. In recent years, the application of composite repairing technology has been widely extended in the engineering field, such as aviation, bridge, RC frame and so on [1–6]. Numerous experiments were carried out to study the cracked aluminum-alloy plate repaired with the FRP patches. Seo et al. [7] conducted numbers of tests on the fatigue performances of repaired type-7075-T6 plates. Test results indicated that the fatigue life of aluminum-alloy specimen increased by 3–5 times after being repaired with CFRP. Schubbe et al. [8] performed a test on the type-2024-T3 plate repaired with the boron/epoxy composite patch, aiming to explore the impacts on thick aluminum-alloy plates. It is concluded that fatigue properties of aluminum-alloy plates increased by 4.33– 7.12 times after being repaired. Denney et al. [9] also found that the fatigue life of cracked aluminum-alloy plate was extended by 900% after being repaired by the boron-epoxy patch. Brighenti et al. [10] ⁎
proposed a procedure to study the optimal shape of a patch repairing the cracked plate by a biology-based method. The procedure is implemented in a finite element code and validated by numerical simulations. Kumar et al. [11] proposed an optimum design of symmetric (balanced) composite patch to repair a center cracked aluminum plate after theoretical and numerical comparison. It is concluded that the skewed plan-form is the most optimum, which has the least volume. The order of merit of plan-form of composite patch is skewed, rectangular, elliptical, square and circular. Jones et al. [12] examined the crack growth history of a range of test plate and service cracking, for cracks repaired with a composite patch and shows that for small to medium crack lengths there is a linear relationship between the log of the crack length and the fatigue life. Sabelkin et al. [13] further studied the influences of debonding location and patch size on the fatigue life of cracked aluminum-alloy plates. In addition, Karatzas et al. [14] also carried out similar fatigue experiments on composite patch repaired steel plates with cracks. However, the abovementioned researches mainly focused on the plate, but not the structural component such as the aluminum-alloy pipes those are widely used in the manufacturing engineering. The pipelines in aircraft are basically constructed with aluminum-alloy
Corresponding author. E-mail address: [email protected] (L. Chen).
http://dx.doi.org/10.1016/j.tws.2016.11.008 Received 7 July 2016; Received in revised form 4 November 2016; Accepted 8 November 2016 0263-8231/ © 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. Detailed dimensions of the specimen.
machine produced by MTS company was employed to conduct the fatigue test. The input fatigue loading was a sine wave with the frequency of 5.6 Hz and stress ratio of 0.1, indicating that the maximum load was 90 kN. Fig. 2 shows the in-situ loading scenario of the fatigue test.
pipes [15]; and the load-bearing members in the military bridge are mostly supported by the aluminum-alloy pipes [16]. Actually, the stress mechanism of repaired aluminum-alloy pipe is different from that of repaired plate. The pipe specimen usually appears necking in the process of axial tension, whilst a patch not only shares the axial stress, but also provides a certain radial constraint to the inhibit necking. Nevertheless, the patch only shares the axial stress in repairing the aluminum-alloy plate. The shaped CFRP patch was employed in this study to repair the cracked type-7005 aluminum-alloy pipe. The screw-thread method was firstly proposed to treat the surface of cracked pipe, compensating some inherent deficiencies in traditional methods [17]. Systematical fatigue tests were conducted on the repaired specimens. The fatigue life and fatigue failure appearances were reported. Numerical analyses were also carried out in the commercial software ANSYS in order to real the failure mechanism. Optimized design parameters of CFRP patches in repairing the cracked aluminum-alloy pipe was finally suggested according to the numerical and test results.
2.2. Bonding surface treatment Impurities on the bonding surface of aluminum-alloy pipes, such as the oxide layer and the greasy dirt, significantly degrades the repairing performances by the CFRP patches, while these impurities were mixed into the sandwich layer between adhesive layer and bonding surface of the pipe. It is very important to pretreat the surface of aluminum-alloy pipe before repairing. There were three existing typical methods
2. Test program 2.1. Specimen preparation All the specimens were made of the type-7005 aluminum-alloy pipe. The external diameter of the pipe specimen was 75 mm, and the internal diameter was 60 mm. A groove for bonding the composite patch was preprocessed in the middle of the specimen. The detailed dimension of the groove part was 160 mm length by 5 mm depth, as shown in Fig. 1(a). The screw threads were preprocessed on the internal surfaces of the specimen at the two ends to facilitate fixing the loading clamps. The external and internal diameters of threads were 52 mm and 50 mm respectively, with the depth of thread pitch of 2 mm. The length of each thread was 96 mm. A pair of holes with the diameter of 2 mm were drilled in the opposite sides of the groove part. Pulling molybdenum wires through the two holes to cut two artificial cracks on the shell of specimen. The length of each crack was 14 mm, as shown in Fig. 1(c). The repairing patch was made of the unidirectional carbon fiber composite, which was 80 mm long (along the axial direction of the pipe) and eight layers thick, as shown in Fig. 1(b). A multi-channel coordination-testing
Fig. 2. Loading scenario of the fatigue test.
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Fig. 3. Screw-thread on the aluminum-alloy pipe.
them uniformly. (c) Preheat the patch piece on the pressure-heating machine. The heating process was maintained about 15 min, and the temperature was maintained at 37°C . Preheating the patch was helpful to soften the resin in each prepreg, and to merge the pieces together. Moreover, preheated CFRP patch was easily to be winded.
available to treat the surface, i.e. anodic oxidation method, chromium sulfate method and polishing treatment. However, the anodic oxidation method is limited in practical operation, which is actually not applicable out of the laboratory. The chromium sulfate method is probably at a toxin risk, and the mechanical polishing method is inefficient. Thus, a new surface treatment method, screw thread, was proposed to improve the bonding performances, except for the existing polishing treatment by sandpapers.
2.3.2. Patch repairing Two pieces of normal adhesive tape were firstly glued on the opposite ends of groove surface to determine and control the position of wrapped CFRP patch. Epoxy adhesive was smeared evenly on the groove between the two adhesive tape pieces. In following, wrapped the shaped CFRP patch around the specimen, as shown in Fig. 4(c). After that, delivered the repaired specimen into an oven to solidify. The solidification process was divided into two phases. The first phase lasted for 1 h at 80°C, which solidified the resin in the prepreg. The second phase lasted for 1 h at 130°C, aiming to solidify the epoxy adhesive layer.
2.2.1. Polishing surface treatment Type-#320 sandpaper was firstly used to grind the pipe surface uniformly until it turned bright white. In following, the pipe specimen was immerged into the acetone for 3–5 mins, then flushed with free water for 5–10 mins, and finally placed into an oven to dry at a consistent temperature of 60°C . It is noted that the specimen should be re-polished after 4 h, when it was not used yet. 2.2.2. Screw-thread surface treatment Blade in the milling machine was applied to prefabricate threads on the outside surface of the specimen. The depth of thread pitch was 0.5 mm, and the pitch space was 1 mm, as shown in Fig. 3. In order to avoid the impacts on the artificial cracks, each boundary of the screwed region was 10 mm away from the artificial crack. The length of each thread region was 30 mm.
3. Fatigue life and failure mode The cracked pipe specimens were divided into three groups those were bare cracked pipe, wrapped pipe with screw-thread surface treatment and wrapped pipe with polishing surface treatment, to conduct the fatigue tests. Each group is consisted of three specimens. A constant sine wave force ranged from 9 kN to 90 kN was axially loaded on the specimen in the fatigue test. The loading frequency was 5.6 Hz. Table 1 presents the fatigue life and failure performances of each group.
2.3. Wrapping CFRP patch 2.3.1. Preparation of the shaped CFRP Patch The bearing capacity of a single layer of CFRP prepreg was actually very weak in the direction perpendicular to the fiber, thus the patch fabricated by continuous winding a single layer of prepreg is easy to crack. Thus, a sandwich patch fabricating method was proposed in this study to avoid cracking in the repairing process.
3.1. Bare cracked pipe and wrapped pipe with screw-thread treatment Compare the results of bare cracked pipe to that of wrapped pipe with screw-thread surface treatment, as listed in 0. It is found that the average fatigue life of wrapped pipe is 22.18 times of that of bare pipe. It is because that the stress concentration in the cracked region of bear pipe is significant, however, the CFRP composite patch shares some stress of the crack front via the adhesive layer that acts as a "bridge" in the repaired specimen. The results prove that the proposed CFRP patch
(a) Cutout a piece of prepreg in consistent with the predetermined size, as shown in Fig. 4(a). X-axis represents the fiber direction (direction of 0°). Y-axis represents the winding direction (direction of 90°). (b) Fabricate the sandwich patch; fold up eight layers of shaped prepreg into a monolithic piece, as shown in Fig. 4(b), and compact
Fig. 4. Wrapping specimen.
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Table 1 Fatigue performances in the test (9 KN~90 KN, 5.6 Hz).
Patch cracks and debonding
Specimen status
No. of specimen
Fatigue Life / thousand cycles
Average life/ thousand cycles
Typical failure modes
Bare pipe
PL1–1 PL1–2 PL1–3
7.186 7.023 7.199
7.136
SeeFig. 5
Wrapped pipe (screw-thread, 80 mm length, 8 layers)
PBC1–1 PBC1–2 PBC1–3
156.342 159.783 158.850
158.325
SeeFig. 6
Wrapped pipe (polishing, 80 mm length, 8 layers)
PBG1–1 PBG1–2
127.637 63.891(bad data) 129.527
128.582
PBG1–3
Fig. 6. Typical failure modes of wrapped pipe with screw-thread.
screw-thread surface treatment is 23.13% longer than that of wrapped pipe with polishing surface treatment. It was indicated that the screwthread method improved the fatigue properties of wrapped aluminumalloy pipe more effectively. It was mainly attributed to that the rough quality of specimen surface obtained by artificial mechanical polishing with sandpaper was not always stable, which affects fatigue performance seriously. Rather, the screw thread greatly increased the contact area between the patch and the pipe, and the quality of screw thread could be precisely controlled by digital machine. The typical failure modes of wrapped pipe with screw-thread surface treatment was shown in Fig. 6. As observed in the wrapped specimen PBG1-1 during the testing process, the first axial crack emerged on the patch between the two artificial cracks after loading 127.495 thousand cycles, accompanied with a loud crash. Four cracks emerged on the patch accompanied with four continuous crisp sounds after 128.505 thousand cycles. Compared with the wrapped specimen with screw thread, the failure omen of the specimen with polishing surface treatment was more obvious. However, the shape of fracture surface was irregular, as shown in Fig. 7. It was attributed to that the damage path of specimen with artificial polishing was random.
SeeFig. 7
repairing technology is capable of improving the fatigue performances of cracked aluminum-alloy pipe effectively. As observed in the bare specimen PL1-1 during the testing process, a tip of an artificial crack began to expand after 5.774 thousand cycles loading, and a tip of the other artificial crack began to expand after 5.965 thousand cycles loading. The color in the middle region of the specimen turned bright after being loaded 7.186 thousand cycles, and then the specimen fractured accompanied with a dull sound. In contrast, a crisp sound was heard instantaneously when the wrapped specimen with screw-thread treatment being loaded 152.498 thousand cycles; an axial crack was observed on the wrapping patch near an artificial crack. The patch debonded locally, as shown in Fig. 6. Another axial crack occurred on the patch after the specimen being loaded 157.090 thousand cycles. The wrapped specimen eventually fractured after being loaded 158.325 thousand cycles, accompanied with a series of sound. A total of six cracks emerged on the patch eventually. The fracture surfaces of bare pipes and wrapped pipes both remained plane. The fatigue region occupied smaller area proportion on the fracture surface of bare pipe than that of the wrapped pipe. The shape of fracture surface of bare pipe turned oval, as shown in Fig. 5. The pipe wall obviously thinned that was necking. On the contrary, the fracture surface of wrapped maintained a circular shape, and no obvious necking phenomena was observed. Actually, on the one hand, the patch shared some stress in the repairing area, bringing a much smaller stress concentration near the crack. It distinctly improved the fatigue properties of specimen, leading to much larger fatigue area on the facture surface. On the other hand, the stiffness of specimen was enhanced by wrapping the patch, and the lateral deformation was constrained by the patch. Thereby, the necking was significantly alleviated.
4. Fatigue failure mechanism Actually, there are three failure modes for the repaired aluminumalloy plate. Those are plate cracking dominant failure, patch cracking dominant failure and patch debonding dominant failure. However, it is completely different for the repaired aluminum-alloy pipe with the composite patch. Test results showed that the wrapped pipe presented a joint failure mode of pipe cracks and interface debonding. The failure mechanism of bare pipe and wrapped pipe are discussed in this section. For the bare pipe, it referred to the crack initiation, crack growth and instability failure; for the wrapped pipe, it mainly focused on the interface debonding. Because the cracking process was shielded by the patch. 4.1. Crack growth in the bare pipe
3.2. Wrapped pipe with different surface treatment
Fig. 8 demonstrates the entire process of crack growth in the bare pipe. After 5711 cycles loading, the main crack appeared in the crack front of one side of an artificial crack, as shown in Fig. 8(a), the crack
As listed in Table 1, the average fatigue life of wrapped pipe with
Crack grows
Fig. 5. Typical failure modes of bare pipe.
Fig. 7. Typical failure modes of wrapped pipe with polishing.
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Fig. 8. The entire crack growth process of bare pipe specimen.
4.2.1. Dyeing method The crisp sound in the test indicated the debonding between the patch and the aluminum-alloy pipe. After hearing the crisp sound, injected the red dye into debonding parts at a rate of 0.1 ml/s until overflow, as shown in Fig. 10(b). Repeat the injection every half minute until the specimen broken, as shown in Fig. 10(c). Due to the good fluidity, the dye quite fits for marking the debonding region. Because the specimen was always releasing heat continuously during the loading process. It promoted the evaporation of free water in the dye, and only left the pigment. The pigment accumulated in the debonding region with the loading time. Thus, the earlier debonding occurred, the deeper the color of debonding region exhibited. In order to describe the test results more conveniently, the specimen was divided into four parts around the circumference. The two parts with artificial cracks are designated as front and back, respectively. The rest two parts are designated as the side. Fig. 11 shows the dyeing result of specimen PBC1-1 in the fatigue test, where an obvious necking performance was observed. As shown in Fig. 11(a), the front part presents the deepest red zone with an "inverted triangle" shape, indicating that the front part is the earliest debonding zone. The red pigment on the back part, as shown in Fig. 11(b), is a little lighter, widespread and uniform, indicating that debonding occurred in this region later. The side part presents the lightest red, especially in the middle segment that has not even been dyed yet, indicating that little debonding occurred in the region without artificial cracks. The main sketch of debonding zone is summarized in Fig. 12, and the red color indicates the debonding zone. The joint failure mode of pipe cracking and interface debonding mentioned above was validated by the testing performances. It was mainly attributed to the “member effect”, which played a bridging role between the pipe and the patch. Actually, the necking effect of pipe promoted the patch debonding under axial tensile, and the debonding leaded to the degradation of
Crack initiation Crack growth Instability failure
Fig. 9. The proportion of three stages in bare pipe.
initiation stage was from 0 to 5711 cycles loading. After 5823 cycles loading, the main crack developed on the other side of the artificial crack. The growth speed of the latter crack was significantly greater than that of the former one, and very soon, the latter crack length was close to the first crack length, as shown in Fig. 8(b). The growth stage of crack was from 5711 to 7009 cycles loading. In this stage, the crack extended very slowly, as shown in Fig. 8(c). After that, followed with the instability failure stage that was from 7010 to 7136 cycles loading, the crack extended rapidly in this stage. The specimen also exhibited a significant necking, as shown in Fig. 8(d). After 7136 cycles loading, the instability failure occurred. Fig. 9 shows the proportion of three stages in the entire fatigue life. This figure shows that the crack initiation is the longest stage, followed by the crack growth stage, and the instability failure is the shortest stage.
4.2. Debonding of the wrapped pipe The dyeing method was applied to study patch debonding of wrapped pipe. The prepared tools such as red ink and medical injector are shown in Fig. 10(a).
Red ink
medical injector
(a) Prepared tools
(b) Injecte the dye Fig. 10. Dyeing process.
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(c) Specimen broken
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Fig. 11. Dyeing effect in the test of PBC1-1.
load-sharing capability. It thereby caused sharply increasing stress on the crack front of aluminum-alloy pipe that accelerated the growth of cracks.
Table 2 Material properties.
5. Optimization of patch length and layer number In order to maximize the benefit of patch repairing, it is particularly important to find out the most optimal scheme in designing a shaped CFRP patch.
Material
Parameters
Aluminum-alloy pipe Adhesive
E=71.028 GPa ν =0.32 G=0.936 GPa ν =0.32
Patch
E1=130 GPa E2=E3=11.2 GPa G23=3.5 GPa G12=G13=5.8 GPa ν 23=0.054 ν12 =ν13=0.33
5.1. Stress intensity factor Fig. 13 shows the stress cloud of σz obtained in the numerical analysis. The axis Z is along the pipe direction. The maximum stress, σz-max , in the pipe before and after being repaired with composite patch is 549 MPa and 458 MPa, respectively. It means that the maximum stress in the crack front decreased by 16.58% after being repaired by 80 mm thick patch with 8 layers. It also proves that the patch is able to share the stress in the crack front. J-integral is an integral path around the crack tip proposed by Rice [21], aiming to avoid the solving difficulty in the crack tip of stress/ strain fields in the two-dimensional space. If it is expanded into the three-dimensional space, the crack tip changes to the crack front [22]. It was pointed out that J-integral was the strain energy release rate G in the linear elastic state [23]. In another word, J-integral is the released energy in per unit cracking area. In order to calculate the J-integral in the crack front, four paths were picked in the crack front, then the integral calculation was carried out respectively. The J-integral in the crack front is the average value. Eq. (1) gives the relationship among J, KI and G.
The stress intensity factor KI was usually employed to describe the elastic stress field intensity of the crack front (or crack tip). Diminishing the KI would significantly benefit improving the fatigue performances of cracked specimen [18–20]. Since it was really difficult to be measured in the test, the stress intensity factor KI was simulated in ANSYS to find out the affecting rule of the patch length and layer number. It also provides a theoretical basis for predicting the fatigue life in the test. An eighth model was established according to the geometrical symmetry. Since the stresses and strains in the crack front were singular, and change with 1/ r . The higher-order type-3D 186 solid singular element with 20 nodes were adopt to simulate the crack front, where the mesh was refined with the intensive ratio 0.1. A total of 10160 elements were meshed in the bare pipe model, and 15320 elements were meshed in the wrapped pipe model. The adjacent nodes of pipe and patch were merged together to model the adhesive effects. Table 2 lists the material parameters used in the` analyses.
J =G=K2I / E′
(1)
Where E′ is the elastic modulus, E′ equals to E in the plane stress condition; E′= E /(1-v2) in the plane strain condition. The calculated relationships between KI and the patch length, as well as the layer number, in the wrapped aluminum-alloy pipe are shown in Fig. 14. As shown in Fig. 14(a), the stress intensity factor KI decreases rapidly when the patch length increases from 0 mm to 100 mm, and then stabilizes after that. As show in Fig. 14(b), the stress intensity factor KI decreases with the layer number until 10, and then increases a little after that. It reveals that increasing the patch length and layer number benefits releasing the stress intensity in the crack front. Similar trend could be found on the curves of patch length versus KI and layer number versus KI. There exists a limit for each of them, after that, the KI improves slightly. In this case, the KI reaches the lowest value with patch length 100 mm and layer number 10, which could markedly promote the fatigue performances of specimens. Parametric tests and analyses were also conducted to find out and validate the influences of patch length and layer number. Fig. 15 shows the relationship between fatigue lives and patch length as well as patch
Fig. 12. Sketch of debonding zone.
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Fig. 13. Stress cloud of σz . 11.0
11.0
Patch layer number is 8
10.5
10.0
9.5
9.5
KI/MPa*m
10.0
K I / M Pa *m
Patch length is 80 mm
10.5
9.0 8.5
9.0 8.5 8.0
8.0
7.5
7.5 7.0
7.0 6.5
0
20
40
60
80
100
0
120
2
4
6
8
10
12
Patch layer number
Patch length /mm
(a) KI versus patch length
(b) KI versus patch layer number
Fig. 14. Relationship between KI and the patch length as well as layer number. 25
25
Patch length is 80 mm
Patch layer number is 8 20
Fatigue life /10000 cycles
Fatigue life /10000 cycles
20
15
10
15
10
5
5
0
0 0
20
40
60
80
100
120
140
Patch length /mm
0
2
4
6
8
10
12
Patch layer number
(a) Patch length
(b) Patch layer number
Fig. 15. Relationship curve of fatigue life versus patch variables obtained in the fatigue tests.
that on. It is obvious that the numerical results on the stress intensity factor KI are perfectly validated and supported by the fatigue test data. The optimum patch length is 100 mm and the optimum layer number is 10. Actually, the local stiffness of the repaired zone increases significantly with the patch layer number. Debonding is more likely
layer number obtained in the fatigue test. As shown in Fig. 15(a), the fatigue life of the repaired specimen extends with the patch length monotonously until 100 mm, and then stabilizes from then on. As shown in Fig. 15(b), fatigue life increases with the composite layer number until 12, and decrease a little from
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1.6
68
1.4
66
1.2
64
Stiffness /KN mm
D is p la c e m e n t /m m
J. Liu et al.
1.0 0.8 0.6 0.4
62 60 58 56
0.2
PBC1-2(80mm, 8 layers, 90KN) PBC2-1(40mm, 8 layers, 90KN)
54
0.0 0
20
40
60
80
0
100
2
4
8
10
12
14
16
18
Fig. 19. Curves of N-K with different patch length.
Fig. 16. Curve of P-S (32.11 thousand cycles). 68
66
PBC1-1I, 80mm, 8 layers, 60KN PBC1-2, 80mm, 8 layers, 90KN
PBC4-2, 80mm, 4 layers PL1-4, bare
64
66 64
Stiffness /KN.mm
62
Stiffness /KN.mm
6
cycles /ten thousand
Load /KN
60 58 56
62 60 58 56
54 52
54 0
1
2
3
4
5
-20
6
0
20
40
60
80
100
120
140
160
Cycles /ten thousand
Cycles /ten thousand
Fig. 20. N-K curves of wrapped specimen under different maximum forces.
Fig. 17. N-K curves of wrapped and bare pipe.
of the slope of the fitted line in Fig. 16.
66
K=P/S
(2)
Stiffness /KN mm
64
where P is the quasi-static load; S is the axial displacement. Fig. 17 shows the tested N-K curves of wrapped specimen PBC4-2 and bare specimen PL1-4, where N denotes the number of loading cycles. As shown in Fig. 17, it is obvious that the residual stiffness of pipe specimen improves significantly after being repaired by the CFRP patch. Fig. 18 presents the N-K curves of wrapped specimens PBC1-2 and PBC4-2 with different patch layer number. It is observed that the residual stiffness decreases with the number of loading cycles. The residual stiffness of PBC4-2 wrapped with 4 patch layers is smaller than that of PBC1-2 wrapped with 8 patch layers, after the same loading cycles. The N-K curves are basically consisted with two branches. The critical transition point of curve PBC4-2 appears after 51.123 thousand loading cycles, while that appears after 148.694 thousand loading cycles in curve of PBC1-2, indicating that the stiffness maintaining capacity of PBC1-2 is much better than that of PBC 4-2. Fig. 19 shows the N-K curves of wrapped specimen with different patch length. As compared in Fig. 19, it is obvious that the stiffness of PBC 2-1 with 40 mm patch length is much smaller than that of PBC1-2 with 80 mm patch length. Similar as those shown in Fig. 17, PBC 2-1 also shows better stiffness maintaining capacity than PBC1-2. Increasing the patch length and layer number, not only enhances the stiffness of wrapped specimens, but also improves the stiffness maintaining ability. It is found that the fatigue life of the wrapped specimen PBC1-1I with patch of 80 mm length and 8 layers under the maximum fatigue force of 60KN is 1439.541 thousand cycles, meanwhile, the same specimen PBC1-2 under the maximum fatigue force of 90KN is 158.325 thousand cycles. The fatigue life of specimen PBC1-1I is 9.1
62 60 58 56
PBC1-2(80mm,8 layers, 90KN) PBC4-2(80mm,4 layers, 90KN)
54 52 0
2
4
6
8
10
12
14
16
18
cycles /ten thousand Fig. 18. Curves of N-K with different layer number.
to occur while the stiffness of the patch and pipe mismatches with each other. That is why the fatigue life decreases slightly while the patch layer number is larger than the optimum value. 5.2. Residual stiffness The residual stiffness is another important index for the fatigue characteristics of the repaired specimen. Quasi-static axial loading tests were also conducted after some cycles of fatigue loading to obtain the load-displacement curve. Fig. 16 shows the obtained load-displacement curve of wrapped specimen PBC1-2 after N=32.11 thousand cycles loading. According to the Eq. (2), the residual stiffness is the reciprocal 162
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100
3.243 thousand cycles 135 thousand cycles 157.09 thousand cycles
80
60
60
Load /KN
Load /KN
80
1.110 thousand cycles 6.048 thousand cycles 6.886 thousand cycles
40
20
40
20
0
0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0
0.5
Displacement /mm
1.0
1.5
2.0
2.5
Displacement /mm
(a)Wrapped pipePBC1-2
(b) Bare pipe PL1-4
Fig. 21. Typical load-displacement curves under different loading cycles. 100
100
80
80
60
Load /KN
L o a d /K N
60
40
Fracture
40
20
Fracture
20 0
0
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
1
2
3
(a) Wrapped pipePBC1-2
4
Displacement /mm
Displacement /mm
5
6
(b) Bare pipe PL1-4
Fig. 22. Hysteretic curves before fracture.
and the displacement increases significantly, then instability failure occurs. However, as shown in Fig. 22(b), hysteretic curve of the bare specimen shifts to right in a steady step before fracture, the displacement is much larger than that of wrapped specimen after the same loading cycles. That is why the stiffness maintaining capacity of wrapped specimen is stronger than that of the bare pipe significantly. The use of aluminum-alloy material was usually limited due to the low stiffness properties in many engineering areas. Retrofitting with the lightweight composites provides the aluminum-alloy material a good chance for a wide application in the military bridge.
times that of the specimen PBC1-2. It indicates that the fatigue life of the repaired specimen declines greatly with the maximum fatigue force. Fig. 20 shows N-K curves of wrapped specimens under different maximum fatigue forces. As compared in Fig. 20, the initial stiffness of the wrapped specimen maintains approximately the same under different maximum fatigue forces. However, the stiffness of PBC1-2 under larger fatigue force of 90KN declines much more significantly with the loading cycles. 5.3. Cyclic creep
6. Conclusions
The cyclic creep was defined as the appearance of ever-increasing strain in the specimen under constant cycle stress. It reveals a transient characteristic of the cyclic stress-strain [24], which is also a typical fatigue property of the repaired specimen. Fig. 21 presents the typical cyclic load-displacement curves of wrapped pipe PBC1-2 (80 mm long, 8 layers) and bare pipePL1-4. As shown in Fig. 21, the axial displacements of wrapped pipe PBC12 and bare pipe PL1-4 both increase monotonously with the loading cycles. Since the fatigue loading was applied in a constant sine wave form, thus, distinct cyclic creep appearances could be found on the two specimens. Fig. 22 presents the recorded completely hysteretic curves of specimen PBC1-2 and PL1-4 before fracture. It is observed that the two hysteretic curves in Fig. 22 both change from dense to sparse. As shown in Fig. 22(a), for the wrapped specimens, the hysteretic curve overlaps constantly and the displacement changes only a little before 158.32 thousand loading cycles. After that, the space between adjacent hysteretic loops gradually increases
Fatigue performances of the cracked aluminum-alloy pipe repaired with a shaped CFRP patch were studied in this paper. A screw-thread method was proposed to treat the pipe surface. The effects of different patch parameters on the fatigue life of the specimen were tested by a series of experiments. Some conclusions were drawn as follows, The average fatigue life of the repaired specimen was 22.18 times of that of bare pipe, indicating that the proposed technique was able to improve fatigue properties of cracked aluminum-alloy pipe significantly. The proposed screw-thread method was more efficient to treat the surface of cracked aluminum-alloy pipe. Fatigue life of the repaired specimen treated by screw-thread method was 23.13% longer than that treated by polishing method. Unlike the cracked aluminum-alloy plate, the cracked aluminumalloy pipe repaired by a shaped CFRP patch showed only one failure 163
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[5] C.C. Choo, T. Zhao, I. Harik, Flexural retrofit of a bridge subjected to overweight trucks using CFRP laminates[J], Compos. Part B Eng. 38 (5–6) (2007) 732–738. [6] H.R. Ronagh, A. Eslami, Flexural retrofitting of RC buildings using GFRP/CFRP – A comparative study[J], Compos. Part B Eng. 46 (46) (2013) 188–196. [7] D.C. Seo, J.J. Lee, Fatigue crack growth behavior of cracked aluminum plate repaired with composite patch[J], Compos. Struct. 57 (s 1–4) (2002) 323–330. [8] J.J. Schubbe, S. Mall, Investigation of a cracked thick aluminum panel repaired with a bonded composite patch[J], Eng. Fract. Mech. 63 (3) (1999) 305–323. [9] J.J. Denney, S. Mall, Characterization of disbond effects on fatigue crack growth behavior in aluminum plate with bonded composite patch[J], Eng. Fract. Mech. 57 (5) (1997) 507–525. [10] R. Brighenti, Patch repair design optimisation for fracture and fatigue improvements of cracked plates[J], Int. J. Solids Struct. 44 (3–4) (2007) 1115–1131. [11] A.M. Kumar, S.A. Hakeem, Optimum design of symmetric composite patch repair to centre cracked metallic sheet[J], Compos. Struct. 49 (2000) 285–292. [12] R. Jones, S.A. Barter, L. Molent, S. Pitt, Crack patching: an experimental evaluation of fatigue crack growth[J], J. Compos. Struct. 67 (2) (2004) 229–238. [13] V. Sabelkin, S. Mall, J.B. Avram, Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch[J], Eng. Fract. Mech. 73 (11) (2006) 1553–1567. [14] V.A. Karatzas, E.A. Kotsidis, N.G. Tsouvalis, Experimental Fatigue Study of Composite Patch Repaired Steel Plates with Cracks, Appl. Compos. Mater. 22 (5) (2015) 507–523. [15] B. Liu, C. Peng, R. Wang, et al., Present research situation and prospect of aluminum-alloy used in large aircraft[J], Chin. J. Nonferrous Met. 20 (9) (2010) 1705–1715. [16] J. Fan, Lightweight and flexible folding aluminum military temporary bridge[J]. Light Alloy Fabrication, Technology 24 (4) (1996) 29–30. [17] Z. Sun, M. Huang, G. Hu, Surface treatment of new type aluminum lithium alloy and fatigue crack behaviors of this alloy plate bonded with Ti–6Al–4V alloy strap[J], Mater. Des. 35 (2012) 725–730. [18] R. Jones, M. Davis, R.J. Callinan, et al., Crack Patching: analysis and Design[J], J. Struct. Mech. 10 (2) (1982) 177–190. [19] R. Jones, R.J. Callinan, Finite Element Analysis of Patched Cracks[J], J. Struct. Mech. 7 (2) (1979) 107–130. [20] Y. Mi, Three-Dimension Analysis of Crack Growth[M], Computational Mechanics Publication, Southampton(UK)/Boston, 1996. [21] J.R. Rice, A path independent integral and the approximate analysis of strain concentration by notches and cracks[J], J. Appl. Mech. 35 (2) (1967) 379–386. [22] H. Zhang, B. He, Finite Element Analysis: ansys13.0 from entry to combat [M], Machinery Industry Press, 2011. [23] C. Lalanne, Fatigue Damage: Mechanical Vibration and Shock Analysis [M], Second edition 4, John Wiley & Sons, Ltd, 2015. [24] W. Yao, Structural Fatigue Life Analysis[M], Defense Industry Press, Beijing, 2003.
mode, that was the pipe cracking and interface debonding joint dominant mode. The stress intensity factor of crack front was calculated by numerical analyses. The patch layers and length have a significant influence on the stress intensity factor. The calculated optimal patch layers and length was 10 layers and 100 mm, respectively, when the corresponding stress intensity factor achieved the minimum. The calculated results were also strongly supported by the test data. The residual stiffness of the wrapped specimen was improved by increasing the patch length or layer number, as well as that the stiffness maintaining capacity was enhanced dramatically simultaneously. The fatigue life and residual stiffness declined significantly with the loading cycles with large maximum force. The cyclic creep appearance occurred both in the wrapped and bare pipe under fatigue loadings. Compared with the bare pipe, the facture omens of wrapped pipe under fatigue loading was not significant. Acknowledgements The authors deeply appreciate the support from National Basic Research Program of China (973 Program, Grant No. 2015CB058002), National Natural Science Foundation of China (Grant No. 51321064, 51622812). References [1] Z.Y. Wang, Q.Y. Wang, Fatigue strength of CFRP strengthened welded joints with corrugated steel plates[J], Compos. Part B Eng. 72 (2015) 30–39. [2] A. Baker, Bonded composite repair of fatigue-cracked primary aircraft structure[J], Compos. Struct. 47 (S1–4) (1999) 431–443. [3] R. Al-Safy, R. Al-Mahaidi, G.P. Simon, A study of the practicality and performance of CFRP applications using post-curing at moderately elevated temperatures[J], Compos. Part B Eng. 48 (8) (2013) 140–157. [4] G. El-Saikaly, O. Chaallal, G. El-Saikaly, et al., Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates[J], Compos. Part B Eng. 68 (2015) 100–112.
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Fatigue performances of the cracked aluminum-alloy pipe repaired with a shaped CFRP patch ⁎
Jinchun Liua, Meijun Qina, Qilin Zhaob, Li Chena, , Pengfei Liub, Jiangang Gaoa a State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact, PLA University of Science and Technology, Nanjing, Jiangsu 210007, China b College of Field Engineering, PLA University of Science and Technology, Nanjing, Jiangsu 210007, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Aluminum-alloy pipe CFRP patch Fatigue Stress intensity factor Surface treatment
To fully understand performances of the cracked aluminum-alloy pipe repaired with CFRP patches, a series of fatigue and quasi-static tests were conducted on commercial available type-7005 pipes. The pipe damaged with artificial cracks was wrapped around by shaped CFRP patches. Two bonding surface treatment methods, i.e. screw-thread and mechanical grinding, were proposed and applied in the repairing process. The fatigue life, residual stiffness and cyclic creep of the repaired specimen were tested, respectively. A fine numerical model was built in ANSYS and validated by the test data. The stress intensity factor was calculated by the developed FE model to reveal the failure mechanism of fatigue tests. The influences of patch length and number of layers were also evaluated and discussed. Test results showed that the fatigue life of the wrapped specimen with the screwthread surface treatment was 22.18 times of that of the bare specimen, and 23.13% longer than that of specimen treated by mechanical grinding. It is concluded that the CFRP patch is capable of improving fatigue performances of the cracked aluminum-alloy pipe. The screw-thread surface treatment provides better fatigue effects than mechanical grinding. Optimized design in the shaped CFRP patch benefits achieving the best fatigue performances of repaired aluminum-alloy pipes.
1. Introduction Bonding with the composite material is a popular technology to repair the damaged aluminum-alloy components, which has the advantages of high quality, efficient and low cost. This method is capable of significantly improving fatigue performances of the repaired component. In recent years, the application of composite repairing technology has been widely extended in the engineering field, such as aviation, bridge, RC frame and so on [1–6]. Numerous experiments were carried out to study the cracked aluminum-alloy plate repaired with the FRP patches. Seo et al. [7] conducted numbers of tests on the fatigue performances of repaired type-7075-T6 plates. Test results indicated that the fatigue life of aluminum-alloy specimen increased by 3–5 times after being repaired with CFRP. Schubbe et al. [8] performed a test on the type-2024-T3 plate repaired with the boron/epoxy composite patch, aiming to explore the impacts on thick aluminum-alloy plates. It is concluded that fatigue properties of aluminum-alloy plates increased by 4.33– 7.12 times after being repaired. Denney et al. [9] also found that the fatigue life of cracked aluminum-alloy plate was extended by 900% after being repaired by the boron-epoxy patch. Brighenti et al. [10] ⁎
proposed a procedure to study the optimal shape of a patch repairing the cracked plate by a biology-based method. The procedure is implemented in a finite element code and validated by numerical simulations. Kumar et al. [11] proposed an optimum design of symmetric (balanced) composite patch to repair a center cracked aluminum plate after theoretical and numerical comparison. It is concluded that the skewed plan-form is the most optimum, which has the least volume. The order of merit of plan-form of composite patch is skewed, rectangular, elliptical, square and circular. Jones et al. [12] examined the crack growth history of a range of test plate and service cracking, for cracks repaired with a composite patch and shows that for small to medium crack lengths there is a linear relationship between the log of the crack length and the fatigue life. Sabelkin et al. [13] further studied the influences of debonding location and patch size on the fatigue life of cracked aluminum-alloy plates. In addition, Karatzas et al. [14] also carried out similar fatigue experiments on composite patch repaired steel plates with cracks. However, the abovementioned researches mainly focused on the plate, but not the structural component such as the aluminum-alloy pipes those are widely used in the manufacturing engineering. The pipelines in aircraft are basically constructed with aluminum-alloy
Corresponding author. E-mail address: [email protected] (L. Chen).
http://dx.doi.org/10.1016/j.tws.2016.11.008 Received 7 July 2016; Received in revised form 4 November 2016; Accepted 8 November 2016 0263-8231/ © 2016 Elsevier Ltd. All rights reserved.
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Fig. 1. Detailed dimensions of the specimen.
machine produced by MTS company was employed to conduct the fatigue test. The input fatigue loading was a sine wave with the frequency of 5.6 Hz and stress ratio of 0.1, indicating that the maximum load was 90 kN. Fig. 2 shows the in-situ loading scenario of the fatigue test.
pipes [15]; and the load-bearing members in the military bridge are mostly supported by the aluminum-alloy pipes [16]. Actually, the stress mechanism of repaired aluminum-alloy pipe is different from that of repaired plate. The pipe specimen usually appears necking in the process of axial tension, whilst a patch not only shares the axial stress, but also provides a certain radial constraint to the inhibit necking. Nevertheless, the patch only shares the axial stress in repairing the aluminum-alloy plate. The shaped CFRP patch was employed in this study to repair the cracked type-7005 aluminum-alloy pipe. The screw-thread method was firstly proposed to treat the surface of cracked pipe, compensating some inherent deficiencies in traditional methods [17]. Systematical fatigue tests were conducted on the repaired specimens. The fatigue life and fatigue failure appearances were reported. Numerical analyses were also carried out in the commercial software ANSYS in order to real the failure mechanism. Optimized design parameters of CFRP patches in repairing the cracked aluminum-alloy pipe was finally suggested according to the numerical and test results.
2.2. Bonding surface treatment Impurities on the bonding surface of aluminum-alloy pipes, such as the oxide layer and the greasy dirt, significantly degrades the repairing performances by the CFRP patches, while these impurities were mixed into the sandwich layer between adhesive layer and bonding surface of the pipe. It is very important to pretreat the surface of aluminum-alloy pipe before repairing. There were three existing typical methods
2. Test program 2.1. Specimen preparation All the specimens were made of the type-7005 aluminum-alloy pipe. The external diameter of the pipe specimen was 75 mm, and the internal diameter was 60 mm. A groove for bonding the composite patch was preprocessed in the middle of the specimen. The detailed dimension of the groove part was 160 mm length by 5 mm depth, as shown in Fig. 1(a). The screw threads were preprocessed on the internal surfaces of the specimen at the two ends to facilitate fixing the loading clamps. The external and internal diameters of threads were 52 mm and 50 mm respectively, with the depth of thread pitch of 2 mm. The length of each thread was 96 mm. A pair of holes with the diameter of 2 mm were drilled in the opposite sides of the groove part. Pulling molybdenum wires through the two holes to cut two artificial cracks on the shell of specimen. The length of each crack was 14 mm, as shown in Fig. 1(c). The repairing patch was made of the unidirectional carbon fiber composite, which was 80 mm long (along the axial direction of the pipe) and eight layers thick, as shown in Fig. 1(b). A multi-channel coordination-testing
Fig. 2. Loading scenario of the fatigue test.
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Fig. 3. Screw-thread on the aluminum-alloy pipe.
them uniformly. (c) Preheat the patch piece on the pressure-heating machine. The heating process was maintained about 15 min, and the temperature was maintained at 37°C . Preheating the patch was helpful to soften the resin in each prepreg, and to merge the pieces together. Moreover, preheated CFRP patch was easily to be winded.
available to treat the surface, i.e. anodic oxidation method, chromium sulfate method and polishing treatment. However, the anodic oxidation method is limited in practical operation, which is actually not applicable out of the laboratory. The chromium sulfate method is probably at a toxin risk, and the mechanical polishing method is inefficient. Thus, a new surface treatment method, screw thread, was proposed to improve the bonding performances, except for the existing polishing treatment by sandpapers.
2.3.2. Patch repairing Two pieces of normal adhesive tape were firstly glued on the opposite ends of groove surface to determine and control the position of wrapped CFRP patch. Epoxy adhesive was smeared evenly on the groove between the two adhesive tape pieces. In following, wrapped the shaped CFRP patch around the specimen, as shown in Fig. 4(c). After that, delivered the repaired specimen into an oven to solidify. The solidification process was divided into two phases. The first phase lasted for 1 h at 80°C, which solidified the resin in the prepreg. The second phase lasted for 1 h at 130°C, aiming to solidify the epoxy adhesive layer.
2.2.1. Polishing surface treatment Type-#320 sandpaper was firstly used to grind the pipe surface uniformly until it turned bright white. In following, the pipe specimen was immerged into the acetone for 3–5 mins, then flushed with free water for 5–10 mins, and finally placed into an oven to dry at a consistent temperature of 60°C . It is noted that the specimen should be re-polished after 4 h, when it was not used yet. 2.2.2. Screw-thread surface treatment Blade in the milling machine was applied to prefabricate threads on the outside surface of the specimen. The depth of thread pitch was 0.5 mm, and the pitch space was 1 mm, as shown in Fig. 3. In order to avoid the impacts on the artificial cracks, each boundary of the screwed region was 10 mm away from the artificial crack. The length of each thread region was 30 mm.
3. Fatigue life and failure mode The cracked pipe specimens were divided into three groups those were bare cracked pipe, wrapped pipe with screw-thread surface treatment and wrapped pipe with polishing surface treatment, to conduct the fatigue tests. Each group is consisted of three specimens. A constant sine wave force ranged from 9 kN to 90 kN was axially loaded on the specimen in the fatigue test. The loading frequency was 5.6 Hz. Table 1 presents the fatigue life and failure performances of each group.
2.3. Wrapping CFRP patch 2.3.1. Preparation of the shaped CFRP Patch The bearing capacity of a single layer of CFRP prepreg was actually very weak in the direction perpendicular to the fiber, thus the patch fabricated by continuous winding a single layer of prepreg is easy to crack. Thus, a sandwich patch fabricating method was proposed in this study to avoid cracking in the repairing process.
3.1. Bare cracked pipe and wrapped pipe with screw-thread treatment Compare the results of bare cracked pipe to that of wrapped pipe with screw-thread surface treatment, as listed in 0. It is found that the average fatigue life of wrapped pipe is 22.18 times of that of bare pipe. It is because that the stress concentration in the cracked region of bear pipe is significant, however, the CFRP composite patch shares some stress of the crack front via the adhesive layer that acts as a "bridge" in the repaired specimen. The results prove that the proposed CFRP patch
(a) Cutout a piece of prepreg in consistent with the predetermined size, as shown in Fig. 4(a). X-axis represents the fiber direction (direction of 0°). Y-axis represents the winding direction (direction of 90°). (b) Fabricate the sandwich patch; fold up eight layers of shaped prepreg into a monolithic piece, as shown in Fig. 4(b), and compact
Fig. 4. Wrapping specimen.
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Table 1 Fatigue performances in the test (9 KN~90 KN, 5.6 Hz).
Patch cracks and debonding
Specimen status
No. of specimen
Fatigue Life / thousand cycles
Average life/ thousand cycles
Typical failure modes
Bare pipe
PL1–1 PL1–2 PL1–3
7.186 7.023 7.199
7.136
SeeFig. 5
Wrapped pipe (screw-thread, 80 mm length, 8 layers)
PBC1–1 PBC1–2 PBC1–3
156.342 159.783 158.850
158.325
SeeFig. 6
Wrapped pipe (polishing, 80 mm length, 8 layers)
PBG1–1 PBG1–2
127.637 63.891(bad data) 129.527
128.582
PBG1–3
Fig. 6. Typical failure modes of wrapped pipe with screw-thread.
screw-thread surface treatment is 23.13% longer than that of wrapped pipe with polishing surface treatment. It was indicated that the screwthread method improved the fatigue properties of wrapped aluminumalloy pipe more effectively. It was mainly attributed to that the rough quality of specimen surface obtained by artificial mechanical polishing with sandpaper was not always stable, which affects fatigue performance seriously. Rather, the screw thread greatly increased the contact area between the patch and the pipe, and the quality of screw thread could be precisely controlled by digital machine. The typical failure modes of wrapped pipe with screw-thread surface treatment was shown in Fig. 6. As observed in the wrapped specimen PBG1-1 during the testing process, the first axial crack emerged on the patch between the two artificial cracks after loading 127.495 thousand cycles, accompanied with a loud crash. Four cracks emerged on the patch accompanied with four continuous crisp sounds after 128.505 thousand cycles. Compared with the wrapped specimen with screw thread, the failure omen of the specimen with polishing surface treatment was more obvious. However, the shape of fracture surface was irregular, as shown in Fig. 7. It was attributed to that the damage path of specimen with artificial polishing was random.
SeeFig. 7
repairing technology is capable of improving the fatigue performances of cracked aluminum-alloy pipe effectively. As observed in the bare specimen PL1-1 during the testing process, a tip of an artificial crack began to expand after 5.774 thousand cycles loading, and a tip of the other artificial crack began to expand after 5.965 thousand cycles loading. The color in the middle region of the specimen turned bright after being loaded 7.186 thousand cycles, and then the specimen fractured accompanied with a dull sound. In contrast, a crisp sound was heard instantaneously when the wrapped specimen with screw-thread treatment being loaded 152.498 thousand cycles; an axial crack was observed on the wrapping patch near an artificial crack. The patch debonded locally, as shown in Fig. 6. Another axial crack occurred on the patch after the specimen being loaded 157.090 thousand cycles. The wrapped specimen eventually fractured after being loaded 158.325 thousand cycles, accompanied with a series of sound. A total of six cracks emerged on the patch eventually. The fracture surfaces of bare pipes and wrapped pipes both remained plane. The fatigue region occupied smaller area proportion on the fracture surface of bare pipe than that of the wrapped pipe. The shape of fracture surface of bare pipe turned oval, as shown in Fig. 5. The pipe wall obviously thinned that was necking. On the contrary, the fracture surface of wrapped maintained a circular shape, and no obvious necking phenomena was observed. Actually, on the one hand, the patch shared some stress in the repairing area, bringing a much smaller stress concentration near the crack. It distinctly improved the fatigue properties of specimen, leading to much larger fatigue area on the facture surface. On the other hand, the stiffness of specimen was enhanced by wrapping the patch, and the lateral deformation was constrained by the patch. Thereby, the necking was significantly alleviated.
4. Fatigue failure mechanism Actually, there are three failure modes for the repaired aluminumalloy plate. Those are plate cracking dominant failure, patch cracking dominant failure and patch debonding dominant failure. However, it is completely different for the repaired aluminum-alloy pipe with the composite patch. Test results showed that the wrapped pipe presented a joint failure mode of pipe cracks and interface debonding. The failure mechanism of bare pipe and wrapped pipe are discussed in this section. For the bare pipe, it referred to the crack initiation, crack growth and instability failure; for the wrapped pipe, it mainly focused on the interface debonding. Because the cracking process was shielded by the patch. 4.1. Crack growth in the bare pipe
3.2. Wrapped pipe with different surface treatment
Fig. 8 demonstrates the entire process of crack growth in the bare pipe. After 5711 cycles loading, the main crack appeared in the crack front of one side of an artificial crack, as shown in Fig. 8(a), the crack
As listed in Table 1, the average fatigue life of wrapped pipe with
Crack grows
Fig. 5. Typical failure modes of bare pipe.
Fig. 7. Typical failure modes of wrapped pipe with polishing.
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Fig. 8. The entire crack growth process of bare pipe specimen.
4.2.1. Dyeing method The crisp sound in the test indicated the debonding between the patch and the aluminum-alloy pipe. After hearing the crisp sound, injected the red dye into debonding parts at a rate of 0.1 ml/s until overflow, as shown in Fig. 10(b). Repeat the injection every half minute until the specimen broken, as shown in Fig. 10(c). Due to the good fluidity, the dye quite fits for marking the debonding region. Because the specimen was always releasing heat continuously during the loading process. It promoted the evaporation of free water in the dye, and only left the pigment. The pigment accumulated in the debonding region with the loading time. Thus, the earlier debonding occurred, the deeper the color of debonding region exhibited. In order to describe the test results more conveniently, the specimen was divided into four parts around the circumference. The two parts with artificial cracks are designated as front and back, respectively. The rest two parts are designated as the side. Fig. 11 shows the dyeing result of specimen PBC1-1 in the fatigue test, where an obvious necking performance was observed. As shown in Fig. 11(a), the front part presents the deepest red zone with an "inverted triangle" shape, indicating that the front part is the earliest debonding zone. The red pigment on the back part, as shown in Fig. 11(b), is a little lighter, widespread and uniform, indicating that debonding occurred in this region later. The side part presents the lightest red, especially in the middle segment that has not even been dyed yet, indicating that little debonding occurred in the region without artificial cracks. The main sketch of debonding zone is summarized in Fig. 12, and the red color indicates the debonding zone. The joint failure mode of pipe cracking and interface debonding mentioned above was validated by the testing performances. It was mainly attributed to the “member effect”, which played a bridging role between the pipe and the patch. Actually, the necking effect of pipe promoted the patch debonding under axial tensile, and the debonding leaded to the degradation of
Crack initiation Crack growth Instability failure
Fig. 9. The proportion of three stages in bare pipe.
initiation stage was from 0 to 5711 cycles loading. After 5823 cycles loading, the main crack developed on the other side of the artificial crack. The growth speed of the latter crack was significantly greater than that of the former one, and very soon, the latter crack length was close to the first crack length, as shown in Fig. 8(b). The growth stage of crack was from 5711 to 7009 cycles loading. In this stage, the crack extended very slowly, as shown in Fig. 8(c). After that, followed with the instability failure stage that was from 7010 to 7136 cycles loading, the crack extended rapidly in this stage. The specimen also exhibited a significant necking, as shown in Fig. 8(d). After 7136 cycles loading, the instability failure occurred. Fig. 9 shows the proportion of three stages in the entire fatigue life. This figure shows that the crack initiation is the longest stage, followed by the crack growth stage, and the instability failure is the shortest stage.
4.2. Debonding of the wrapped pipe The dyeing method was applied to study patch debonding of wrapped pipe. The prepared tools such as red ink and medical injector are shown in Fig. 10(a).
Red ink
medical injector
(a) Prepared tools
(b) Injecte the dye Fig. 10. Dyeing process.
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(c) Specimen broken
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Fig. 11. Dyeing effect in the test of PBC1-1.
load-sharing capability. It thereby caused sharply increasing stress on the crack front of aluminum-alloy pipe that accelerated the growth of cracks.
Table 2 Material properties.
5. Optimization of patch length and layer number In order to maximize the benefit of patch repairing, it is particularly important to find out the most optimal scheme in designing a shaped CFRP patch.
Material
Parameters
Aluminum-alloy pipe Adhesive
E=71.028 GPa ν =0.32 G=0.936 GPa ν =0.32
Patch
E1=130 GPa E2=E3=11.2 GPa G23=3.5 GPa G12=G13=5.8 GPa ν 23=0.054 ν12 =ν13=0.33
5.1. Stress intensity factor Fig. 13 shows the stress cloud of σz obtained in the numerical analysis. The axis Z is along the pipe direction. The maximum stress, σz-max , in the pipe before and after being repaired with composite patch is 549 MPa and 458 MPa, respectively. It means that the maximum stress in the crack front decreased by 16.58% after being repaired by 80 mm thick patch with 8 layers. It also proves that the patch is able to share the stress in the crack front. J-integral is an integral path around the crack tip proposed by Rice [21], aiming to avoid the solving difficulty in the crack tip of stress/ strain fields in the two-dimensional space. If it is expanded into the three-dimensional space, the crack tip changes to the crack front [22]. It was pointed out that J-integral was the strain energy release rate G in the linear elastic state [23]. In another word, J-integral is the released energy in per unit cracking area. In order to calculate the J-integral in the crack front, four paths were picked in the crack front, then the integral calculation was carried out respectively. The J-integral in the crack front is the average value. Eq. (1) gives the relationship among J, KI and G.
The stress intensity factor KI was usually employed to describe the elastic stress field intensity of the crack front (or crack tip). Diminishing the KI would significantly benefit improving the fatigue performances of cracked specimen [18–20]. Since it was really difficult to be measured in the test, the stress intensity factor KI was simulated in ANSYS to find out the affecting rule of the patch length and layer number. It also provides a theoretical basis for predicting the fatigue life in the test. An eighth model was established according to the geometrical symmetry. Since the stresses and strains in the crack front were singular, and change with 1/ r . The higher-order type-3D 186 solid singular element with 20 nodes were adopt to simulate the crack front, where the mesh was refined with the intensive ratio 0.1. A total of 10160 elements were meshed in the bare pipe model, and 15320 elements were meshed in the wrapped pipe model. The adjacent nodes of pipe and patch were merged together to model the adhesive effects. Table 2 lists the material parameters used in the` analyses.
J =G=K2I / E′
(1)
Where E′ is the elastic modulus, E′ equals to E in the plane stress condition; E′= E /(1-v2) in the plane strain condition. The calculated relationships between KI and the patch length, as well as the layer number, in the wrapped aluminum-alloy pipe are shown in Fig. 14. As shown in Fig. 14(a), the stress intensity factor KI decreases rapidly when the patch length increases from 0 mm to 100 mm, and then stabilizes after that. As show in Fig. 14(b), the stress intensity factor KI decreases with the layer number until 10, and then increases a little after that. It reveals that increasing the patch length and layer number benefits releasing the stress intensity in the crack front. Similar trend could be found on the curves of patch length versus KI and layer number versus KI. There exists a limit for each of them, after that, the KI improves slightly. In this case, the KI reaches the lowest value with patch length 100 mm and layer number 10, which could markedly promote the fatigue performances of specimens. Parametric tests and analyses were also conducted to find out and validate the influences of patch length and layer number. Fig. 15 shows the relationship between fatigue lives and patch length as well as patch
Fig. 12. Sketch of debonding zone.
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Fig. 13. Stress cloud of σz . 11.0
11.0
Patch layer number is 8
10.5
10.0
9.5
9.5
KI/MPa*m
10.0
K I / M Pa *m
Patch length is 80 mm
10.5
9.0 8.5
9.0 8.5 8.0
8.0
7.5
7.5 7.0
7.0 6.5
0
20
40
60
80
100
0
120
2
4
6
8
10
12
Patch layer number
Patch length /mm
(a) KI versus patch length
(b) KI versus patch layer number
Fig. 14. Relationship between KI and the patch length as well as layer number. 25
25
Patch length is 80 mm
Patch layer number is 8 20
Fatigue life /10000 cycles
Fatigue life /10000 cycles
20
15
10
15
10
5
5
0
0 0
20
40
60
80
100
120
140
Patch length /mm
0
2
4
6
8
10
12
Patch layer number
(a) Patch length
(b) Patch layer number
Fig. 15. Relationship curve of fatigue life versus patch variables obtained in the fatigue tests.
that on. It is obvious that the numerical results on the stress intensity factor KI are perfectly validated and supported by the fatigue test data. The optimum patch length is 100 mm and the optimum layer number is 10. Actually, the local stiffness of the repaired zone increases significantly with the patch layer number. Debonding is more likely
layer number obtained in the fatigue test. As shown in Fig. 15(a), the fatigue life of the repaired specimen extends with the patch length monotonously until 100 mm, and then stabilizes from then on. As shown in Fig. 15(b), fatigue life increases with the composite layer number until 12, and decrease a little from
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Fig. 17. N-K curves of wrapped and bare pipe.
of the slope of the fitted line in Fig. 16.
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where P is the quasi-static load; S is the axial displacement. Fig. 17 shows the tested N-K curves of wrapped specimen PBC4-2 and bare specimen PL1-4, where N denotes the number of loading cycles. As shown in Fig. 17, it is obvious that the residual stiffness of pipe specimen improves significantly after being repaired by the CFRP patch. Fig. 18 presents the N-K curves of wrapped specimens PBC1-2 and PBC4-2 with different patch layer number. It is observed that the residual stiffness decreases with the number of loading cycles. The residual stiffness of PBC4-2 wrapped with 4 patch layers is smaller than that of PBC1-2 wrapped with 8 patch layers, after the same loading cycles. The N-K curves are basically consisted with two branches. The critical transition point of curve PBC4-2 appears after 51.123 thousand loading cycles, while that appears after 148.694 thousand loading cycles in curve of PBC1-2, indicating that the stiffness maintaining capacity of PBC1-2 is much better than that of PBC 4-2. Fig. 19 shows the N-K curves of wrapped specimen with different patch length. As compared in Fig. 19, it is obvious that the stiffness of PBC 2-1 with 40 mm patch length is much smaller than that of PBC1-2 with 80 mm patch length. Similar as those shown in Fig. 17, PBC 2-1 also shows better stiffness maintaining capacity than PBC1-2. Increasing the patch length and layer number, not only enhances the stiffness of wrapped specimens, but also improves the stiffness maintaining ability. It is found that the fatigue life of the wrapped specimen PBC1-1I with patch of 80 mm length and 8 layers under the maximum fatigue force of 60KN is 1439.541 thousand cycles, meanwhile, the same specimen PBC1-2 under the maximum fatigue force of 90KN is 158.325 thousand cycles. The fatigue life of specimen PBC1-1I is 9.1
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to occur while the stiffness of the patch and pipe mismatches with each other. That is why the fatigue life decreases slightly while the patch layer number is larger than the optimum value. 5.2. Residual stiffness The residual stiffness is another important index for the fatigue characteristics of the repaired specimen. Quasi-static axial loading tests were also conducted after some cycles of fatigue loading to obtain the load-displacement curve. Fig. 16 shows the obtained load-displacement curve of wrapped specimen PBC1-2 after N=32.11 thousand cycles loading. According to the Eq. (2), the residual stiffness is the reciprocal 162
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Fig. 22. Hysteretic curves before fracture.
and the displacement increases significantly, then instability failure occurs. However, as shown in Fig. 22(b), hysteretic curve of the bare specimen shifts to right in a steady step before fracture, the displacement is much larger than that of wrapped specimen after the same loading cycles. That is why the stiffness maintaining capacity of wrapped specimen is stronger than that of the bare pipe significantly. The use of aluminum-alloy material was usually limited due to the low stiffness properties in many engineering areas. Retrofitting with the lightweight composites provides the aluminum-alloy material a good chance for a wide application in the military bridge.
times that of the specimen PBC1-2. It indicates that the fatigue life of the repaired specimen declines greatly with the maximum fatigue force. Fig. 20 shows N-K curves of wrapped specimens under different maximum fatigue forces. As compared in Fig. 20, the initial stiffness of the wrapped specimen maintains approximately the same under different maximum fatigue forces. However, the stiffness of PBC1-2 under larger fatigue force of 90KN declines much more significantly with the loading cycles. 5.3. Cyclic creep
6. Conclusions
The cyclic creep was defined as the appearance of ever-increasing strain in the specimen under constant cycle stress. It reveals a transient characteristic of the cyclic stress-strain [24], which is also a typical fatigue property of the repaired specimen. Fig. 21 presents the typical cyclic load-displacement curves of wrapped pipe PBC1-2 (80 mm long, 8 layers) and bare pipePL1-4. As shown in Fig. 21, the axial displacements of wrapped pipe PBC12 and bare pipe PL1-4 both increase monotonously with the loading cycles. Since the fatigue loading was applied in a constant sine wave form, thus, distinct cyclic creep appearances could be found on the two specimens. Fig. 22 presents the recorded completely hysteretic curves of specimen PBC1-2 and PL1-4 before fracture. It is observed that the two hysteretic curves in Fig. 22 both change from dense to sparse. As shown in Fig. 22(a), for the wrapped specimens, the hysteretic curve overlaps constantly and the displacement changes only a little before 158.32 thousand loading cycles. After that, the space between adjacent hysteretic loops gradually increases
Fatigue performances of the cracked aluminum-alloy pipe repaired with a shaped CFRP patch were studied in this paper. A screw-thread method was proposed to treat the pipe surface. The effects of different patch parameters on the fatigue life of the specimen were tested by a series of experiments. Some conclusions were drawn as follows, The average fatigue life of the repaired specimen was 22.18 times of that of bare pipe, indicating that the proposed technique was able to improve fatigue properties of cracked aluminum-alloy pipe significantly. The proposed screw-thread method was more efficient to treat the surface of cracked aluminum-alloy pipe. Fatigue life of the repaired specimen treated by screw-thread method was 23.13% longer than that treated by polishing method. Unlike the cracked aluminum-alloy plate, the cracked aluminumalloy pipe repaired by a shaped CFRP patch showed only one failure 163
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[5] C.C. Choo, T. Zhao, I. Harik, Flexural retrofit of a bridge subjected to overweight trucks using CFRP laminates[J], Compos. Part B Eng. 38 (5–6) (2007) 732–738. [6] H.R. Ronagh, A. Eslami, Flexural retrofitting of RC buildings using GFRP/CFRP – A comparative study[J], Compos. Part B Eng. 46 (46) (2013) 188–196. [7] D.C. Seo, J.J. Lee, Fatigue crack growth behavior of cracked aluminum plate repaired with composite patch[J], Compos. Struct. 57 (s 1–4) (2002) 323–330. [8] J.J. Schubbe, S. Mall, Investigation of a cracked thick aluminum panel repaired with a bonded composite patch[J], Eng. Fract. Mech. 63 (3) (1999) 305–323. [9] J.J. Denney, S. Mall, Characterization of disbond effects on fatigue crack growth behavior in aluminum plate with bonded composite patch[J], Eng. Fract. Mech. 57 (5) (1997) 507–525. [10] R. Brighenti, Patch repair design optimisation for fracture and fatigue improvements of cracked plates[J], Int. J. Solids Struct. 44 (3–4) (2007) 1115–1131. [11] A.M. Kumar, S.A. Hakeem, Optimum design of symmetric composite patch repair to centre cracked metallic sheet[J], Compos. Struct. 49 (2000) 285–292. [12] R. Jones, S.A. Barter, L. Molent, S. Pitt, Crack patching: an experimental evaluation of fatigue crack growth[J], J. Compos. Struct. 67 (2) (2004) 229–238. [13] V. Sabelkin, S. Mall, J.B. Avram, Fatigue crack growth analysis of stiffened cracked panel repaired with bonded composite patch[J], Eng. Fract. Mech. 73 (11) (2006) 1553–1567. [14] V.A. Karatzas, E.A. Kotsidis, N.G. Tsouvalis, Experimental Fatigue Study of Composite Patch Repaired Steel Plates with Cracks, Appl. Compos. Mater. 22 (5) (2015) 507–523. [15] B. Liu, C. Peng, R. Wang, et al., Present research situation and prospect of aluminum-alloy used in large aircraft[J], Chin. J. Nonferrous Met. 20 (9) (2010) 1705–1715. [16] J. Fan, Lightweight and flexible folding aluminum military temporary bridge[J]. Light Alloy Fabrication, Technology 24 (4) (1996) 29–30. [17] Z. Sun, M. Huang, G. Hu, Surface treatment of new type aluminum lithium alloy and fatigue crack behaviors of this alloy plate bonded with Ti–6Al–4V alloy strap[J], Mater. Des. 35 (2012) 725–730. [18] R. Jones, M. Davis, R.J. Callinan, et al., Crack Patching: analysis and Design[J], J. Struct. Mech. 10 (2) (1982) 177–190. [19] R. Jones, R.J. Callinan, Finite Element Analysis of Patched Cracks[J], J. Struct. Mech. 7 (2) (1979) 107–130. [20] Y. Mi, Three-Dimension Analysis of Crack Growth[M], Computational Mechanics Publication, Southampton(UK)/Boston, 1996. [21] J.R. Rice, A path independent integral and the approximate analysis of strain concentration by notches and cracks[J], J. Appl. Mech. 35 (2) (1967) 379–386. [22] H. Zhang, B. He, Finite Element Analysis: ansys13.0 from entry to combat [M], Machinery Industry Press, 2011. [23] C. Lalanne, Fatigue Damage: Mechanical Vibration and Shock Analysis [M], Second edition 4, John Wiley & Sons, Ltd, 2015. [24] W. Yao, Structural Fatigue Life Analysis[M], Defense Industry Press, Beijing, 2003.
mode, that was the pipe cracking and interface debonding joint dominant mode. The stress intensity factor of crack front was calculated by numerical analyses. The patch layers and length have a significant influence on the stress intensity factor. The calculated optimal patch layers and length was 10 layers and 100 mm, respectively, when the corresponding stress intensity factor achieved the minimum. The calculated results were also strongly supported by the test data. The residual stiffness of the wrapped specimen was improved by increasing the patch length or layer number, as well as that the stiffness maintaining capacity was enhanced dramatically simultaneously. The fatigue life and residual stiffness declined significantly with the loading cycles with large maximum force. The cyclic creep appearance occurred both in the wrapped and bare pipe under fatigue loadings. Compared with the bare pipe, the facture omens of wrapped pipe under fatigue loading was not significant. Acknowledgements The authors deeply appreciate the support from National Basic Research Program of China (973 Program, Grant No. 2015CB058002), National Natural Science Foundation of China (Grant No. 51321064, 51622812). References [1] Z.Y. Wang, Q.Y. Wang, Fatigue strength of CFRP strengthened welded joints with corrugated steel plates[J], Compos. Part B Eng. 72 (2015) 30–39. [2] A. Baker, Bonded composite repair of fatigue-cracked primary aircraft structure[J], Compos. Struct. 47 (S1–4) (1999) 431–443. [3] R. Al-Safy, R. Al-Mahaidi, G.P. Simon, A study of the practicality and performance of CFRP applications using post-curing at moderately elevated temperatures[J], Compos. Part B Eng. 48 (8) (2013) 140–157. [4] G. El-Saikaly, O. Chaallal, G. El-Saikaly, et al., Fatigue behavior of RC T-beams strengthened in shear with EB CFRP L-shaped laminates[J], Compos. Part B Eng. 68 (2015) 100–112.
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