Fatigue Crack Repair Using Drilled Holes and Externally Bonded CFRP Strips

Transcription

1 Bridge Maintenance, Safety, Management, Reilience and Sutainability Biondini & Frangopol (Ed) 212 Taylor & Franci Group, London, ISBN Fatigue Crack Repair Uing Drilled Hole and Externally Bonded CFRP Strip F. Lin Tokyo Metropolitan Univerity, Tokyo, Japan J.G. Sun Tokyo Metropolitan Univerity, Tokyo, Japan H. Nakamura Tokyo Metropolitan Univerity, Tokyo, Japan K. Maeda Tokyo Metropolitan Univerity, Tokyo, Japan ABSTRACT: Fatigue crack in teel bridge are often repaired by contructing drilled hole at the crack tip, becaue thee drilled hole can top or delay crack propagation. Repair method uing drilled hole have been actively invetigated, with technique uch a the tightening of high-trength bolt in the drilled hole ued to enhance the repair effect. However, it may be difficult to tighten high-trength bolt in narrow location. On the other hand, the reult of ome tudie have demontrated that Carbon-Fiber-Reinforced Platic (CFRP) trip, which are lightweight, have high trength, diplay excellent corroion reitance, and are eay to ue, are ueful for the repair of fatigue crack. Therefore, an improvement in the effect of fatigue-crack repair by combining drilled hole with externally bonded CFRP trip i expected. In thi tudy, a repair method combining drilled hole with externally bonded CFRP trip i propoed, and tenile tet and fatigue tet are carried out to verify the effect of the fatigue-crack repair uing pecimen with welded web guet plate. A hown by the reult of the tenile tet, the train concentration of the drilled hole i greatly reduced by the externally bonded CFRP trip. The reult of the fatigue tet alo how that the fatigue life i improved ignificantly. 1 INTRODUCTION Hitorically, fatigue damage often occurred at the weld joint of tructure, becaue a weld joint i a tructural dicontinuity, and i ubjected to tre concentration at the weld toe, welded reidual tree, and o on. In recent year, fatigue damage at welded web guet joint, which are ued to connect lateral bracing member, ha been reported frequently. Fatigue crack at a welded web guet cannot be neglected, becaue the fatigue crack might actually propagate and finally caue the brittle fracture of the main girder. Fatigue crack in teel bridge are uually repaired by forming drilled hole at the crack tip, becaue thee drilled hole top or delay crack propagation. There have been active invetigation on uch repair method uing drilled hole, uch a the tightening of high-trength bolt in drilled hole to enhance the repair effect (Fiher 1984). However, it may be difficult to tighten high-trength bolt in narrow location. On the other hand, the reult of ome tudie have demontrated that a Carbon-Fiber-Reinforced Platic (CFRP) trip i ueful for the repair of fatigue crack. A CFRP trip i lightweight, and ha high trength and excellent corroion reitance. In addition, it i a very imple tructure and i eay to ue. Therefore, we propoed to improve the repair effect of drilled hole by combining thi method with the ue of externally bonded CFRP trip (Nakamura et al. 29). In pat tudie (Nakamura et al. 29), after the fatigue crack initiated, they were repaired uing only the CFRP trip. The fatigue-tet reult howed that the fatigue limit wa not reached, and the propoed method wa poitioned a a firt-aid repair for prolonging the fatigue life. Hence, a repair method combining drilled hole with externally bonded CFRP trip i expected to provide a permanent countermeaure. Although the improvement in fatigue trength uing the repair method of drilling hole at the crack tip and tightening high-trength bolt in the hole ha been clarified experimentally (Mori et al. 21), there were not ufficient data available on the repair effect achieved by combining drilled hole with externally bonded CFRP trip. For example, it ha been proved that in comparion with drilled hole, the fatigue trength i greatly increaed when uing both drilled hole and CFRP trip on mall coupon pecimen of flat teel plate (Suzuki et al. 23). Therefore, examination uch a modeling of fullcale drilled hole and fatigue-crack initiation at 138

2 weld joint are neceary for the practical ue of thi method. In thi tudy, the effect of repair uing both drilled hole and externally bonded CFRP trip i examined experimentally uing a cracked pecimen with welded web guet plate. In the experimental procedure, firt, the effect of the lamination number of CFRP trip on the reduction in train concentration i invetigated through tenile tet. Second, the improvement in fatigue life i invetigated by fatigue tet with varying tre range. 2.2 Repair method uing drilled hole (DH erie) The drilled hole of the DH erie are hown in Figure 2 (a). Auming that fatigue crack are initiated at the weld toe and then propagate, firt, two circular hole of 25 mm in diameter were intalled on the predicated crack path. The outer-edge ditance between the two drilled hole, c, wa et to 8 mm, conidering the workability of the portable drilling machine and hand grinder. Second, the bae plate between two drilled hole wa penetrated uing a aber aw. Finally, in order to prevent crack initiation at the other ide of the weld toe, thi part wa moothly treated uing the hand grinder. The train gage were intalled a hown in Figure 2 (a). The number refer to the ditance from the center of the pecimen (left ide i - and right ide i + ). Firt, a hown in Figure 2 (a), train gage were intalled on the chamfer of the hole edge; thee had a gage length of 1 mm and interval of 2 mm (named ±4Sa, m, b). The train gage of ±4Sm were located at the center of the plate thickne. In addition, near the hole edge, train gage (±41.5Sa, b) were intalled on ide a and ide b of the pecimen. 2 TEST PROCEDURES 2.1 Tet pecimen and material propertie A hown in Figure 1, the pecimen were fabricated with two guet plate ( mm) welded to both ide of the central part of the bae teel plate ( mm). The mechanical propertie of the teel plate, the CFRP trip, and the epoxy rein adheive are hown in Table 1. The CFRP trip i 1.2 mm in thickne, and the carbon fiber i arranged unidirectionally. For comparion of the repair effect, two erie of repair method were invetigated experimentally. One wa the repair method uing only drilled hole (hereafter called DH), and the other wa the repair method uing both drilled hole and externally bonded CFRP trip (hereafter called DHC). Figure 1. Specimen with welded web guet plate Table 1. Material propertie (a) Steel plate and CFRP trip Steel plate (JIS SM4A) CFRP trip Yield point σ Y (MPa) Tenile trength σ tu (MPa) Breaking elongation (%) 29 - Elatic modulu E (GPa) (b) Epoxy rein adheive Compreive elatic modulu E (GPa) 1.5 Tenile trength σ tu (MPa) 3 Bending trength σ bu (MPa) Repair method uing both drilled hole and externally bonded CFRP trip (DHC erie) A hown in Figure 2 (b) and (c), the drilled hole of the DHC erie are imilar to thoe of the DH e- (a) DH erie (b) DH5C erie (c) DH7C erie Figure 2. Repair method and train gage location 139

3 rie. Five (DH5C) or even (DH7C) layer of CFRP trip (5 mm in width) were bonded to the teel plate. In order to prevent debonding of the CFRP trip from the end, the total and lap length of the CFRP trip were deigned a multiple-tepped lamination on the bai of finite element analyi (FEA) conidering the hear lag (Ihikawa et al. 21). Before bonding, the urface of the teel plate and the CFRP trip were roughly treated with the hand grinder. The bonding procedure wa a follow: Firt, the CFRP trip were multiple-tepped and laminated, and cured for one day at room temperature (approx. 25 C). Second, a hown in Figure 2 (b) and (c), the multiple-tepped CFRP trip were externally bonded at the drilled hole, and cured for even day at room temperature (approx. 25 C). The reult of the previou tudy (Nakamura et al. 29) howed that the CFRP trip debonded earlier when the drilled hole were covered with the CFRP trip becaue the opening diplacement of the drilled hole were conidered to be rather large. Therefore, in thi tudy, the bonding method of the CFRP trip hown in Figure 2 (b) and (c) and Figure 3 wa adopted. The train gage were intalled a hown in Figure 2 (b) and (c). The train gage on the chamfer of the hole edge were imilar to thoe of the DH erie. Moreover, the train gage were only intalled on the CFRP trip of ide a (±42Ca, ±65Ca, ± 88Ca). 2.4 Experimental condition The experimental condition are hown in Table 2. In the firt column of thi table, the firt mark indicate the kind of repair method, the number in the middle i the tre range, and the lat number give the pecimen number of the ame repair method and the ame tre range. Under fatigue-tet condition, the minimum nominal tre of the teel plate, σ n, wa 2 MPa, and the nominal tre range of the teel plate, σ n, wa et to 8, 1, 12, or 14 MPa by changing the maximum nominal tre. In thi tudy, the fatigue limit wa et a 1,, cycle. If fatigue crack Figure 3. Repair method of DHC erie Figure 4. Tet etup Table 2. Experimental condition Experimental Max. tre Min. tre Stre range Loading peed Repair method erie σ max (MPa) σ min (MPa) σ n (MPa) f (Hz) DH-8 Drilled hole DH-1-1 Drilled hole DH-1-2 Drilled hole DH-12-1 Drilled hole DH-12-2 Drilled hole DH-14-1 Drilled hole DH-14-2 Drilled hole DH5C-8 Drilled hole+cfrp DH5C-1 Drilled hole+cfrp DH5C-12-1 Drilled hole+cfrp DH5C-12-2 Drilled hole+cfrp DH5C-12-3 Drilled hole+cfrp DH5C-14-1 Drilled hole+cfrp DH5C-14-2 Drilled hole+cfrp DH7C-8 Drilled hole+cfrp DH7C-1 Drilled hole+cfrp DH7C-12 Drilled hole+cfrp DH7C-14 Drilled hole+cfrp

4 wa not initiated, the repaired pecimen could reach the fatigue limit, and the fatigue tet wa carried out again, increaing the tre range by 2 MPa until fatigue-crack initiation. The loading peed f wa dependent on the limit of the teting machine. Thi wa et lower when the tre range wa higher. Before the fatigue tet, the tenile tet wa carried out, but only the tenile tet wa carried out for the DH7C erie. Figure 4 how the tet etup. 3 DISCUSSION OF REDUCTION IN STRAIN CONCENTRATION IN DRILLED HOLES The tenile tet wa carried out before the fatigue tet. In order to eliminate the influence of in-plane and out-of-plane bending on the meaured train, the average value from ix train gage on the chamfer of the hole edge (hereafter called the train in the drilled hole) i ued for the dicuion in thi ection. The tre- or train-concentration factor of drilled hole in a finite plate i given by Mori et al. (21) a follow: { 166. ( c/m ) c 119} π c α ec 2 = +. (1) w M where α i the tre- or train-concentration factor, Μ i the diameter of the drilled hole, w i the width of the teel plate, and c i the outer-edge ditance between two drilled hole. Here, α i calculated to be A an example of the tenile tet reult, Figure 5 how a comparion between the DH and DH5C erie for σ n = 12 MPa. Thi figure include the tenile tet reult from the firt to the fourth time, a well a the reult calculated from Eq. (1). A hown in thi figure, in the firt tenile tet, becaue of the effect of tre concentration, although the tenile tre of the teel plate σ n wa under the yield point, the train in the drilled hole became large and platic train wa generated, and after unloading, reidual train remained both in the DH and DH5C erie. In the econd and ubequent tenile tet, the train in the drilled hole wa maller than that of the firt time, and after unloading, a light reidual train remained in both erie, but that of the DH5C erie wa maller than that of the DH erie. The reidual train after unloading decreaed with an increaing number of cycle. Before the platic train wa generated, the train in the drilled hole wa equivalent to the theoretical value calculated from Eq. (1). The other erie howed imilar reult (not hown). A mentioned above, the train variation in the drilled hole became table after everal tenile tet. In Figure 6, the reult after everal tenile tet in each tre range are hown for all the erie. It wa found that the relationhip between the tenile nominal tre and the train in the drilled hole did not how elatic behavior, and were hyterei loop. However, reidual train wa not generated o much after unloading. It wa conidered that the yield point of the teel plate ued (JIS SM4A) wa low, and the edge of the drilled hole eemed to be trongly influenced by the welding reidual train and tre concentration. It ha alo been hown that the tre-train relationhip in teel plate with higher yield point howed almot elatic behavior (Uchida 27). In the DH5C and DH7C erie, although light hyterei behavior were oberved compared with that of the DH erie, and the tre-train relationhip in the drilled hole howed almot elatic behavior with an increae in the number of layer of CFRP trip, it wa found that the tenile tiffne near the drilled hole by the bonded CFRP trip became higher, and that the train concentration wa Nominal tenile tre σ n (MPa) Yield train ε y Nominal train ε n ε =α ε n DH 12 2 DH5C Strain in the drilled hole ε (x1 6 ) Figure 5. Relationhip between tenile nominal tre and train in drilled hole during everal tenile tet Nominal tenile tre σ n (MPa) Nominal train ε n ε =α ε n 4 DH DH5C 2 DH7C Yield train ε y Strain in the drilled hole ε (x1 6 ) Figure 6. Relationhip between tenile nominal tre and train in drilled hole after everal tenile tet 1311

5 dratically reduced. The effect of the externally bonded CFRP trip on the nominal tre and train reduction of the teel plate wa calculated on the bai of the tenile tiffne ratio S R, a in the following equation (Liu et al. 29): R ( E A E A ) S = E A + (2) c c where E and A are the elatic modulu and croectional area, and the ubcript and c refer to the teel plate and the CFRP trip, repectively. The value of S R wa.5 for DH5C and.42 for DH7C. The train in the drilled hole ε DHmax for the maximum nominal tenile tre σ max of all the erie are hown in Table 3. The train of ε DHmax are the averaged value in the drilled hole after everal tenile tet. The train concentration factor of the tenile tet wa calculated by α =ε DHmax /ε max, where ε max i the maximum nominal train. Compared to the DH erie, the train concentration factor of the DHC erie wa greatly reduced, and the reduction of the train concentration increaed with an increaing number of laminate layer. It wa alo found that the train concentration factor α increaed with increaing nominal tenile tre. It i conidered that the reaon for thi wa that a large platic train ε DHmax wa generated at the hole edge under the elatic nominal train ε max, and thu, the train concentration factor α =ε DHmax /ε max increaed naturally. The repair effect are alo hown in Table 3. Thee were calculated by comparing the train in the drilled hole ε DHmax with the maximum nominal train ε max for the DH and DHC erie. Firt, the experimental reult did not agree with the value obtained from Eq. (2) becaue Eq. (2) conidered elatic behavior. However, a large platic train wa generated near the drilled hole becaue of the tre concentration, reidual train, and o on. Thi alo how that the repair effect increaed with increaing tenile tre, becaue the platic train wa reduced Nominal tenile tre σ n (MPa) ±42Ca ±65Ca ±88Ca Strain on the CFRP trip ε (x1 6 ) Figure 7. Strain variation in the CFRP trip (DH5C-14-2) by the externally bonded CFRP trip. A mentioned above, inelatic behavior wa oberved in the DH erie, and almot elatic behavior in the DHC erie. Figure 7 how the tenile tet reult for DH5C The train on the CFRP trip are plotted under the nominal tenile tre. In order to eliminate the influence of in-plane bending, the average value of the left and right ide are ued here. A hown in thi figure, the train of ±42Ca, which wa cloe to the drilled hole, wa the highet. It wa found that the CFRP trip cloe to the drilled hole were alo ubjected to the influence of the tre concentration. Therefore, the externally bonded CFRP trip bore a part of the tre concentration of the edge of the drilled hole, and a a reult, the train concentration of the hole edge were reduced. 4 DISCUSSION OF REPAIR EFFECTS BASED ON FATIGUE TEST During the fatigue tet, the dynamic train were Table 3. Comparion of the maximum train in drilled hole Experimental erie DH DH5C DH7C Maximum nominal tre σ max (MPa) Maximum nominal train ε max ( 1-6 ) Maximum train in drilled hole (ave.) ε DHmax ( 1-6 ) Strain concentration factor α (=ε DHmax /ε max ) Tenile tiffne ratio S R Maximum train ratio to DH DH DH DH DH DH5C DH5C DH5C DH5C DH7C DH7C DH7C DH7C

6 Strain range ε (x1 6 ) N p =7,752 +4Sa +4Sm +4Sb +41.5Sa +41.5Sb Number of cycle N (x1 3 ) Figure 8. Relationhip between meaured train range and number of cycle (DH-14-2) recorded at a ampling frequency of 1 Hz. The train difference i defined a the difference between the maximum and minimum train (hereafter called ε). Figure 8 how an example of the DH erie (DH- 14-2). Firt, the train range of all gage maintained contant value up to approximately 18, cycle. The train range of +4Sa and +4Sm uddenly increaed at around 18, and 26, cycle, repectively, and their value immediately became zero and the train gage failed. Then, only the train range of +41.5Sa gradually decreaed with an increae in the number of cycle. At about 47, cycle, a fatigue crack wa detected by viual obervation in the right drilled hole of ide a (located near +4Sa). Moreover, we oberved viually that thi crack propagated to ide b and in the width direction when the train range of +4Sb and +41.5Sb moved up and down. Finally, the pecimen failed when thee value became zero. Therefore, it wa found that the train range became almot zero when the (a) Debonding from the end (b) After removing CFRP trip Figure 1. Debonding of CFRP trip (DH5C-12-2) fatigue crack penetrated near the target train gage. The experimental reult of the DH5C erie, that i, the relationhip between the meaured train range and number of cycle of DH5C-12-2, DH5C- 12-3, and DH5C-14-1, are hown in Figure 9 (a) and (b), repectively. Firt, in DH5C-12-2, during the fatigue tet, one of the CFRP trip, located on the left ide and ide b, wa debonded from the end after approximately 88, cycle. The debonding progreed to the center, and approached the drilled hole at around 1,18, cycle, a hown in Figure 1 (a). Then, the fatigue tet wa topped after 1,214,167 cycle, and the CFRP trip were removed, a hown in Figure 1 (b). After removal of the CFRP trip, no fatigue crack in the drilled hole wa oberved. A hown in Figure 9 (a), the train range of all gage maintained contant value even at approximately 88, cycle, when one of the CFRP trip wa debonded from the end. Moreover, when the debonding approached the drilled hole after about 1,18, cycle, the train range in the drilled hole (-4Sm) uddenly increaed and maintained a higher Strain range ε (x1 6 ) DH5C 12 2 N=1,214,167 4Sm 42Ca +4Sm +42Ca DH5C 12 3 N=1,214,167+1,333,821 =2,547,988 Strain range ε (x1 6 ) Sm 42Ca +4Sm +42Ca N d =89, N p =289, N d =1,18, N d =2,38, Number of cycle N (x1 3 ) (a) DH5C-12-2 and DH5C-12-3 Figure 9. Meaured train range v. number of cycle Number of cycle N (x1 3 ) (b) DHC5C

7 contant value. The train range in the other drilled hole (+4Sm), where the CFRP trip were not debonded, alo increaed. However, it value wa lower than that of the other drilled hole on the debonding ide. Therefore, it can be conidered that the train range of both drilled hole increaed and maintained higher contant value when the CFRP trip debonded near the drilled hole. In addition, the CFRP trip were bonded again in DH5C-12-2, which wa newly named a DH5C- 12-3, and a retet wa carried out under the ame loading condition. A a reult, the other CFRP trip, which were located on the right ide and ide a, debonded 1,94,633 cycle after the retart, and the total life until debonding wa approximately 2,38,8 cycle, a hown in Figure 9 (a) and Table 4. Table 4. Debonding life and failure life from fatigue tet Experimental erie Debonding life Failure life N d (cycle) N p (cycle) DH-8* - >1,, DH-1-1* - 419,19 DH ,612 DH ,646 DH ,3 DH ,529 DH ,752 DH5C-8** - >1,, DH5C-1** - >1,, DH5C-12-1** 5,5, 6,15,127 DH5C-12-2*** 1,18, - DH5C-12-3*** 2,38,8 - DH5C , 289,81 DH5C , 54,719 The ame number of aterik indicate the ame pecimen ued (retet) Figure 9 (b) how the reult for DH5C The CFRP trip debonded near the drilled hole after about 89, cycle, then the fatigue crack initiated from around 22, to 27, cycle. Finally, the pecimen failed after 289,91 cycle. On the other hand, in the DH5C erie, fatigue tet under the tre range σ n of 8 and 1 MPa reached the fatigue limit. The pecimen failed under the tre range σ n of 12 and 14 MPa. However, in all the erie, the fatigue crack initiated after debonding of the CFRP trip. A mentioned above, two kind of failure mode, namely the debonding of the CFRP trip and the fatigue failure of the pecimen, were oberved in thi tudy. The number of cycle until the debonding of the CFRP trip near the drilled hole wa defined a the debonding life, N d, and the number of cycle until the pecimen failed wa defined a the failure life, N p. Table 4 and Figure 11 how the N d and N p value of all the pecimen. The fatigue limit were reached with tre range of 8 MPa for the DH erie and 1 MPa for the DH5C erie. In the tre range of 12 MPa or over, the debonding of the CFRP trip became dominant in the durability aement. However, the fatigue life wa dratically improved compared to the DH erie. A hown in Figure 6, the train in the drilled hole wa lower in all the repaired erie (except DH5C-14) than in the DH-8 erie, which reached the fatigue limit. Therefore, the fatigue limit might be achieved in the tre range of 12 MPa if the debonding of the CFRP trip can be prevented. Stre range σ (MPa) DH Failure life DH5C Failure life Debonding life Retet Retet Retet Nubmer of cycle N Figure 11. S-N curve 5 CONCLUSION In thi tudy, a repair method combining drilled hole with externally bonded CFRP trip wa propoed, and tenile tet and fatigue tet were carried out in order to verify the effect of the fatigue-crack repair uing pecimen with welded web guet plate. The following concluion were obtained: (1) The externally bonded CFRP trip bore a part of the tre of the edge of the drilled hole, and the train concentration of the hole edge wa reduced dratically. (2) Since the platic train of the hole edge wa reduced by the externally bonded CFRP trip, the repair effect increaed with increaing tenile tre. (3) In thi tudy, the fatigue limit wa achieved in the tre range of 1 MPa when uing the drilled hole and five layer of externally bonded CFRP trip. (4) In the tre range of 12 MPa and over, the debonding of the CFRP trip became dominant in the durability aement. However, 1314

8 the fatigue life wa improved dratically compared to the ue of drilled hole only. (5) Fatigue crack were not initiated before the CFRP trip debonded near the drilled hole. The repair effect would be greater if the debonding of the CFRP trip could be prevented. Although the durability of the adheive joint became dominant in higher tre range, a ignificant effect on the fatigue-crack repair wa confirmed when combining drilled hole with the application of externally bonded CFRP trip. REFERENCES Fiher, J.W Fatigue and Fracture in Steel Bridge. New York, USA: John Wiley & Son. Nakamura, H., Jiang, W., Suzuki, H., Maeda, K., Irube, T. 29. Experimental tudy on repair of fatigue crack at welded web guet joint uing CFRP trip. Thin-Walled Structure, 47(1): : Elevier. Mori, T. & Uchida, D. 21. Fatigue Strength of Out-of-Plane Guet Welded Joint Repaired by Bolting-Stop-Hole Method, Steel Contruction Engineering, JSSC. 8(29): (in Japanee) Suzuki, H. & Okamoto, Y. 23. Repair of Steel Member with a Fatigue Crack uing the Carbon Fiber Reinforced Polymer Strip, Journal of Contructional Steel, JSSC. 11: (in Japanee) Ihikawa, T. & Okura, I. 21. Required Length and Optimum Stiffne of Multiple-Stepped CFRP Strip Bonded to Steel Plate, Journal of Japan Society of Civil Engineer, Ser. A, JSCE. 66(2): (in Japanee) Uchida, D. 27. Fatigue Strength Evaluation of Out-of-Plane Guet Welded Joint Repaired by Bolting-Stop-Hole Method, Mitui Zoen Technical Review, 19: (in Japanee) Liu, H., Xiao, Z., Zhao, X. L. & Al-Mahaidi, R. 29. Prediction of Fatigue Life for CFRP-Strengthened Steel Plate, Thin-Walled Structure, 47(1): : Elevier. 1315