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Inspection and Maintenance of Bridge Stay Cable Systems (2005)

Chapter: Chapter Four - Maintenance and Repair of Stay Cables

« Previous: Chapter Three - Inspection and Monitoring Techniques
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Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Page 53
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
Page 53
Page 54
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
Page 54
Page 55
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
Page 55
Page 56
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
Page 56
Page 57
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
Page 57
Page 58
Suggested Citation:"Chapter Four - Maintenance and Repair of Stay Cables." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Page 58

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50 REPAIR AND RETROFIT STRATEGIES AND METHODS The maintenance manuals of 11 bridges were reviewed in this study. There were major differences between the approaches and contents of the different manuals. Although a few manu- als included procedures for the repair of cable sheathing and replacement of cables, others did not provide such information. There were major differences as to the level of detail between different manuals, even for the coverage of the same topics. The as-designed and as-built cable forces, deck profile eleva- tions, and others, are typically not included in the manuals and are not required to be measured during inspections. Procedures for checking for moisture and evidence of vibration problems are generally lacking in many maintenance manuals. The following is a list of the items found in the different manuals that are related to stay cables: • Bridge description; • Design considerations; • Loads; • Stay cable details: identification numbers, number of strands and wires, diameter of cable, mass per unit length, inclination angles, length, estimated cable tension at the end of construction and after creep and shrinkage effects are taken into account; • Stay cable shop drawings including as-built anchorage design, materials used, any repairs done during construc- tion, and history of problems during construction; • Inspection and maintenance frequencies, and qualifica- tions of inspection teams; • Information on access: platforms, ladders, and snooper trucks; • Cable retensioning procedures; • Cable replacement procedures including traffic patterns and specific replacement procedures; • Inspection procedures for anchorages, guide pipes, neo- prene boots, neoprene washers, sheathing, cross cables, dampers, and so forth, including identification of criti- cal areas and how and where to look for moisture and corrosion; • Listing of designers, contractors, and suppliers of stay cables and components; • Summary of qualification test results for MTEs; • Summary of qualification test results for the entire stay cable system; • Deck elevation surveys; • Repair procedures including sheathing repair, PVF tape repair, repair of damage to guide pipes, and welded connections; • Safety and traffic control during inspections; • Description of methods for measuring cable forces; • Inspection forms; and • Deck elevation survey forms. The examination of the maintenance manuals did not iden- tify cases where any of the following methods discussed in the literature were included in the manuals: • Recommendations for baseline measurements of cable frequency, damping ratio, cable sag, and cable inclina- tion angles (at specific points accessible by inspectors). Such measurements can be taken when the effects of creep and shrinkage have dissipated. Such measurements could also include air and structure temperatures. • The designer’s estimated (calculated) cable frequencies, sag, and inclination angles (at a specific point) with and without the effects of cross cables or dampers (if used). This information could be provided for different ambient temperatures. • The designer’s estimated (calculated) bending stiffness and damping of cable in the free length and in the anchorage zones. • The designer’s estimated (calculated) stiffness of neo- prene rings and/or proprietary dampers in contact with the cable. • Procedures for checking if viscous or other dampers are actually working as intended including maintenance pro- cedures for dampers. • The designer’s estimated (calculated) wind speeds at which vibrations owing to vortex shedding would be expected. • The designer’s calculated values of the “precursor trans- formation matrix.” This matrix would be required if the damage detection methodology, Precursor Transforma- tion Method (Tabatabai et al. 1998b), is employed in the future. This method uses a linearly elastic finite-element model of the bridge. In the computer model, the temper- ature of the cables are, one by one, raised by say 100 degrees, and the force changes in all other cables are noted. Each column in the transformation matrix would consist of cable force changes associated with tempera- CHAPTER FOUR MAINTENANCE AND REPAIR OF STAY CABLES

51 ture increase in a particular cable. Temperature increases are meant to represent loss of stiffness of individual cables without the need to modify the cable stiffness. Future measured cable force changes can then be used together with the transformation matrix to identify cables that have suffered stiffness losses. A similar transforma- tion matrix can be formed that is related to deck eleva- tions instead of cable forces. Other sources of damage, such as support settlements, can also be incorporated. There are very few components of the common stay cables (i.e., those that have been designed over the last 30 years) that could be considered repairable. Practically, the only items that the inspectors and maintenance engineers can realistically repair are the HDPE cable sheathings, neoprene boots, and possibly the elastomeric rings. Retrofitting for vibration con- trol can also be done. However, repair of corrosion or fatigue damage to MTEs in the free length or anchorages of older cables (not the newer designs) is practically impossible, short of removal of the entire cable. The removal process itself is a major challenge and a significant undertaking, especially on older bridges. The main task of the maintenance engineer and inspectors is therefore prevention, especially control of mois- ture (from internal and external sources) and elimination of excessive vibrations. If preventive measures fail, the mainte- nance engineer must then have a reliable tool to determine if a cable or cables must be replaced and when they should be replaced. A number of options are available with regard to repair of damaged or cracked HDPE sheathing. For minor localized damage, conventional wrapping with PVF tape is typically done, although this is believed by some not to be effective. When the HDPE has cracked or has more widespread dam- age, then a more extensive repair must be considered. The options include an elastomeric wrap system and a two-piece HDPE pipe that snaps together to form a cover for the original pipe. The elastomeric wrap is installed with an automatic wrapping device with 50% overlap. Within 24 h after wrap- ping, the wrap is heated to fuse the seams and shrink the wrap against the cable. The ends of the wrap must be secured firmly to prevent lifting. The maintenance manual for the James River Bridge in Vir- ginia included procedures for the repair of longitudinal splits in PE sheathing. This involves removal of the existing film tape, cleaning of the damaged area at least 3 ft above and below the split, filling of the crack with a suitable polyurethane grout or other compatible material to obtain a smooth surface, using 8 mil polyester film tape with fiberglass reinforcement to wrap the cable from 2 ft below to 2 ft above the split with minimum of 50% overlap, and wrapping again with PVF film. The available choices for the repair of steel sheathings are far more limited, and there is no known track record for the effectiveness of such repairs. A report prepared for the Delaware DOT recommends application of flexible liquid mastic to the cracks on a steel saddle pipe and continual inspections. Figure 66 shows the results of the survey as related to the repair of stay cables. Approximately 30% of cables in the United States and Canada have had some form of repair. MITIGATION OF STAY CABLE VIBRATIONS A wide variety of solutions to the problem of stay cable vibra- tions have been proposed and/or implemented. These mitiga- tion approaches can be categorized as modifications to the surface of HDPE pipe, cross cables, viscous dampers, visco- elastic dampers, friction dampers, tuned mass dampers, semi- active and active dampers, and others. In this section, a brief summary of each approach is given. Modifications to the Surface of HDPE Pipes As discussed earlier, the formation of rivulets on the surface of the cable is believed to be the cause of rain–wind vibrations. Therefore, a very popular and effective approach has been to modify the surface of the cable to break up and disrupt the flow of water, thus not allowing the formation of rivulets. A very common form of this modification is helical or spiral marks, fillets, or ribs on the surface of HDPE pipe as shown in Figure 67. Figure 68 provides wind tunnel results with and FIGURE 66 Percentage of bridges that have had cables repaired. 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 yes no not known no answer Cables Repaired? Pe rc en t o f B rid ge s U.S. Canada

without surface modifications. According to the wind tunnel tests by Larose and Wagner Smitt (1999), in some cases, the rain–wind vibrations persisted with limited amplitudes even with the helical fillets. Other, less frequently used options are dimples or longi- tudinal ribs on the surface (Figure 69). Surface dimples were used on the Tatara Bridge in Japan, and Yamaguchi et al. (1999) reported that they were effective in controlling rain– wind vibrations. Cross Cables Cross cables, secondary cables, cable restrainers, or cross ties are used to connect different stay cables within a cable plane. Figure 70 shows a cross cable installed on SR-46 over the East Fork White River in Indiana. These transverse connections reduce the effective length of the cable and increase cable frequency (Ito 1999). They also somewhat increase cable damping (Lankin et al. 2000). Yamaguchi and Nagahawatta (1995) performed experimental and analytical research on the damping effects of cable cross ties. The experiments consisted of two cables connected with two cross ties. They concluded that “there exists a more or less 52 damping effect” from cross ties, which can be increased by using more flexible and dissipative ties. Cable restrainers have also been used as a temporary solu- tion to rain–wind vibrations (Poston 2002). Figure 71a shows the restrainer system with three lines of cables, and Figures 71b and c, respectively, show the measured vibration ampli- tudes before and after installation of restrainers. During the construction of the Burlington Bridge over the Mississippi in Iowa, several incidences of rain–wind vi- brations were observed with amplitudes of up to 0.6 m (2 ft) (Bierwagen no date). Bierwagen reports that temporary ropes in the form of 25-mm or 1-in.-diameter Manila ropes were first used to help tie the cables down. However, the Manila rope broke during a subsequent occurrence of vibrations. There- FIGURE 67 Spiral strakes on the surface of HDPE pipe to control rain–wind vibrations. FIGURE 68 Effect of surface modifications on vibration amplitudes (Stubler 1999). Duct dynamic efficiency 5m/s 10m/s Vibration amplitude Wind velocity 15m/s Filet No Filet 0.05 0.5 1 m FIGURE 69 Surface modifications on the HDPE (Matsumoto 2000). FIGURE 70 Cross cable installed on a bridge in Indiana (courtesy: Indiana DOT).

53 fore, a cross cable system was designed and implemented. Figure 72 shows the layout of cross cables (top) and the method of connection to the cables (bottom). According to Bierwagen, the restraint system included 12.6-mm or 0.5-in.- diameter zinc-coated wire ropes that crisscross through the cables and are attached to them using friction clamps. Similar cross cables have also been used on the Clark Bridge in Alton, Illinois. It is reported that the cross cables should be tensioned pro- perly to prevent slacking of the restrainers (Bournand 1999). Bournand reported that the cross cables on the Fred Hartman Bridge in Texas failed one year after installation as a result of fatigue and fretting. He suggests that “the cables must be designed using a flexible wire rope or similar system (with high internal damping) and with good fatigue and wear resistance.” This system was installed on the bridge. Some observers also FIGURE 71 (a) Cable restrainer, (b) vibrations before installation of cross cables, (c) vibrations after installation of restrainers (Poston 2002). (a) (b) (c) 0 0 5 10 15 20 25 30 35 40 0.5 1 1.5 2 2.5 3 3.5 1- m in ut e RM S ac ce le ra tio n (g ) 1-minute mean deck-level wind speed (mph) 4 0 0 5 10 15 20 25 30 35 40 0.5 1 1.5 2 2.5 3 3.5 1- m in ut e RM S ac ce le ra tio n (g ) 1-minute mean deck-level wind speed (mph) 4

believe that cross cables reduce the aesthetic quality of cable- stayed bridges (Johnson et al. 2002). Bloomstine and Stoltzner (1999) reported on the failure of a wire cross cable on the Faroe cable-stayed bridge in Denmark. The original system consisted of steel brackets with neoprene linings attached to the cables with stainless steel wire connected in between them. The wires “were wrapped around a thick washer in the bracket and secured by two wire locks.” Abrasion between wire and the washer caused the first wire failure after 4 years. A new system using 10-mm marine grade stainless steel wire and turnbuckles was used. Many respondents to the survey had positive views con- cerning cross cables, with the ability to inspect them and know whether they are working given as an important factor. Figure 73 shows the results of the survey as related to the use of cross cables. 54 Viscous Dampers In this section, the application of mechanical viscous dampers for suppression of stay cable vibrations is discussed. In gen- eral, the term “viscous damper” used here refers to a mechan- ical damper that generates force proportional to the velocity of piston movements (i.e., it can be idealized as a dashpot). Other investigators sometimes prefer to use the terms “oil damper” or “hydraulic damper,” and distinguish them from viscous damper. In this discussion, they are all referred to as viscous damper as long as they meet the definition given. Viscous dampers for stay cables have been installed on a large number of cable-stayed bridges worldwide, including the Sunshine Skyway Bridge, Cochrane Bridge, and Erasmus Bridge. Figure 74 shows a schematic of a cable of length L, with a viscous damper positioned at a distance of Ld from one end. FIGURE 72 Cable restraint system for the Burlington Bridge in Iowa (Bierwagen no date). 660' SPAN MS1 CABLE TIE-DOWNS (TYPICAL) 405' SPAN MS2

55 Several researchers have proposed numerical approaches for determining the contribution of a viscous damper to the overall cable damping. Some of the earlier works were by Kovacs (1982), Yoneda and Maeda (1989), and Pacheco et al. (1993). Each idealized the cable as a taut string when deriving their formulations. In 1999, Xu et al. presented results of their experimental study on control of cable vibrations using vis- cous dampers. Tabatabai and Mehrabi (2000) presented a nondimensio- nal formulation that included the effects of cable sag and bending stiffness, and performed parametric studies (using cable parameter ranges from a database of stay cables) to develop an equation for calculating the first mode damping contribution by a viscous damper. This study indicated that the influence of cable sag was insignificant for the range of parameters found in stay cables. However, the influence of cable bending stiffness was found to be important, as dampers are typically located close to the anchorages. Although their formulation was applicable to higher modes as well, their proposed equation was optimized for the first mode only. Main and Jones (2002) investigated the multi-mode contri- bution of a linear viscous damper attached to a taut string. They pointed out that damper performance at higher modes is of particular interest, because vibrations occur over a wide range of cable modes. The influences of sag and bending stiff- ness were ignored. Main and Jones (2001) discussed the installation of two viscous dampers on the Fred Hartman Bridge in Texas. They analyzed the pre- and post-damper installation response of the cables, and showed that although the dampers were designed for the first mode, they were very effective in controlling all of the high-amplitude vibrations that had been observed be- fore damper installation. There is a rough “rule-of-thumb” that can be used to esti- mate the maximum achievable damping (in fraction of criti- cal damping). The maximum damping is approximated as 0.5(Ld/L) (Lankin et al. 2000). Therefore, if a damper is located at 2% of the length of the cable, then the maximum achievable damping is 1%. It is important to realize that the theoretical end of the cable from which Ld is calculated is generally dif- ferent from the actual end. The complicating factors are the varying bending stiffness of the cable at the end, the presence of neoprene dampers, and the presence of steel sockets. Tabatabai et al. (1998b) presented approximate relationships that allow determination of an equivalent effective length for different end conditions. FIGURE 73 Frequency of the use of cross cables. FIGURE 74 Idealized cable with viscous damper (Tabatabai and Mehrabi 2000). 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 yes - from the beginning yes - retrofitted to correct vibrations no no answer Cross Cables Pe rc en t o f B rid ge s U.S. Canada

The respondents to the questionnaire indicated seven bridges with viscous dampers, most of which were installed to correct observed vibrations. There is some concern expected about potential leakage of fluids in such dampers. Viscoelastic Dampers The commonly used neoprene ring can essentially serve as a viscoelastic damper; however, because of difficulty with con- sistent installation and their variability, their level of damping contribution is difficult to estimate. Tabatabai and Mehrabi (2000) performed tests on a scale stay cable model with and without neoprene rings. The neoprene rings increased damp- ing by more than 10-fold to 0.6% of critical damping. There is at least one cable supplier that is supplying pro- prietary viscoelastic and hydraulic damping systems. These systems are placed between the HDPE and the guide pipe (or an extension of guide pipe). The viscoelastic damper uses a shaped elastomeric material to damp cable vibrations (Fig- ure 75). It is expected that most suppliers would have their own damping systems in the near future. Another form of viscoelastic damper is the Super High- Damping Rubber Damper (SDR). Mizoe et al. (1999) pre- sented a damping device that is installed between the cable and the guide pipe (or anchor pipe). When the cable moves, a relative displacement occurs between the cable and the guide pipe causing shear deformations in the damping ma- terial. A high-damping material is developed by combining styrene butadiene rubber, high-damping carbon, and some plastics to achieve its properties. Figure 76 shows the SDR damper. These dampers were first installed on two cables of the Meiko East Bridge in Japan for testing. The authors reported that the damping level achieved was confirmed with calculated values. Subsequently, these dampers were installed on most cables of this bridge. They have been in service since 1998, and wind-induced vibrations have reportedly not been observed. 56 Friction Dampers Bournand (1999) reported on the development of a friction damper for stay cables (see Figure 77). This damper system has two parts; a movable part that is attached to the strands by a bolted collar and a fixed part that is bolted to the steel support pipe. The bolted collar has several “friction wings,” and the fixed part has several “spring ring blades supporting several friction screws.” The ring blades are deflected to have a steady friction contact of the friction screws. This damper type has reportedly been installed on the Uddevalla Bridge in Sweden. Semi-Active Dampers Johnson et al. (2002) presented a theoretical discussion and described the development of semi-active damping for stay cables. A semi-active damper can be a variable-orifice vis- cous damper, a controllable friction damper, or a controllable fluid damper (Johnson et al. 2000). Computational simula- tions were used to examine the effectiveness of semi-active damping. The authors reported that the potential for using semi-active dampers to control stay cable vibrations “has been demonstrated” in comparison with passive viscous dampers. FIGURE 75 Viscoelastic and hydraulic dampers (Stubler et al. 1999). FIGURE 76 SDR damper (Mizoe et al. 1999).

57 Using an optimal control algorithm, the authors stated that a simulated semi-active damper located at 2% of the distance from the end of cable reduced responses by 71% compared with an optimal viscous damper and 72% compared with fully active devices. Johnson et al. (2000) reported on laboratory experiments on scaled stay cables with a magnetorheological (MR) fluid damper. MR dampers are a type of semi-active damper (con- trollable fluid) in which the yield stress of the fluid is change- able through variations in magnetic field strength. Laboratory results indicated that the damper was able to achieve “signif- icant” response reductions, but not to the level expected from simulations. Recommendations were made for addressing this problem in future studies. Ko et al. (2002) reported on field tests of stay cables with MR dampers on the Dongting Lake Bridge in China. Field measurements were taken before and after damper installa- tions. The equivalent damping level was found to be depen- dent on damper location, voltage applied to damper, and the level of vibration. Under optimum voltage input, the damping ratios for the second and third modes can reportedly be greater than 0.8% of critical damping. These semi-active dampers are commercially available. Figure 78 shows MR damper installation on a bridge cable in China. Tuned Mass Dampers The tuned mass damper (TMD) is tuned to a particular fre- quency of interest; for example, the first mode of the cable. The TMD, in its basic mathematical representation, consists of a mass, a spring, and a damping component. By changing the basic properties of the damper, the TMD can be tuned to the right frequency. TMDs have been applied to a variety of structures including power line cables. Tabatabai and Mehrabi (1999) patented a shaped viscoelastic TMD for stay cables. The main advantage of the TMD is that it is not restricted to the cable ends. The main disadvantage is that it can only be tuned to a particular frequency, and its effectiveness is reduced at other frequencies. Jensen et al. (2002) proposed using a TMD between two cables at mid-length. In their article, the authors present an analytical formulation for their concept. Other Damping Systems Tabatabai and Mehrabi (2000) reported on damping tests on a scale model of a stay cable. They tested a number of approaches for cable damping including using common neoprene rings, latex grout as filler inside HDPE, a liquid damper, application of spiral adhesive damping tapes around HDPE, and filling of the guide pipes with a low stiffness polyurethane. They concluded that the conventionally used neoprene ring improved cable damping significantly to 0.4% to 0.6% of critical damping (compared with a damping of 0.05% for cable without neoprene ring). They suggested that the effectiveness of neoprene rings is influenced by the degree of precompression in the neoprene ring and any re- straint of ring movement in the transverse direction. The use of latex grout increased cable damping by 60%, but not to the level needed for control of rain–wind vibrations. They also concluded that the liquid damper and damping tapes did not significantly improve damping. Filling of guide pipes FIGURE 78 MR damper installation on bridge cable in China (Ko et al. 2002). Cable Support 2. 25 0. 30 0. 24 0.1 8 Foundation MR damper0.14 54° FIGURE 77 Friction damper (Bournand 1999). Friction Damper Anchorage Head Sta y P ipe Damper Steel Support Pipe

around HDPE with polyurethane improved cable damping somewhat, but not to the level achieved by a properly in- stalled neoprene ring. The authors suggested experimenting with higher stiffness polyurethanes as a possible effective approach. It is interesting to note however that Yamaguchi et al. (1999) reported that filling “rubber seals into the entrance of cable in the girder” in some cables of the Tatara Bridge in Japan reduced incidences of vortex-induced vibrations, and they consider this to be an efficient solution for long cables. There are a number of other patented concepts for damp- ing stay cable vibrations including flexible damper bands by Sarkar et al. (2002) and two separate patents on active damper bands with shiftable mass by Phelan et al. (2002, 2004). 58 CHALLENGES IN MAINTENANCE, REPAIR, AND RETROFIT The main challenges in maintenance and repair are: • Determining the condition of the anchorage elements, especially those that include cement grout or epoxy fillers, at reasonable cost and with reasonable confidence. • Proper and safe access for inspection of cables. • Methods and procedures to replace existing cables on a number of aging bridges, when needed. • Control and elimination of moisture and corrosion inside cable components. • Vibration control and fatigue issues associated with vibrations. • Insufficient sharing of knowledge and training for those responsible for maintaining cable-stayed bridges.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 353: Inspection and Maintenance of Bridge Stay Cable Systems identifies and explains various inspection and maintenance techniques for bridge stay cable systems. It discusses both short- and long-term approaches. The report information on methods for inspections and assessments, including nondestructive testing and evaluation procedures; repair and retrofit; methods for control of cable vibrations, including rain–wind vibrations; stay cable fatigue and failure; effectiveness of various inspection and repair methods; limitations of available technologies; and trends and recommendations for future study.

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