Injection repair of fatigue cracks in composite materials

Abstract

A system and method for repairing fatigue cracks in a multilayer composite (140). The method includes repairing a fatigue crack at a first interface between a first layer of the multilayer composite and a second layer of the multilayer composite. The method further includes repairing a fatigue crack at a second interface between a second layer of the multilayer composite and a third layer of the multilayer composite. The method further includes anchoring the second interface with a third layer of the multilayer composite to prevent failure caused by fatigue cracks.

Description

Injection Repair of Fatigue Cracks in Composite Materials Background Technology

[0001] Turbine blades are the primary components in wind turbines that convert wind energy into electrical energy. Turbine blades typically consist of suction-side and pressure-side shell components, which are bonded together at adhesive joints at the leading and trailing edges of the blade. Adhesive joints are generally formed by applying an appropriate width of adhesive paste or compound along the adhesive joint between the shell components at a minimum designed adhesive joint width.

[0002] Turbine blade defects arise from either the original manufacturing process or the operating conditions of the blade. For example, fatigue cracks are common in the multi-layered composite structures within turbine blades, such as the root bushing. These defects can cause the turbine blade to detach from its root bushing and separate from the rotor hub. Structurally, the metal bushing at the turbine blade root may become loose as it wears and may fail to withstand the loads of the rotating blade. The rotating blade typically slides down from the bottom of the root bushing during downward oscillations and impacts the root bushing during upward oscillations, subjecting the root bushing to alternating cycles of tensile and compressive forces. As a result, a gap may develop between the blade and the root bushing. The root bushing, which is firmly attached to the rotor hub, may eventually yield and release the blade, leading to catastrophic turbine failure.

[0003] Existing solutions in the wind energy industry include completely scrapping the faulty blade and procuring a new replacement. Alternatively, the damaged blade must be removed, repair holes drilled into the damaged sleeve, and the sleeve treated with adhesive before reinstallation. In this case, the ridges inside the damaged sleeve are typically drilled out, and the repair method relies entirely on the strength of the newly introduced adhesive.

[0004] Another common industry solution for subsurface defects in blade laminates requires sanding the affected blade area and reapplying the laminate material. Defects are removed by sanding / grinding the laminate layer until they are exposed. The laminate layer is then reapplied and sanded smooth. Typically, an "over-laminate" layer is added to the repaired area to enhance strength. However, this extra laminate layer extends above the planar surface of the surrounding blade area, disrupting airflow over the blade and reducing aerodynamic performance. Furthermore, the sanding / overlay repair procedure requires extensive surface preparation and skill in applying the repair laminate material, followed by sanding, lamination, and painting—all of which are laborious and time-consuming.

[0005] Fatigue cracking is prevalent in the wind energy industry across many different blade designs and / or types, and transferring loads from metal to composite materials has always been a challenge. The wind turbine blade manufacturing industry has consistently failed to satisfactorily address manufacturing defects and / or operational condition deficiencies. In fact, wind turbine blade design and development have focused primarily on the blade itself rather than the root sleeve. As a result, performance suffers, and the failure rate of wind turbine blades at the root sleeve is higher than it should be.

[0006] In addition, on-site repair of blade defects is usually required to ensure efficient operation of the wind turbine at its design life and rated power.

[0007] Therefore, the industry will benefit from an improved wind turbine blade repair procedure that is less time-consuming, particularly suitable for on-site repairs, and enables consistent and structurally reliable repairs. Attached Figure Description

[0008] The accompanying drawings, included to provide a further understanding of the disclosed subject matter, are incorporated into and form part of this specification. The drawings also illustrate embodiments of the disclosed subject matter and, together with the detailed description, explain the principles of these embodiments. No attempt is made to demonstrate more detailed structural details than are necessary for understanding the disclosed subject matter and the various ways in which it can be practiced.

[0009] Figure 1 is a schematic perspective view of a conventional wind turbine.

[0010] Figure 2 is a schematic cross-sectional view of a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure.

[0011] Figure 3 is a schematic overview of the formation of defects in the root sleeve of a wind turbine rotor blade according to an embodiment of the present disclosure.

[0012] Figure 4 is a schematic overview of the process for repairing a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure.

[0013] Figure 5 is a schematic cross-sectional view of a repaired wind turbine rotor blade root sleeve according to an embodiment of the present disclosure.

[0014] Figure 6 is a schematic flowchart of composite material fatigue crack injection repair according to an embodiment of the present disclosure.

[0015] Figure 7 is a schematic illustration of the defects in a cross-sectional view of a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure.

[0016] Figure 8A is a schematic top view of a repaired root sleeve for a damaged wind turbine rotor blade according to an embodiment of the present disclosure.

[0017] Figure 8B is a schematic front view of the repaired damaged wind turbine rotor blade root sleeve, corresponding to the top view of Figure 8A.

[0018] Figure 9 is a schematic view of a drill template according to an embodiment of the present disclosure.

[0019] Figure 10A is a schematic top view of a C-type repair of a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure.

[0020] Figure 10B is a schematic front view of a C-type repair of a damaged wind turbine rotor blade root sleeve, corresponding to the top view of Figure 10A.

[0021] Figure 11 is a schematic method for repairing root sleeves installed in the root region of wind turbine blades.

[0022] Figure 12 is a schematic method for repairing fatigue cracks at the first interface between the first and second layers of a multilayer composite.

[0023] Figure 13 is a schematic method for repairing fatigue cracks at the first interface between the first and second layers of a multilayer composite.

[0024] Figure 14 is a schematic method for repairing fatigue cracks at the first interface between the first and second layers of a multilayer composite.

[0025] Figure 15 is a schematic view of a root sleeve repair kit for damaged wind turbine blades. Detailed Implementation

[0026] Various aspects or features of this disclosure are described with reference to the accompanying drawings, wherein like reference numerals are used to refer to like elements. Numerous specific details are set forth in this specification to provide a thorough understanding of this disclosure. However, it should be understood that certain aspects of this disclosure may be practiced without these specific details, or with other methods, components, materials, etc. In other instances, well-known structures and devices are shown in block diagram form to facilitate the description of the disclosed subject matter.

[0027] Various embodiments of the disclosed subject matter generally relate to and can provide improvements to apparatuses, systems, and methods in the field of wind turbines, and relate to methods, systems, and kits for repairing damaged wind turbine blade root sleeves. More specifically, the disclosed subject matter relates to a process for repairing fatigue cracks in composite laminate structures by utilizing an injection method that injects a structural adhesive into cracked and porous composite laminates. As a non-limiting example, the adhesive may be a self-etching thermoplastic adhesive that facilitates bonding to surfaces contaminated by hydraulic pitch system oil or grease leaks. Furthermore, the thermoplastic adhesive can be heated and reheated and is essentially “self-healing.” In other words, if damage is found during post-repair inspection, a heat blanket can be applied and the defective repair can be repaired again without re-injecting new adhesive. The process also uses the same method to inject cleaning chemicals and surface preparation chemicals to prepare multiple composite bonding surfaces for adhesive injection application.

[0028] In one aspect of the disclosed subject matter, a method for repairing fatigue cracks in a multilayer composite is disclosed. The method includes repairing fatigue cracks at a first interface between a first and a second layer of the multilayer composite, and repairing fatigue cracks at a second interface between a second and a third layer of the multilayer composite. The method may further include anchoring the second interface to a third layer of the multilayer composite to prevent failure caused by the fatigue cracks.

[0029] A method for repairing fatigue cracks at the first interface between the first and second layers of a multilayer composite may include drilling a first series of flushing holes and a first series of seepage holes extending from the outer surface of the composite to the first interface. The method may further include flushing the first series of flushing holes and the first series of seepage holes with a high-pressure flushing fluid and drying them with an air jet. The method may also include drilling a first series of filling holes extending from the outer surface of the composite to the first interface and installing fluid injection ports in the first series of filling holes. The method further includes injecting crack repair fluid through the injection ports to fill the first series of filling holes and thermally curing the crack repair fluid deposited in the first series of filling holes.

[0030] Fatigue cracks at the second interface between the second and third layers of a multilayer composite can be repaired by drilling a second series of flushing holes and a second series of seepage holes extending from the outer surface of the composite to the second interface, flushing the second series of flushing holes and the second series of seepage holes with high-pressure flushing fluid, and drying the second series of flushing holes and the second series of seepage holes with air jets. Furthermore, a second series of filling holes can be drilled from the outer surface of the composite to the second interface, and fluid injection ports can be installed in the second series of filling holes. The method can also include injecting crack repair fluid through the injection ports to fill the second series of filling holes, and thermally curing the crack repair fluid deposited in the second series of filling holes. The second series of filling holes can be arranged and spaced relative to the first series of filling holes in a predetermined force transmission pattern.

[0031] Anchoring the second interface to prevent failure due to fatigue cracking of the composite may include tilting some of the second series of filler holes to make them transverse to the first series of filler holes, and extending at least one of the tilted second series of filler holes from the third layer through the second layer to the first layer.

[0032] The method may also include repairing fatigue cracks at the third interface between the third and fourth layers of the multilayer composite and anchoring the third interface to the fourth layer of the multilayer composite to prevent failure caused by fatigue cracks.

[0033] Repairing fatigue cracks at the third interface between the third and fourth layers of a multilayer composite may include drilling a third series of flushing holes and a third series of seepage holes extending from the outer surface of the composite to the third interface; flushing the third series of flushing holes and the third series of seepage holes with a high-pressure flushing fluid; and drying the third series of flushing holes and the third series of seepage holes with an air jet. The method may also include drilling a third series of filling holes extending from the outer surface of the composite to the third interface and installing fluid injection ports in the third series of filling holes. The method may further include injecting crack repair fluid through the injection ports to fill the third series of filling holes and thermally curing the crack repair fluid deposited in the third series of filling holes. The third series of filling holes may be arranged and spaced relative to the second series of filling holes in a predetermined force transmission pattern.

[0034] Anchoring the third interface to prevent failure due to fatigue cracking of the composite may include tilting some of the third series of filler holes to make them transverse to the second series of filler holes, and extending at least one of the tilted third series of filler holes from the fourth layer through the third layer to the second layer.

[0035] In one aspect of the disclosed subject matter, a wind turbine blade with a repaired root sleeve is disclosed. The wind turbine blade may include a blade body having a pressure side and a suction side joined at the leading and trailing edges. The blade body may extend longitudinally from a root region through a transition region to a tip region, the transition region extending between the root and tip regions. The blade body may be mechanically connected to a rotor hub via a plurality of root sleeves mounted in the root region. The repaired root sleeve may include a multilayer composite having fatigue cracks repaired at a first interface between a first and second layer of the multilayer composite. Additional fatigue cracks may be repaired at a second interface between a second and third layer of the multilayer composite. The second interface is anchored to a third layer of the multilayer composite.

[0036] In operation, fatigue cracks at the first interface between the first and second layers of a multilayer composite can be repaired by drilling a series of flushing holes from the outer surface of the composite through the root sleeve to the first interface. The flushing holes can be flushed first with a high-pressure flushing fluid, followed by air jet drying. Alternatively, a series of exudation holes can be drilled from the outer surface of the composite to the first interface. Similar to the flushing holes, the exudation holes can be flushed with a high-pressure flushing fluid and dried with air jet drying. Furthermore, a series of filling holes can be drilled from the outer surface of the composite to the first interface, and fluid injection ports can be installed in these filling holes. Crack repair adhesive fluid can be injected through the injection ports and deposited in the first series of filling holes. The crack repair adhesive fluid deposited in the first series of filling holes can be thermosetting.

[0037] In a similar structure, fatigue cracks at the second interface between the second and third layers of a multilayer composite can be repaired by drilling a second series of flushing holes from the outer surface of the composite through the root sleeve to the second interface. These flushing holes can be flushed first with a high-pressure flushing fluid, followed by air jet drying. A second series of exudation holes can also be drilled from the outer surface of the composite to the second interface. Similar to the flushing holes, these exudation holes can be flushed with a high-pressure flushing fluid and dried with air jet drying. Furthermore, a second series of filling holes can be drilled from the outer surface of the composite to the second interface, and fluid injection ports can be installed in these filling holes. Crack repair adhesive fluid can be injected through the injection ports and deposited in the second series of filling holes. The crack repair adhesive fluid deposited in the second series of filling holes can be thermosetting.

[0038] The second series of filling holes can be arranged and spaced relative to the first series of filling holes in a predetermined force transmission pattern. Furthermore, some of the second series of filling holes can be inclined laterally to the first series of filling holes and extend from the third layer through the second layer to the first layer to anchor the second interface to the third layer of the multilayer composite.

[0039] Wind turbine blades may include additional fatigue cracks repaired at the third interface between the third and fourth layers of a multilayer composite. The third interface may be anchored to the fourth layer of the multilayer composite.

[0040] In a similar structure, fatigue crack repair at the third interface between the third and fourth layers of a multilayer composite can be achieved by drilling a third series of flushing holes from the outer surface of the composite's root sleeve to the third interface. These flushing holes can be first flushed with a high-pressure flushing fluid, followed by air jet drying. Alternatively, a third series of exudation holes can be drilled from the outer surface of the composite to the third interface. Similar to the flushing holes, these exudation holes can be flushed with a high-pressure flushing fluid and dried with air jet drying. Furthermore, a third series of filling holes can be drilled from the outer surface of the composite to the third interface, and fluid injection ports can be installed in these filling holes. Crack repair fluid can be injected through these injection ports and deposited in the third series of filling holes. The crack repair fluid deposited in the third series of filling holes can then be thermosetting.

[0041] The third series of filling holes can be arranged and spaced relative to the second series of filling holes in a predetermined force transmission pattern. Furthermore, some of the third series of filling holes can be inclined laterally to the second series of filling holes and extend from the fourth layer through the third layer to the second layer to anchor the third interface to the fourth layer of the multilayer composite.

[0042] In one aspect of the disclosed subject matter, a repair kit for repairing damaged root sleeves in wind turbine blades is disclosed. The repair kit may include a drill template configured and arranged around a multilayer composite. The drill template may define the drilling locations for a series of flushing holes, seepage holes, and filling holes in the multilayer composite. The repair kit may also include a quantity of crack repair fluid suitable for repairing cracks in the multilayer composite, and an injection tool configured and arranged to inject the crack repair fluid through the filling holes.

[0043] Figure 1 is a schematic perspective view of a conventional wind turbine 100. As shown, the wind turbine 100 may include a tower 112 on which a nacelle 114 is mounted. The wind turbine 100 may also include a rotatable hub (also referred to as a “rotor hub”) 118 on which a plurality of rotor blades 116 are mounted, the rotor hub 118 being connected in turn to a main flange that rotates a main rotor shaft (not shown). The power generation and control components of the wind turbine are typically housed within the nacelle 114. The views of Figure 1 are provided for illustrative purposes to place this disclosure within an exemplary field of use. It should be understood that this disclosure is not limited to any particular type of wind turbine configuration.

[0044] A wind turbine blade 116 typically includes a blade body 120 having a pressure side 122 and a suction side 124 joined at a leading edge 126 and a trailing edge 128. The blade body 120 extends longitudinally from a root region 132 through a transition region 136 to a tip region 134, the transition region 136 extending between the root region 132 and the tip region 134. The blade body 120 is mechanically connected to a rotor hub 118 via multiple root sleeves (described in more detail below in relation to Figures 2-15) mounted in the root region 132 of the wind turbine blade 116. The root sleeves can typically be made of a multi-layered composite structure. Fatigue cracks can develop at multiple interfaces between the multiple layers of the multi-layered composite structure; timely and on-tower repair of fatigue cracks can extend the life and operational efficiency of the wind turbine blade 116.

[0045] Figure 2 is a cross-sectional view of a damaged wind turbine rotor blade root sleeve 140 according to an embodiment of the present disclosure. Referring to Figure 2, the root sleeve 140 may include an impact plate 142 for mechanically connecting the root sleeve to the rotor hub 118 (Figure 1) via an exemplary end threaded rod 144. The root sleeve 140 may also include an outer sleeve laminate 146, an inner sleeve laminate 148, a sleeve tail portion 152 with a chamfered core, and a sleeve body 154. Due to fatigue wear and possible oil leakage in the hydraulic system of the wind turbine blade assembly, oil deposits 156 may be found in the outer sleeve laminate 146 or the inner sleeve laminate 148, or in the space between the two laminates, or between the inner sleeve laminate 148 and the sleeve body 154. It is well known that some wind turbines use hydraulic systems to pitch the blades, while others use electric pitch systems. Most of the affected blades happened to have a hydraulic pitch system, in which the hydraulic oil was prone to leakage. As a result, the root sleeve was coated with deposits of leaked hydraulic oil 156.

[0046] According to one embodiment of this disclosure, FIG3 is a schematic overview 160 of the defect formation process in an exemplary root sleeve 140 (FIG. 2). Multiple stress zones 162 may exist in the outer sleeve laminate 146 or the inner sleeve laminate 148, or in the space between the two laminates, or between the inner sleeve laminate 148 and the sleeve body 154, due to fatigue caused by continuous blade operation. For example, when the blade is at the 6 o'clock position 164, the weight of the blade may exert a tensile force 166 on the root sleeve 140, as shown in enlarged view 168. Furthermore, when the blade is at the 12 o'clock position 172, the weight of the blade may exert a compressive force 174 on the root sleeve 140, as shown in enlarged view 176. Over time, deposited oil 178 (as shown in enlarged view 182) and wear may lead to failure at multiple sleeve-fiber interfaces.

[0047] Figure 4 is a schematic process overview 190 of repairing a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure. Referring to Figure 4, deposited oil and contaminants 192 (also shown in enlarged view 194) may cause failure of the sleeve's connection to the fibers. The deposited oil and contaminants 192 can be removed using a rinsing method, as shown in 196 (also shown in enlarged view 198), using a specific cleaning solution. Subsequently, an adhesive repair material can be injected into the cleaned area, as shown in 202 (also shown in enlarged view 204), to repair and reinforce the damaged portion within the root sleeve 140.

[0048] Figure 5 is a cross-sectional view of a repaired wind turbine rotor blade root sleeve 210 according to an embodiment of the present disclosure. As shown in Figure 2, the root sleeve 210 may include an impact plate 142, an end threaded rod 144, an outer sleeve laminate 146, an inner sleeve laminate 148, a sleeve tail portion 152 with a chamfered core, and a sleeve body 154. The oil deposits 156 in Figure 2 have been flushed away and replaced with clean adhesive deposits 212.

[0049] Figure 6 is a schematic flowchart 220 of fatigue crack injection repair in composite materials according to an embodiment of the present disclosure. The present disclosure provides a method for repairing fatigue cracks in a multilayer composite after detecting and inspecting defects, as shown in 222. There are typically three repair types, illustratively referred to as "Type A" as shown in 224, "Type B" as shown in 226, and "Type C" as shown in 228, depending on the defect type. At the end of the Type A repair process 224 and / or the Type B repair process 226 and / or the Type C repair process 228, the repaired wind turbine blade can be inspected, as shown in 232, and is ready for operation.

[0050] To further illustrate, an exemplary Type A repair process 224 can be performed at the first interface between the first layer (typically a metal sleeve) and the second layer (typically a glass-wound layer) of the root sleeve 140. Fatigue cracks may occur in multiple failure zones located at the first interface. The fatigue cracks at the first interface between the first and second layers of the root sleeve 140 can be repaired by mapping and drilling a series of flushing holes (also referred to as “Type A flushing holes”) from the outer surface of the root sleeve 140 to the first interface, as shown in 234. The series of flushing holes can be flushed first with a high-pressure flushing fluid and then dried with an air jet, as shown in 236. A series of filling holes (also referred to as “Type A filling holes”) can be drilled from the outer surface of the root sleeve 140 to the first interface, as shown in 236, and fluid injection ports can be installed in the filling holes, as shown in 242. Crack repair adhesive fluid can be injected through the injection ports and deposited in the filling holes, as shown in 244. The crack repair adhesive fluid deposited in the filling holes can be thermosetting, as shown in 246. In fact, the repair at the first interface will transfer the faulty area from the first interface to the second interface.

[0051] Fatigue cracks can also occur at multiple other failure sites located at the second interface between the second layer (glass-wound layer) and the third layer (typically a pultruded part) of the root sleeve 140. Fatigue cracks at the second interface can be repaired using a crack repair adhesive fluid. Repair at the second interface reinforces and anchors the second interface to the third layer of the root sleeve 140 to prevent failure due to fatigue cracks.

[0052] Referring back to Figure 6, an exemplary Type B repair 226 can be performed by mapping and drilling a second series of flushing holes (also referred to as "Type B flushing holes") and a second series of filling holes from the outer surface of the composite structure of the root sleeve to the second interface. Similar to the Type A repair, the second series of flushing holes and filling holes can be flushed with a high-pressure flushing fluid and subsequently dried with an air jet. A fluid injection port can be installed in the filling hole, as shown in 252. Crack repair adhesive fluid can be injected through the injection port and deposited in the second series of filling holes, as shown in 254. The crack repair adhesive fluid deposited in the second series of filling holes can be thermosetting, as shown in 256. After the adhesive has cured, the injection port can then be removed, as shown in 258.

[0053] Referring again to Figure 6, an exemplary Type C repair 228 can be performed on fatigue cracks at the third interface between the third and fourth layers (typically glass laminates) of the root sleeve 140. A third series of flushing holes (also referred to as “Type C flushing holes”) and a third series of filling holes can be mapped and drilled, extending from the outer surface of the composite of the root sleeve to the third interface, as shown in Figure 262. Similar to Type A and Type B repairs, the third series of flushing holes and filling holes can be flushed with a high-pressure flushing fluid and subsequently dried with an air jet. Fluid injection ports can be installed in the filling holes, as shown in Figure 264. Crack repair fluid can be injected through the injection ports and deposited in the third series of filling holes, as shown in Figure 266. The crack repair fluid deposited in the third series of filling holes can be thermosetting, as shown in Figure 268. After the adhesive has cured, the injection ports can then be removed, as shown in Figure 272.

[0054] Figure 7 is a schematic illustration of a summary of defects in a cross-sectional view 280 of a damaged wind turbine rotor blade root sleeve 140 (Figure 2) according to an embodiment of the present disclosure. These views represent root-to-tip and tip-to-root cross-sections. The exemplary Type A repair 282 performed first, at the interface between the metal sleeve 284 and the glass winding layer 286, may be crucial for restoring approximately 80% of the original strength. An exemplary Type B repair 288 can then be performed at the interface between the glass winding layer 286 and the pultruded part 292. Type B repair 288 can be built upon Type A repair 282 such that a second series of infill holes (described in the context of Figure 6) can be arranged and spaced relative to a first series of infill holes (Figure 6) in a predetermined force transmission pattern to transfer the fatigue crack failure region of the first composite from the first interface to the second interface. Furthermore, some of the second series of infill holes can be inclined transversely to the first series of infill holes and extend from the third layer through the second layer to the first layer to anchor the second interface to the third layer of the multilayer composite.

[0055] The exemplary C-type repair 294 can be a third and final repair performed at the interface between the pultruded part 292 and the glass laminate 296. The C-type repair 294 can be built upon the A-type repair 282 and the B-type repair 292, such that the third series of filler holes (FIG. 6) can be arranged and spaced relative to the second series of filler holes (FIG. 6) in a predetermined force transmission pattern to transfer the fatigue crack failure region of the second composite from the second interface to the third interface. Furthermore, some of the third series of filler holes can be inclined transversely to the second series of filler holes and extend from the fourth layer through the third layer to the second layer to anchor the third interface to the fourth layer of the multilayer composite.

[0056] Figure 8A is a schematic top view 300 of a repaired wind turbine rotor blade root sleeve 140 (Figure 2) according to an embodiment of the present disclosure. Figure 8B is a schematic front view of the repaired wind turbine rotor blade root sleeve 140 (Figure 2) corresponding to the top view of Figure 8A. In general, the present disclosure provides a method for repairing fatigue cracks in a multilayer composite (such as root sleeve 140). Returning to Figure 8B, a first interface (282, Figure 7) exists between the first layer (284, Figure 7) and the second layer (286, Figure 7) of the multilayer composite (280, Figure 7). Fatigue cracks may occur in multiple failure regions located at the first interface 282. The fatigue cracks at the first interface 282 can be repaired using a crack repair adhesive fluid.

[0057] Furthermore, a second interface (288, Figure 7) may exist between the second layer (286, Figure 7) and the third layer (292, Figure 7) of the multilayer composite (280, Figure 7). Fatigue cracks may occur in multiple failure regions located at the second interface (288, Figure 7). Fatigue cracks at the second interface (288, Figure 7) can be repaired using a crack repair adhesive fluid. In effect, repair at the first interface (282, Figure 7) transfers the failure region from the first interface (282, Figure 7) to the second interface (288, Figure 7). Moreover, repair at the second interface (288, Figure 7) can reinforce the second interface and anchor the second interface (288, Figure 7) to the third layer (296, Figure 7) of the multilayer composite to prevent failure caused by fatigue cracks.

[0058] Returning to Figure 8A, the root sleeve can be inspected using non-destructive testing / torque testing methods to identify defective sleeves. Once a defective sleeve is identified, exemplary flushing holes 302 (for Type A repair) and 312 (for Type B repair), effluent holes 304 (for Type A repair) and 314 (for Type B repair), and filling holes 306 (for Type A repair) and 316 (for Type B repair) can be mapped and drilled using a pre-designed, optionally 3D-printed drill guide (also known as a "drill template," described in detail in the context of Figure 9) with reference to the root sleeve body. This ensures that the drill hole location corresponds to the center of the most likely defect location and the localized failure area. The drill template can slide across the root sleeve body and the laminate and can help map the location to be drilled. The drill template can also guide holes drilled perpendicular to the sleeve body. Once the holes are mapped and drilled, an exemplary zerk fitting 318 (Figure 8B) can be installed into the drilled holes. The drill bit used can typically be the appropriate size for a 318 grease nipple.

[0059] Exemplary drill holes 302, 304, 306, 312, 314, 316, etc., can be flushed with a modified high-pressure washer and cleaning fluid until the outflowing liquid appears clean. Optionally, compressed air can also be used to blow out any residual cleaning fluid in the cavities. The cleaning and flushing process can typically begin at the tip of the wind turbine blade and proceed to the root, as there may often be a solid laminate behind the tip to block the outflow of cleaning and flushing fluid.

[0060] As previously mentioned, the root sleeve may be coated with leaked hydraulic oil, which may require the application of a cleaning fluid such as mineral oil to remove the leaked hydraulic oil. In practice, the cleaning fluid can treat the applied surface and create a chemically active surface. During operation, flushing holes and seepage holes can be drilled near cracks and microcracks; after drilling, these holes and seepage holes can be flushed with the cleaning fluid. The cleaning fluid is typically volatile and non-flammable. Any remaining residual cleaning fluid may evaporate. In one embodiment, compressed air can be blown to promote the evaporation of any residual cleaning fluid.

[0061] The flushing holes and oozing holes are flushed with high-pressure flushing fluid, typically using a high-pressure washer, with the correctly oriented nozzles inserted into the flushing holes and oozing holes. As the flushing process continues, the contaminated dirty solution flows out of the oozing holes; flushing continues until the solution is clean. The flushing holes and oozing holes are then dried with a correctly oriented jet of compressed air into the corresponding holes. The flushing holes and oozing holes are then allowed to stand for a period of time to allow all cavities to dry completely and be free of flushing solution.

[0062] Alternatively, a vacuum pump can be used at the root side to remove the flushing and cleaning fluid. The vacuum generated by the pump can effectively boil away any remaining cleaning fluid in the cavity and dry it. However, vacuum has a limiting pressure of 14.7 psi, less than the thousands of psi pressure of a grease fitting. Vacuum suction is likely most effective when a complete seal is possible. Furthermore, if repairs are being performed down-tower, boreholes can be drilled along the sleeve in different directions. Other alternative methods may exist to generate flushing and cleaning pressure, such as using pressurized gas. Additionally, flushing and cleaning can be performed using threaded nozzles after the rotor blade bolts have been removed.

[0063] Returning to Figures 8A and 8B, structural adhesive can be injected from the tip to the root of the wind turbine blade to fill and clean the holes 302, 304, 306, 312, 314, and 316 after flushing and cleaning. In effect, the structural adhesive bonds the root sleeve to the corresponding roving, as explained in more detail below.

[0064] Sometimes, blade repair at the 3 o'clock or 9 o'clock position, where the load is neutral, may be more advantageous because these positions offer a favorable load distribution. Typically, non-destructive testing and inspection methods can be performed before and after repair to assess the range and condition of porosity and to evaluate how to adjust the repair work for better results.

[0065] In operation, a structural adhesive (also referred to as "adhesive") can be injected into the fractured laminate to repair the root sleeve of a wind turbine blade. As a non-limiting example, the adhesive used in various embodiments of the disclosed subject matter may be methyl methacrylate (MMA) 8120, which is commercially available. This adhesive is self-etching, compatible with fatigue conditions, and effective for both metals and composites. MMA 8120 has a low viscosity and can reach laminate cracks and microcracks under pressure without dilution. In addition to MMA 8120, other polymers such as epoxy resins, polyesters, vinyl esters, polyurethanes, and general-purpose polyurethanes can be used to treat and repair composite fatigue cracks. Furthermore, other polymer compounds commonly used for the repair of structural concrete can also be used to treat and repair composite fatigue cracks.

[0066] Structurally, using internal ridges to distribute the weight load of the suspension blades (whether at the commonly known 6 o'clock or 12 o'clock position) is more effective than relying solely on the shear strength of adhesives or resins. The operation is safe, simple, and uncomplicated. Holes can be drilled in the damaged laminate, and adhesive can be injected into the holes. In this way, by reinforcing mechanical connections that may have worn down over time, the damaged laminate can be restored to its original, or even better, condition. This is achieved by reinforcing shear strength not only by relying on the shear strength of adhesives and resins, but also by distributing tensile and compressive loads across the ridges within the root sleeve.

[0067] In the case of Type B repair (explained in more detail below), offset fill holes are drilled to repair defects between the glass and the pultruded part, and flushing may be impossible because cracks and microcracks may be too small for any cleaning fluid to penetrate. In the case of Type C repair (explained in more detail below), it can be done in a smaller cavity using the same structural approach. However, a change in materials may be necessary, using resin instead of adhesive.

[0068] Figure 9 is a schematic view of a drill template 320 for drilling exemplary Type A and Type B flushing holes, seepage holes, and filler holes in Figures 8A and 8B, according to an embodiment of the present disclosure. Once a defective sleeve is identified, the flushing hole can be mapped and drilled using the drill template 320, referenced to the root sleeve 140 (Figure 2) body. This ensures that the drill hole location corresponds to the center of the most likely defect location and localized failure area. The drill template 320 can be configured and arranged around the root sleeve 140 and define the drill hole locations corresponding to the localized stress centers that may lead to fatigue concentration and eventual failure of the root sleeve 140. The drill template 320 can slide over the root sleeve body and laminate, aiding in mapping the location to be drilled. The drill template 320 can also guide holes drilled perpendicular to the sleeve body. Once the hole is mapped and drilled, a grease fitting can be installed into the filler hole. The drill bit used can typically be appropriately sized relative to the grease fitting.

[0069] The pre-configured dimensions of the drill template 320, such as the lengths of the first series of holes 322 on the exemplary first root sleeve, the second series of holes 324 on the exemplary first root sleeve, the first series of holes 326 on the exemplary second root sleeve, and the second series of holes 328 on the exemplary second root sleeve (i.e., the longer of the two planar dimensions), the total length of the template 332 (i.e., the longer of the two planar dimensions), the total width of the template 334 (i.e., the shorter of the two planar dimensions), the distance 336 from the farthest hole to the baseline, the distance 338 from the exemplary intermediate hole to the baseline, the distance 342 from the nearest hole to the baseline, the distance 344 between two rows of holes corresponding to two different sleeves, the distance 346 between two rows of holes corresponding to one exemplary sleeve, the distance 348 from the first row of holes to the side line, and the distance 352 from the second row of holes to the side line, can be optimized through trial and error and / or parameterized according to the original dimensions of the sleeves. The drilling pattern, the distance between the filling holes, and the adhesive injection mechanism affect the performance and effectiveness of the repair. In the exemplary setup, four holes are drilled at 80mm intervals.

[0070] Figure 10A is a schematic top view 380 of an exemplary C-type repair of a damaged wind turbine rotor blade root sleeve according to an embodiment of the present disclosure. Figure 10B is a schematic front view of a C-type repair of a damaged wind turbine rotor blade root sleeve corresponding to the top view of Figure 10A. Referring to Figure 10B, a third interface (294, Figure 7) may exist between the third layer (292, Figure 7) and the fourth layer (296, Figure 7) of the multilayer composite (280, Figure 7). Fatigue cracks may occur in multiple failure areas located at the third interface 294. Fatigue cracks at the third interface 294 can be repaired with a crack repair fluid. A third series of flushing holes (also referred to as “C-type flushing holes”) 382 can be marked and drilled such that the flushing holes 382 extend from the outer surface of the composite to the third interface. A third series of seepage holes 384 can be marked and drilled such that the seepage holes 384 extend from the outer surface of the composite to the third interface.

[0071] Returning to Figures 10A and 10B, the root sleeve can be inspected using non-destructive testing / torque testing methods to identify defective sleeves. Once defective sleeves are identified, exemplary flushing holes 382 (for Type C repair), bleed holes 384 (for Type C repair), and fill holes 386 (for Type C repair) can be mapped and drilled using a pre-designed, optionally 3D-printed drill template 320 (Figure 9). Flushing holes 382 and bleed holes 384 can be flushed with high-pressure flushing fluid. Subsequently, flushing holes 382 and bleed holes 384 can be dried with air jets and allowed to stand for a period of time.

[0072] Filling holes 386 can be marked and drilled, extending from the outer surface of the composite to the third interface. Fluid injection ports can be installed in the third series of filling holes 386. Crack repair fluid (structural adhesive) can be injected through the injection ports until the filling holes 386 are completely filled with crack repair fluid. The crack repair fluid deposited in the third series of filling holes can be thermo-cured for a period of time.

[0073] In practice, the repair at the third and final interface 294 reinforces and anchors the third interface 294 to the fourth layer (glass laminate) 296 to prevent failure due to fatigue cracking. Specifically, some of the third series of filler holes can be arranged in an inclined direction transverse to the third interface, such that the inclined filler holes extend from the fourth layer to the third layer as anchoring supports. The drilled and filled holes 386 extend through the pultruded part into the metal sleeve and can structurally serve as anchors to reinforce the third interface. Adhesive material is dispensed into a grease gun using a glue gun.

[0074] Figure 11 illustrates a schematic method 400 for repairing a root sleeve 140 (Figure 2) installed in the root region of a wind turbine blade. The root sleeve 140 typically includes a multi-layered composite having fatigue cracks repaired at a first interface between a first and second layer, as shown in 402. Additional fatigue cracks may be repaired at a second interface between a second and third layer, as shown in 404. This repair effectively transfers the fatigue crack failure region of the composite from the first interface to the second interface, and the second interface is anchored to a third layer of the multi-layered composite, as shown in 406.

[0075] Figure 12 illustrates a schematic method 420 for repairing fatigue cracks at the first interface between the first and second layers of an exemplary root sleeve 140 (Figure 2). The fatigue cracks at the first interface can be repaired by drilling a series of flushing holes, seepage holes, and filling holes from the outer surface of the composite to the first interface, as shown in 422. The flushing holes and seepage holes can be flushed first with a high-pressure flushing fluid, as shown in 424, followed by air jet drying, as shown in 426. Furthermore, fluid injection ports can be installed in the filling holes, as shown in 428. Crack repair fluid can be injected through the injection ports and deposited in the filling holes, as shown in 432. The crack repair fluid deposited in the filling holes can be thermosetting, as shown in 434.

[0076] Figure 13 illustrates a schematic method 460 for repairing fatigue cracks at the second interface between the third and second layers of an exemplary root sleeve 140 (Figure 2). The fatigue cracks at the second interface can be repaired by drilling a second series of flushing holes, seepage holes, and filling holes from the outer surface of the composite to the second interface, as shown in Figure 462. The second series of flushing holes and seepage holes can be flushed first with a high-pressure flushing fluid, as shown in Figure 464, followed by air jet drying, as shown in Figure 466. Furthermore, fluid injection ports can be installed in the second series of filling holes, as shown in Figure 468. Crack repair fluid can be injected through the injection ports and deposited in the second series of filling holes, as shown in Figure 472. The crack repair fluid deposited in the second series of filling holes can be thermosetting, as shown in Figure 474.

[0077] The second series of filling holes can be arranged and spaced relative to the first series of filling holes in a predetermined force transmission pattern to transfer the fatigue crack failure region of the first composite from the first interface to the second interface. Furthermore, some of the second series of filling holes can be inclined transversely to the first series of filling holes and extend from the third layer through the second layer to the first layer to anchor the second interface to the third layer of the multilayer composite.

[0078] Wind turbine blades may include additional fatigue cracks repaired at the third interface between the third and fourth layers of a multilayer composite. This repair transfers the fatigue crack failure region of the composite from the second interface to the third interface, and the third interface is anchored to the fourth layer of the multilayer composite.

[0079] Figure 14 illustrates a schematic method 480 for repairing fatigue cracks at the third interface between the third and fourth layers of an exemplary root sleeve 140 (Figure 2). The fatigue cracks at the third interface can be repaired by drilling a third series of flushing holes, seepage holes, and filling holes from the outer surface of the composite to the first interface, as shown in Figure 482. The third series of flushing holes and seepage holes can be flushed first with a high-pressure flushing fluid, as shown in Figure 484, followed by air jet drying, as shown in Figure 486. Furthermore, fluid injection ports can be installed in the third series of filling holes, as shown in Figure 488. Crack repair fluid can be injected through the injection ports and deposited in the third series of filling holes, as shown in Figure 492. The crack repair fluid deposited in the third series of filling holes can be thermosetting, as shown in Figure 494.

[0080] The third series of filling holes can be arranged and spaced relative to the second series of filling holes in a predetermined force transmission pattern to transfer the fatigue crack failure region of the second composite from the second interface to the third interface. Furthermore, some of the third series of filling holes can be inclined transversely to the second series of filling holes and extend from the fourth layer through the third layer to the second layer to anchor the third interface to the fourth layer of the multilayer composite.

[0081] Figure 15 is a schematic view 500 of a root sleeve repair kit 502 for damaged wind turbine blades. The repair kit 502 may include a drill template 504 configured and arranged around a multi-layer composite. The drill template 504 may define the drilling locations for a series of flushing holes, seepage holes, and filler holes in the multi-layer composite. The repair kit 502 may also include a flushing gun 506 for flushing the flushing holes and seepage holes, and an air dryer 508 for drying the flushing holes and seepage holes. The air dryer 508 can provide a directional supply of dry compressed air by applying it to the cracked cavity through a mechanical nozzle that focuses the air onto the cavity, forcing solvents, cleaning fluids, and any contaminants that may affect the bonding ability in subsequent bonding steps to evaporate.

[0082] Repair kit 502 may also include a quantity of crack repair fluid 512 suitable for repairing cracks in multilayer composites, and an injection tool 514 configured and arranged to inject the crack repair fluid through a filling hole. The crack repair fluid 512 may be a self-etching adhesive capable of surface preparation of the composite material while providing potentially stronger chemical bonding than the original design. The injection tool 514 may be a drill bit designed to drill injection holes of specified dimensions without tearing the composite laminate and ensuring the holes are installed onto the blade surface at specified angles and spacing. Kit 502 may optionally include an instruction manual 516 describing the intended user's operating instructions for kit 502.

[0083] However, the above-described repair method is not limited to wind turbine blade root sleeves and can generally be applied to fatigue / laminate crack repair in any composite material. In operation, an adhesive can be injected under pressure and / or a predetermined volume to repair the cracked laminate and restore it to its nominal strength. The adhesive can be self-etching and acidic, enabling it to etch the application surface and enhance surface adhesion. First, multiple pores are prepared, and a surfactant is injected into the pores as a dispersant to flush out any residual contaminants in the cavities. Furthermore, the adhesive can be heated for rebonding. In one embodiment, a resin can be used instead of the adhesive to re-bond the laminate layers. A predetermined level of pressure can be applied to allow the adhesive to penetrate cracks and microcracks in the laminate and repair manufacturing defects.

[0084] The foregoing description sets forth numerous specific details, such as resource partitioning / sharing / copying embodiments, types and interrelationships of system components, and logical partitioning / integration choices, to provide a more thorough understanding. The embodiments disclosed herein can be practiced without these specific details. In other instances, control structures, logic implementations, opcodes, means of specifying operands, and complete sequences of software instructions are not shown in detail because those skilled in the art will be able to implement the described content without excessive experimentation using the included description.

[0085] The use of terms such as "an embodiment," "an exemplary embodiment," and "an exemplary model" in the specification indicates that the described embodiment may include a specific feature, structure, or characteristic, but each embodiment does not necessarily include that specific feature, structure, or characteristic. Furthermore, these phrases do not necessarily refer to the same embodiment. Additionally, when a specific feature, structure, and / or characteristic is described in connection with an embodiment, those skilled in the art will understand that combining it with other embodiments affects that feature, structure, and / or characteristic, whether explicitly described or not.

[0086] For example, a flowchart diagram may sometimes refer to a block diagram diagram, and vice versa. Whether explicitly described or not, alternative embodiments discussed in connection with a block diagram diagram also apply to discussions related to a flowchart diagram diagram, and vice versa. Furthermore, the scope of this specification includes embodiments for performing the flowchart, other than those discussed in connection with a block diagram, and vice versa.

[0087] The detailed description and claims may use the term "coupling" and its derivatives. "Coupling" is used to refer to two or more elements, whether they are in direct physical or electrical contact with each other, or whether they cooperate or interact with each other.

[0088] Although the flowcharts in the figures show a specific order of operations performed in some embodiments, this order is illustrative and not restrictive (e.g., alternative embodiments may perform operations in a different order, combine certain operations, perform certain operations in parallel, perform certain operations in overlapping order so that they are partially parallel, etc.).

[0089] While the above description includes several exemplary embodiments, the invention is not limited to the described embodiments and may be practiced with modifications and variations within the spirit and scope of the appended claims. This description is therefore illustrative and not restrictive.

Claims

1. A method for repairing fatigue cracks in a multilayer composite, the method comprising: Repair the fatigue crack at the first interface between the first layer and the second layer of the multilayer composite; Repair fatigue cracks at the second interface between the second layer and the third layer of the multilayer composite; and anchor the second interface to the third layer of the multilayer composite to prevent failure caused by fatigue cracks.

2. The method according to claim 1, wherein, Repairing fatigue cracks at the first interface between the first layer and the second layer of the multilayer composite includes: drilling a first series of flushing holes extending from the outer surface of the composite to the first interface; drilling a first series of seepage holes extending from the outer surface of the composite to the first interface; flushing the first series of flushing holes and the first series of seepage holes with high-pressure flushing fluid; drying the first series of flushing holes and the first series of seepage holes with air jet; drilling a first series of filling holes extending from the outer surface of the composite to the first interface; installing fluid injection ports in the first series of filling holes; injecting crack repair fluid through the injection ports to fill the first series of filling holes; and thermally curing the crack repair fluid deposited in the first series of filling holes.

3. The method according to claim 2, wherein, Repairing fatigue cracks at the second interface between the second layer and the third layer of the multilayer composite includes: drilling a second series of flushing holes extending from the outer surface of the composite to the second interface; drilling a second series of seepage holes extending from the outer surface of the composite to the second interface; flushing the second series of flushing holes and the second series of seepage holes with high-pressure flushing fluid; drying the second series of flushing holes and the second series of seepage holes with air jet; drilling a second series of filling holes extending from the outer surface of the composite to the second interface; installing fluid injection ports in the second series of filling holes; injecting crack repair fluid through the injection ports to fill the second series of filling holes; and thermally curing the crack repair fluid deposited in the second series of filling holes.

4. The method of claim 3, further comprising: The second series of filling holes are arranged and spaced relative to the first series of filling holes in a predetermined force transmission pattern.

5. The method according to claim 3, wherein, Anchoring the second interface to prevent failure due to fatigue cracking of the composite includes: tilting at least one of the second series of filling holes to be transverse to the first series of filling holes; and extending at least one of the tilted second series of filling holes from the third layer through the second layer to the first layer.

6. The method of claim 1, further comprising: Repair the fatigue crack at the third interface between the third layer and the fourth layer of the multilayer composite; And anchoring the third interface to the fourth layer of the multilayer composite to prevent failure caused by fatigue cracks.

7. The method according to claim 6, wherein, Repairing fatigue cracks at the third interface between the third layer and the fourth layer of the multilayer composite includes: drilling a third series of flushing holes extending from the outer surface of the composite to the third interface; drilling a third series of seepage holes extending from the outer surface of the composite to the third interface; flushing the third series of flushing holes and the third series of seepage holes with a high-pressure flushing fluid; drying the third series of flushing holes and the third series of seepage holes with an air jet; drilling a third series of filling holes extending from the outer surface of the composite to the third interface; installing fluid injection ports in the third series of filling holes; injecting crack repair fluid through the injection ports to fill the third series of filling holes; and thermally curing the crack repair fluid deposited in the third series of filling holes.

8. The method of claim 7, further comprising: The third series of filling holes are arranged and spaced relative to the second series of filling holes in a predetermined force transmission pattern.

9. The method according to claim 7, wherein, Anchoring the third interface to prevent failure due to fatigue cracking of the composite includes: tilting at least one of the third series of filling holes so that it is transverse to the second series of filling holes; and extending at least one of the tilted third series of filling holes from the fourth layer through the third layer to the second layer.

10. A wind turbine blade, comprising: The blade body includes a pressure side and a suction side joined at the leading and trailing edges, the blade body extending longitudinally from a root region through a transition region to a tip region, the transition region extending between the root region and the tip region; The blade body is mechanically connected to the rotor hub via a repaired root sleeve installed in the root region. The repaired root sleeve comprises a multilayer composite, the multilayer composite including a first plurality of fatigue cracks repaired at a first interface between a first layer and a second layer of the multilayer composite; and a second plurality of fatigue cracks repaired at a second interface between a second layer and a third layer of the multilayer composite, wherein the second interface is anchored to the third layer of the multilayer composite.

11. The wind turbine blade according to claim 10, wherein, The first plurality of fatigue cracks repaired at the first interface between the first layer and the second layer of the multilayer composite include: a first series of flushing holes drilled from the outer surface of the composite to the first interface, the first series of flushing holes being flushed with high-pressure flushing fluid and dried with air jet; a first series of seepage holes drilled from the outer surface of the composite to the first interface, the first series of seepage holes being flushed with high-pressure flushing fluid and dried with air jet; a first series of filling holes drilled from the outer surface of the composite to the first interface; a plurality of fluid injection ports installed in the first series of filling holes; and crack repair fluid injected through the injection ports and deposited in the first series of filling holes, the crack repair fluid being thermo-cured after being deposited in the first series of filling holes.

12. The wind turbine blade according to claim 10, wherein, The second plurality of fatigue cracks repaired at the second interface between the second layer and the third layer of the multilayer composite include: a second series of flushing holes drilled from the outer surface of the composite to the second interface, the second series of flushing holes being flushed with high-pressure flushing fluid and dried with air jet; a second series of seepage holes drilled from the outer surface of the composite to the second interface, the second series of seepage holes being flushed with high-pressure flushing fluid and dried with air jet; a second series of filling holes drilled from the outer surface of the composite to the second interface; a plurality of fluid injection ports installed in the second series of filling holes; and crack repair fluid injected through the injection ports and deposited in the second series of filling holes, the crack repair fluid being thermosetting after being deposited in the second series of filling holes.

13. The wind turbine blade according to claim 12, wherein, The second series of filling holes are arranged and spaced relative to the first series of filling holes in a predetermined force transmission pattern.

14. The wind turbine blade according to claim 12, wherein, At least one of the second series of filling holes is inclined transversely to the first series of filling holes and extends from the third layer through the second layer to the first layer to anchor the second interface to the third layer of the multilayer composite.

15. The wind turbine blade according to claim 10, further comprising: The third plurality of fatigue cracks are repaired at the third interface between the third layer and the fourth layer of the multilayer composite. The third interface is anchored to the fourth layer of the multilayer composite.

16. The wind turbine blade according to claim 15, wherein, The third plurality of fatigue cracks repaired at the third interface between the third layer and the fourth layer of the multilayer composite include: a third series of flushing holes drilled from the outer surface of the composite to the third interface, the third series of flushing holes being flushed with high-pressure flushing fluid and dried with air jet; a third series of seepage holes drilled from the outer surface of the composite to the third interface, the third series of seepage holes being flushed with high-pressure flushing fluid and dried with air jet; a third series of filling holes drilled from the outer surface of the composite to the third interface; a third plurality of fluid injection ports installed in the third series of filling holes; and crack repair fluid injected through the injection ports and deposited in the third series of filling holes, the crack repair fluid being thermosetting after being deposited in the third series of filling holes.

17. The wind turbine blade according to claim 16, wherein, The third series of filling holes are arranged and spaced relative to the second series of filling holes in a predetermined force transmission pattern.

18. The wind turbine blade according to claim 16, wherein, At least one of the third series of filling holes is inclined transversely to the second series of filling holes and extends from the fourth layer through the third layer to the second layer to anchor the third interface to the fourth layer of the multilayer composite.

19. A kit comprising: A drilling template configured and arranged around a multi-layer composite, the drilling template defining the drilling locations of a plurality of flushing holes, seepage holes and filling holes in the multi-layer composite; The amount of crack repair fluid suitable for repairing cracks in the multilayer composite; and an injection tool configured and arranged to inject the crack repair fluid through the filling orifice.

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