Composite repairs, such as co-adhesive tapered scarf repairs, are designed to fully restore structural loads so that the components can function properly #troubleshooter #layup #repair
Figure 1. A hand-processed scarf passes through three layers of CF unidirectional (UD) tape and a layer of CF plain weave fabric, each layer is 0.5 inches (nominal) apart. Note the small break at the innermost edge of the cone. Image source: Abaris training resources
The goal of any composite repair is to restore structural loads through or around the damaged area in a manner sufficient to allow the overall composite structure to operate under maximum load without failure. Repair methods include the use of pre-cured (or titanium) bolted multipliers, co-adhesive multipliers, co-adhesive ladder repair and co-adhesive tapered scarf repair. This month, we will focus on co-bonding tapered scarf repair, which is the most effective way to repair thin (<0.2 inch thick) laminate and sandwich structure skins-providing a nearly flush surface on the part. This is ideal for restoring aerodynamics and decorative surfaces.
Due to the uniform shear characteristics of the through joints, tapered ring joints have long been considered the preferred method for repairing composite laminates. It works by first cleaning the area with an approved, uncontaminated solvent, and then removing the damage from the structure. The damage is removed by grinding several layers to remove gouges, or by laminates or, in the case of sandwich structures, by removing the damage from the skin and core. This is usually done using a combination of hole saws, routers or other tools. After the damage is removed, a 90º grinder/abrasive mandrel, jitterbug sander, orbital sander or possibly an automatic ultrasonic five-axis milling machine can be used to machine the scarf around the removed damaged area in preparation for repair. After machining, if applicable, replace the core and prepare the surface for bonding. A stack of adhesive layers and a separate layer of the same (or alternative) material are installed to match the original axial direction of the structure (Figure 1). If applicable, then cure the patch in the vacuum bag.
The scarf angle label is usually expressed as the distance to thickness ratio (d:t) between 20:1 and 120:1, or simply defines the equivalent "per layer" distance (that is, using a nominal layer thickness of 0.01 inches, At a ratio of 50:1, each layer will get 0.01 x 50 = 0.5 inches. For a four-layer laminate, the scarf distance will be 4 x 0.5 = 2.0 inches). Please note that as the scarf distance increases, more of the original structure is removed, further weakening the parent structure. If you can reach the far side of the laminate, it is best to use a double-sided scarf (Figure 2). An engineering repair analysis may be required to determine the slope ratio required for a particular application.
Figure 2. Double-sided scarf repair can be used to minimize the amount removed from the original structure. Image source: Abaris training resources
Figure 3. The technician uses the trailing edge of the abrasive mandrel as the machining surface to "feather" the taper angle from the inner edge of the scarf to the desired size. Image source: Abaris training resources
Machining with a hand-held grinder can be tricky, whether it's a 90º die grinder with a 2-inch or 3-inch grinding spindle, a jitterbug sander, or a larger orbital sander that might be used for blade repairs. In any case, technicians need a lot of practice to master this skill. To be successful, technicians need to learn to "feather" with the trailing edge of a grinder instead of using the leading edge like grinding metal. The goal is to have a uniform and smooth tapered surface, without dents or gouges, proportional to the contour of the part (flat or contoured surface contour) (Figure 3).
The innermost edge of the taper should always be a sharp point without leaving any steps. Even a small step in this position will affect the performance of the scarf joint, which should be reduced by careful processing or preferably by hand sanding to a fine point. A small number of breakthroughs at the edges are acceptable, as shown in Figures 1 and 2. 1 and 3.
When repairing the sandwich structure, it is necessary to perform the core replacement procedure before making the patch. If this is the case, just before preparing the surface to bond the repair patch, glue the slightly thicker core plug in place and machine it flush to the desired contour, as shown in Figure 4. After sanding, use detergent to clean the area. Purchase a vacuum cleaner and approved dry wipes as needed.
Figure 4. Honeycomb core plugs of the same density are glued in place with a core joint adhesive, cured, and then machined or sanded to match the final contour. Image source: Abaris training resources
The first step is to make a template to lay out and cut the repair layer, additional layers and adhesive (if applicable) to the specified size and shape, while matching the orientation of the parent layer. If the repair is round, you can use a pair of ordinary dividers with ultra-fine pen points to arrange a group of concentric circles, and mark the main (or warp) fiber direction layer for each concentric circle on the back film or the applied release film . If a film adhesive is used, it should be cut approximately 0.13 inches beyond the outermost layer to provide rounded corners.
After cutting the layers, reactivate the bonding surface of the mother laminate and add 0.5 inches of ultra-fine ScotchBrite wear around the outermost layer, and use an approved dry rag to remove all remaining dust. (Alternatively, treat the dry-erase surface with blown ion plasma to further increase the surface free energy and promote the wetting of the adhesive.)
Figure 5. Each repair layer is located directly above the adjacent layer in the structure, matching the fiber axial direction. Additional layers (if needed) are placed on top of the repair layer, overlapping the original surface, and oriented to match the outermost repair layer. Image source: Greg Kress and Abaris training resources
Carefully arrange a set of target and direction information on the part (or adhesive layer) using approved contrasting color markings to correctly position each layer. Apply an adhesive layer and subsequent repair layers so that each repair layer overlaps the previous layer, and the specified distance is determined by the scarf angle ratio conversion (Figure 5).
After the repair is completed, apply peeling layer, perforated film, fine glass leakage layer, solid film, backing plate, thermocouple, thermal blanket, breathable layer and vacuum bag (if applicable) for treatment. The patch is cured according to the time/temperature formula specified for the cured resin. As shown in Figure 6, aluminum or composite backing plates can provide better compaction and a nearly flush repair surface.
Figure 6. The backing plate is used to achieve uniform compaction of the repaired area, thereby forming a highly compacted patch and a nearly flush surface. Image source: Greg Kress and Abaris training resources
After the patch is cured, the area can be prepared for painting as needed. Do not sand the structural fibers during this process. If this is a problem, an extra layer of fine glass fabric or adhesive can be added to the previous repair ply.
Figure 7. Classic laminate theory states that composite laminates are orthotropic (usually anisotropic), a homogeneous material composed of multiple layers laminated together, rather than all layers being on top of each other On the "formation flight". Image source: Greg Kress and Abaris training resources
Over the years, the industry has told us that each layer in the repair patch transfers the axial load at the edge to the adjacent layer in the original structure. According to Abaris’ senior engineering lecturer Greg Kress, “I do not support the concept of transferring the load to each individual repair layer through each layer in the original laminate through its separate overlap.” Instead, he said that once repaired, "The laminate acts as a uniform thickness to carry and transfer the load, not as a separate layer to transfer the load" (Figure 7). This makes a lot of sense to me, it just proves that there is still a lot to learn in the field of composite repair.
Author's note: Special thanks to Greg Kress for his contribution to this month's column.
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