After completing initial damage removal, the area around the repair must be prepared. The corners of the repair hole must be rounded off and the hole itself should be tapered to provide the best load transfer when the repair patch is bonded in. Scarfing, or taper sanding, is usually achieved using a compressed-air powered high-speed grinder. This is a gentle process, which prepares the damaged area for application of a repair patch. It is imperative to follow all repair manual guidelines, and significant skill and practice on the part of the repair technician is mandatory.
Note: If the damaged area exceeds allowable repair limits in applicable repair manuals, then specific engineering support is required in order to proceed with the repair.
For scarfing, it is important to know the number of plies in the composite laminate. The ply orientations are not needed for preparing the damaged area but will be needed for cutting repair materials and fabricating the repair patch.
A crude rule of thumb for the amount of material to remove during scarfing is to taper sand approximately 1/2 inch (12.5mm) of area per ply of composite laminate. Typical scarf distances are from 20 to 120 times the thickness of the laminate being scarfed.
The material removed during scarfing is often referred to by angles, rather than distance per ply. The flatter the scarf (more area per ply), the larger the adhesive bond, and the lower the load on the bond. The steeper the scarf, the less undamaged material is removed. Lightly loaded structures may be able to tolerate a smaller, steeper scarf. Heavily loaded structures usually require a larger, more gentle scarf, such as 20:1 up to as much as 100:1.
Scarfing vs. Stepping
Stepping is an alternate method for removing material in preparation for applying a repair patch. In stepping, the overall angle is achieved by removing a precise area of material per ply of composite laminate. Both methods work. Most consider scarfing to be easier, and it is generally considered to be better. Stepping leaves abrupt edges and butt joints in each repaired ply. It is also hard to do without cutting through and damaging the underlying plies.
Published courtesy of Abaris Training Resources, Inc