The edges of the repair patch should be tapered and all plies should have rounded corners. The repair patch is attempting to replace the damaged area in the composite laminate exactly, restoring it as much as possible to original. Thus, the number of plies and orientations of each ply must match, layer for layer, that of the original structure.
So, in an exact ply-by-ply replacement, will the repaired structure be as strong as the original? No. Depending on many details, the repaired structure is typically about 60-80% as strong as the original undamaged structure.
Is it possible to make a repaired structure as strong as the original? Yes. However, extra repair plies must be added to compensate for the loss of strength caused by the repair. This means the repair will not be perfectly flush, and also that the repaired structure will be stiffer than the original.
Is the extra stiffness a problem? If the original structure is not stiffness critical but is primarily loaded in straight tension or compression, then a stiffer repair will most likely be fine. However, if the structure flexes significantly under load, the “stiff spots” caused by a full-strength repair can cause failure at the edges of the repair. Some repairs may therefore need to be deliberately under-strength, in order to match the stiffness of that original area in the structure.
Published courtesy of Abaris Training Resources, Inc