Make Sure that Patch is Reliable
Aircraft, both commercial and military fleets, see decades of service. The stress and strain from thousands of take-offs and landings, as well as in-flight turbulence, all contribute to component fatigue. Every mechanical component has a finite fatigue life. Aircraft skin is just as susceptible to cracking as the airframe itself. It is absolutely critical to make proper repairs and ensure those repairs are at least as strong as the original structure.
For years cracks in aircraft skin required, in many cases, complete replacement. In recent years a material referred to as GLARE has become the material of choice for repairing aircraft skin. GLARE (glass laminate aluminum reinforced epoxy) is composed of several very thin layers of metal (usually aluminum) with alternating layers of glass-fiber/epoxy laminate. GLARE is applied as a patch over the crack and adds a parallel load-carrying path. It has been proven to arrest cracks, therefore eliminating the need for skin replacement.
Extensive strain gage testing has been performed on thousands of sample coupons, as well as on the repairs themselves. The procedures for gage selection and application are similar for both composite and aluminum specimens. Gage selection follows the same process as when picking a gage for composite testing. The active gage length should be sufficient to span 3-5 repetitions of the composite fiber structure. In most cases, an active grid length of 6 mm (0.25 in) is sufficient. Since the top surface is normally aluminum, then surface preparation and gage bonding techniques are the same as for any aluminum substrate.
Below are examples of 0.250 in gages that are commonly used for this application:
Linear gage: http://www.vishaypg.com/docs/11312/250uw.pdf;
Rectangular rosette: http://www.vishaypg.com/docs/11299/250ur.pdf.