The SCB geometry can't be used because the skins are too flexible.
Instead, a centre-notch flexure (CNF) test can be used. A hole is drilled above the lower skin and a bar is inserted. A force is applied to ends of bar and the crack extends towards the supports.
The P- δ curve is similar to the SCB and P, δ and ‘a' are measured. The equations used for the thick skinned sandwich structures are used to determine Gc.