A test method is described for measuring cohesive laws for interfaces in sandwich structures. It is proposed to increase the bending stiffness of the sandwich faces by adhering steel bars onto the sandwich faces. This stiffening reduces rotations and ensures that the method is applicable for thin face sandwich specimens. Crack growth along the face/core interface is obtained by a stiffened double cantilever beam specimen loaded with uneven bending moments (DCB-UBM). The J integral is employed and the opening of the pre-crack tip is measured using a commercial optical measurement system, from which mixed-mode cohesive laws are extracted.