Most composite materials have a coefficient of thermal expansion much lower than aluminum

Most composite materials have a coefficient of thermal expansion much lower than aluminum. When the aluminum, adhesive, and composite patch system are brought up to temperature, the aluminum will have expanded much more than the composite patch. After the cure time, the system is then cooled to room temperature. As of now, the aluminum and composite are coupled as one structure owing to the bond. This causes the specimen to warp and inducing bending stresses into the structure. Because of the tendency of the brittle adhesives to disbond around the damage area, ductile adhesives such as FM-73 are preferred over brittle adhesives such as FM-400 which is reducing the load transfer to the repair patch.