Nonlinear structural surface intensity: An application of contact acoustic nonlinearity to power flow based damage detection

The concept of structural surface intensity is extended to the nonlinear domain introducing a physical quantity denominated nonlinear structural surface intensity (NSSI). This quantity is formulated for mechanical systems whose dynamic response is governed by nonlinear harmonic frequencies due to contact nonlinearities. A specific experiment simulating a "loose joint" type of damage in a stiffened bolted aluminum panel was designed to validate the NSSI formulation. Damage was introduced in the form of loose bolts characterized by a contact nonlinear response. The nonlinear response resulted from separation and impact at closure of the joined structures due to active interrogation by an external dynamic force. The results prove that NSSI is a monotonic function of the damage size.