Magnetic pressure stressing of lap joints: Modeling and experimental verification

A novel electromagnetic stressing/optical detection method has been developed in response to the need for better nondestructive evaluation techniques for the detection of disbonds in aging aircraft lap joints. This technique uses magnetic pressure to pull the top surface of a thin conductive bonded sheet and senses the out-of-plane displacement of the surface with an optical lever fiber bundle detector. This method of inspection has the advantages of being noncontacting, relatively inexpensive, and because it pulls on the top surface, is a promising candidate for the detection of “kissing” disbonds—a condition in which there is no material missing from the joint, but the bond has failed. A series of three models was developed and implemented to simulate system performance from the driving circuitry to the measured response of the sample. Using a computer model of the driving circuit, component value variations could be analyzed to optimize the current through the electromagnetic coil as a function of time. An analytical pressure model was developed to predict pressure on the sample as a function of time for a given current waveform input. The predicted pressure was then used as the driving function in a finite element structural model which predicted displacement of the sample surface. Laboratory experiments were conducted on simple bonded and unbonded samples, and the two cases exhibited large differences in amplitude, resonant frequency, and damping. Test results compared favorably to the predicted displacement data. The close correspondence between measured and predicted results indicates that the models are useful not only in the system design but also as a means to predict performance.