Stress Wave Measurements in an Electromagnetic Launcher

This paper discusses strain measurements that were conducted on the electromagnetic rail launcher at the Georgia Institute of Technology. The dynamic strain in the rails of the launcher was measured using fiber Bragg grating strain sensors with an interrogation system developed by micron optics. The fiber optic sensors are immune to the large pulsed electromagnetic fields in the bore of the launcher. The immunity of this system to electromagnetic interference allowed for dynamic strain measurements to be made during launch. These measurements show changes in the dynamic strain that are indicative of a critical velocity of the rails. These changes include wave radiation, wave reflection and strain amplification above the critical velocity. The magnitude of the stress waves are large enough for plastic deformation of the rails to occur. This plastic deformation could directly affect the lifetime of the rails in the electromagnetic launcher. The apparent critical velocity seen in these experiments is lower than the value from the Bernoulli-Euler beam theory. This is believed to be caused by a combination of the nonlinear load-deflection curve and effective mass of the containment. In addition to these phenomena, jerk effect waves and asymmetries between the two different rails were also observed.