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Solenoids, relays, and other magnetically activated devices are frequently operated in modes that permit repeated impacting of an armature against a core. Such operation not only produces wear of the contacting faces but results in stresses that can change the magnetic behavior of the materials employed. The magnitude of this change was evaluated for typical core operation of grain-oriented silicon iron, 2 V-Permendur and Hipernik alloys. Hysteresis loop properties were measured at intervals during the operating sequence. Pronounced deterioration of structure-sensitive properties (permeability, residual induction, and coercive force) was observed in silicon iron and Hipernik with relatively little change noted in the 2 V-Permendur. The changes appear to be clearly related to the cumulative microdamage within the metal due to impact fatigue effects. The results indicate that significant property deterioration does result from repeated impacting and this may present a design limitation on the lifetime of devices having critical magnetic demands.