The causes of armature surface fracturing within helical flux-compression generators

Aluminum and copper tubes filled with explosive were tested during this study of high strain rate effects, as an adjunct to helical flux-compression generator research at the University of Missouri-Rolla, directly affecting the understanding of flux cutoff and high strain-rate changes in generator armatures. Longitudinal cracks characteristically developed in the outer surface of armatures at a smaller expansion ratio than predicted. These cracks occurred within two diameters of the detonator end of the armature but did not extend when the tubing expanded under explosive pressurization. Such cracks appear to cause magnetic flux cutoff, and flux losses seriously affect energy conversion efficiency. Energy, timing, and structural analyzes showed that detonation pressurization was not the cause of fracturing. A two-dimensional Lagrangian finite-difference numerical model was used to analyze the effect of detonation waves on the armature, and demonstrated that the cracking resulted from the stress field caused by the waves. Compressive detonation waves cause both compressive and tensile regions in armatures. This complex stress field causes low-cycle metal fatigue, affecting how the tube fractures when it is impulsively loaded by high-pressure detonation gases. Isolation of shock wave effects during operation is demonstrated in the paper, allowing for more efficient generators in practice.